Taste acuity in response to zinc supplementation in older Europeans
Barbara J. Stewart-Knox1*, Ellen E.A. Simpson1,2, Heather Parr1, Gordon Rae2, Angela Polito3,
Federica Intorre3, Maud Andriollo Sanchez4, Natalie Meunier5, Jacqueline M. O’Connor1,
Giuseppe Maiani3, Charles Coudray5and J. J. Strain1
1Northern Ireland Centre for Food and Health, University of Ulster, Coleraine BT52 1SA, UK
2School of Psychology, University of Ulster, Coleraine, UK
3National Institute for Food and Nutrition Research, INRAN, Rome, Italy
4Laboratoire de Nutrition Vieillissement et Maladies Cardiovasculaires (NVMC), Faculte ´ de Pharmacie,
Universite ´ de Joseph Fourier, Grenoble, France
5Centre de Recherche en Nutrition Humaine d’Auvergne, Unite ´ des Maladies Me ´taboliques et Micronutrients, INRA,
(Received 5 September 2006 – Revised 29 May 2007 – Accepted 31 May 2007)
Taste acuity declines with age and may be dependent upon Zn status. The aim of the present double-blind, randomised controlled intervention trial
has been to determine taste acuity in response to Zn supplementation (placebo, or 15 or 30mg Zn/d). Healthy older European adults aged 70–87
years were recruited within Italy (Rome) (n 108) and France (Grenoble) (n 91) to the European Commission-funded Zenith project. A signal detec-
tion theory approach was adopted for taste assessment. The data were converted to R indices and analysed by repeated-measures ANOVA con-
trolling for baseline taste acuity as well as serum and erythrocyte Zn. Serum Zn increased post-intervention, indicating compliance with the
intervention. Results differed across geographical region. Salt taste acuity was greater in response to Zn (30mg) than placebo post-intervention
among those recruited in Grenoble. There was no apparent change in acuity for sweet, sour or bitter taste in response to Zn. Supplemented Zn
may have potential to enhance salt taste acuity in those over the age of 70 years. Further research is required to determine if enhanced salt
taste acuity is reflected in the eating experiences of older individuals.
Zinc: Elderly: Taste acuity: Signal detection: Basic tastes
Serum Zn levels tend to decrease with advancing age1,2, poss-
ibly because of lower consumption of Zn-rich foods, or foods
that adversely affect the bioavailability of Zn, by older indi-
viduals3,4. Taste acuity also appears to decline in normal
ageing5–8. Previous taste threshold studies have indicated
that the extent of decline in taste acuity differs for each of
the basic tastes. Consistent evidence implies an age-related
decline in salt taste acuity6,8–10, which may be associated
with putative indicators of Zn status11. Zn is present in
saliva12as well as in the salivary gland13and is linked to gus-
tatory nerve activity14. Zn is bound to gustin, which is a pro-
tein and, like Zn, a component of saliva15. Zn deficiency can
be associated with a lack of gustin16and can potentially lead
to impaired taste acuity7. Zn may be involved in the control of
appetite through interaction with leptin17. Depleted Zn can be
associated with reduced appetite generally and may result
from, or lead to, decreased intake of Zn-rich foods by older
individuals2,18–20. Zn deficiency, therefore, could be both a
cause and an effect of age-related decline in taste acuity.
Taste impairment has been observed in cases of Zn
deficiency21,22and supplemental Zn has been shown to both
prevent23and treat24taste disorder and enhance taste acuity
in Zn-depleted individuals25–27. It is important to consider
the degree to which age-impaired taste acuity is responsive
to Zn supplementation given the potentially detrimental
effect upon food preference and choice and, ultimately, overall
dietary quality28,29. Older individuals are at risk of sub-opti-
mal Zn intake for physiological, psychosocial and economic
reasons30. Age-related decline in taste acuity, therefore,
could potentially result from failure to meet Zn requirement.
There is convincing evidence that would lead us to assume
that a large proportion of individuals over the age of 60
years experience some degree of age-related decline in taste
acuity31–33. The present study, therefore, meets a need to
establish whether supplemental Zn can improve taste acuity
in normal ageing9. The aim of the present double-blind, ran-
domised controlled intervention trial was to compare taste
acuity for the four basic tastes (sweet, sour, bitter and salt)
using a signal detection approach, in response to elemental
Zn supplementation (placebo, or 15mg or 30mgZn/d) in
healthy older European adults aged 70–87 years recruited in
Grenoble (France) and Rome (Italy).
*Corresponding author: Dr Barbara Stewart-Knox, fax þ44 28 70324965, email B.email@example.com
British Journal of Nutrition (2008), 99, 129–136
q The Authors 2008
British Journal of Nutrition
The research was carried out from centres in Grenoble (Uni-
versity de Joseph Fourier) and Rome (Istituto Nazionale di
Ricerca per gli Alimenti e la Nutrizione; INRAN). Ethical
approval was obtained independently by both research
centres to conduct the present study. All volunteers gave
informed written consent to take part in the screening and
research study. A randomised, placebo-controlled, double-
blind clinical controlled trial design was employed in
which taste acuity (detection thresholds) for the four basic
tastes was assessed in response to elemental Zn (15mg
and 30mg) or placebo over a 6-month period. Because the
present study aimed to determine the impact of Zn upon
healthy ageing, it was necessary to employ dosages that
would not be toxic to healthy individuals and available
Older individuals were recruited through community groups
and organisations. Prospective volunteers then initiated con-
tact with the research group. Approximately 10–15% of
groups, etc volunteered to take part in the study. Volunteers
underwent screening provided they met none of the follow-
ing exclusion criteria: undergoing treatment for acute or
chronic disease (including cancer, diabetes, renal or hepatic
disease, malabsorption and chronic inflammatory pathol-
ogies); having a BMI of ,20 and .33kg/m2; consuming
a special prescribed diet or having unusual dietary habits
(vegetarian or vegan); consuming .30galcohol/d (more
than twenty-one units/week) for men or .20g alcohol/d
(more than fourteen units/week) for women; smoking more
than ten cigarettes per d (.10gtobacco/d); taking more
than four prescription drugs; using hormone replacement
therapy; taking medication that could affect Zn absorption;
taking nutritional supplements containing minerals or trace
elements within6 months
study34. Zn at doses of 30mg and above can be toxic,
especially if Cu intake is sub-optimal35,36. Individuals with
chronic intestinal disorders and any who were prescribed
medication, nutritional supplements or dietary regimes that
could affect Zn absorption or inhibit Zn bioavailability
were also excluded from taking part in the study. Full medi-
cal screening was provided which included liver and kidney
function tests, full blood profiles, and blood pressure and
heart rate measures. Qualified phlebotomists and nurses
took blood for these tests. Volunteers were assessed for
depression by means of the Geriatric Depression Scale37
and for dementia by means of the Mini Mental State Exam-
ination38. Those with cognitive deficit or mental health pro-
blems were subsequently excluded from the study. There
was some sample attrition subsequent to recruitment (n 23).
This was because a number of volunteers did not arrive for
initial testing, while others refused supplementation. The
eventual sample subjects were apparently healthy commu-
nity-dwelling older adults aged 70–87 years (n 199).
Approximately equal numbers of males and females were
randomly allocated to either the placebo, or 15mg or
30mg Zn-supplemented groups.
Putative indicators of zinc status
Both serum and erythrocyte Zn determinations were carried
out at the Laboratoire Nutrition, Vieillissement et Maladies
Cardiovasculaires (NVMC), Grenoble, France. Details of
sample storage and transport have been reported elsewhere
by Andriollo-Sanchez et al.39. Serum, erythrocyte and urinary
Zn concentrations were determined by flame atomic absorption
spectrometry using a PerkinElmer 560 (PerkinElmer Life And
Analytical Sciences, Inc., Waltham, MA, USA) and employing
a previously described method40. Seronormwtrace element
serum was used as an internal quality control (Serow, Billing-
stad, Norway). Dietary Zn intake was assessed by means of a
semi-structured standardised 4d food diary. The information
in the food diaries was analysed using the NetWISP (version
3.0; Tinuviel Software, Anglesey, UK) database.
Taste threshold assessment
Taste acuity refers to the range of taste stimuli that an indi-
vidual is able to detect41and is usually assumed from taste
threshold assessment. Taste acuity is reflected in ability to
detect and/or recognise the four to five basic tastes –
sweet, sour, salty, bitter, and umami (monosodium glutamate)
– and can be assessed in a variety of different ways42. The
reported study adopts a signal detection theory approach43
adapted for sensory taste threshold measurement44
which it has been shown to produce repeatable results45.
Signal detection theory views sensory taste threshold detec-
tion as the outcome of a cognitive decision-making process46.
The procedure adopted required the panellist to decide
whether ‘noise’ (i.e. water) or ‘noise and signal’ (i.e. water
and tastant) was present in each trial comprised of three
samples (one signal and two noise stimuli)47. As a measure
of decision criteria and to control for guessing, panellists
were also required to indicate whether they were sure or
unsure of their decision.
The taste solutions were made up using glucose (sweet),
sodium chloride (salt), citric acid (sour) and quinine hydro-
chloride (bitter). Both research centres acquired equipment
and ingredients for the taste solutions from the same suppli-
ers. All chemicals were ‘food’ grade. Sodium chloride and
glucose were both supplied by Provincial Butchers Supplies
Ltd (Lisburn, County Antrim, UK). Sigma Aldrich, UK,
supplied the citric acid and quinine hydrochloride. Each
tastant was suspended in a series of six solutions of increas-
ing concentration using ultra-purified water (deionisation
and reverse osmosis to purity greater than 15MV). With
exception of glucose, these initial solutions were based on
concentrations previously presented to older adults and
reported by Mojet et al.6. A pilot study (n 4) employing
three females and one male, aged over 55 years, was carried
out to establish that the range of strengths required for each
basic taste was adequate to prevent ceiling or floor effects.
This resulted in the final ranges (g/l): sweet (glucose),
4 £ 10212 51; sour (citric acid), 1 £ 10232 39 £ 1022;
salty (sodium chloride), 6 £ 10232 1·75; bitter (quinine
hydrochloride), 1 £ 10242 7·5 £ 1022.
B. J. Stewart-Knox et al.130
British Journal of Nutrition
The solutions were prepared weekly. For each taste concen-
tration, the compounds were weighed out, added to 800ml
purified water in a Pyrex beaker, stirred until dissolved, then
poured into a volumetric 1 litre flask, to which further purified
water was added to make up 1 litre of the solution and
then stirred well. Solutions were stored at 58C for use within
3–4d. Polypropylene cups were used to contain each tastant
and/or purified water, the volume of which presented was
2·5ml. The solutions were then presented at room temperature
for the purpose of the sensory trials.
The intervention commenced in February 2003 and completed
in September 2004. Subsequent to a 12h overnight fast, blood
samples (5ml) for serum and erythrocyte Zn assay were col-
lected via antecubital venepuncture into EDTA and serum
tubes by a qualified phlebotomist at baseline and again 3
months and 6 months into the intervention period. For Zn
element-free vacutainerwtubes (Becton Dickinson, Le Pont
de Claix, France). Samples were kept on ice immediately
after drawing. Trace element-free pipettes and vials were
also used for the preparation of samples. Fasting venous
blood samples were allowed to sit at room temperature and
clot for 30min before serum was separated from blood cells
by centrifugation (15min; 1000g) and then sampled into
Eppendorfs. EDTA tubes were centrifuged immediately at
1500rpm for 15min and then sampled into Eppendorfs. Eryth-
rocytes were then washed three times in PBS (pH 7·4). Serum
samples were diluted 1:5 in 0·1 M-HCl. Erythrocytes were
diluted 1:100 in deionised water. Serum and erythrocytes
were stored at 2808C. Carriage conditions for the frozen
samples were identical across the two regions.
Dietary Zn was assessed at baseline and 6 months later at
the conclusion of the intervention using 4d food diaries that
included estimates of portion size. Full standardised written
and verbal instructions were provided. All food and drink con-
sumed, including snacks, was recorded in the food diary over
4 consecutive days comprising two weekdays (Thursday and
Friday) and two weekend days (Saturday and Sunday).
Participants were required to take elemental Zn (either 15 or
30mg) per d (as zinc gluconate) or placebo over a period of 6
months. Supplements were supplied by E-Pharma and issued
in 7d pill organiser boxes (Carepac, Farringdon, Oxon, UK).
The study required that all participants ingested two tablets
daily. Those in the 15mg Zn group received two capsules of
7·5mg daily and those in the 30mg Zn group received two
15mg capsules daily. Written instructions as to when and
how to take the tablets were provided. The pill-boxes were
checked for compliance and replenished every 6 weeks. Par-
ticipants also kept a diary to record any supplements that
they forgot to take.
Taste acuity (threshold) assessment also took place initially
at baseline and was repeated at 3 and 6 months during the
intervention period. Both centres carried out tasting at the
same time of day, in the early morning (09.00 hours) and
employed the same method throughout. Participants were
asked to attend the food sensory testing laboratory early in
the morning having fasted overnight to control for individual
differences in satiety.
was collectedusing trace
The order in which the basic tastes were presented was stan-
dardised as follows: citric acid (sour); glucose (sweet); sodium
chloride (salt); quinine (bitter). Both verbal and written stan-
dardised instructions were given on how to carry out the
test. The aqueous solutions (2·5ml) were presented in the
cups on a tray in six ascending concentrations for each taste
quality. For each basic taste, eighteen cups were set out in
six rows each of three cups, one cup per row contained the
taste solution (signal plus noise) and the other two cups con-
tained purified water (noise). A comparison of results of ‘sip
and spit’ and ‘sip and swallow’ tests have indicated that sip
and spit tests do not relate well to consumption48. This
suggests that results of sip and spit tests should be interpreted
with caution especially when assessments have been made by
untrained panellists. Accordingly, ‘natural’ consumption-
based tasting methods were deemed appropriate for the pre-
sent study. In an effort to mimic normal taste conditions as
far as possible and in doing so to maximise the chances that
the taste be detected, participants were instructed to ‘sip and
swallow’ each solution ‘in their normal way’.
Participants were instructed to sample, in an ascending
series, the contents of each of the three cups (2·5ml in each
cup) in each row once, decide which solution was different
and to record their decision on a standardised response sheet
before moving on to taste the next row containing solutions
of greater strength. The task was to distinguish the ‘signal’
(tastant plus purified water) from the ‘noise’ (purified water)
in each row. In addition, for each row (triad) the participants
were required to indicate whether they were sure or unsure as
to their decision. No rinsing was advocated between taste
samples, merely to sip and swallow purified water following
presentation of each taste (sour, sweet, salt and bitter) and
before moving on to the next, to minimise habituation to the
different taste qualities. No conferring on decisions was per-
mitted. On completion of the research study, a light breakfast
The R index is a data-handling technique developed by
Brown49for the purpose of discrimination studies and is
amenable to parametric analysis44. The R index is the pre-
dicted probability of the correct choice of a signal over
decision-making criteria. For the purpose of the present
study, the R index was derived from the information recorded
during signal detection trials regarding the detection of a
signal or noise also taking into account whether the respondent
was sure or unsure about each decision. Each response was
quantified and tabulated into one of the following categories:
correct-signal-sure; correct-noise-sure; correct-signal-unsure;
sure; incorrect-signal-unsure; incorrect-noise-unsure. Data
were then converted to R indices using a macro statistical
equation created in Microsoft Excel (Microsoft Corp., Red-
mond, WA, USA). The higher the R index the lower the
taste threshold and the greater the taste acuity. R indices
were computed for citric acid, glucose, salt and quinine and
were entered separately as dependent variables in each anal-
ysis. Data were then transferred into Statistical Package for
the Social Sciences (version 11; SPSS Inc., Chicago, IL, USA).
Taste acuity and zinc supplementation131
British Journal of Nutrition
Factorial ANOVA was carried out to determine any change
in serum and erythrocyte Zn levels in response to the Zn inter-
vention (placebo, or 15mg or 30mgZn). Repeated-measures
ANOVA was undertaken to compare dietary Zn and salt
intake between centres (Rome and Grenoble) and by condition
(placebo, or 15mg or 30mgZn) over time (baseline and 6
months). Factorial ANOVA with repeated measures (baseline,
3 months and 6 months) was carried out separately for each
region (Rome and Grenoble) and for each basic taste (sweet,
sour, salt and bitter) to establish differences in taste acuity
in response to Zn (placebo, or 15mg or 30mgZn). Baseline
serum and erythrocyte Zn status values were entered as covari-
ates to control for differences in Zn status before intervention.
Baseline taste acuity was also entered into the analysis as a
covariate to control for individual differences in taste acuity.
Approximately equivalent proportions of each sex were
recruited in Rome, Italy (fifty-six males and fifty-two females;
mean age 74·5 years) and Grenoble, France (forty-seven males
and forty-four females; mean age 74·2 years). A total of 189
individuals (ninety-one in Grenoble and ninety-eight in
Rome) completed the taste tests. Owing to difficulties in clas-
sifying social class by occupation across cultures, social class
has been collapsed into three groupings (professional, skilled
and unskilled). Grenoble had a higher proportion of pro-
fessional participants (41%) than Rome (27%). Educational
background was evaluated in terms of the percentage that com-
pleted tertiary education in Rome (20%) and Grenoble (15%).
Putative indicators of zinc status
There were no differences in serum Zn between regions or
treatment group at baseline. Mean serum Zn levels were
within the normal range (11–18mmol/l)39for the placebo
(13·20 (SD 1·69) mmol/l), 15mg (13·28 (SD 1·84) mmol/l)
and 30mg (13·13 (SD 1·63) mmol/l) supplemented groups at
baseline and remained so throughout the intervention period.
Serum Zn increased post-intervention (F(4,197)¼ 11·021;
P¼0·000) in both the 15mg (13·99 (SD 2·47) mmol/l;
P¼0·018) and 30mg (15·03 (SD 3·17) mmol/l; P¼0·000) sup-
plemented groups compared with placebo (13·05 (SD 1·66)
mmol/l) (Fig. 1). Serum Zn levels in response to 30mg Zn
were higher among those recruited in Rome (16·26 (SD 3·41)
mmol/l) than Grenoble (13·64 (SD 2·21) mmol/l) post-interven-
tion (F(4,197)¼ 3·526; P¼0·008).
Dietary Zn intake decreased between baseline (12·33 (SE
0·528) mg/d) and 6 months (10·52 (SE 0·349) mg/d) among
those in Grenoble (F(1,186)¼ 4·196; P¼0·042) but not
Rome. Na intake was higher among those in Grenoble
(2433·50 (SE 61·07) g/d) than in Rome (1285·22 (SE 55·73)
g/d) at both baseline and 6 months (F(1,186)¼ 192·874;
P¼0·000). There were no differences in Zn or Na intake
between treatment groups at either centre.
Mean erythrocyte Zn levels were also within normal limits
(120–250mmol/l) at baseline and remained normal throughout
the intervention period. The sample recruited in Grenoble
(188·54 (SD 49·38) mmol/l) had lower erythrocyte Zn levels
than those in Rome at baseline (213·79 (SD 60·28) mmol/l)
(F(2,189)¼ 15·579; P¼0·000). Erythrocyte Zn did not alter in
either sample (Rome or Grenoble) in response to the
Zinc and taste acuity
Acuity for salt taste was greater in the 30mg supplemented
group (0·84409 (SD 0·13349)) than the placebo group
(0·75045 (SD 0·210)) post-intervention in the Grenoble
sample (F(2,91)¼ 3·632; P¼0·031) (Fig. 2). There were no
apparent differences in taste acuity in response to the
30mgZn intervention for sweet, sour or bitter tastes among
those recruited in Grenoble or for any of the four basic
tastes in the Rome sample. There were no apparent differences
in acuity for any of the tastes in response to 15mgZn.
Salt taste acuity increased in response to 30mg Zn in older
adults recruited in Grenoble. Many factors may influence
salt taste acuity. Sensory threshold for salty tastes can
Fig. 1. Serum Zn in response to Zn supplementation in older adults (70–87
years) from Rome and Grenoble (n 197). (
), 30mg zinc. Values are means.
), Placebo; ( ), 15mg Zn;
Fig. 2. Salt taste acuity in response to Zn supplementation in older adults
(70–87 years) from Grenoble (n 91). (
30mg zinc. Values are means. Baseline taste acuity was entered into the
analysis as a covariate.
), Placebo; (), 15mgZn; ( ),
B. J. Stewart-Knox et al.132
British Journal of Nutrition
become elevated with age6–8,10,41. That Zn enhanced salt taste
acuity was, therefore, not surprising. The finding that salt taste
acuity increased in response to Zn among those recruited in
Grenoble but not in Rome is more difficult to explain.
Although mean erythrocyte Zn was within normal limits in
both groups throughout the study, participants recruited in
Grenoble, on average, had lower levels of erythrocyte Zn
than those in Rome at baseline. This apparent baseline differ-
ence in long-term putative erythrocyte Zn status could explain
why salt taste acuity improved subsequent to 30mgZn among
those in Grenoble but not Rome. Given that differences in
putative measures of Zn status (serum and erythrocyte Zn)
and taste acuity at baseline were controlled for in the analysis,
however, suggests that this baseline difference in long-term
putative Zn status was unlikely to have influenced the
Differences in reported Zn and Na intake between those
recruited in Grenoble and Rome may provide clues as to the
disparate response of the two centres to the Zn intervention.
Reported dietary Zn intake decreased between baseline and
6 months into the intervention among those recruited in Gre-
noble but not Rome. That a similar reduction in dietary Zn
intake occurred between baseline and 6 months in the placebo
and the treatment groups, however, suggests that the decrease
in reported dietary Zn was not a response to the Zn sup-
plementation and may reflect seasonal variation in dietary
habits. Whether the apparent decrease in Zn intake over the
intervention period among those in Grenoble was enough to
bring about the improvement in salt taste acuity observed
exclusively in this sample is a matter for further study.
Sodium intake was reportedly higher among those in Greno-
ble than in Rome. Previous research has suggested that higher
dietary Zn intake is associated with better taste acuity for salt
because Na and Zn intake are related51. It is unfortunately dif-
ficult to draw firm conclusions on the basis of food records.
Not only are diaries subject to reporting bias52–54but dietary
information gathered over 4d may not be representative of
habitual intakes55. Na intake is especially difficult to assess
with any degree of accuracy56,57. Improved salt taste acuity
in response to the Zn intervention among those in Grenoble
could, nevertheless, be associated with higher Zn and Na
intake reported at baseline by this group.
Inadequate Zn intake and potentially Zn status is more
common among socio-economically deprived groups58,59.
The apparent improvement in taste acuity subsequent to inter-
vention with Zn (30mg) in Grenoble but not Rome cannot,
however, easily be explained by socio-demographic-related
differences in Zn status between the two groups. The two
samples recruited to the present study, although derived
from different European regions, were proportionally similar
in terms of age, sex and only marginally different in terms
of education and social class. The finding that supplemented
Zn only benefited salt taste acuity among those in Grenoble
and not Rome, therefore, provides further evidence to imply
that differences in taste response to Zn were to some degree
dependent upon culturally determined differences in dietary
habits between the two countries60and, accordingly, taste
response to Zn. Future research should consider culturally
determined lifestyle differences including dietary Zn intake
in relation to taste acuity, putative measures of Zn status
and response to Zn supplementation.
Serum Zn, as expected, increased in the treated groups over
the course of the study, indicating good compliance with the
intervention. Erythrocyte Zn, however, did not increase in
either group during the intervention period. Expressing eryth-
rocyte Zn concentrations in absolute values rather than in
terms of mg per g Hb increased the possibility that any rise
in erythrocyte Zn levels in response to the intervention
could have been missed if changes in the Hb content of the
blood samples occurred between baseline and 6 months of
supplementation. Unfortunately, no interpretative criteria for
erythrocyte Zn have been established and there are no standar-
dised units for the expression of Zn concentration in erythro-
cytes, making any comparison between different methods and
Another limitation of the present study is that, on the basis
of the biochemical measures taken (serum, erythrocyte and
dietary Zn), it has not been possible to establish conclusively
that the participants in the present study were Zn replete. Var-
ious methods are available to assess Zn status. Diagnosis of Zn
deficiency, nevertheless, is hampered by the lack of a single,
specific and sensitive biochemical index that reflects the
entire spectrum of Zn status from deficiency through adequacy
to excess and toxicity. Besides serum and erythrocyte Zn, leu-
cocyte, neutrophil, urine, hair and salivary Zn levels could
have been assessed, none of which have proven useful in iden-
tifying marginal Zn deficiency in man16,61. Given the lack of a
single valid biochemical measure of Zn status62we cannot
establish conclusively that the participants in the present
study were Zn replete.
The dietary records indicate that the subjects in the study
group were Zn replete. Reported mean daily dietary Zn
intake in our sample of older adults was estimated at 10·53
(SD 5·24) mg/d for females and 12·04 (SD 5·01) mg/d for
males39. Recommended daily intake for Zn varies between
countries. Based on the Nutrition Board and Institute of Medi-
cine63figures (8mg/d for females and 11mg/d for males),
according to 4d food diaries, the proportion of those recruited
to the present study who were consuming below two-thirds of
the RDA for Zn was small (0% of females and 3·55% of
males)39. These dietary Zn intake levels appear higher than
those reported in other studies, for example, 10·9 (SD 0·21)
mg/d for men and 7·8 (SD 0·14) mg/d for women in the
Third National Health and Nutrition Examination Survey
(NHANES III) population aged .60 years64, 12 (SD 6·4)
mg/d for males and 8·0 (SD 4·0) mg/d for females observed
in the Continuing Survey of Food Intakes by Individuals
(CSFII) study of those aged 60 þ59and 8·4mg/d for the
Iowa 65 þ Rural Health Study (IRHS)65. The 4d dietary
intake records indicate that our sample of healthy older
adults were dietary Zn replete.
Assuming that the sample was healthy and dietary Zn
replete and in view of the negative finding in response to
15mgZn, it could be argued that the observed effect for
30mgZn was pharmacological rather than nutritional. The
highest daily nutrient intake level likely to pose no risk of
adverse health effects for adults is 25mg/d, including dietary
and supplemental Zn66. It has been suggested that higher than
recommended doses of Zn supplemented over prolonged
periods can induce toxicity by inhibiting Cu bioavailabil-
ity58,67. Other research involving postmenopausal women,
however, has suggested that inadequate intake of Zn is more
Taste acuity and zinc supplementation133
British Journal of Nutrition
likely than high intake of Zn to be associated with decreased
Cu status35. In view of this, it was not surprising that putative
indices of Cu status did not appear to alter in response to Zn
intervention in the healthy older adults recruited to the present
study68, suggesting that it was unlikely that they experienced
toxic effects as a consequence of any decrease in Cu status
as a result of the Zn intervention.
The present study is unusual in having explored taste acuity
in response to Zn in a healthy, apparently Zn-replete, group of
older individuals. Most previous studies that have observed an
improvement in taste acuity in response to supplemented Zn
have tended to employ a small number of Zn-depleted individ-
uals derived from clinical populations23–25,27,69. The present
research is also novel in that it has investigated the impact
of Zn supplementation upon taste acuity in doses that reflect
recommended daily amounts. Other studies have tended to
intervene with higher doses than the 15 and 30mgZn used
for the purpose of the present study. Sweet, sour and bitter
taste thresholds have been apparently enhanced subsequent
to 45mg zinc sulfate27and all four basic tastes observed to
improve in response to 66mg69and 140mg zinc sulfate per
d25. Although the present research study investigated the
impact of lower doses of Zn upon taste acuity in healthy
ageing, making it difficult to compare findings across studies,
these findings agree with those of previous studies using
higher doses in clinical populations25,27,69in suggesting that
Zn is beneficial to taste acuity in doses of 30mg or above.
The specific protocol employed for the sensory testing in
the present study is also different in some respects from that
employed in other studies. Participants were instructed to sip
and swallow rather than sip and spit the taste stimuli. Given
that the panel were untrained, it was considered important
that the solutions be tasted in the normal way, lending greater
validity to the findings. Sip and spit would not have reflected
‘normal’ tasting conditions. The present study is also unusual
in employing a signal detection approach to taste threshold
assessment in older adults. Signal detection theory has been
previously applied to the detection of salt45,50in younger
but not older individuals. Signal detection is perceived to be
more robust than other sensory threshold techniques in that
it enables individual differences in the way individuals make
decisions to be taken into account. Signal detection theory
could, therefore, be considered ideal for studies employing
diverse samples. The use of the ‘basic taste’ paradigm to
study taste acuity could also be perceived as controversial41.
The method has nevertheless provided a well-controlled
model that is replicable and which enables comparison
across different studies and that appears to be particularly
useful for clinical intervention trials looking at taste acuity
in qualitatively different study populations.
The present study appears to be the first randomised clinical
controlled intervention trial that has considered taste acuity
in response to Zn supplementation in healthy ageing. Sup-
plementation with 30mg Zn may have potential to enhance
salt taste acuity in those over the age of 70 years in certain
cultures. Meanwhile, further research is required to determine
if Zn supplementation, in improving salt taste acuity, can
stimulate appetite or enhance the eating experience in older
individuals. Although above the upper limit of 25mg, 30mg
Zn is generally available to the consumer ‘over the counter’;
however, in view of potential adverse effects on Cu status
of prolonged daily intake of Zn, we should refrain from
recommending such high doses to the consumer public.
ZENITH is supported by the European Commission ‘Quality
of Life and Management of Living Resources’ Fifth Frame-
work Programme, contract no. QLK1-CT-2001-00168.
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