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Effect of caffeine on human health

Authors:

Abstract

Caffeine is probably the most frequently ingested pharmacologically active substance in the world. It is found in common beverages (coffee, tea, soft drinks), in products containing cocoa or chocolate, and in medications. Because of its wide consumption at different levels by most segments of the population, the public and the scientific community have expressed interest in the potential for caffeine to produce adverse effects on human health. The possibility that caffeine ingestion adversely affects human health was investigated based on reviews of (primarily) published human studies obtained through a comprehensive literature search. Based on the data reviewed, it is concluded that for the healthy adult population, moderate daily caffeine intake at a dose level up to 400 mg day(-1) (equivalent to 6 mg kg(-1) body weight day(-1) in a 65-kg person) is not associated with adverse effects such as general toxicity, cardiovascular effects, effects on bone status and calcium balance (with consumption of adequate calcium), changes in adult behaviour, increased incidence of cancer and effects on male fertility. The data also show that reproductive-aged women and children are 'at risk' subgroups who may require specific advice on moderating their caffeine intake. Based on available evidence, it is suggested that reproductive-aged women should consume </=300 mg caffeine per day (equivalent to 4.6 mg kg(-1) bw day(-1) for a 65-kg person) while children should consume </=2.5 mg kg(-1) bw day(-1).
Effects of caffeine on human health
P. Nawrot*, S. Jordan, J. Eastwood, J. Rotstein, A.
Hugenholtz and M. Feeley
Toxicological Evaluation Section, Chemical Health Hazard Assess-
ment Division, Bureau of Chemical Safety, Food Directorate,
Health Canada, Tunney’s Pasture, PL 2204D1, Ottawa, Ontario,
Canada K1A 0L2
(Received 19 November 2001; revised 17 June 2002; accepted
18 June 2002)
Caffeine is probably the most frequently ingested
pharmacologically active substance in the world. It is
found in common beverages (coffee, tea, soft drinks), in
products containing cocoa or chocolate, and in medica-
tions. Because of its wide consumption at different
levels by most segments of the population, the public
and the scientific community have expressed interest in
the potential for caffeine to produce adverse effects on
human health. The possibility that caffeine ingestion
adversely affects human health was investigated based
on reviews of (primarily) published human studies
obtained through a comprehensive literature search.
Based on the data reviewed, it is concluded that for
the healthy adult population, moderate daily caffeine
intake at a dose level up to 400 mg day
1
(equivalent to
6mgkg
1
body weight day
1
in a 65-kg person) is not
associated with adverse effects such as general toxicity,
cardiovascular effects, effects on bone status and
calcium balance (with consumption of adequate cal-
cium), changes in adult behaviour, increased incidence
of cancer and effects on male fertility. The data also
show that reproductive-aged women and children are ‘at
risk’ subgroups who may require specific advice on
moderating their caffeine intake. Based on available
evidence, it is suggested that reproductive-aged women
should consume 4 300 mg caffeine per day (equivalent
to 4.6 mg kg
1
bw day
1
for a 65-kg person) while
children should consume 4 2.5 mg kg
1
bw day
1
.
Keywords : behaviour, bone, caffeine, calcium bal-
ance, cardiovascular effects, children, coffee, congeni-
tal malformations, development, fertility, foetal
growth, pregnancy, spontaneous abortion, tea
Introduction
Caffeine (1,3,7-trimethylxanthine) is a natural alka-
loid found in coffee beans, tea leaves, cocoa beans,
cola nuts and other plants. It is probably the most
frequently ingested pharmacologically active sub-
stance in the world, found in common beverages
(coffee, tea, soft drinks), products containing cocoa
or chocolate, and medications, including headache or
pain remedies and over-the-counter stimulants
(Murphy and Benjamin 1981, IARC 1991b, Dlugosz
and Bracken 1992, Carrillo and Benitez 1996).
The possibility that caffeine consumption can have
adverse effects on human health was assessed based
on the results of (primarily) published human studies
obtained through a comprehensive literature search.
The results of this assessment are summarized here.
Sources and prevalence of caffeine consumption
In North America, coffee (60–75%) and tea (15–30%)
are the major sources of caffeine in the adult diet,
whereas caffeinated soft drinks and chocolate are the
major sources of caffeine in the diet of children.
Coffee is also the primary source of caffeine in the
diet of adults in some European countries, such as
Finland, Sweden, Denmark and Switzerland. Brewed
coffee contains the most caffeine (56–100 mg/100 ml),
followed by instant coffee and tea (20–73 mg/100 ml)
and cola (9–19 mg/100 ml). Cocoa and chocolate
products are also important sources of caffeine (e.g.
5–20 mg/100 g in chocolate candy), as are a wide
variety of both prescription (30–100 mg/tablet or
capsule) and non-prescription (15–200 mg/tablet or
capsule) drugs (Dlugosz and Bracken 1992, Barone
and Roberts 1996, Shils et al. 1999, Tanda and
Goldberg 2000).
Food Additives and Contaminants, 2003, Vol. 20, No. 1, 130
*
To whom correspondence should be addressed. e-mail: peter_nawrot@
hc-sc.gc.ca
Food Additives and Contaminants ISSN 0265–203X print/ISSN 1464–5122 online # 2003 Taylor & Francis Ltd
http://www.tandf.co.uk/journals
DOI: 10.1080/0265203021000007840
In Canada, published values for the average daily
intake of caffeine from all sources is about
2.4 mg kg
1
body weight (bw) for adults and
1.1 mg kg
1
bw for children 5–18 years old (Chou
1992). Recently, Brown et al. (2001) reported daily
caffeine intakes ranging from 288 to 426 mg (equiva-
lent to 4.5–6.5 mg kg
1
bw in a 65-kg person) in the
adult population (481 men and women aged 30–75
years) residing in southern Ontario, Canada.
Elsewhere, mean daily caffeine intake for adults
among the general population has been given as
approximately 3 mg kg
1
bw in the USA, 4 mg kg
1
bw in the UK and 7 mg kg
1
bw in Denmark. For
high-level consumers, daily intakes range from 5 to
15 mg kg
1
bw. For children, daily caffeine intakes
have been given as 1 mg kg
1
bw in the USA,
<3mgkg
1
bw in the UK and <2.5 mg kg
1
bw in
Denmark (IARC 1991b, Ellison et al. 1995, Barone
and Roberts 1996, Hughes and Oliveto 1997).
Note that the caffeine content of coffee and tea is
dependent on their method of preparation and the
product brand. In addition, variations in caffeine
intake can occur due to differences in the size of the
serving ‘cup’ (Stavric et al. 1988). The impact of these
variations should be considered in the interpretation
and comparison of clinical studies, particularly when
cultural differences may be involved.
Pharmacokinetics
Following ingestion, caffeine is rapidly and essentially
completely absorbed from the gastrointestinal tract
into the bloodstream. Maximum caffeine concentra-
tions in blood are reached within 1–1.5 h following
ingestion. Absorbed caffeine is readily distributed
throughout the entire body. It passes across the
blood–brain barrier, through the placenta into am-
niotic fluid and the foetus, and into breast milk.
Caffeine has also been detected in semen (Berger
1988, Arnaud 1999).
The liver is the primary site of caffeine metabolism
(Stavric and Gilbert 1990, Arnaud 1999). In adults,
caffeine is virtually completely metabolized to 1-
methylxanthine and 1-methyluric acid from the para-
xanthine intermediate. Only 1–5% of ingested caf-
feine is recovered unchanged in the urine. Infants up
to the age of 8–9 months have a greatly reduced
ability to metabolize caffeine, excreting about 85%
of the administered caffeine in the urine unchanged
(Nolen 1989, Stavric and Gilbert 1990).
The elimination half-life of caffeine ranges between 3
and 7 h and can be influenced by many factors,
including sex, age, use of oral contraceptives, preg-
nancy and smoking. Caffeine’s half-life has been
reported to be 20–30% shorter in females than in
males. The half-life in newborns ranges from 50 to
100 h, but it gradually approaches that of an adult by
6 months of age. The half-life in females using oral
contraceptive steroids is approximately twice that
observed for ovulatory females. During pregnancy,
the metabolic half-life increases steadily from 4 h
during the first trimester to 18 h during the third
trimester. Cigarette smoking is associated with about
a twofold increase in the rate at which caffeine is
eliminated (Aranda et al. 1979, Dalvi 1986, Gilbert
et al. 1986, Stavric and Gilbert 1990, James 1991a,
Dlugosz and Bracken 1992, Eskenazi 1993, Hinds
et al. 1996, Arnaud 1999, Karen 2000).
General toxicity
Death due to excessive caffeine ingestion is not com-
mon, and only a few cases have been reported in the
literature. The acute lethal dose in adult humans has
been estimated to be 10 g/person. Death has been
reported after ingestion of 6.5 g caffeine, but survival
of a patient who allegedly ingested 24 g caffeine was
also reported (Stavric 1988, James 1991b).
Caffeine toxicity in adults can present a spectrum of
clinical symptoms, ranging from nervousness, irrit-
ability and insomnia to sensory disturbances, diuresis,
arrhythmia, tachycardia, elevated respiration and
gastrointestinal disturbances. Caffeine toxicity in chil-
dren is manifested by severe emesis, tachycardia,
central nervous system agitation and diuresis.
Chronic exposure to caffeine has been implicated in
a range of dysfunctions involving the gastrointestinal
system, liver, renal system and musculature (Stavric
1988, James 1991b).
The most important mechanism of action of caffeine
is the antagonism of adenosine receptors. Adenosine
is a locally released purine which acts on different
receptors that can increase or decrease cellular con-
centrations of cyclic adenosine monophosphate
(cAMP). Caffeine selectively blocks adenosine recep-
tors and competitively inhibits the action of adeno-
2 P. Nawrot et al.
sine at concentrations found in people consuming
caffeine from dietary sources. Caffeine results in the
release of norepinephrine, dopamine and serotonin in
the brain and the increase of circulating catechola-
mines, consistent with reversal of the inhibitory effect
of adenosine (Benowitz 1990).
It is now widely believed that habitual daily use of
caffeine >500–600 mg (four to seven cups of coffee or
seven to nine cups of tea) represents a significant
health risk and may therefore be regarded as ‘abuse’.
Sustained abuse may in turn result in ‘caffeinism’,
which refers to a syndrome characterized by a range
of adverse reactions such as restlessness, anxiety,
irritability, agitation, muscle tremor, insomnia, head-
ache, diuresis, sensory disturbances (e.g. tinnitus),
cardiovascular symptoms (e.g. tachycardia, arrhyth-
mia) and gastrointestinal complaints (e.g. nausea,
vomiting, diarrhoea) (James and Paull 1985).
Excessive caffeine intake (>400 mg day
1
) may in-
crease the risk of detrusor instability (unstable blad-
der) development in women. For women with pre-
existing bladder symptoms, even moderate caffeine
intake (200–400 mg day
1
) may result in an increased
risk for detrusor instability (Arya et al. 2000).
Cardiovascular effects
Clinical studies have investigated the effects of caf-
feine or coffee on cardiac arrhythmia, heart rate,
serum cholesterol and blood pressure. Epidemi-
ological studies have largely focused on the associ-
ation between coffee intake and cardiovascular risk
factors, including blood pressure and serum choles-
terol levels, or the incidence of cardiovascular disease
itself.
Clinical studies have shown that single doses of
caffeine <450 mg do not increase the frequency or
severity of cardiac arrhythmia in healthy persons,
patients with ischaemic heart disease or those with
serious ventricular ectopia (Myers 1998). Studies
conducted in healthy or hypertensive subjects suggest
that when a change in heart rate is observed, it is
typically a decrease at doses >150 mg/person (James
1991c, Green et al. 1996, Myers 1998). The rapid
development of tolerance to the heart rate effect of
caffeine (Green et al. 1996) complicates data inter-
pretation. The generally modest decrease in heart rate
is likely not clinically relevant (Myers 1998).
Several clinical and epidemiological studies have
suggested that coffee consumption is associated with
significant increases in total and low-density lipo-
protein cholesterol levels. Recent studies, however,
suggest that it is not the caffeine in coffee that is
responsible for its hypercholesterolaemic effect
(Thelle et al. 1987, James 1991c, d, Thelle 1993,
1995, Gardner et al. 1998). Two diterpenoid alcohols,
cafestol and kahweol, found at significant levels in
boiled coffee have been identified as hypercholester-
olaemic components. Although these components are
largely trapped by the use of a paper filter in coffee
preparation, there is some evidence that consumption
of filtered coffee is associated with small increases in
serum cholesterol levels (Thelle 1995).
The effect of caffeine on blood pressure in habitual
caffeine consumers and abstainers has been investi-
gated in more than 50 acute and 19 repeated-dose
clinical trials with healthy or hypertensive subjects
(reviewed by Myers 1988, 1998, James 1991c, Green
et al. 1996). The results of the acute studies indicate
that caffeine induces an increase in systolic (5–15
mmHg) and/or diastolic (5–10 mmHg) blood pres-
sure, most consistently at doses >250 mg/person, in
adults of both sexes, irrespective of age, race, blood
pressure status, or habitual caffeine intake. The effect
is most pronounced in elderly, hypertensive or caf-
feine-naive individuals. The pressor effect of caffeine
was also observed in many of the repeated-dose
studies, but not as consistently as in the acute studies.
It is generally agreed that tolerance to these pressor
effects develops within 1–3 days, but is partially lost
after abstinence for as little as 12 h. The clinical
significance of caffeine’s pressor effects and the devel-
opment of tolerance continues to be discussed in the
literature (James 1991c, Green et al. 1996, Myers
1998).
Epidemiological studies investigating associations be-
tween caffeine and blood pressure (reviewed by Myers
1988, 1998, James 1991c, 1997, Green et al. 1996)
have yielded conflicting results (i.e. positive, negative
or no association). These inconsistencies may reflect
methodological problems, including misclassification
resulting from the use of dietary recall data, tolerance
to the pressor effects of caffeine and the effect of
smoking on the plasma half-life of caffeine. While
James (1991c, 1997) and Green et al. (1996) indicated
that further research was needed, Myers (1998) con-
cluded that there was no epidemiological evidence to
support any relationship between caffeine use and
blood pressure.
3Effects of caffeine on human health
Epidemiological studies addressing the possible asso-
ciation between consumption of caffeine-containing
beverages, usually coffee, and coronary heart disease
include case-control, longitudinal cohort and pro-
spective studies (reviewed by James 1991d, Lynn
and Kissinger 1992, Myers and Basinski 1992,
Franceschi 1993, Thelle 1995, Myers 1998); meta-
analyses of case-control and/or prospective study
data were published by Greenland (1987, 1993) and
Kawachi et al. (1994); and a recent case-control was
published by Palmer et al. (1995) and two recent
prospective studies were published by Stensvold and
Tverdal (1995) and Hart and Smith (1997). Most
relied on self-administered questionnaires to deter-
mine intakes of caffeinated beverages. Cardiovascular
disease was assessed by a variety of outcome vari-
ables, including death from myocardial infarction or
coronary heart disease, non-fatal myocardial infarc-
tion or coronary event, angina pectoris and/or hospi-
talization for coronary heart disease. The results of
both case-control and prospective epidemiological
studies yielded inconsistent results, although case-
control studies were more likely to show a significant
relationship between coffee consumption and cardio-
vascular disease, with an increased risk generally
observed at intakes of five or more cups of coffee
per day ( 5 500 mg caffeine day
1
). Longitudinal
cohort studies published from 1986 yielded more
consistent positive associations than those published
up to 1981 (Greenland 1993). The inconsistencies
both within and between case-control and prospective
studies have resulted in controversies regarding study
methodologies and data interpretation (James 1991d,
Myers and Basinski 1992, Franceschi 1993,
Greenland 1993, Myers 1998). While recognizing the
ambiguity of the epidemiological data, Greenland
(1993) and Franceschi (1993) concluded that the
possibility of heavy coffee consumption (defined as
10 or more cups per day by Greenland 1993; probably
four or more cups per day in Franceschi 1993)
adversely affecting the incidence of coronary heart
disease or mortality cannot be ruled out.
None of the epidemiological data determine whether
it is caffeine per se or other components of coffee that
are responsible for coffee’s association with cardio-
vascular disease. Although no significant association
has been found between tea consumption and cardi-
ovascular disease (Franceschi 1993, Thelle 1995,
Myers 1998), it has been suggested that the beneficial
effects of the flavonoids present in tea may offset any
adverse effect of caffeine (Thelle 1995). Support for
the idea that caffeine in coffee is not responsible for
cardiovascular effects comes from epidemiological
studies showing an increased risk of coronary events
with consumption of decaffeinated coffee (Grobbee
et al. 1990, Gartside and Glueck 1993).
In summary, the data currently available indicate that
moderate caffeine intake (four or fewer cups of coffee
per day, or 4 400 mg caffeine day
1
) does not ad-
versely affect cardiovascular health. There are insuffi-
cient epidemiological data to draw any conclusions
about the risk for coronary heart disease or mortality
associated with consumption of 10 or more cups of
coffee per day ( 5 1000 mg caffeine day
1
).
Effects on bone and calcium balance
The database on caffeine’s potential to adversely
influence bone metabolism includes epidemiological
studies investigating the relationship between caffeine
and/or coffee intake and the risk of osteoporosis as
characterized by low bone mineral density and in-
creased susceptibility to fractures, as well as metabolic
studies examining the effect of caffeine on calcium
homeostasis.
Caffeine intake of 150–300 mg after a 10-h fast in-
creased urinary calcium excretion 2–3 h after expo-
sure in adolescent men and women (Massey and
Hollingbery 1988), women 22–30 years of age
(Massey and Wise 1984, Massey and Opryszek
1990), men 21–42 years of age (Massey and Berg
1985), and women 31–78 years of age consuming
5 200 mg caffeine day
1
(Bergman et al. 1990).
Tolerance to the renal effects of caffeine does not
develop, as habitual coffee intake had no effect on the
increase in calcium excretion associated with an acute
caffeine dose (Massey and Opryszek 1990). Caffeine-
induced hypercalciuria was not affected by oestrogen
status (Bergman et al. 1990), gender or age (Massey
and Wise 1992). Barger-Lux et al. (1990) reported
that caffeine intakes of 400 mg person
1
day
1
for 19
days led to evidence of altered bone remodelling in
healthy premenopausal women between the ages of 35
and 44, but had no effect on fractional calcium
absorption, endogenous faecal calcium or urinary
calcium excretion. An earlier study in the same
population suggested that caffeine consumption of
175 mg person
1
day
1
was positively associated with
increased 24-h urinary calcium excretion (Heaney and
Recker 1982).
4 P. Nawrot et al.
Whether it is through increased urinary calcium ex-
cretion (Massey and Whiting 1993) or decreased
intestinal calcium absorption (Heaney 1998), caffeine
does appear to have a negative effect on calcium
balance (Hasling et al. 1992, Barger-Lux and
Heaney 1995). Barger-Lux et al. (1990) concluded
that a daily intake of 400 mg caffeine by healthy
premenopausal women with a calcium intake of at
least 600 mg day
1
has no appreciable effect on cal-
cium excretion. Hasling et al. (1992) derived a model
from data collected from postmenopausal women
that indicated coffee intakes >1000 ml day
1
(760 mg
caffeine day
1
) could induce excess calcium loss, while
intakes of 150–300 ml coffee day
1
(112–224 mg caf-
feine day
1
) would have little impact on calcium
balance. The biological significance of caffeine’s ne-
gative effect on calcium balance has been debated
(Barger-Lux et al. 1990, Massey and Whiting 1993).
Several epidemiological studies have been conducted
to assess the relationship between caffeine intake and
bone density. Increasing caffeine intakes were not
associated with significant decreases in bone density
in adolescent women (Lloyd et al. 1998), young
women 20–30 years of age (Eliel et al. 1983,
McCulloch et al. 1990, Packer and Recker 1996,
Conlisk and Galuska 2000), premenopausal women
(Picard et al. 1988, Lacey et al. 1991, Lloyd et al.
1991, Hansen 1994), perimenopausal women
(Slemenda et al. 1987, 1990), postmenopausal women
(Slemenda et al. 1987, Hansen et al. 1991, Reid et al.
1994, Lloyd et al. 1997, 2000, Hannan et al. 2000) or
men (Eliel et al. 1983, Glynn et al. 1995, Hannan et al.
2000). Some negative associations between caffeine
intake and bone density have been observed; these
associations disappeared when confounders such as
calcium intake were adjusted for in some studies
(Cooper et al. 1992, Johansson et al. 1992), but not
others (Herna
´
ndez-Avila et al. 1993). Some research-
ers have found that caffeine’s effects on bone density
were dependent on calcium intakes. Harris and
Dawson-Hughes (1994) concluded that two to three
servings of coffee (280–420 mg caffeine day
1
) may
accelerate bone loss in healthy postmenopausal wo-
men with calcium intakes <800 mg day
1
. Barrett-
Connor et al. (1994) found that only postmenopausal
women who did not report drinking at least one glass
of milk per day between the ages of 20 and 50 years
exhibited a coffee-associated decrease in bone mineral
density.
Caffeine intake has been investigated as a potential
risk factor for bone fracture, the major cause of
morbidity and mortality associated with osteoporosis.
In case-control studies, caffeine intakes were not
associated with an increased risk of hip fracture in
women >55 years of age (Nieves et al. 1992), women
18–70 years of age (Tavani et al. 1995), or men or
women >65 years of age (Cumming and Klineberg
1994). In a cross-sectional study, Travers-Gustafson
et al. (1995) were also unable to show that caffeine
intakes were related to an increased incidence of low-
trauma fractures. In contrast, data from the Nurses
Health Study found that women who consumed more
than four cups of coffee per day (>544 mg caffeine
day
1
) had a higher risk of hip fracture than those
who ‘almost never’ consumed coffee (Herna
´
ndez-
Avila et al. 1991). Although other studies have shown
an increase in the risk of hip fracture with dietary
caffeine, it was not clear whether the analysis adjusted
for differences in calcium intake (Holbrook et al.
1988) or whether calcium intake data were unavail-
able (Kiel et al. 1990).
Interpretation of caffeine’s effects on bone metabo-
lism are complicated because coffee intake is asso-
ciated with other risk factors for osteoporosis:
calcium intake (Heaney and Recker 1982, Massey
and Hollingbery 1988, Hasling et al. 1992,
Herna
´
ndez-Avila et al. 1993), age (Barger-Lux and
Heaney 1995), cigarette smoking (Cooper et al. 1992,
Johansson et al. 1992, Barrett-Connor et al. 1994) and
alcohol consumption (Cooper et al. 1992, Barrett-
Connor et al. 1994). Collectively, the available data
suggest that an increased caffeine intake is associated
with a slight but biologically real deterioration in
calcium balance. The majority of evidence indicates
that this effect is through caffeine-induced hypercal-
ciuria. The biological significance of caffeine’s nega-
tive effect on calcium balance continues to be the
topic of scientific debate, as studies on both bone
density and fracture risk have revealed conflicting
results. Bruce and Spiller (1998) suggest that a lifetime
pattern of high caffeine intake (more than four cups
of coffee per day or >400 mg caffeine day
1
)in
women contributes to a negative impact on calcium
and bone metabolism and is correlated with bone loss
or fracture risk, particularly when there is a low
calcium intake. Heaney (1998) suggests that the epi-
demiological studies showing a negative association
between caffeine intake and bone mass may be ex-
plained by an inverse relationship between consump-
tion of milk and consumption of caffeine-containing
beverages, concluding that there is no evidence that
caffeine has any harmful effect on bone status or
5Effects of caffeine on human health
calcium economy in individuals ingesting recom-
mended levels of calcium.
To date, the evidence indicates that the significance of
caffeine’s potential to affect calcium balance and bone
metabolism adversely is dependent on lifetime caf-
feine and calcium intakes and is biologically more
relevant in women. Current data suggest that caffeine
intakes of <400 mg day
1
do not have significant
effects on bone status or calcium balance in indivi-
duals ingesting at least 800 mg calcium day
1
(an
intake that <50% of Canadian women achieve).
Effects on human behaviour
Mood and performance in adults
The results of studies on the effects of caffeine on
various psychomotor tasks (reviewed by James 1991e,
Smith 1998) are sometimes conflicting. For example,
some studies have shown no effects of caffeine on
hand steadiness, whereas others have associated caf-
feine consumption with poorer performance in this
parameter (Bovim et al. 1995). Studies showing both
positive effects (Jacobson and Edgley 1987, Roache
and Griffiths 1987) and no effects (Zahn and
Rapoport 1987) on reaction time have also been
reported.
Inconsistent results can be encountered in the litera-
ture in terms of the impact of caffeine on cognitive
functioning, including alertness, vigilance, memory
and mood. These inconsistencies may be due to
methodological differences, personality differences
(e.g. introverts versus extroverts), the time of day
when tests were conducted, and uncontrolled con-
founding factors (e.g. habitual caffeine, alcohol or
tobacco use) (James 1991e, Smith 1998). In general,
caffeine (100 mg day
1
for 4 days, Leathwood and
Pollet 1982/83; 1.5–3 mg kg
1
bw as single doses, 2 h
apart, or 105–210 mg for a 70-kg adult, Smith et al.
1993; 250 mg day
1
for 2 days, Johnson et al. 1990;
two doses of 200 mg, Regina et al. 1974) has been
shown to increase the alertness of individuals, espe-
cially in situations where arousal is low (e.g. night-
shift workers, early in the morning). Caffeine can also
increase vigilance in the daytime. In a double-blind
placebo-controlled study in males, statistically signif-
icant increases were observed in two of three vigilance
tests, including both visual and auditory tests, at all
caffeine doses employed (as low as 32 mg caffeine up
to 256 mg) (Lieberman et al. 1987). In another in-
vestigation of the effects of caffeine on alertness,
subjects given caffeine (250 mg twice per day) per-
formed significantly better in an auditory vigilance
test than did the placebo group (Zwyghuizen-
Doorenbos et al. 1990).
Most studies on the effects of caffeine on psychomo-
tor and cognitive parameters deal with acute admin-
istration. In a study on regular consumers of coffee
and tea (Jarvis 1993), higher levels of coffee consump-
tion were associated with improved performance in
reaction time, verbal memory and visuospatial rea-
soning. The consumption of tea was related to an
improved performance in one test of reaction time
and in visuospatial reasoning, but not in the other
tests. The best performance was noted at an intake of
five to six cups of coffee or tea per day.
Although the results of studies on the effects of
caffeine on alertness, vigilance and memory are some-
times contradictory in terms of whether caffeine
produces beneficial effects or no effects, there is little
indication that intake of caffeine (up to approxi-
mately 250 mg in a single dose or over a few days)
affects these processes in a negative manner (Smith
1998). However, a single caffeine dose of 100 mg was
shown to affect short-term memory adversely in one
study (Terry and Phifer 1986).
Some studies have noted little or no change in mood
after the consumption of single doses of caffeine of
32 mg (Lieberman et al. 1987), 100 mg (Svensson et al.
1980) or 200 mg (Swift and Tiplady 1988). Larger
amounts of caffeine (200, 400 or 600 mg as a single
dose) have been associated not only with slight in-
creases on an anger/hostility scale, but also with
reduced ratings for drowsiness and incoordination
(Roache and Griffiths 1987). Caffeine has little effect
in producing depression, even at the consumption of
more than eight cups of coffee per day (James 1991f ).
It is unclear why some studies have found effects on
mood and others have not.
The consumption of caffeine by adults has been
associated with an increase in anxiety in several
studies. Many studies conducted on psychiatric in-
patients, for example, have shown significantly in-
creased anxiety levels in heavier users of caffeine
(James 1991f ); however, some of these studies did
not control for alcohol and tobacco use, and patients
may have been primed to report more symptoms.
James et al. (1987) remedied these methodological
6 P. Nawrot et al.
problems in a survey of 173 psychiatric in-patients,
reporting no association between the consumption of
caffeine and anxiety. In patients with generalized
anxiety disorder, the administration of caffeine in-
creased their already high anxiety level in a dose-
related manner (Bruce et al. 1992). Note that the
results of studies using psychiatric patients or patients
with anxiety disorders may not be applicable to the
general population (James and Crosbie 1987). Other
studies have shown no effects of caffeine (e.g. regular
consumption of up to seven or more cups of coffee or
tea per day) on anxiety in psychiatric patients, non-
clinical subjects or patients with anxiety disorders
(Lynn 1973, Hire 1978, Eaton and McLeod 1984,
Mathew and Wilson 1990, James 1991f, Smith 1998).
The literature suggests that caffeine can produce
anxiety or exacerbate anxiety in adults with pre-
existing anxiety disorders; however, the doses asso-
ciated with these effects are large (1–2 g caffeine
day
1
) and would likely be consumed only by a small
segment of caffeine consumers. In addition, it has
been suggested that people experiencing the anxio-
genic effects of caffeine are likely to avoid the use of
this substance (James 1991f ); thus, the self-limiting
nature of caffeine intake would reduce any potential
that caffeine had to produce anxiety in adults.
Studies have shown that caffeine can increase the time
taken to fall asleep (sleep latency) and reduce sleep
duration, especially if large amounts of caffeine
(>3mgkg
1
bw, >210 mg for a 70-kg person) are
ingested close to the usual bedtime of the individual
(Smith 1998). High consumers of caffeine are less
likely to report sleep disturbances than individuals
consuming caffeine more infrequently (Snyder and
Sklar 1984, Zwyghuizen-Doorenbos et al. 1990), sug-
gesting the development of tolerance to the effects of
caffeine on this parameter. It is apparent that if
caffeine ingestion (especially in the late evening)
affects the sleep of the individual, a self-limiting
reduction in caffeine intake will likely occur to avoid
any effects on sleep.
In summary, the moderate consumption of caffeine in
normal adults has not been associated with any major
adverse effects on mood or performance, and most
effects associated with higher consumption rates
would be self-limiting. However, in light of inconsis-
tent results in the literature and individual differences
in sensitivity to caffeine, some people (e.g. those with
anxiety disorders) need to be aware of the possible
adverse effects of caffeine and to limit their intake
accordingly.
Tolerance, physical dependence, and withdrawal
The literature on the development of tolerance to the
effects of caffeine during prolonged ingestion is sparse
and inconsistent (James 1991e). Any tolerance that
may be present is likely to be dependent on the
biological or behavioural effect produced by caffeine
and by the level and pattern of caffeine consumption.
Cessation of caffeine ingestion has been associated
with a wide variety of mainly subjective effects, in
particular headache (Rubin and Smith 1999) and
fatigue, characterized by such symptoms as mental
depression, weakness, lethargy, apathy, sleepiness and
decreased alertness (Griffiths and Woodson 1988).
The general caffeine withdrawal pattern appears to
be an onset from 12 to 24 h after cessation, a peak at
20–48 h, and a duration of about 1 week (Griffiths
and Woodson 1988). The strength of the association
between caffeine cessation and withdrawal is sup-
ported by the fact that symptoms can be ameliorated
by administration of caffeine tablets in a dose-depen-
dent manner (Griffiths and Woodson 1988). The
intensity of the symptoms has been described as mild
to extreme. The presence or absence of withdrawal
symptoms is not always predictable, as some heavy
users have ceased ingestion of caffeine with no appar-
ent withdrawal (Griffiths and Woodson 1988).
Symptoms associated with caffeine withdrawal have
been noted in studies involving the cessation of
regular consumption of high ( 4 1250 mg day
1
,
Griffiths et al. 1986; 4 2548 mg day
1
, Strain et al.
1994) and much lower doses (100 mg day
1
, Griffiths
et al. 1990; 235 mg day
1
, Silverman et al. 1992;
290 mg day
1
, Weber et al. 1993; 428 mg day
1
,
Bruce et al. 1991; four to six cups of coffee per day,
van Dusseldorp and Katan 1990; five cups of coffee
per day, Hughes et al. 1991). While some studies have
shown a dose-dependent increase in the effects of
withdrawal (increased headaches after the stoppage
of regular consumption of >700 mg caffeine day
1
compared with 4 700 mg day
1
; Weber et al. 1993),
others have shown little correlation between daily
intake and withdrawal symptoms (in a range of
regular intake of 231–2548 mg day
1
; Strain et al.
1994). In Strain et al. (1994), the most severe effects
upon cessation were noted with the lowest consump-
tion, while the individual with the highest regular
consumption reported only moderate effects.
Dews et al. (1998) hypothesized that bias and priming
of the subjects in caffeine withdrawal studies led to
7Effects of caffeine on human health
the exaggeration of the incidence and severity of
symptoms of caffeine withdrawal. They suggested
that the prevalence and severity of withdrawal symp-
toms have been exaggerated in the literature, as
illustrated by the variability among published reports
of both the symptoms associated with caffeine with-
drawal and the incidence rates, and concluded that
the true level of caffeine withdrawal is low and near
background levels. Also, there are reports of caffeine
withdrawal continuing for long periods, which may
be the result of a return of performance and alertness
to pre-caffeine conditions. Since caffeine has been
shown to improve these parameters, the return to
normalcy may be associated with reduced perfor-
mance and alertness compared with caffeine use,
and these effects may be attributed to a caffeine
withdrawal syndrome or as a sign that physical
dependence has been produced during caffeine con-
sumption.
In a blinded study by Dews et al. (1999), subjects were
given coffee and then subjected to continued caffeine
intake, abrupt caffeine cessation or gradual caffeine
cessation (from 100 to 0% over 7 days). Subjects in
the gradual cessation group reported no adverse
effects of caffeine cessation, while females (but not
males) in the abrupt cessation group had adverse
effects, as evidenced by reduced mood/attitude scores
on no-caffeine days (reductions in scores were small).
This study showed that the blinding of subjects to
caffeine cessation reduced the incidence of reported
symptoms of caffeine withdrawal, as about half of the
subjects reporting severe withdrawal symptoms in a
prior telephone interview experienced no symptoms
of withdrawal in the blinded study.
The literature thus supports the existence of caffeine
withdrawal in some individuals, with variability in the
severity of symptoms. When withdrawal occurs, it is
short-lived and relatively mild in the majority of
people affected.
Effects on children
Scientific studies have shown a variety of effects of
caffeine consumption in children, although it is sur-
prising that so few studies have specifically addressed
effects in this population.
At low doses, an increased performance in attention
tests has been noted in children. A double-blind and
placebo-controlled study was conducted in which 21
children (mean body weight 38.1 12.5 kg; average
age 10.6 1.3 years) were administered a placebo, a
low dose of caffeine (single dose of 2.5 mg kg
1
bw) or
a high dose of caffeine (single dose of 5.0 mg kg
1
bw)
(Bernstein et al. 1994). The authors noted a statisti-
cally significant, dose-dependent improvement in a
performance test of attention after caffeine adminis-
tration compared with the placebo group. A signifi-
cant but non-dose-related improvement in a manual
dexterity test was also noted. In a double-blind
placebo-controlled cross-over study (Elkins et al.
1981, Rapoport et al. 1981b), a group of 19 pre-
adolescent boys were tested for a number of para-
meters after the ingestion of a placebo or a single
caffeine dose of 3 or 10 mg kg
1
bw on three separate
occasions (each separated by 48 h). The children in
the high-dose group showed a significant increase in
motor activity compared with the control and low-
dose groups, an increase in speech rate compared with
the low-dose group, a significant reduction in reaction
time in a vigilance test, and a reduced number of
errors in a sustained attention measure test compared
with the placebo group. Stratification of usual, pre-
study caffeine use was not conducted for the subjects
in this study.
Anxiety, measured both subjectively and objectively,
has also been associated with the administration of
low doses of caffeine in children in a number of
studies. In the Bernstein et al. (1994) study described
above, there was a trend (although it was statistically
non-significant) towards a higher level of anxiety in
one of the subsets of the Visual Analogue Scale for
state anxiety (‘how I feel right now’) just after caffeine
administration. There was a statistically significant
correlation between salivary caffeine concentration
and the severity of the state anxiety as measured by
the Visual Analogue Scale. It was noted in this study
that the levels of salivary caffeine were significantly
correlated with the dose of caffeine administered.
Other anxiety measurements conducted in this study
(all self-reported, including other measurements of
state and trait anxiety) showed no difference after
caffeine administration. While this study randomized
the order of testing, there was a lack of participant
stratification based on regular, pre-study caffeine
consumption. Even so, the level of caffeine adminis-
tered to children in the Bernstein et al. (1994) study is
the lowest in the available literature, and this study
should be considered along with the wider body of
evidence.
8 P. Nawrot et al.
Other reviewed studies showing manifestations of
anxiety in children associated with caffeine were those
by Rapoport et al. (1981a) (10 mg kg
1
bw day
1
),
Rapoport et al. (1981b) (3 and 10 mg kg
1
bw day
1
)
and Rapoport et al. (1984) (10 mg kg
1
bw day
1
). In
all of these studies, effects on anxiety were noted at all
doses tested. Other effects in these studies included
being nervous, fidgety, jittery, and restless and experi-
encing hyperactivity and difficulty sleeping. Positive
dose–responses were noted for skin conductance (a
measure of anxiety) as well as for nervous/jittery
behaviour in the children in the Rapoport et al.
(1981b) study. When subjects were stratified by pre-
study caffeine intake (Rapoport et al. 1981a), differ-
ences between low and high dose consumers (pre-
study intake of <50 and 5 300 mg caffeine day
1
,
respectively) were apparent. High dose consumers
were more easily frustrated, with a greater feeling of
nervousness on baseline tests, than the low consumer
group, possibly pointing to caffeine withdrawal dur-
ing this period of testing. In terms of reported side-
effects, the low users could distinguish between the
placebo and the caffeine treatment (according to a
variety of self-reported side-effects), while the high
users could not. The high users given placebo and
then caffeine experienced more side-effects during the
initial placebo administration than they did when
administered caffeine. The study by Rapoport et al.
(1981a) appears to provide evidence of tolerance in
the high regular consumers, and this group also
appeared to show withdrawal in the baseline and
placebo conditions. In Rapoport et al. (1984), a
number of differences were noted between high and
low consumers in terms of behaviour. During the
screening, baseline and initial pre-study caffeine-free
periods, the high consumers reported significantly
more symptoms of anxiety and were reported to be
more ‘disobedient’ than the low consumers. There
appeared to be many differences between the groups
when caffeine was administered for 2 weeks. Low
consumers exhibited a significant increase in restless-
ness and fidgety behaviour, while the high-dose group
showed a decrease in this behaviour. Statistically
significant differences between the groups were mood
changes, excitability, inattentiveness, restlessness and
crying (the direction of these changes between the two
groups was not mentioned in the paper). In terms of
side-effects during this period, the low consumers
reported headache, stomach-ache and nausea. These
effects were not noted in the high consumers. A
feeling of faintness and of being flushed was signifi-
cantly increased in the low consumers and signifi-
cantly decreased in the high consumers. Also, the low
consumers had difficulty sleeping and a decreased
appetite compared with the high consumer group. It
was suggested by the authors that child consumers of
high-caffeine diets differ inherently from those con-
suming low-caffeine diets in certain ways, namely
having lower autonomic arousal and being more
impulsive, leading to the self-administration of caf-
feine. In this study the initial pre-study stratification
of subjects into high and low consumers (18 18 and
641 350 mg day
1
, respectively) was based on a 24-h
recall; however, based on a 7-day food diary for the
pre-study baseline period, it was observed that there
was a large overlap between the low and high con-
sumer groups (95 84 and 290 275 mg week
1
or
about 41.4 and 13.6 mg day
1
, respectively). The over-
lap in pre-study caffeine intake may reduce the ability
to evaluate the differential effects of caffeine on high
and low consumers that were noted.
Other studies dealing with the effects of caffeine on
children were those by Baer (1987), Hale et al. (1995)
and Davis and Osorio (1998). The study by Baer
(1987) used six 5-year-old children who were admi-
nistered either a caffeine-free or a caffeinated soft
drink each day for 2 weeks, resulting in a dose of
1.6–2.5 mg kg
1
bw day
1
when caffeine was adminis-
tered. Drink conditions were reversed at the end of
the 2 weeks. Effects noted on behaviour (e.g. off-task
behaviour, motor activity, continuous performance)
were inconsistent and small. No testing for anxiety
was conducted. Hale et al. (1995) examined the self-
administration of caffeine in 18 adolescent children of
both sexes (age 11–15) in a double-blind, placebo-
controlled study. Soft drinks containing either caf-
feine (33.3 mg/8 ounce serving) or a placebo were
supplied to the participants. The children consumed
a particular drink one day (either caffeinated or
placebo) followed by another drink (either the same
as the previous day or different) the next day.
Consumption of all drinks was ad libitum. Four
children met the criteria for repeatable self-adminis-
tration, preferring the caffeinated drink to the place-
bo; however, only one child had a statistically
significant self-administration. In these four children,
the average intake of caffeine was 169 mg day
1
com-
pared with 62 mg day
1
in those where self-adminis-
tration was not evident. No behavioural symptoms
were consistently reported in any participant. When
the results were analysed across all participants, it was
noted that on caffeine-free beverage days, there was
significantly more depression, drowsiness and fatigue.
No differences between the consumption of caffei-
9Effects of caffeine on human health
nated or non-caffeinated drinks were observed in the
children when a parent rating scale for anxiety,
hyperactivity or impulsivity was employed. No infor-
mation was provided in this study about the pre-study
intake of caffeine. Davis and Osorio (1998) reported
that caffeine intake can worsen and trigger the ap-
pearance of tics in children, based on two children
aged 11 and 13. The authors concluded that con-
sumption of caffeine can trigger the appearance of tics
in susceptible children, although they made no indi-
cation of how the determination of a ‘susceptible’
child could be made. It is possible that genetic factors
play a role, since the two children in this study were
related. It should be recognized that with only two
children, this study is only suggestive of a problem;
however, it is an area that deserves further research.
In a meta-analysis of nine studies (Stein et al. 1996),
caffeine showed no significant deleterious acute effects
on behaviour or cognition in children. The results of
the meta-analysis with respect to anxiogenic effects
are difficult to interpret, for several reasons. For
example, tests of anxiety were grouped with a number
of other tests to form an ‘internalizing’ category. This
may have diluted any effects of anxiety. In addition,
the tests used to assess anxiety were not the same in
each study, making comparisons between these stu-
dies more difficult. Of the nine studies used for the
meta-analysis, four dealt with normal children, while
the remainder used children who had attention deficit
hyperactivity disorder. Again, this makes the inter-
comparison of studies difficult.
The cessation of caffeine intake in normally high
consuming children ( 5 300 mg day
1
) or those admi-
nistered larger amounts of caffeine (10 mg kg
1
bw
day
1
) over a period of weeks has resulted in the
production of symptoms associated with caffeine
withdrawal (Rapoport et al. 1981a). Bernstein et al.
(1998) studied the single-blinded withdrawal of caf-
feine in 30 normal pre-pubertal children (mean age 10
years) having an average pre-study consumption of at
least 20 mg caffeine day
1
. Children were adminis-
tered 150 mg caffeine day
1
for 13 days followed by a
non-caffeinated drink for 1 day, then resumed their
normal diet. While on caffeine, the subjects responded
significantly faster in the test of attention than in the
withdrawal period and resumption to normal diet
period. During the withdrawal period, the response
time was significantly increased compared with the
pre-caffeine (baseline) period. This increased response
time was still significantly elevated 1 week post-
caffeine cessation. The authors suggested that the
children had developed a physical dependence on
the caffeine and exhibited withdrawal effects upon
removal of the caffeine. Anxiety was observed to be
higher during the baseline period in this study, with
scores decreasing over time, possibly related to an
increasing familiarity of the children with the testing
procedure.
Caffeine has been tested for use in the treatment of
hyperactivity/attention deficit disorder in children
(James 1991e, Leviton 1992). A few early studies
showed beneficial effects of caffeine intake at doses
ranging from 175 to 600 mg day
1
; in these studies,
few adverse effects were noted, although some effect
on sleep (dose-dependent insomnia) was noted in one
study (100–400 mg caffeine day
1
), and minor group
increases in blood pressure and heart rate were noted
in another (300 mg day
1
). Many other studies, how-
ever, have shown no benefit of caffeine use in children
with attention deficit disorder. Some studies, in fact,
suggest that caffeine ingestion can lead to symptoms
of hyperactivity in natural low consumers. In a study
in which the 7-day food diaries from 30 low- and 30
high-caffeine-consuming school children were ana-
lysed, 30% of the high consumers met criteria for
attention deficit disorder with hyperactivity, and the
high consumers were perceived as being more restless
than the low consumers (Rapoport et al. 1984).
Problems with this study in terms of overlap between
the low and high consumers’ pre-study intake of
caffeine have been noted above.
The studies reviewed here and their sometimes con-
flicting results can be difficult to compare, since they
employed either different endpoints or different ways
to assess similar endpoints. In addition, most studies
used a small number of subjects. The problems asso-
ciated with differing groups of caffeine consumers
within the population of children and the potential
differential susceptibility to caffeine of certain sub-
populations need to be clarified. Another difficulty
with some studies is the non-stratification of children
based on their usual (pre-study) caffeine intake, since
high consumers and low consumers may not always
respond in the same manner to additional adminis-
tered caffeine. In addition, no studies have been
designed to test for potential chronic effects of caf-
feine consumption by children.
In conclusion, it is unknown if long-term daily con-
sumption of caffeine would produce effects similar
to those observed in the studies reviewed above.
However, it is known that the human nervous system
(including the brain) continues to develop and mature
10 P. Nawrot et al.
throughout childhood. It is possible that the pro-
tracted development of the nervous system may ren-
der children more sensitive to any adverse effects of
caffeine.
Mutagenicity/genotoxicity
Caffeine not only induces mutations in bacteria in the
absence of mammalian metabolic activation, but also
can exhibit weak antimutagenic activity in some
microorganisms (Legator and Zimmering 1979,
Brusick et al. 1986, Rosenkranz and Ennever 1987,
Pons and Muller 1990). In eukaryotic organisms,
including fungi and yeasts (Legator and Zimmering
1979, Osman and McCready 1998), higher plants
(Gonzalez-Fernandez et al. 1985, Manandhar et al.
1996), rodent cell lines (Jenssen and Ramel 1980,
Aeschbacher et al. 1986, Brusick et al. 1986, Haynes
et al. 1996, Kiefer and Wiebel 1998), and human cell
lines (Lachance 1982, Bernhard et al. 1996, Roldan-
Reyes et al. 1997), caffeine inhibits cell cycle-depen-
dent DNA repair induced by a variety of physical and
chemical mutagens, leading to the potentiation of
clastogenic effects (D’Ambrosio 1994, Puck et al.
1998, Harish et al. 2000, Jiang et al. 2000). In chick
embryo cells, DNA damage was induced (Mu
¨
ller et al.
1996) at dose levels in the 5 1mm range, not con-
sidered toxicologically relevant (Tempel and von
Zallinger 1997). Genotoxic activity in Drosophila
was weakly positive or inconclusive for chromosomal
effects, dominant lethals, the somatic mutation and
recombination test, and chromatid aberrations
(Legator and Zimmering 1979, Graf and Wu
¨
rgler
1986, 1996), while X-ray damage was enhanced (De
Marco and Cozzi 1980). Only at high levels of caffeine
were clastogenic effects reported in somatic cells of
rodents (Jenssen and Ramel 1980, Aeschbacher et al.
1986, Haynes et al. 1996), while no specific locus
mutations or chromosomal effects were induced in
germ cells or embryonic cells (Legator and Zimmering
1979, Mailhes et al. 1996, Mu
¨
ller et al. 1996).
Antigenotoxic activity on somatic or germ cells ex-
posed to a variety of physical and chemical mutagens,
following ingestion of caffeinated or decaffeinated
coffees was weak or negative (Legator and
Zimmering 1979, Everson et al. 1988, Reidy et al.
1988, Chen et al. 1989, MacGregor 1990, Smith et al.
1990, Robbins et al. 1997, Vine et al. 1997, Abraham
and Singh 1999).
Although evidence for the mutagenic potential of
caffeine is conflicting (Lachance 1982, Grice 1987,
Rosenkranz and Ennever 1987, D’Ambrosio 1994),
it appears to be unlikely that at normal, physiologi-
cally relevant levels of consumption (i.e. at less than
systemic toxicity ranges), caffeine would result in
mutagenic effects in humans.
Carcinogenicity
The evidence from several oral oncogenicity/chronic
toxicity studies in mice (Bauer et al. 1977, Macklin
and Szot 1980, Stalder et al. 1990) and rats (Wurzner
et al. 1977, Johansson 1981, Takayama and
Kuwabara 1982, Mohr et al. 1984) indicate that
caffeine is not a carcinogen, up to dose levels of 391
and 230 mg kg
1
bw day
1
, respectively. The most
common clinical sign observed in these studies was a
decrease in body weight, with no concomitant de-
crease in food consumption.
Epidemiological studies on the carcinogenicity of
caffeine as present in coffee have consistently shown
that caffeine is not associated with cancer develop-
ment at several tissue and organ sites. For example,
caffeine consumption, from three or more cups of
coffee per day ( 5 300 mg caffeine day
1
) was
not associated with cancer development in the follow-
ing sites: large bowel in 13 case-control studies (cited
in IARC 1991a, Lee et al. 1993, Olsen and Kronborg
1993); stomach in six case-control studies (cited in
IARC 1991a, Agudo et al. 1992); prostate in one case-
control study (cited in IARC 1991a); liver in one case-
control study (cited in IARC 1991a); lung in two
cohort studies and one case-control study (cited in
IARC 1991a); and vulva in one case-control study
(Sturgeon et al. 1991). Higher caffeine consumption,
specifically drinking seven or more cups of coffee per
day ( 5 700 mg caffeine day
1
) was not associated
with breast cancer in 11 case-control studies (cited
in Rohan et al. 1989, IARC 1991a, McLaughlin et al.
1992, Folsom et al. 1993, Smith et al. 1994, Tavani
et al. 1998).
On the other hand, caffeine intake, as measured by
coffee consumption, was occasionally associated with
cancer development at some sites. In the urinary
bladder, four cohort studies showed no effect with
doses of five or more cups of coffee per day ( 5 500 mg
caffeine day
1
) (cited in IARC 1991a, Chyou et al.
11Effects of caffeine on human health
1993, Stensvold and Jacobsen 1994). In 26 case-con-
trol studies, 17 studies showed no effect with doses of
five or more cups of coffee per day. Nine studies were
positive, and three of these studies showed a dose–
response (cited in IARC 1991a, Vena et al. 1993,
Donato et al. 1997). Of these three studies, two
showed a positive increase with any coffee consump-
tion, and the third study was significant only when
consumption was five or more cups of coffee per day.
In the pancreas, out of nine cohort studies, eight
showed no significant effect with doses of five or more
cups of coffee per day (500 mg caffeine day
1
), while
one study was positive for any coffee consumption
(cited in IARC 1991a, Stensvold and Jacobsen 1994,
Harnack et al. 1997). Of 24 case-control studies, 21
showed no effect on pancreas with doses of five or
more cups per day. In one of the three positive case-
control studies, a significant effect was observed only
when four cups of coffee per day were drunk (400 mg
caffeine day
1
). In a second study, doses exceeding
two cups of coffee per day (200 mg caffeine day
1
)
were associated with an increase. In the third positive
study, any level of coffee drinking resulted in an
increased risk. Of the three positive studies, two
studies showed a dose-related response. When smok-
ing was taken into consideration, the positive re-
sponses in these studies were weakened (cited in
IARC 1991a, Bueno de Mesquita et al. 1992, Lyon
et al. 1992, Partanen et al. 1995, Nishi et al. 1996). In
the ovary, two case-control studies showed a sig-
nificant increase in cancer incidence with doses of
more than one cup of coffee per day, while five case-
control studies showed no effect with doses of five
or more cups per day (cited in IARC 1991b,
Polychronopoulou et al. 1993). In the skin, a case-
control study showed that the risk of basal cell
carcinoma was increased with doses of more than
two-and-a-half cups of coffee per day (>250 mg caf-
feine day
1
) (Sahl et al. 1995).
Overall, the evidence indicates that caffeine, as
present in coffee, is not a chemical that causes breast
or bowel cancer. Results on the association between
caffeine and the development of urinary bladder
and pancreatic cancer are inconsistent and the
data are not conclusive. At other sites (e.g. ovary,
stomach, liver) the data are insufficient to conclude
that caffeine consumption is related to carcinogenesis.
Based on the studies reviewed in this report, caffeine
is not likely to be a human carcinogen at a dose less
than five cups of coffee per day (<500 mg caffeine
day
1
).
Reproductive and developmental effects
There is evidence that many women spontaneously
reduce their caffeine intake during pregnancy, some
apparently developing a temporary ‘loss of taste’ for
the substance. Nevertheless, caffeine consumption in
this group can remain relatively high. About 98% of
women of reproductive age regularly consume caf-
feine in the form of caffeinated beverages or in
caffeine-containing medications, while 72% of them
continue to do so during pregnancy (James 1991g).
Epidemiological investigations reviewed for this pa-
per showed that a majority of women consumed
caffeine during pregnancy in a range of 100–
300 mg day
1
(Fenster et al. 1991a, Fortier et al.
1993, Mills et al. 1993, Dominguez-Rojas et al.
1994, Rondo et al. 1996). A small proportion of
pregnant women in the population may ingest a much
greater amount, 5 400 mg caffeine day
1
(Kurppa
et al. 1983, Toubas et al. 1986, Olsen et al. 1991,
Armstrong et al. 1992, McDonald et al. 1992a).
During the past 20 years, a great deal of evidence has
accumulated concerning the effects of caffeine con-
sumption on reproduction and pre- and postnatal
development. Although the results from studies re-
viewed for this publication have not been entirely
consistent, the bulk of evidence suggests that caffeine
intake at dose levels of 5 300 mg day
1
may have
adverse effects on some reproductive/developmental
parameters when exposure takes place during certain
periods (Dlugosz and Bracken 1992).
Christian and Brent (2001) reviewed published animal
and human epidemiological studies investigating the
association between caffeine ingestion and adverse
reproductive/developmental effects and concluded
that pre-pregnant or pregnant women who do not
smoke or drink alcohol and who consume moderate
amounts of caffeine ( 4 5–6 mg kg
1
bw day
1
spread
throughout the day) will be unlikely to develop
reproductive problems.
The effects of caffeine on the outcome of pregnancy
appear biologically plausible. Published data suggest
that the human foetus and neonate may be exposed to
substantial amounts of caffeine or its metabolites, as
caffeine ingested by the mother is rapidly absorbed
from the gastrointestinal tract, readily crosses the
placenta and is distributed to all foetal tissues, in-
cluding the central nervous system. Caffeine is also
excreted in mother’s milk. In addition, exposure of
the foetus and newborn to caffeine is enhanced due to
12 P. Nawrot et al.
the half-life of caffeine being markedly increased in
the foetus (the enzymes involved in the oxidation of
methylated xanthines are absent in the foetus), new-
born infant and pregnant woman in comparison with
non-pregnant adults and older children (James and
Paull 1985, James 1991g, Dlugosz and Bracken 1992).
Effects on conception and female fertility
Caffeine consumption is one of many factors impli-
cated in the reduction of fecundity, or the capacity
to reproduce. There are several plausible biological
mechanisms by which caffeine could delay concep-
tion. Caffeine consumption has been associated with
alteration of hormone levels (e.g. oestradiol), with
tubal disease or endometriosis, with altered tubal
transport time, and with reduced viability of the
fertilized ovum (Alderete et al. 1995). Caffeine meta-
bolism varies during the menstrual cycle, with re-
duced clearance during the luteal phase, resulting in
greater accumulation during the period of implanta-
tion and early embryonic development. Caffeine con-
sumption may lead to pregnancy loss, which might
result in prolongation of the waiting time required to
achieve a clinically recognized pregnancy (Stanton
and Gray 1995).
Thirteen epidemiological studies (retrospective and
prospective data collection) investigating the relation-
ship between coffee/caffeine consumption and time to
conception (fecundability) present conflicting results.
Five studies reported no delay in conception in
women who consumed up to 5 700 mg caffeine day
1
before pregnancy. In a multicentre study conducted in
the USA and Canada, caffeine consumption was not
associated with decreased fertility in a group of 2817
women whose caffeine consumption from all sources
ranged from 100 to 5 240 mg day
1
(Joesoef et al.
1990). Results of a study by Olsen (1991) showed no
association between subfecundity and consumption
of coffee or tea at any dose level (none to eight cups
per day) among non-smoking women. Florack et al.
(1994) showed that participants (male and female
partners) with caffeine intake of 400–700 mg day
1
had a higher fecundability than those with a lower
intake level; only heavy caffeine intake
(>700 mg day
1
) among partners was negatively re-
lated to fecundability when compared with the lowest
intake level (<300 mg day
1
). Caan et al. (1998) found
no association between caffeine intake at a mean dose
level of about 90 mg day
1
and a reduction in fertility
of women trying to conceive for at least 3 months.
Alderete et al. (1995) examined the independent and
combined effects of smoking and coffee consumption
on time to conception in 1341 primigravid women
and found that women who consumed more than
three cups of coffee per day (>300 mg caffeine day
1
)
but did not smoke showed no decrease in fertility
when compared with non-coffee-drinking women (ad-
justed odds ratio [OR] ¼ 1.0–1.2) who did not smoke.
Results from two studies showed a significant de-
crease in monthly probability of pregnancy among
women who consumed the equivalent of three or
more cups of coffee per day ( 5 300 mg caffeine
day
1
). In a retrospective study of 2465 women,
Stanton and Gray (1995) found that the adjusted
OR of delayed conception for >1 year was not
increased among women who consumed 4 300 mg
caffeine day
1
, but the OR was 2.65 (95% confidence
interval [CI] ¼ 1.38–5.07) among non-smokers who
consumed 5 301 mg caffeine day
1
. (In this study,
no effect of high caffeine consumption was observed
among women who smoked.) In a study of 430
Danish couples planning their first pregnancy,
Jensen et al. (1998) found that compared with non-
smoking couples with caffeine intake <300 mg day
1
,
non-smoking females and males who consumed 300–
700 mg caffeine day
1
had fecundability ORs of 0.88
and 0.87 (95% CI ¼ 0.60–1.31 and 0.62–1.22), respec-
tively, whereas females and males with a higher
caffeine intake (>700 mg day
1
) had ORs of 0.63
and 0.56 (95% CI ¼ 0.25–1.60 and 0.31–0.89), respec-
tively. No dose–response relationship was found
among smokers. Smoking women whose only source
of caffeine was coffee (>300 mg day
1
) had a reduced
fecundability OR ¼ 0.34 (95% CI ¼ 0.12–0.98), and
non-smoking women with a caffeine intake of
>300 mg day
1
from other sources had a low, but
non-significant, OR ¼ 0.43 (95% CI ¼ 0.16–1.13)
compared with non-smoking women consuming
<300 mg caffeine day
1
. The authors concluded that
the results indicated a possible association between
male and female caffeine intake and decreased fe-
cundability only among non-smokers.
Another four studies reported delayed conception in
women who consumed 5 400, 5 500, or 5 800 mg
caffeine day
1
. Data collected by Christianson et al.
(1989) showed a dose-related effect of coffee con-
sumption on reported difficulties in becoming preg-
nant. Women who were heavy coffee drinkers before
pregnancy (four to seven or more cups of coffee per
day) experienced almost double the time in becoming
13Effects of caffeine on human health
pregnant compared with women who consumed none
or one cup of coffee per day. Williams et al. (1990)
examined data from a large cross-sectional study on
3010 postpartum women, finding that times to con-
ception for women who consumed three, two, one or
no cups of coffee per day were similar (ranging from
4.8 to 5.0 months), whereas time to conception was
longer (6.6 months) for the 129 women who con-
sumed four or more cups of coffee per day (approxi-
mately 400 mg caffeine day
1
). In a retrospective
study by Bolumar et al. (1997), a significantly in-
creased OR (1.45, 95% CI ¼ 1.03–2.04) for subfe-
cundity in the first pregnancy was observed among
women consuming >500 mg caffeine day
1
. Women
in this highest level of consumption had an increase of
11% in the time leading to the first pregnancy. (The
effect of drinking >500 mg caffeine day
1
was rela-
tively stronger in smokers [OR ¼ 1.56, 95%
CI ¼ 0.92–2.63] than in non-smokers [OR ¼ 1.38,
95% CI ¼ 0.85–2.23].) In Olsen (1991), a statistically
significant association was observed (OR ¼ 1.35, 95%
CI ¼ 1.02–1.48) for a delay of 5 1 year in women
who smoked and also consumed at least eight cups of
coffee per day (or an equivalent amount of caffeine
from 16 cups of tea).
Three studies found modest positive associations with
delayed conception from maternal consumption of
more than one caffeinated beverage per day. A pro-
spective study by Wilcox et al. (1988) showed that
women who consumed more than one cup of coffee
per day (126 mg caffeine day
1
) were half as likely to
conceive during a given menstrual cycle. In a cross-
sectional study, Hatch and Bracken (1993) found that
intake of caffeine from coffee, tea and caffeinated soft
drinks was associated with an increased risk of a
delay of conception of 5 1 year. Compared with no
caffeine use, consumption of 1–150 mg caffeine day
1
resulted in an OR for delayed conception of 1.39
(95% CI ¼ 0.90–2.13), consumption of 151–
300 mg day
1
was associated with an OR ¼ 1.88
(95% CI ¼ 1.13–3.11), and consumption of
>300 mg day
1
resulted in an OR ¼ 2.24 (95%
CI ¼ 1.06–4.73). Women who reported drinking
>300 mg caffeine day
1
had a 27% lower chance of
conceiving for each cycle, and those who reported
drinking <300 mg day
1
had a 10% reduction in
conception rates per cycle compared with women
who consumed no caffeine. Hakim et al. (1998)
examined the effects of caffeine consumption on con-
ception in a prospective study of 124 women, finding
that the consumption of the equivalent of more than
one cup of coffee per day among the sample of
women who neither smoked nor drank alcohol was
associated with a decreased risk of conception
(18.0%, adjusted OR ¼ 0.56, 95% CI ¼ 0.23–1.33),
which did not reach statistical significance.
In one of the above-described studies, delayed con-
ception was observed among non-smoking women
who consumed >300 mg caffeine day
1
, but not
among women who smoked (Stanton and Gray
1995). Also, Jensen et al. (1998) found no dose–
response relationship among smokers at caffeine
doses of up to 5 700 mg day
1
, whereas non-smoking
males and females who consumed 300–700 mg day
1
exhibited decreased fecundability compared with
non-smoking couples with caffeine intake of
<300 mg day
1
. However, in Olsen (1991), no asso-
ciation was found among non-smokers at any dose
level of caffeine, just for women who smoked and also
consumed at least eight cups of coffee per day.
Bolumar et al. (1997) also found that the effect of
drinking >500 mg caffeine day
1
was relatively stron-
ger in smokers than in non-smokers. An interaction
between caffeine and smoking is biologically plaus-
ible. Reports in the literature have shown that cigar-
ette smoking significantly increases the rate of caffeine
metabolism (see ‘Pharmacokinetics’). The enhanced
caffeine metabolism in smokers also accelerates caf-
feine clearance and, as a result, reduces the duration
and magnitude of the exposure.
Most epidemiological studies reviewed here were
affected by methodological issues, including inade-
quate measurement of caffeine intake, failure to dis-
tinguish among different types of preparation and
different strengths of coffee, inadequate control for
possible confounding effects, recall bias in retrospec-
tive studies, lack of data on frequency of unprotected
intercourse, and, in some studies, inadequate sample
size. Despite these limitations, epidemiological studies
are an important source of information on potential
adverse effects of caffeine on fertility (delayed con-
ception) in humans.
The evaluated epidemiological studies generally indi-
cate that consumption of caffeine at dose levels of
>300 mg day
1
may reduce fecundability in fertile
women.
Effects on sperm and male fertility
Although ingested caffeine is capable of crossing the
blood–testis barrier, caffeine consumption as a factor
14 P. Nawrot et al.
that could alter male reproductive function has not
been investigated extensively. Data from in vitro
studies suggest that caffeine has variable, dose-related
effects on human sperm motility, number and
structure (Dlugosz and Bracken 1992). It has been
reported that women undergoing artificial insemina-
tion were twice as likely to become pregnant if their
husbands’ semen had been treated with caffeine than
if it had not. Scanning electron microscopic examin-
ation of fresh semen showed no morphological
changes caused by in vitro treatment with caffeine
(IARC 1991b, Dlugosz and Bracken 1992).
In an investigation of semen quality and its associa-
tion with coffee drinking, cigarette smoking and
alcohol consumption in 445 men attending an inferti-
lity clinic, coffee drinking was correlated with in-
creases in sperm density and percentage of
abnormal forms, but not in a dose-dependent man-
ner. Men who drank one to two cups of coffee per day
had increased sperm motility and density compared
with subjects who drank no coffee. However, men
who drank more than two cups per day had decreased
sperm motility and density. The combination of
drinking more than four cups of coffee per day
(>400 mg caffeine day
1
) and smoking >20 cigarettes
per day diminished spermatozoan motility and in-
creased the percentage of dead spermatozoa. No
alteration in the fertility of individuals who consumed
these substances was observed (Marshburn et al.
1989, IARC 1991b, Dlugosz and Bracken 1992).
Jensen et al. (1998) found no association between
caffeine intake and semen quality in men exposed to
caffeine for an extended period at dose levels as high
as 5 700 mg day
1
.
Based on the limited data, it is concluded that caffeine
consumption at dose levels of >400 mg day
1
may
decrease sperm motility and/or increase the percen-
tage of dead spermatozoa (only in heavy smokers),
but will be unlikely to adversely affect male fertility in
general.
Spontaneous abortion (miscarriage)
The influence of caffeine on the risk of spontaneous
abortion in humans is difficult to assess. A number of
studies have been conducted that show either a
positive effect or a lack of effect of caffeine on this
pregnancy outcome. Shortcomings in the literature
include small sample size and inadequate adjustment
for potential confounders. A major potential con-
founder is the presence of nausea in the first trimester
of pregnancy, as a lack of nausea early in pregnancy
has been associated with a significantly increased risk
of miscarriage (Stein and Susser 1991). Nausea in
pregnancy may cause a reduction in the consumption
of coffee/caffeine, while a lack of nausea may lead to
continued ingestion. This may result in an erroneous
association of caffeine intake with increased risk of
spontaneous abortion. Another drawback is the gen-
eral lack of accurate measurement of actual caffeine
consumption by the participants in the epidemiologi-
cal studies. Stavric et al. (1988), for example, found a
marked variation in caffeine content of coffee and tea
depending on the method of preparation and brand,
and errors also arise from differences in the size of the
serving ‘cup’ used by different participants. Another
serious limitation is the potential for poor identifica-
tion of foetal loss due to enrolment of women later in
the pregnancy or only those who presented to hospi-
tals, as many early foetal losses go unnoticed by
women. Studies measuring human chorionic gonado-
trophin levels, such as those of Wilcox et al. (1990),
Mills et al. (1993) and Hakim et al. (1993), should
reduce any bias in this factor. In addition, the ma-
jority of the studies showing positive associations
between caffeine and spontaneous abortion are
retrospective in nature, and at least one study de-
pended on information recalled after several preg-
nancies (Armstrong et al. 1992).
Most of the studies have shown no association be-
tween a caffeine intake of <300 mg day
1
and an
increased risk of spontaneous abortion (Watkinson
and Fried 1985, Wilcox et al. 1990, Armstrong et al.
1992, Mills et al. 1993, Dlugosz et al. 1996, Wen et al.
2001). In the one study that accurately assessed
caffeine intake (the prospective study by Watkinson
and Fried 1985), 284 mothers were interviewed about
their caffeine intake from coffee, tea, caffeinated soft
drinks, chocolate bars, chocolate drinks and caffeine-
containing medicines 3 years before pregnancy, dur-
ing each trimester of pregnancy and the year after
pregnancy. Caffeine consumption was measured and
categorized into <100, 100–300 and >300 mg day
1
.
There was no association between caffeine consump-
tion and risk of miscarriage. In this study, there was a
long period for which the women had to recall their
caffeine consumption, so all recalled intakes may not
have been accurate. Another study that found no
association between caffeine consumption at levels
of 5 300 mg day
1
and an increase in spontaneous
15Effects of caffeine on human health
abortion was the prospective study by Mills et al.
(1993).
The meta-analysis conducted by Fernandes et al.
(1998), using data from six original epidemiological
studies (including 42 988 pregnancies), showed a po-
sitive association (small but statistically significant) of
spontaneous abortion with the consumption of
>150 mg caffeine day
1
(OR ¼ 1.36, 95% CI ¼ 1.29–
1.45). No other more definitive consumption cate-
gories were used in this study, and adjusting for
confounders was not possible. The authors described
the increased risk as small and noted that ‘a possible
contribution to these results of maternal age, smok-
ing, ethanol use or other confounders could not be
excluded’.
Srisuphan and Bracken (1986) conducted a prospec-
tive cohort study with 3135 pregnant women whose
caffeine consumption was estimated from their re-
ported consumption of coffee, tea, caffeinated soft
drinks and caffeine-containing drugs. In terms of a
crude association, the rate of spontaneous abortion
was 1.8% for those who did not use caffeine
(<1 mg day
1
), 1.8% for the light users (1–
150 mg day
1
) and 3.1% for the moderate/heavy users
( 5 151 mg day
1
). When exposure was divided into
50-mg increments, there was a ‘marked increase’ in
the relative risk for spontaneous abortion at use levels
of >150 mg day
1
, but no dose–response was noted,
as no further risk was associated with exposures
>200 mg caffeine day
1
. This study also pointed out
that coffee consumption rather than caffeine con-
sumption per se may have contributed to the risk of
spontaneous abortion, as those who had a caffeine
consumption from coffee alone had an increased
crude relative risk compared with those consuming
tea or caffeinated soft drinks alone, although the
differences were not statistically significant. In this
study, there was no more definitive categorization of
intake >150 mg day
1
.
Al-Ansary and Babay (1994) conducted a retrospec-
tive case-control study with 226 women in Saudi
Arabia and found an increased risk of miscarriage
with the consumption of >150 mg caffeine day
1
(OR ¼ 1.0 [referent] and OR ¼ 1.9 [95% CI ¼ 1.2–
3.0] for consumption of 1–150 and >150 mg day
1
,
respectively). No subclassification of intake
>150 mg day
1
was conducted in this study, and it
appears that no confounders were taken into consid-
eration in the analysis. Only cases that had presented
to a hospital were included, which may not give a
complete picture of all possible miscarriages.
The retrospective case-control study by Infante-
Rivard et al. (1993) is one of the better papers of
those showing an association between lower levels of
caffeine consumption and the risk of spontaneous
abortion. In total, there were 331 cases and 993
controls. The investigators found significant increases
in OR for the risk of foetal loss in high consumers of
caffeine when it was ingested before and during
pregnancy (>321 mg caffeine day
1
before pregnancy,
OR ¼ 1.85, 95% CI ¼ 1.18–2.89; 163–321 and
>321 mg caffeine day
1
during pregnancy,
OR ¼ 1.95, 95% CI ¼ 1.29–2.93, and OR ¼ 2.62,
95% CI ¼ 1.38–5.01, respectively). For caffeine con-
sumption before pregnancy, the OR increased by a
factor of 1.10 for each 100 mg caffeine ingested per
day. For consumption during pregnancy, the OR
increased by a factor of 1.22 for each 100 mg ingested
per day. The conclusion was that the incidence of
spontaneous abortions was strongly associated with
caffeine intake during pregnancy and moderately
associated with caffeine use before pregnancy.
The majority of papers that showed an increased risk
of spontaneous abortion with caffeine consumption
showed associations at levels of 5 300 mg caffeine
day
1
. In a prospective cohort study by Dlugosz et al.
(1996), for example, only the highest use of coffee and
tea (three or more cups per day, about 5 300 mg
caffeine day
1
) was associated with an increased risk
of spontaneous abortions (OR ¼ 2.63, 95%
CI ¼ 1.29–5.34, for coffee; OR ¼ 2.33, 95%
CI ¼ 0.92–5.85, for tea). Armstrong et al. (1992), in
a retrospective study of 35 848 pregnancies in Quebec,
Canada, found the percentage of subjects with spon-
taneous abortions to be 20.4, 21.3, 24.1, 28.1 and
30.9% for persons consuming none, one to two, three
to four, five to nine and 10 or more cups per day,
respectively. The ORs in these consumption cate-
gories were 1.00 (referent), 0.98 (95% CI ¼ 0.93–
1.04), 1.02 (0.94–1.12), 1.17 (1.03–1.32) and 1.19
(0.97–1.45), respectively. In this paper, the time lag
between the actual abortion and the interview may
have introduced errors in recall about the amount of
coffee consumed in previous pregnancies. Subjects in
this paper were questioned about the incidence of
spontaneous abortion and caffeine intake in all pre-
vious pregnancies.
Wen et al. (2001) studied the association between
caffeine consumption and nausea and the risk of
spontaneous abortion. The categories of caffeine
consumption (based on periodic food frequency
questionnaires) were: <20, 20–99, 100–299 and
16 P. Nawrot et al.
5 300 mg day
1
. Caffeine consumption was calculated
for the periods before pregnancy, in the first trimester
of pregnancy, and up to the date of any spontaneous
abortion if it occurred before the end of the first
trimester. The presence and duration of nausea were
monitored. Potential confounders were analysed, in-
cluding demographic factors, smoking and the con-
sumption of alcohol. Parity and body mass index
were also considered. None of these parameters
caused any important confounding and, therefore,
the data were left unadjusted for these factors.
Overall, 7.2 versus 29.6% of the women who experi-
enced any nausea or no nausea, respectively, had
spontaneous abortions. In this study, no increased
risk of spontaneous abortion was noted with any level
of pre-pregnancy intake of caffeine. The data showed
that the consumption of caffeine did not increase the
risk of spontaneous abortion in women who were
already at risk due to a lack of nausea or a reduced
frequency/duration of nausea. However, in those
women who had nausea in their first trimester and
who were consequently at a reduced risk of sponta-
neous abortion, increased caffeine consumption dur-
ing the first trimester was associated with abortion.
The risk ratios and 95% CIs were: <20 mg caffeine
day
1
, 1.0 (reference category); 20–99 mg day
1
, 1.8
(0.8–3.9); 100–299 mg day
1
, 2.4 (0.9–6.2); and
5 300 mg day
1
, 5.4 (2.0–14.6). The risk of sponta-
neous abortion was elevated significantly with a con-
sumption of caffeine 5 300 mg day
1
.
Klebanoff et al. (1999), using actual serum measure-
ments of paraxanthine, a major caffeine metabolite,
showed an increased risk of spontaneous abortion at
an estimated 600–1100 mg caffeine day
1
. In this
retrospective study of 591 women who had sponta-
neous abortions and 2558 matched controls, women
with spontaneous abortions had significantly higher
serum paraxanthine levels than the controls (752 and
583 ng ml
1
in women having spontaneous abortions
and controls, respectively). The increased risk of
spontaneous abortions (OR ¼ 1.9, 95% CI ¼ 1.2–2.8)
was noted only in those women with serum para-
xanthine concentrations >1845 ng ml
1
. The authors
concluded that the daily intake of caffeine needed to
reach 1845 ng paraxanthine/ml serum in a 60-kg
woman would be about 600 mg for those who do
not smoke and 1100 mg in those who smoke. This
would correlate with about six and 11 cups of coffee
per day, respectively.
Some studies have revealed the possibility that con-
stituents in coffee or tea other than caffeine may be
related to an increased risk of spontaneous abortion
in women (Watkinson and Fried 1985, Srisuphan and
Bracken 1986, Dlugosz et al. 1996). The one study
that accurately measured caffeine consumption
(Watkinson and Fried 1985) found no association
between caffeine intake and spontaneous abortion,
but did find a statistically significant larger propor-
tion of coffee and tea drinkers in the group of women
who had spontaneous abortions. Dlugosz et al. (1996)
found that caffeinated soft drink use (up to three or
more cans per day) did not increase the risk of
spontaneous abortions. Tea and coffee (at consump-
tion of up to three or more cups of either drink per
day) produced similar risks, despite these products
having differing caffeine contents.
Although much epidemiological work has been con-
ducted, additional prospective studies that measure
actual caffeine intake in the participants and that
adjust for potential confounders such as nausea and
vomiting during pregnancy would be beneficial. In the
absence of these data, however, there appear to be
reasonable grounds for limiting the consumption of
caffeine to <300 mg day
1
in women who are, or who
are planning to become, pregnant.
Foetal growth
The potential adverse impact of caffeine consumption
during pregnancy on foetal growth has been a con-
cern for many years. Caffeine increases the levels of
cAMP through inhibition of phosphodiesterases, and
the rise in cAMP might interfere with foetal cell
growth and development (Karen 2000). Caffeine
may also block specific adenosine receptors. As ade-
nosine is involved in maintaining the balance between
the availability and the use of tissue oxygen, blockage
of its receptors could increase the susceptibility of the
cell to hypoxia. Consumption of two cups of coffee
has been reported to increase maternal epinephrine
concentration and decrease intervillous placental
blood flow (Fortier et al. 1993). As smoking is closely
associated with caffeine consumption, it is important
to stress that caffeine and smoking impose similar
adverse physiological effects on foetal development
(Fortier et al. 1993).
Results from epidemiological studies investigating the
association between caffeine consumption and foetal
growth have been conflicting. Of 18 original epide-
miological studies, three indicate an association be-
17Effects of caffeine on human health
tween either low birth weight (body weight <2500 g at
birth) or intrauterine growth retardation (defined as
birth weight <10th percentile of the sex-specific and
gestation age-specific distribution of birth weight) and
caffeine consumption <300 mg day
1
. In a popula-
tion-based study by Fortier et al. (1993), caffeine
intake by 7025 women living in the Quebec City,
Canada, area was not related to low birth weight
but was associated with an increased risk of intra-
uterine growth retardation. For women whose aver-
age daily caffeine consumption was 0–10, 11–150,
151–300 or >300 mg, the adjusted ORs for deliver-
ing a newborn with growth retardation were 1.00,
1.28 (95% CI ¼ 1.04–1.59), 1.42 (1.07–1.87) and 1.57
(1.05–2.33), respectively. In a Brazilian unmatched
case-control study by Rondo et al. (1996), results
showed that the proportion of mothers who delivered
babies with intrauterine growth retardation increased
as the average consumption of coffee increased during
pregnancy. Compared with mothers whose babies’
growth was appropriate for gestation age, the ORs
of mothers with babies with intrauterine growth
retardation were 1.55 (95% CI ¼ 0.99–2.44), 2.25
(1.34–3.78) and 2.07 (1.14–3.78) for caffeine con-
sumption levels of approximately < 140, 141–280
and 5 281 mg caffeine day
1
, respectively, following
adjustment for confounders such as cigarette smok-
ing, alcohol intake and per capita income. Vlajinac
et al. (1997), in an investigation of the effect of
caffeine consumption during the third trimester on
birth weight, found that birth weight decreased as
caffeine consumption increased at levels ranging from
71 to 5 140 mg day
1
in non-smokers.
Five studies reported an increased risk for foetal
growth retardation in infants whose mothers were
exposed to caffeine at dose levels of 5 300 mg day
1
during pregnancy after adjustment for potential con-
founders, including cigarette smoking and alcohol
consumption (especially binge drinking). In the pro-
spective study by Watkinson and Fried (1985) in
which data were collected on maternal use of tea,
coffee, caffeinated soft drinks, chocolate bars, choco-
late drinks and caffeinated medication, the most
marked effects associated with heavy caffeine use
(>300 mg day
1
) were reduced birth weight and small
head circumference; the associations were still signifi-
cant after adjustment for maternal nicotine use. The
mean weight of babies born to 12 heavy users was
3158 compared with 3537 g for the remaining sample.
The results suggest that daily caffeine intake of
5 300 mg can interfere with normal foetal growth.
In a prospective study investigating the effects of
caffeine consumption on intrauterine growth retarda-
tion, Martin and Bracken (1987) found that low birth
weight was most common among offspring of women
consuming >300 mg caffeine day
1
, the rate being
7.3% compared with the unexposed group rate of
4.1%. Heavy caffeine intake (>300 mg day
1
) was
associated with a 120-g reduction in birth weight
compared with the untreated group. Moderate use
of caffeine (151–300 mg day
1
) was also associated
with a decrease in birth weight, but to a lesser extent.
When a comparison was made with women who had
no caffeine exposure, the relative risks (RR) of low
birth weight after adjustment for confounding factors
(maternal age, ethnicity, education, previous sponta-
neous abortions, previous stillbirth, weight gain, body
mass index, smoking and alcohol intake) were 1.4
(95% CI ¼ 0.70–3.00) for 1–150 mg caffeine day
1
, 2.3
(1.1–5.2) for 151–300 mg, and 4.6 (2.0–10.5) for
>300 mg. Beaulac-Baillargeon and Desrosiers (1987)
found that birth weight was significantly less for
women who consumed > 300 mg caffeine day
1
and
who smoked 15 or more cigarettes per day. In a case-
control study by Caan and Goldhaber (1989), the
data showed no increased risk of low birth weight
with light to moderate consumption of caffeine
(<300 mg day
1
) (adjusted OR ¼ 0.90, 95%
CI ¼ 0.4–1.92) but a small but measurable increased
risk with heavy consumption of caffeine
(>300 mg day
1
) (adjusted OR ¼ 2.94, 95% CI ¼
0.89–9.65). One limitation of this study was its small
sample size (131 cases, 136 controls). Fenster et al.
(1991b) found that heavy caffeine consumption of
>300 mg day
1
significantly increased the risk for
foetal growth retardation. The mean birth weights
for no, light (1–150 mg day
1
), moderate (151–
300 mg day
1
) and heavy (>300 mg day
1
) caffeine
use were 3327, 3311, 3288 and 3170 g (reduction of
0, 0.5, 1.2 and 4.7%), respectively. Adjusted ORs for
low birth weight for women consuming 1–150, 150–
300 and 300 mg caffeine day
1
were 0.78 (95%
CI ¼ 0.45–1.35), 1.07 (0.51–2.21) and 2.05 (0.86–
4.88), respectively.
Three studies reported a reduction in birth weight for
infants born to mothers who consumed caffeine dur-
ing gestation at 400, 500 or 5 800 mg caffeine day
1
.
Olsen et al. (1991), in a study of 11 858 pregnant
women in Denmark, found that maternal coffee con-
sumption of four or more cups per day (400 mg
caffeine day
1
) was associated with a moderate de-
crease in birth weight. The adjusted OR for women
consuming 400–700 mg caffeine day
1
was 1.4 (95%
CI ¼ 1.10–1.70); for those consuming 5 800 mg day
1
,
18 P. Nawrot et al.
the OR was 1.2 (0.90–1.80). No dose–response rela-
tionship was observed. One explanation for the re-
sults might be that individuals who drink many cups
of coffee may tend to drink weaker coffee, and there-
fore the caffeine intake may have been overestimated
in the group drinking more coffee. In this study, the
women assigned to the control group consumed 0–
300 mg caffeine day
1
. McDonald et al. (1992a), in a
study of 40 455 pregnancies in Montreal, Canada,
found that coffee consumption at levels of 10 or more
cups per day was associated with low birth weights
and that consumption at levels of five to nine cups per
day was associated with lower birth weight for gesta-
tional age, after adjusting for such confounders as
maternal age, smoking and alcohol consumption.
Adjusted ORs for low birth weight at one to two,
three to four, five to nine and 10 or more cups per day
were 1.05 (95% CI ¼ 0.95–1.16), 1.08 (0.93–1.25), 1.13
(0.92–1.39) and 1.43 (1.02–2.02), respectively. For low
birth weight for gestational age, the ORs at one to
two, three to four, five to nine and 10 or more cups
per day were 1.05 (95% CI ¼ 0.94–1.16), 1.15 (0.99–
1.34), 1.34 (1.10–1.65) and 1.39 (0.97–1.98), respec-
tively, when compared with the controls (no coffee
consumption). Although Larroque et al. (1993) found
no clear relation between caffeine consumption and
birth weight in different groups of maternal tobacco
use, there was a decreasing trend in non-smokers;
women who drank >800 mg caffeine day
1
had in-
fants weighing 187 g less than the infants of those who
drank 4 400 mg day
1
, and this difference was at the
limit of significance. In this study, non-users and users
of <400 mg caffeine day
1
were combined and used as
the control group.
Seven studies reported no association of caffeine
consumption with birth weight or foetal growth
retardation at levels of 300 to 5 400 mg day
1
during
pregnancy. In a study of 12 205 women in the Boston
area in the USA, Linn et al. (1982) found no relation
between low birth weight and coffee consumption of
up to four cups per day after controlling for con-
founders, including smoking and alcohol intake. The
adjusted OR among heavy coffee drinkers (four or
more cups per day) was 1.19 (95% CI ¼ 0.86–1.65).
These negative results suggest that coffee consump-
tion had a minimal effect, if any, on birth weight
under the conditions of this study. Brooke et al.
(1989) found no significant effects of caffeine con-
sumption on birth weight in 1513 women in England
after controlling for smoking with caffeine intakes of
0, 1–200, 201–400 and 5 401 mg day
1
. Barr and
Streissguth (1991) reported no undesirable changes
in birth weight, length or head circumference for
infants born to mothers exposed to caffeine at doses
up to 750 mg day
1
during the entire pregnancy.
Godel et al. (1992) found no association between
caffeine ingestion (>300 mg day
1
) and birth weight,
length or head circumference in the babies of 162
women in northern Canada when the data were
adjusted for smoking and alcohol intake. Mills et al.
(1993), in a prospective study of 423 women in the
USA, found that moderate caffeine consumption
( 4 300 mg day
1
) was not associated with a reduc-
tion in early foetal growth. Although heavy caffeine
consumption (>300 mg day
1
) appeared to have
a negative effect on intrauterine growth and head
circumference, the negative effect was no longer sig-
nificant after adjusting for other risk factors, notably
smoking and maternal age. In a prospective study by
Shu et al. (1995), caffeine consumption at dose levels
up to 300 mg day
1
(three cups of coffee per day)
showed no relation to foetal growth. Although heavy
caffeine consumption ( 5 300 mg day
1
) in the first or
second trimester was related to a reduction of crude
mean birth weight (93 g for the first trimester, 141 g
for the second trimester), the study reported no
decrease in foetal growth in any trimester when the
data were adjusted for parity, pre-pregnancy weight,
income, smoking and nausea. A matched case-control
study by Santos et al. (1998) found no association
between caffeine consumption at an average dose
level of approximately 150 mg day
1
and increased
risk of low birth weight or intrauterine growth retar-
dation.
The interaction of caffeine consumption and smoking
and their association with low birth weight were also
reported. Several studies have found a marked posi-
tive correlation between smoking and caffeine intake,
including Godel et al. (1992), Fortier et al. (1993),
and Vlajinac et al. (1997). Beaulac-Baillargeon and
Desrosiers (1987) found that birth weight was not
statistically different with a caffeine consumption of
>300 mg day
1
for non-smokers and women who
smoked one to 14 cigarettes per day, but the birth
weight of babies of women who consumed 5 300 mg
caffeine day
1
and smoked 15 or more cigarettes per
day was significantly lighter (206 g less) than that of
babies whose mothers consumed less caffeine.
Contradictory results were found by Vlajinac et al.
(1997): that caffeine intake had an effect only in
non-smokers. Among non-smokers, women whose
daily caffeine intake was 71–140 mg day
1
had infants
weighing 116 g less than the infants of women whose
caffeine consumption was 0–10 mg day
1
. For
19Effects of caffeine on human health
those whose caffeine intake was 5 140 mg day
1
, the
decrease in birth weight was 153 g. The authors
suggested that the effect of smoking is more powerful
than that of caffeine, so that caffeine intake does
not produce any noticeable effect in women who
smoke.
It is difficult to establish the cause of the inconsisten-
cies in the results of studies investigating the associa-
tion between caffeine consumption and foetal growth.
They may have resulted from recall bias, particularly
in retrospective studies, incomplete information on
amounts and sources of caffeine consumption, mis-
classification of caffeine exposure, inadequate control
for confounders or simply unknown study bias. In two
studies (Olsen et al. 1991, Larroque et al. 1993),
investigators combined non-users and users (consum-
ing <400 mg caffeine day
1
in Larroque et al. 1993)
and used them as the control group. If, for example,
exposure to caffeine at dose levels <400 mg day
1
is
associated with reduced birth weight, then comparing
this control group with heavier users may obscure any
positive association. Despite inconsistencies in the
results, the persistent association between caffeine
consumption during pregnancy and low birth weight
observed in eight original studies strongly suggests
that caffeine may adversely affect foetal growth. This
conclusion is supported by a meta-analysis study
incorporating seven original studies and involving a
total of 64 268 pregnancies, which reported a statisti-
cally significant increase in the risk for low birth
weight babies in pregnant women consuming >50 mg
caffeine day
1
(Fernandes et al. 1998). It should be
indicated that due to the nature of data presentation in
individual studies used in meta-analysis, the authors
were unable to adjust for potential confounders (ma-
ternal age, smoking, alcohol intake or other confoun-
ders) that may have contributed to the final result.
Based on the above evaluated data, despite inconsis-
tencies in the results, it is concluded that caffeine
consumption during pregnancy at dose levels of
5 300 mg day
1
may interfere with foetal growth
(decrease in birth weight or intrauterine growth re-
tardation), particularly in smokers or heavy alcohol
drinkers.
Preterm delivery
Relatively few epidemiological studies are available
that address an association between caffeine con-
sumption and preterm delivery. Nine of 11 studies
reviewed showed that caffeine consumption at dose
levels up to 5 300 mg day
1
was not an important
risk factor for preterm delivery (Linn et al. 1982,
Watkinson and Fried 1985, Fenster et al. 1991b,
Olsen et al. 1991, McDonald et al. 1992a, Fortier
et al. 1993, Mills et al. 1993, Pastore and Savitz 1995,
Santos et al. 1998). In the case-control study per-
formed by Pastore and Savitz (1995) to investigate
the association between caffeinated beverage con-
sumption and preterm delivery in women from
North Carolina, USA, consumption at the 1–150 mg
caffeine day
1
level was associated with a moderately
increased risk of preterm delivery, although there
was no association between high levels of caffeine
consumption and preterm delivery. The lack of a
dose–response relationship strongly suggests that
there is no association between caffeine consumption
at dose levels as high as 5 400 mg day
1
and preterm
delivery.
Only two studies (Berkowitz et al. 1982, Williams et al.
1992) suggested a possible relation between caffeine
consumption ( 5 300 mg day
1
) and preterm delivery.
Although Berkowitz et al. (1982) observed no asso-
ciation between coffee consumption (four or more
cups per day) and preterm delivery in their case-
control retrospective study, tea drinking, especially
four or more cups per day in the first trimester,
resulted in a slightly increased risk of preterm delivery
(OR ¼ 2.0, 95% CI ¼ 1.0–4.0). The authors postu-
lated that some other component of tea, if consumed
in sufficient amounts, may have an adverse effect on
gestation age. In Williams et al. (1992), women who
consumed three or more cups of coffee per day during
the first trimester had a 2.2-fold increase in risk of
preterm premature rupture of the membranes com-
pared with women who consumed two or fewer cups
of coffee per day (OR ¼ 2.2, 95% CI ¼ 1.5–3.5). When
only coffee drinkers were examined, there appeared to
be a linear trend in the risk of preterm premature
rupture of the membranes as coffee consumption
increased. Maternal coffee consumption had rela-
tively little relation to the risk of spontaneous preterm
labour not complicated by premature rupture of the
membranes. Women who drank three or more cups of
coffee per day experienced a 1.4-fold increase in the
risk of spontaneous preterm labour not complicated
by premature rupture of the membranes compared
with women who drank two or fewer cups of coffee
per day (adjusted OR ¼ 1.4, 95% CI ¼ 1.0–1.9). It
should be pointed out that low socio-economic status,
history of adverse pregnancy outcome and antepar-
20 P. Nawrot et al.
tum haemorrhaging have been reported consistently
as risk factors of preterm delivery (Williams et al.
1992). Other factors, such as young and advanced
maternal age, low maternal weight before pregnancy,
and smoking during pregnancy, may also influence
pregnancy outcome.
Based on the above evaluated data, it is concluded
that caffeine consumption during pregnancy at dose
levels of 4 300 mg day
1
is unlikely to have an
adverse effect on the length of gestation (preterm
delivery).
Congenital malformations
The limited available epidemiological data show no
increase in the incidence of congenital morphological
malformations in infants born to mothers who con-
sumed three to 10 or more cups of coffee per day
(300–1000 mg caffeine day
1
) during the entire preg-
nancy.
Rosenberg et al. (1982) examined the association
between drinking caffeine-containing beverages and
five malformations (inguinal hernia, oral clefts, car-
diac defects, pyloric stenosis, neural tube defects) in a
case-control study of 2030 children in Canada and the
USA. No association was found between coffee con-
sumption at levels up to 5 400 mg caffeine day
1
and
any of the malformations investigated. In a case-
control study of 706 children with birth defects in
Finland (central nervous system defects, orofacial
clefts, musculoskeletal defects, cardiovascular malfor-
mations), coffee consumption (up to 1000 mg caffeine
day
1
) showed no significant association with mal-
formations observed under the conditions of the
study (Kurppa et al. 1983). Linn et al. (1982) reported
no consistent association between coffee consumption
(up to four or more cups per day) and the occurrence
of malformations in a retrospective study of 12 205
women in the Boston area in the USA. Similarly,
Olsen et al. (1991) found no association between
coffee or tea consumption up to four or more cups
per day and the occurrence of malformations in a
Danish study.
Narod et al. (1991) reviewed the results from many
epidemiological studies investigating potential terato-
genic effects of caffeine and found that available data
do not implicate coffee and/or caffeine as a likely
human teratogen in the classical sense (development
of morphological malformations), even at dose levels
up to eight cups of coffee per day.
In one positive study, McDonald et al. (1992b) ana-
lysed the association of coffee consumption with
congenital defects for 80 319 pregnancies in
Montreal, Canada. A significant increase in the in-
cidence of heart defects (RR ¼ 1.52, 95% CI ¼ 1.1–
2.2) was observed among the children of women who
drank three or more cups of coffee per day. However,
no specific type of heart defect was over-represented
in this group when compared with defects in babies
born to women who did not drink coffee.
There is therefore little evidence to support the hy-
pothesis that moderate consumption of caffeine
during pregnancy can present a teratogenic (morpho-
logical malformations) risk in humans. It should,
however, be noted that available data from reviewed
literature show that caffeine can be teratogenic in
animals when ingested at very high dose levels
( 5 80 mg kg
1
bw day
1
) in comparison with the
range of typical human intakes (e.g. Collins et al.
1981, James 1991a, Purves and Sullivan 1993).
Postnatal development
The foetus is exposed to caffeine ingested by the
pregnant mother, since caffeine is rapidly absorbed
from the gastrointestinal tract, readily crosses the
placenta, and is distributed to all foetal tissues. In
addition, exposure of the foetus to caffeine is en-
hanced because caffeine’s half-life is markedly in-
creased in the foetus and pregnant women in
comparison with non-pregnant adults and older chil-
dren (Dalvi 1986, Dlugosz and Bracken 1992,
Eskenazi 1993). Because of the rapid growth that
occurs during the late prenatal period, the impact of
chronic caffeine exposure may be far greater than at
any other time of life.
In a cohort study of 453 infants, caffeine ingested
during pregnancy at dose levels up to 444 mg day
1
did not adversely affect infant size at 8 months of age
(Barr et al. 1984). A prospective study of 123 infants
from three hospitals in Ottawa, Canada, showed that
caffeine consumption at doses of 5 300 mg day
1
had
no adverse effects on postnatal growth at 12 and 24
months of age following adjustment for relevant
confounders (Fried and O’Connell 1987). Barr and
Streissguth (1991) investigated the effects of prenatal
caffeine exposure on postnatal development from
21Effects of caffeine on human health
birth to 7 years of age and found that long-term
prenatal exposure (during the entire pregnancy) to
caffeine at dose levels ranging from 174 to
740 mg day
1
had no adverse effects on the physical
and/or behavioural development (e.g. orientation,
reactivity, IQ, fine and gross motor skills) of children
during the first 7 years of life.
Toubas et al. (1986) demonstrated that maternal ex-
posure to caffeine (350 370 mg day
1
, non-smokers,
185 cases) during gestation resulted in an increased
incidence of central and obstructive infantile apnoea
(cessation of breathing). The incidence of these
symptoms was greater in infants born to mothers
who smoked (85 cases) and consumed caffeine at
dose levels of 610 517 mg day
1
.
Two studies assessed the association between caffeine
consumption and the risk of sudden infant death
syndrome (SIDS). In Ford et al. (1998), heavy
consumption of caffeine ( 5 400 mg day
1
, equivalent
to four or more cups of coffee per day) was associated
with a significantly increased risk for SIDS after
adjustment for likely confounders. Although the
results of this study have been criticized (Leviton
1998) on the grounds that parental smoking was not
properly assessed, the authors responded that supple-
mentary analysis of the data supported their results.
The second study (Alm et al. 1999) found no associa-
tion between caffeine ingestion and increased risk of
SIDS at dose levels up to 800 mg day
1
during and
after pregnancy after adjustment either for smoking
or for maternal age, education, parity and smoking in
the first trimester. Many factors have been identified
that may increase the risk of SIDS including, low
maternal age, high live birth order, foetal prone sleep
position, maternal smoking during pregnancy and
postnatal exposure to passive smoke (MacDorman
et al. 1997, Oyen et al. 1997, l’Hoir et al. 1998). The
two factors, maternal smoking during pregnancy and
infant prone sleeping position, appeared to be the
major risk factors in SIDS (Golding 1997,
MacDorman et al. 1997, Brouillette 2001, Nelson
and Taylor 2001, Paris et al. 2001). Based on the data
presented, it is difficult to establish what risk, if any,
intake of caffeine during pregnancy may play in
SIDS.
Based on limited epidemiological data, it can be
concluded that it is unlikely that moderate intake of
caffeine ( 4 300 mg day
1
) by pregnant and nursing
mothers would pose adverse effects on postnatal
development.
Summary and conclusions
Caffeine is widely consumed at different levels by
most segments of the population. Both the public
and the scientific community have expressed concern
about the potential for caffeine to produce adverse
effects on human health. The possibility that caffeine
ingestion adversely affects human health was investi-
gated based on reviews of published (primarily)
human studies obtained through a comprehensive
literature search. The following potential adverse
effects of caffeine on human health were investigated:
general toxicity, cardiovascular effects, effects on
calcium balance and bone status, behavioural effects
in adults and children, carcinogenic potential, geno-
toxic potential, and reproductive effects, including
pre- and postnatal development. It should be pointed
out that review of some of the epidemiological studies
was complicated by one or more methodological
issues, such as inadequate measurement of caffeine
intake; a lack of consideration of all sources of
caffeine intake; a lack of consideration of caffeine
intake before study; the lack of distinction made
between different types of preparation and different
strengths of coffee in most studies; inadequate control
for the possible confounding effects of variables such
as smoking, alcohol consumption, age, nutrition and
lifestyle factors in some studies; the low response rates
in several studies; biased selection of adequate con-
trols because of self-selection into groups of drinkers
and non-drinkers of coffee; recall bias in retrospective
studies; and insufficient statistical power in some of
the studies. Despite these issues, the majority of the
reviewed studies provided important and useful data
with which to assess the potential effects of caffeine on
human health.
Based on the data reviewed, it can be concluded that
there is ample evidence indicating that for the general
population of healthy adults, moderate caffeine in-
take at a dose level of 400 mg day
1
is not associated
with adverse effects such as general toxicity, cardio-
vascular effects, changes in adult behaviour, increased
incidence of cancer and effects on male fertility. Nor
are moderate intakes of caffeine associated with ad-
verse effects on bone status and/or calcium balance if
adequate intakes of calcium are being consumed.
Data have also shown that reproductive-aged women
can be defined as an ‘at risk’ group who may require
specific advice on moderating their caffeine intake. It
is therefore recommended that caffeine intake for
women who plan to become pregnant and for women
22 P. Nawrot et al.
during gestation should not exceed 300 mg day
1
,
equivalent to 4.6 mg kg
1
bw day
1
in a 65-kg person.
Children are another at-risk population identified in
the literature. While data are lacking on adolescent
children, some studies exist for pre-adolescents.
Although this literature has its shortcomings, findings
of altered behaviour, including anxiety, are noted in a
variety of studies using caffeine in children. The
existing literature is difficult to compare due to differ-
ing methodologies as well as inadequacies in meth-
odology in some cases; however, effects have been
noted down to the lowest level of administered caf-
feine used (effects on state anxiety, correlated with
salivary caffeine levels at an intake of 2.5 mg kg
1
bw,
in Bernstein et al. 1994). The body of evidence, in
totality, suggests that caffeine can elicit behavioural
effects in children. Owing to these findings, as well as
the fact that the nervous system in children is con-
tinually developing and the lack of available informa-
tion on the longer-term effects of caffeine in this
population, a cautious approach is warranted. It is
judged that in the absence of more robust data
associated with low levels of administered caffeine,
an upper intake of 2.5 mg kg
1
bw day
1
is an amount
on which to base risk assessments of caffeine con-
sumption in children.
Acknowledgements
The authors gratefully acknowledge the assistance of
Dr Sheila Dubois for statistical analysis, Elizabeth
Vavasour for critical comments, Betty Anne
Morrison for clerical assistance and Marla Sheffer
for editorial assistance.
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