Tea and health—A review of the current state of knowledge

Abstract

Until recently, tea has been numbered among stimulants, i.e. products of no nutritional value. Nowadays, with advance of research studies, the amount of data suggesting beneficial effect of tea on health is increasing. Polyphenols are the basic tea ingredients to which positive effect on human body is attributed. Their wide spec- trum of biochemical activity, including a strong antioxidant potential, contributes to the situation in which tea may have various beneficial functions in the body. Research studies focus mostly on green tea which is believed to reduce the risk of many modern diseases. However, so far the preventive effect of tea has not been confirmed yet. Despite it being a natural product, too much tea in a diet carries the risk of excessive caffeine intake and decreased absorption of non-heme iron which may be of detrimental consequences for some groups of consumers.

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PRZEGL EPIDEMIOL 2014; 68: 501 - 506 Public health
© National Institute of Public Health – National Institute of Hygiene
Regina Wierzejska
TEA AND HEALTH – A REVIEW OF THE CURRENT STATE OF KNOWLEDGE
Department of Nutrition and Dietetic with Clinic of Metabolic Diseases and Gastroenterology,
National Food and Nutrition Institute in Warsaw
ABSTRACT
Until recently, tea has been numbered among stimulants, i.e. products of no nutritional value. Nowadays,
with advance of research studies, the amount of data suggesting beneficial effect of tea on health is increasing.
Polyphenols are the basic tea ingredients to which positive effect on human body is attributed. Their wide spec-
trum of biochemical activity, including a strong antioxidant potential, contributes to the situation in which tea
may have various beneficial functions in the body. Research studies focus mostly on green tea which is believed
to reduce the risk of many modern diseases. However, so far the preventive effect of tea has not been confirmed
yet. Despite it being a natural product, too much tea in a diet carries the risk of excessive caffeine intake and
decreased absorption of non-heme iron which may be of detrimental consequences for some groups of consumers.
KEY WORDS: tea, polyphenols, caffeine, health
INTRODUCTION
Tea is considered to be one of the most popular
beverages. It is estimated that the mean consumption
of tea per inhabitant worldwide amounts to 120 mL/
day while for the Great Britain, which since centuries
is famous of afternoon tea and high consumption of tea,
it is 540 ml per day on average (1). In Poland, 76-90%
of population drinks tea with the majority consuming
2-3 cups in a day (2).
Tea is produced from leaves, buds or delicate stems
of the plant of genus Camellia. Dependent on the
manufacturing process, three main types of tea may
be enumerated, different in terms of flavour, colour
and composition, i.e. green tea which is not subject
to fermentation, oolong tea which undergoes semi-
fermentation and black tea which results from full
fermentation of plant. So far, the highest percentage of
worldwide production and consumption has been at-
tributed to black tea (ca 80%), which is very popular in
North America and Europe and then to green tea which
is mainly consumed in China and Japan (3, 4).
In folk medicine of Asian countries, tea was con-
sidered to be a medicine effective for many ailments.
Furthermore, drinking tea was treated as a ceremony
and philosophy of life (4, 5). Nowadays, tea and its
impact on health attracts the attention of public and
literature provides optimistic data regarding tea’s ben-
eficial impact on cardiovascular system, reduction of
body mass and even decreasing the risk of cancer and
neurodegenerative diseases (3, 6).
The objective of the paper is to present the current
data regarding the nutritional value of tea and its effect
on health.
NUTRITIONAL VALUE OF TEA
The sugar free tea infusion is of no energetic value.
Consequently, the amount of tea in a diet does not affect
the daily calories intake. The energetic value of sweet-
ened tea is dependent on the type of sugar substitutes
used. The tea sweetened with table sweeteners does
not provide calories while a cup of tea with a teaspoon
of sugar or honey added has ca 20 kcal. Tea is a good
source of ingredients having physiological properties,
including polyphenols, caffeine and certain minerals.
Polyphenols
Having regarded the climate conditions of tea cul-
tivation, including insolation and humidity as well as
the age of picked leaves, the concentration of polyphe-
nols in tea leaves varies. Their concentration in young
tea leaves is higher compared to older ones which

Regina Wierzejska
502 No 3
are picked later. The type of polyphenols and their
properties are associated with tea manufacturing pro-
cess. Green tea contains mainly catechins while black
tea - tannins, produced from catechins as the result of
biochemical changes in the fermentation process. Hav-
ing considered the concentration of catechins in green
tea, the most predominant is epigallocatechin gallate
(EGCG) which constitutes more than 50% of the total
amount of catechins. This compound, compared to
other catechins, is of the highest antioxidant potential.
Thus, it is considered to be the most biologically active
substance (3, 7-9).
From the nutritional perspective, the important
issue is the amount of polyphenols in tea infusion
intended for consumption. Besides the natural amount
of polyphenols in tea leaves, their concentration may
be highly affected by the process of infusion, including
the amount of tea used for a cup, water temperature and
brewing time. From the studies regarding the extent of
polyphenols extraction in the infusion transpires that
their highest concentration (170 mg/L) is observed in
teas brewed for 10 minutes. Such infusions, however,
are not preferred by consumers due to the worse organo-
leptic properties, especially bitterness, resulting from
high concentration of polyphenols (10). According to
the European Food Safety Authority (EFSA), 100 ml of
green tea infusion contains 126 mg of catechins. In the
light of data of Food and Drug Administration (FDA),
such amount of green tea contains 71 mg of epigallo-
catechin gallate (11, 12). Other authors state that a cup
of black tea provides 200 mg of flavonoids (9).
A lot of data support the statement that green tea is
the most optimal source of catechins among the food
products, followed by chocolate, red grapes, wine and
apples (4). The American Heart Association recom-
mends the Americans to increase the consumption of
flavonoids, including those contained in tea. So far,
however, no recommendations regarding the amount
of polyphenols in a diet have been issued (13).
Caffeine
Tea, accompanied by coffee and cocoa seed, is the
main vegetable source of caffeine. Caffeine is a meth-
ylxanthine of group of alkaloids, belonging to psycho-
active substances. Its action consists in stimulating the
central nervous system and cardiac action, which may
positively affect the psychophysical capacity of a hu-
man (2). In teas such as Camellia sinensis and Camellia
assamica, caffeine predominates (ca 90%) over other
methylxanthines. However, there are also teas, e.g.
Camellia ptilophylla, which contain high amount of
theobromine (6%), thus, they are called cacao tea. The
concentration of caffeine in tea leaves ranges from 2 to
5%, with higher values observed in young tea leaves (5).
The concentration of caffeine in tea infusions is
dependent on the amount of tea used for a cup as well
as brewing conditions, including water temperature and
brewing time. The extraction efficiencies of caffeine
from tea leaves is assessed at 61–85% (14). Having
considered the multitude of factors, the concentration
of caffeine in a cup of tea ranges from 14 to 65 mg (15).
The highest concentration of caffeine is present in teas
which are brewed for a long time. The studies on the
caffeine concentration in black teas, purchased on the
Warsaw markets, revealed that a cup of tea brewed for
a short time (15 seconds), one minute and long time
(5 minutes) provides 15, 22 and 34-38 mg of caffeine,
respectively (16). EFSA states that a cup of black tea,
brewed for 3–5 minutes, contains 40–45 mg of caffeine
(11).
Having considered the caffeine concentration in
black and green tea infusions, the study results vary.
Several authors claims that the caffeine concentration
in green tea is twofold lower compared to black tea
while others state that their concentration is comparable
(16, 17).
Furthermore, the study results on caffeine concen-
tration in tea bags and loose leaf tea infusions are not
coherent either. Pursuant to the certain results, caffeine
extraction from tea bags is 30% poorer compared to
loose leaf teas which results in lower concentration of
caffeine. Others claim that the tea bags infusion is of
higher caffeine concentration due to higher fragmenta-
tion of tea leaves (4). The studies conducted in Poland
did not reveal any disparities between the caffeine
concentration in both types of teas (16, 17).
Minerals
Dry tea leaves contain numerous elements such
as fluoride, manganese, chrome, selenium, calcium,
magnesium and zinc. Their concentrations are highly
differentiated, ranging from 25 to 1000 µg/g and depen-
dent on many factors, including conditions of cultiva-
tion, fermentation and even leaves size (4). From the
nutritional perspective, however, more important is the
concentration of these elements in tea infusions intended
for consumption. Having considered the amount of tea
required for preparation of tea infusion and extraction of
minerals, tea infusion is not a good source of calcium,
zink and magnesium in a diet. According to the country
data, 100 ml of tea provides 0.07 mg of zink, 2 mg of
magnesium and 5 mg of calcium which means that a
cup of tea covers only 1-1.5 % of recommended daily
intake of these elements in adults (18).
The situation is different in case of fluoride and
manganese which are highly prevalent in tea infusions.
The fluoride concentration in 100 ml of infusion, pre-
pared using non-fluoridated water is 0.15 mg on average
(1). Thus, a cup of tea covers 7-10% and 25-30% of

Tea and health 503No 3
the daily intake of this element in adults and children,
respectively. Having considered manganese, the western
countries data specify that the concentration of this ele-
ment in tea infusion ranges from 0.17 to 0.66 mg/100
ml of green tea and from 0.08 to 0.51 mg/100 ml of
black tea (19, 20). According to the country tables of
food nutritional value, 100 ml of tea infusion contains
0.34 mg of manganese on average (18). Due to its
high concentration in tea, it becomes a major source of
manganese in a diet of persons who drink tea. In the
Great Britain, the average consumption of manganese
in persons drinking ≥ 1 litre of tea per day was assessed
at 5.5–10 mg, while in persons who do not drink tea
– manganese consumption was 3.2 mg on average. In
case of high consumption of strong tea, the intake of
manganese may achieve Tolerable Upper Intake Level,
which for adults was established at 11 mg/day (4, 19).
Irrespective of high intake of manganese with tea, the
clinical trials do not reveal significant differences in its
concentration in blood and expression of Mn-dependent
enzymes in persons who drink tea and non-tea drinkers
(19). It may suggest that the organism is able to sustain
quite constant concentration of manganese (homeostasis
of manganese) as well as the lack of direct impact on its
concentration in organism with higher intake in a diet.
Remaning ingredients
Tea belongs to the products containing siginifcant
amount of oxalates. These compounds form insoluble
salts with minerals. Consequently, they hinder their
absorption and increase the risk of nephrolithiasis.
The concentration of oxalates in tea infusions is highly
dependent on water temperature and brewing time. The
study of Jabłońska-Ryś revealed that the concentration
of oxalates in teas brewed in the temperature of 100 °C
for 5 minutes ranged from 2.5 to 6.57 mg/100 ml of in-
fusion. Green teas, brewed in lower temperature (75°C)
contain lower concentration of oxalates compared to
teas prepared with boiling water (21).
Tea infusion also contains small amounts of niacin
(0.1 mg/100 ml) and folates (5 µg/100 ml) which com-
pared to the average daily requirement does not con-
stitute their significant source in a diet (18). According
to Spanish data, tea provides 3-5% of daily intake of
vitamins C and E in this population. However, Polish
data suggest that tea infusion does not contain aforesaid
vitamins (4, 18).
The leaves of tea contain also polysaccharides
(13%), fats (3%), amino acids (4%), including mainly
L-theanine, which as the data suggest may have a posi-
tive effect on mental capacity. However, the literature
does not provide information on concentration of these
compounds in tea infusions. A separate group of tea
ingredients constitute numerous aromatics, saponins
and colour additives (1, 11).
TEA AND HEALTH
For the last 20 years, tea was an object of many
researches considering its health benefits. However,
it has not been determined yet whether conventional
amount of tea in a diet is of prophylactic properties (5,
6). The hypothesis regarding health-oriented effect of
tea is based on the high concentration of polyphenols
which increase the antioxidant potential of blood and
thus can protect cells and tissues from detrimental
effects of oxidation (9). Besides antioxidant activity,
anti-inflammatory properties are also attributed to poly-
phenols consisting in stimulating the immune system,
decreasing the absorption of cholesterol in blood and
having positive effect on intestinal microflora by inhibit-
ing the colonization with pathogenic bacteria (1, 22).
In the worldwide literature regarding the impact of
tea on health, the publications from the Asian countries
predominate, where the most popular is green tea and
the data on black tea is considerably less abundant.
Green tea
Such type of tea is characterized by high concen-
tration of catechins which are of stronger antioxidant
potential compared to tannins present in black tea. From
the epidemiological studies transpires that persons who
drink more than 2 cups of green tea per day have lower
concentration of total cholesterol and LDL cholesterol,
which is also confirmed by the results of meta-analysis
of randomized clinical trials. The authors claim that it
may be interpreted into 22-33% lower risk of death due
to cardiovascular diseases. Thus, drinking green tea is
considered to be an element of prophylaxis of heart
diseases (23). Having referred to other data, only higher
consumption of green tea – at least 7 cups per day may
have a prophylactic effect on cardiovascular diseases.
However, it is emphasized that this assumption requires
further studies (3).
Furthermore, the relation between drinking tea and
progression of cancer has not been determined yet.
Several studies involving animals suggest the beneficial
effect of tea on decreasing the risk of cancer. However,
it should be considered that tea dose or its ingredients
administered to animals determined by body mass in
kg are much higher compared to those consumed by
humans (24). One of the argument cited in the literature,
which confirms the effect of green tea on carcinogenesis
is the low percentage of lung cancers in Japan. Japa-
neses who smoke tobacco to a larger extent compared
to Americans, develop lung cancer significantly more
rarely which may result from disparities in green tea
consumption between these populations (22).
Having referred to the progression of stomach can-
cer, the current literature provides two meta-analyses of

Regina Wierzejska
504 No 3
epidemiological studies. From one of them transpires
that drinking 5 cups of green tea per day may have a
protective effect. Nevertheless, the authors emphasized
that these results should be interpreted cautiously due to
the presence of numerous life style factors affecting the
progression of disease (25). The second meta-analysis
suggests that persons who drink high amount of green
tea (5-10 cups per day) do not suffer from stomach
cancer less frequently compared to those who drink its
small amount (1-2 cups per day) (26).
Several studies regarding the effect of tea on cancer
focus on the correlation between green tea consumption
and colorectal cancer. Some of them suggest that drink-
ing green tea may have a slight effect on decreasing the
risk of this cancer occurrence while others do not reveal
any relation (1, 27). Such situation is also observed in
case of breast and prostate cancer. According to the
Food and Drug Administration, the study results are so
ambiguous that based on the current state of knowledge
it is not feasible to acknowledge that green tea may
decrease the risk of aforesaid cancers (12).
The studies conducted in Chinese population indi-
cate that drinking tea may slightly protect from Parkin-
son’s disease. The mechanism of such action has not
been recognized, however, it is presumed that it may
result from the action of polyphenols and caffeine which
increase the circulation of blood in brain. These conlu-
sions are of such initial nature that nowadays they are
exclusively subject to scientific deliberations (28, 29).
Several studies suggest that green tea reduces the
body mass as the effect of the action of caffeine and
epigallocatechin gallate which decrease the absorption
of fat, increase the energetic expenditure and burn-
ing of adipose tissue (4, 7-8, 23). Pursuant to some
epidemiological studies, persons who drink more than
400 ml of tea per day for many years (mainly green
tea) have lower body mass and waist circumference
compared to non-tea drinkers. Contrary conlusions are
drawn from the meta-analysis of randomized clinical
trials which suggests that in patients who were admin-
istered catechins and caffeine derived from green tea
for 12 weeks, the observed reduction of body mass was
inconsiderable (<0,5 kg) and statistically insignificant
(7). Irrespective of the contrary results, some experts
claim that drinking green tea may be considered as an
element of prevention of obesity. Since years, produc-
ers use the ingredients derived from green tea for the
production of weight loss supplements (8). However,
here a fact should be mentioned. Based on the evalua-
tion of studies results intended to issue health-oriented
statements regarding the food, EFSA claimed that the
beneficial effect of caffeine on body mass has not been
scientifically proved. The works of EFSA concerning
the assessment of health aspects of teas and polyphenols
are in progress.
Black tea
Black tea is not subject to analyses to such an extent
as green tea is. However, it may be presumed that some
properties of green tea can also be referred to black tea
with the example being the effect of tea on reduction
of body mass resulting from caffeine action. If some
studies suggest that green tea reduces the body mass,
then it should be remarked that it does not contain higher
concentration of caffeine than black tea. Thus, such ac-
tion should be associated with these two types of teas.
The beneficial conclusions are also drawn from the
relations between drinking black tea and heart action.
From the review of epidemiological studies transpires
that drinking at least 3 cups of black tea per day reduces
the risk of cardiovascular diseases which results mainly
from the presence of polyphenols, mainly tannins (1).
The meta-analysis of epidemiological studies also
presents interesting and optimistic data which specify
that drinking at least 4 cups of tea per day (irrespec-
tive of its type) may reduce the risk of type 2 diabetes
by 20% (30). Similar results were obtained for coffee.
However, in both cases, it is not linked with the con-
centration of caffeine but polyphenols (31).
SAFE TEA AMOUNT IN A DIET
Tea which is consumed in conventional amounts
does not negatively affect the human health. It may
have detrimental affects when tea consumption in a diet
is high. It results from excessive absorption of caffeine
and polyphenols binding non-heme iron which reduce
its absorption in the gastrointestinal tract even by 79%.
Polyphenols which are present in black tea, especially
tannins inhibit the absorption of iron to a larger extent
compared to catechins of green tea. Thus, persons di-
agnosed with iron deficiency anaemia or being at risk
of developing anaemia should refrain from consuming
tea, especially black tea at meals (1, 4, 5). Having con-
sidered its effect on decreasing the absorption of iron,
black tea is recommended in a diet of persons with
haemochromatosis in which the excessive accumulation
of iron is observed.
From the perspective of caffeine, the safe amount
of tea is dependent on the consumption of other prod-
ucts such as coffee, cola or energy drink. In the light
of current state of knowledge, the safe consumption of
caffeine by healthy adults and children aged up to 12
years should not exceed 400 mg per day and 2.5 mg/
body mass in kg/day, respectively. Provided there are no
other sources of caffeine in a diet, an adult could even
consume several cups of tea.
Several experts claim that having considered
the concentration of caffeine, its diuretic properties

Tea and health 505No 3
observed with the high consumption as well as the
unfavourable effect on iron absorption, the maximum
consumption of black tea should not exceed 8 cups per
day (1).
SUMMARY
Tea is a natural, low-processed and calorie-free
beverage. Thus, it is much better for the majority of
population, including children compared to sweetened
and carbonated beverages. Drinking tea is not only
pleasureable but also may have beneficial effect on
health by strengthening the antioxidant potential of
human organism.
The study results in this respect are promising but
they should be interpreted cautiously as the tea infusion
composition is affected by many factors. Irrespective of
that fact, nowadays the producers of food are increas-
ingly interested in tea extracts intended to be used for
functional food.
In the light of current state of knowledge, it may be
presumed that green tea is of higher nutritional value
compared to black tea. However, the irrefutable argu-
ments indicating which type of tea is the most optimal
and what is their effect on health are still to be uncovered
in the future.
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Received: 24.01.2014
Accepted for publication: 25.06.2014
Address for correspondence:
Regina Wierzejska
Department of Nutrition and Dietetic with Clinic
of Metabolic Diseases and Gastroenterology
National Food and Nutrition Institute in Warsaw
Powsińska 61/63 street
02-903 Warsaw
Tel. (22) 55 09 747
rwierzejska@izz.waw.pl