Vitamin D status and its determinants in adolescents from the Northern
Ireland Young Hearts 2000 cohort
Tom R. Hill1, Alice A. Cotter1, Sarah Mitchell1, Colin A. Boreham3, Werner Dubitzky4,
Liam Murray6, J. J. Strain5, Albert Flynn1, Paula J. Robson5, Julie M. W. Wallace5, Mairead Kiely1
and Kevin D. Cashman1,2*
1Department of Food and Nutritional Sciences and2Department of Medicine, University College, Cork, Republic of Ireland
3UCD Institute for Sport and Health, University College, Dublin, Republic of Ireland
4Systems Biology Research Group and5Northern Ireland Centre for Food and Health, University of Ulster, Coleraine, UK
6Department of Epidemiology and Public Health, Queens University, Belfast, UK
(Received 21 June 2007 – Revised 11 September 2007 – Accepted 13 September 2007 – First published online 15 January 2008)
Despite recent concerns about the high prevalence of sub-clinical vitamin D deficiency in adolescents, relatively few studies have investigated the
underlying reasons. The objective of the present study was to investigate the prevalence and predictors of vitamin D inadequacy among a large
representative sample of adolescents living in Northern Ireland (54–558N). Serum concentrations of 25-hydroxyvitamin D (25(OH)D) were ana-
lysed by enzyme-immunoassay in a subgroup of 1015 of the Northern Ireland Young Hearts 2000 cohort; a cross-sectional study of 12 and 15 year-
old boys and girls. Overall mean 25(OH)D concentration throughout the year was 64·3 (range 5–174) nmol/l; 56·7 and 78·1nmol/l during winter
and summer, respectively. Reported intakes of vitamin D were very low (median 1·7mg/d). Of those adolescents studied, 3% and 36% were vita-
min D deficient and inadequate respectively, as defined by serum 25(OH)D concentrations ,25 and ,50nmol/l. Of the subjects, 46% and 17%
had vitamin D inadequacy during winter and summer respectively. Gender differences were also evident with 38% and 55% of boys and girls
respectively classified as vitamin D inadequate during winter (P, 0·001). Predictors of vitamin D inadequacy during winter were vitamin D
intake and gender. In conclusion, there is a high prevalence of vitamin D inadequacy in white-skinned adolescents in Northern Ireland, particularly
during wintertime and most evident in girls. There is a clear need for dietary recommendations for vitamin D in this age group and for creative
strategies to increase overall vitamin D status in the population.
Serum 25-hydroxyvitamin D: Vitamin D insufficiency: Determinants: Season: Gender: Adolescents
It is well established that prolonged and severe clinical vita-
min D deficiency (represented as serum 25-hydroxyvitamin
D (25(OH)D)) concentrations ,10–25nmol/l) leads to rickets
in children and osteomalacia in adults(1). Currently in the UK,
a plasma level of 25nmol/l 25(OH)D is used as the lower
threshold for vitamin D status(1). There is also a growing
body of evidence to suggest that less severe degrees of
deficiency, or sub-clinical deficiency (serum 25(OH)D con-
centrations ,50nmol/l), may be associated with increased
risk of a wide range of chronic diseases, including tuberculo-
sis, rheumatoid arthritis, multiple sclerosis, inflammatory
bowel diseases, CVD, hypertension and certain cancers(2,3).
With this in mind, it is of concern that a high prevalence of
sub-clinical deficiency has been reported in adults from
many countries (see reviews(4–7)).
Until recently, it had been assumed that children and adoles-
cents were not at risk of low vitamin D status because of their
outdoor activities and dietary intake(3). However, a number of
recent studies in adolescents have revealed a high prevalence
of vitamin D insufficiency in Europe(8–15), the USA(16–19)and
elsewhere (the Lebanon(20), New Zealand(21,22), Tasmania(23)),
especially during the winter months. Several of the afore-men-
effect of low vitamin D status for adolescent bone growth and
D deficiency highlights evidence that living at latitudes above
358Nforthe first10years oflife increasesrisk ofmultiple scler-
osis by 100%, as well as increasing risk of several other auto-
immune diseases(24). Thus, failure to address low vitamin D
status among adolescents could have serious long-term impli-
cations for public health.
Despite concerns about the high prevalence of low vitamin
D status in adolescents, relatively few studies have investi-
gated the potential determinants of vitamin D status in this
life-stage group. A small number of studies of non-nationally
representative samples of adolescent girls in Europe and USA
have shown that season, sun exposure, ethnicity and race, as
well as body weight, are determinants of vitamin D
status(11,13,18). Rockell et al. reported that ethnicity, season
and, to lesser degrees, obesity and gender were significant
*Corresponding author: Professor Kevin D. Cashman, fax þ353 21 4270244, email email@example.com
Abbreviations: 25(OH)D, 25-hydroxyvitamin D; YH2000, Young Hearts 2000.
British Journal of Nutrition (2008), 99, 1061–1067
q The Authors 2008
British Journal of Nutrition
predictors of year-round vitamin D status in a nationally repre-
sentative sample of New Zealand children and adolescents(22).
Knowledge of such determinants is of importance for the
development of public health strategies for prevention of
low vitamin D status. Despite our relatively northerly latitude
(51–558N), there are almost no data on vitamin D status and
its determinants among adolescents in Ireland. Therefore, we
examined vitamin D status and its determinants during
winter and summer in a representative sample of adolescents
in Northern Ireland, a region with a high prevalence of dis-
eases associated with low vitamin D status, including
CVD(25), multiple sclerosis(26)and colon cancer(27).
Subjects and methods
The Young Hearts 2000 (YH2000) survey is the second in a
series of cross-sectional studies examining a representative
sample of adolescents in Northern Ireland. The primary aim
of YH2000 was to identify the prevalence of risk factors for
CHD in adolescents aged 12 and 15 years. Details of subject
recruitment, inclusion and exclusion criteria, response rate
and ethical approval have been described elsewhere(28–30).
During the months of January, February, March, April, May,
June, September, October, November and December, 11%,
16%, 16%, 6%, 2%, 7%, 10%, 11%, 11% and 11% of
the group were sampled respectively. None of the subjects
was sampled during July or August owing to summer
vacation. Given that Northern Ireland is at a latitude where
UVB intensity is insufficient to promote dermal synthesis of
vitamin D between November to March(31), we defined
winter as November to March, and summer as April to Octo-
ber. Complete records were available for 1015 adolescents
who had provided a blood sample and for whom data on pub-
ertal status, anthropometry, habitual physical activity and food
intakes were also available.
Anthropometry, lifestyle and dietary data
Standing height and body weight were measured as described
previously(29). Pubertal status of each subject was assessed
by a paediatrician using visual signs such as non-genital
secondary hair growth, vocal timbre, body habitus, general
muscular development and overall breast development in
girls. Lifestyle data and physical activity data were obtained
from questionnaires, as described previously(29,32). Dietary
data were collected by a nutritionist-administered 7 d diet
history method(33). Energy and nutrient intakes were calcu-
lated using a computer program (WISP; Tinuviel Software,
Warrington, UK) based on McCance and Widdowson’s
Composition of Foods(34).
Collection and preparation of samples
Blood was collected by venipuncture into a vacutainer tube
with no additive and processed to serum, which was immedi-
ately stored at 2808C until required for analysis.
Serum 25-hydroxyvitamin D.
measured in serum samples using ELISA (OCTEIAw25-
Hydroxy Vitamin D; Immuno Diagnostic Systems, Ltd.,
Boldon, UK). The intra- and inter-assay CV for the ELISA
method was 5·9% and 6·6%, respectively. The quality and accu-
ongoing basis by participation in the Vitamin D External Quality
Assessment Scheme (Charing Cross Hospital, London, UK).
25(OH)D concentrations were
Statistical analysis of the data was conducted using SPSSqfor
WindowsTMVersion 12.0 (SPSS, Inc., Chicago, IL, USA).
Serum 25(OH)D concentrations and dietary vitamin D intakes
were not normally distributed and, therefore, values were log-
arithmically (natural log) transformed prior to statistical anal-
ysis, to achieve near-normal distributions. Differences in age,
height, weight, BMI, physical activity, serum 25(OH)D and
dietary vitamin D and Ca between the genders and age-group-
ings were examined using unpaired Student’s t tests. Tests for
independence were used to compare demographic variables
such as age-grouping, gender, season of sampling and pubertal
status between those included in current analysis (n 1015) and
the complete YH2000 dataset (n 2017). Differences in pro-
portion of vitamin D deficiency/inadequacy between genders
and age-groupings were assessed by x2tests. Two-way
ANOVA was used to investigate gender–season interactions
on serum 25(OH)D concentrations. Tests for independence
were used to compare serum 25(OH)D concentrations between
boys and girls during winter and summer, separately. Multiple
logistic regression was used to investigate predictors of risk
of vitamin D inadequacy (serum 25 (OH)D ,50nmol/l),
whilst adjusting for possible confounding factors. The
following categorical variables were included: season (coded
as: 0, winter, November–March; 1, summer, April–October);
gender (coded as: 0, female; 1, male); pubertal status (coded
as: 0, pre-puberty; 1, post-puberty); vitamin D intake above
or below the median (coded as: 0, greater than median; 1,
less than median); Ca intake above or below the median
(coded as: 0, greater than median; 1, less than median); physi-
cal activity level above or below the median (coded as: 0,
greater than median; 1, less than median). The continuous
numerical variable BMI (kg/m2) was also included. Multiple
linear regression analysis was performed to identify indepen-
dent predictors of serum 25(OH)D. The following categorical
variables were included: season (coded as: 0, winter, Novem-
ber–March; 1, summer, April–October); gender (coded as: 0,
female; 1, male); age-grouping (coded as: 0, aged 12 years;
1, aged 15 years); pubertal status (coded as 0, pre-puberty;
1, post-puberty). The continuous numerical variables, BMI
(kg/m2), vitamin D intake (mg/d), Ca intake (mg/d) and physi-
cal activity score were also included. P values ,0·05 were
considered statistically significant.
Baseline characteristics of adolescents
Characteristics of the adolescents included in the current anal-
ysis (n 1015) were compared with those of all participants in
T. R. Hill et al.1062
British Journal of Nutrition
the YH2000 study (n 2017), which was a representative
sample of adolescents in Northern Ireland (Table 1). No sig-
nificant differences were evident between these two groups
Vitamin D status and vitamin D intakes
The mean, standard deviation and distribution of serum
25(OH)D concentrations throughout the year in the sample
of 1015 adolescents, and in each age and gender group, separ-
ately, are shown in Table 2. While all girls had a slightly
lower mean year-round serum 25(OH)D concentration com-
pared with boys (P¼ 0·054), a difference between the genders
was significant in 15 year-olds (P¼ 0·018) but not in 12 year-
olds (P¼ 0·81). Two-way ANOVA showed that serum
25(OH)D concentration in the entire group was significantly
affected by season (mean 56·7 (SD 24·3) nmol/l and 78·1
(SD 27·1) nmol/l, for winter (n 657) and summer (n 358),
respectively; P, 0·0001) but not gender (P.0·05; data in
Table 2); there was, however, a significant interaction between
these two factors (P, 0·001). While all girls (n 326) had a
significantly lower mean wintertime serum 25(OH)D concen-
tration compared with all boys (n 331) (mean 52·3 (SD 22·6)
nmol/l and 61·1 (SD 25·1) nmol/l, respectively; P, 0·001),
all girls (n 184) had significantly higher mean summertime
serum 25(OH)D concentrations compared with all boys
(n 174) (mean 81·5 (SD 23·9) nmol/l and 74·7 (SD 29·7)
nmol/l, respectively; P¼ 0·002).
The mean, standard deviation, median and 5th and 95th per-
centiles of vitamin D and Ca intakes (from food and sup-
plements) were 2·6, 3·2, 1·7, 0·6 and 10·5mg and 973, 390,
907, 467 and 1676mg/d, respectively. Vitamin D and Ca
intakes were significantly higher (P, 0·0001) in boys than
girls (3·0 v. 2·2mg/d (vitamin D); 1089 v. 858mg/d (Ca)).
There were no significant differences in vitamin D or Ca
intakes between 12 and 15 year-olds (data not shown).
(P, 0·05) in boys than girls (30 v. 18, respectively). In
addition, mean physical activity score was significantly
higher (P, 0·001) in boys than girls during summertime
(data not shown).
Prevalence of vitamin D deficiency and inadequacy
Cumulative percentages of adolescents with serum 25(OH)D
concentrations ranging from ,10nmol/l to ,120nmol/l,
which have been variably suggested as cut-off values for
defining vitamin D deficiency to insufficiency(1,2,3), are
shown in Fig. 1. Stratification of the adolescents by sampling
period showed that the proportion of subjects with serum
25(OH)D concentrations ,50nmol/l (the cut-off value that
defines sub-clinical deficiency) was 46% during winter and
17% during summer. During winter, 38% of boys and 55%
of girls had serum 25(OH)D ,50nmol/l (P, 0·001).
concentrations ,25 and $25nmol/l were 1·5 and 2·6mg/d,
respectively (P¼ 0·003). Mean daily vitamin D intakes in sub-
jects with serum 25(OH)D concentrations ,50 and $50nmol/l
were 2·1 and 2·9mg/d, respectively (P, 0·001).
Determinants of serum 25-hydroxyvitamin D levels
Potential determinants of serum 25(OH)D levels throughout
the year were assessed using multiple linear regression anal-
ysis. Ca intake, gender and physical activity score were non-
significant (P.0·05) determinants of serum 25(OH)D levels.
Being sampled during the sunnier half of the year (i.e.
summer to autumn) (b 0·370; CI 0·318, 0·.423; P, 0·0001),
vitamin D intake (b 0·083; CI 0·050, 0·115; P, 0·0001) and
increasing BMI (b 0·008; CI 0·001, 0·015; P¼ 0·035) were
positively associated with serum 25(OH)D levels. Limiting
winter showed that vitamin D intake (b 0·998; CI 0·478,
1·518; P, 0·0001) and gender (b 6·041; CI 2·162, 9·919;
P¼ 0·002) were the only two significant determinants of
serum 25(OH)D levels.
Predictors of low vitamin D status
Multiple logistic regression analysis was used to identify the
,50nmol/l (Table 3). Winter sampling time and low vitamin
D intake were significant predictors of low vitamin D status
(Table 3). Ca intake, gender, age, BMI, pubertal status and
physical activity were not significant predictors of low vitamin
D status (data not shown). For those subjects sampled during
winter, a vitamin D intake below the median of 1·7mg/d
(OR 1·589 (95% CI 1·131, 2·232); P¼ 0·008) and being
female (OR 1·460 (95% CI 1·017, 2·097); P¼ 0·04) were
Table 1. Characteristics of Northern Ireland Young Hearts 2000 Project
participants entire cohort and those in the current analysis of vitamin D
Participants with a
Season of sampling‡
Vitamin D intake (mg/d)
Ca intake (mg/d)
25(OH)D, 25-hydroxyvitamin D.
*In the Young Hearts Project, a participant was defined as an adolescent who com-
pleted a diet history (n 2017).
‡Winter was defined as months November to March and summer was defined as
months April to October.
§For details of subjects and procedures, see Subjects and methods.
Vitamin D status of adolescent boys and girls1063
British Journal of Nutrition
significant predictors of low vitamin D status, after adjusting
for possible confounders. On the other hand, limiting the anal-
ysis to only those subjects sampled during summer did not
reveal any significant predictors of low vitamin D status
among those tested.
It is clearly recognised that serum/plasma 25(OH)D levels
below 12·5nmol/l can result in bone diseases, such as rickets
in children and osteomalacia in adults(35). There is also evi-
dence that circulating 25(OH)D levels below 20–25nmol/l
may result in rickets and osteomalacia in the longer
term(1,36). In the present study, 2·9% of a representative
sample of 12 and 15 year-old adolescents from Northern Ire-
land had serum 25(OH)D concentrations ,25nmol/l through-
out the year. Only three other studies to date have used a
nationally representative sample. Data from the UK NDNS9
showed that 11–16% of 11–18 year-old adolescents in Eng-
land, Wales and Scotland had serum 25(OH)D ,25nmol/l
throughout the year. One fundamental difference between
the NDNS sample and the current cohort is the homogeneity
of the YH2000 sample in terms of race; the overwhelming
majority of the adolescents in the current sample were white
as at the time of sampling, ethnic diversity within the popu-
lation of Northern Ireland was almost non-existent. In con-
trast, the NDNS included a representative sample from
across Great Britain, which has a more ethnically diverse
population profile. Skin colour is an important predictor of
vitamin D status. The efficiency of synthesis of cholecalciferol
is negatively related to the extent of skin pigmentation; thus,
the lower prevalence of vitamin D deficiency observed in
the YH2000 cohort than in the NDNS is most likely related
to differences in the capacity for vitamin D synthesis. In the
Table 2. Serum 25-hydroxyvitamin D (S-25(OH)D) concentrations throughout the year in healthy adolescents in Northern
All subjectsAll 12 year-old 15 year-oldAll12 year-old15 year-old
*v. all boys (P#0·05).
**v. 15 year-old boys (P¼0·018).
†For details of subjects and procedures, see Subjects and methods.
Fig. 1. Cumulative percentage of Northern Ireland adolescents (n 1015) with serum 25-hydroxyvitamin D (25(OH)D) between 10 and 120nmol/l. For details of sub-
jects and procedures, see Subjects and methods.
T. R. Hill et al.1064
British Journal of Nutrition
US-based National Health and Nutrition Examination Survey
study, #1% of 12–19 year-olds had serum 25(OH)D
,25nmol/l(16), although this is likely to be an underestimate
because vitamin D status was measured in northern United
States during summer and in southern United States during
winter. A study in New Zealand found that 4% of adolescents
aged 11–14 years had serum 25(OH)D ,17·5nmol/l(22).
Circulating 25(OH)D concentrations greater than 25nmol/l
but less then 50nmol/l reflect sub-clinical vitaminD deficiency,
which is associated with elevated parathyroid hormone with
consequences for bone turnover rates, at least in adults(37). Fur-
thermore, serum 25(OH)D concentration ,50nmol/l has also
been linked to the development of various chronic diseases
(e.g. hypertension, CVD, diabetes mellitus, as well as some
inflammatory and autoimmune diseases and some forms of
cancer).(2,3)Over one-third (36%) of adolescents in the present
study had 25(OH)D concentrations ,50nmol/l across the year
and the prevalence of vitamin D inadequacy, or low vitamin D
status, was much higher (48–52%) in subjects sampled during
winter (November–March). These findings are broadly in line
with those of the three previously mentioned studies of nation-
ally representative samples of adolescents(9,16,22)and in
keeping with several studies that investigated the prevalence
of vitamin D inadequacy in adolescents in Europe(8,10–14), the
In the current study, girls had lower wintertime mean serum
25(OH)D levels than boys, although girls had higher levels
during summertime. The gender difference during summer-
time might relate to a higher proportion of girls (72%)
being sampled in late summer (September/October; when vita-
min D status would be at its peak) compared with only 43% of
boys being sampled during that time. Those sampled in earlier
summer (April–June) would be expected to have lower vita-
min D status. There was an increased proportion of girls
with low (,50nmol/l) 25(OH)D levels throughout the year
(39% v. 32%, for girls and boys, respectively) as well as in
winter only (55% v. 38%, for girls and boys, respectively).
While not evident in the UK-based NDNS study of adoles-
cents(9), this lower vitamin D status among girls has also
been reported in the national studies of children and adoles-
cents in the USA(16)and New Zealand(22). The reason for
this apparent gender difference is unclear. However, in the
current study, girls had lower vitamin D intakes than boys.
It is also interesting to note that subjects with vitamin D
deficiency and insufficiency had significantly lower vitamin
D intakes compared with those in vitamin D-replete subjects.
Multiple linear regression analysis of subjects sampled in
winter showed that being female and low vitamin D intake
were the only two significant determinants of serum
25(OH)D levels. Rockell et al. suggested that the difference
in vitamin D status between the genders was acting as a sur-
rogate marker for sunlight exposure through an association
with physical activity(22). In the present study, physical
activity was significantly higher in boys than girls, even
during summertime. Physical activity was not a determinant
of serum 25(OH)D levels in these adolescents. The physical
activity score, however, included activity that is both indoor-
To explore the reasons for the high prevalence of low vita-
min D status in the adolescents in Northern Ireland, we inves-
tigated the potential predictors of vitamin D inadequacy
(serum 25(OH)D ,50nmol/l). Not surprisingly, season was
the major determinant of vitamin D status, a finding shown
in many studies of adults(38,39), and in the New Zealand
study of determinants of low vitamin D status in children
and adolescents(22). Potentially of concern was our observation
that 13% of adolescents living in Northern Ireland had low
vitamin D status during summertime, at a time when vitamin
D synthesis and status would be expected to be optimum(4). As
season was such a major predictor, we repeated the analysis in
the subgroups sampled during winter- and summertime
separately. During winter, low vitamin D intake (less than
1·7mg/d) and being female were significant predictors of
having a serum 25(OH)D value of ,50nmol/l.
There is no dietary recommendation for vitamin D for ado-
lescents in the UK, as there is an assumption that sun exposure
will provide for adequate status during summer and allow for
stores to be laid down to support vitamin D status in winter1.
This lack of a dietary recommendation appears unwise when
12% and 47% of UK adolescents (aged 11–18 years) have
plasma 25(OH)D concentrations that are reflective of serious
and mild deficiency, respectively(9). In contrast, the US Insti-
tute of Medicine recommends 5mg/d for adolescents(40), an
intake level that only 10% of the current sample managed
to achieve. These low intakes are in line with similar reports
from several countries(8,9,13,15,41,42). For example, data from
the NDNS show that the mean daily intake of vitamin D for
adolescents (aged 11–18 years) in the UK is 2·6mg(9). For
individuals living in Ireland, the UK and elsewhere in
Europe and North America at latitudes above 378N, low
intakes of vitamin D may take on increased importance
during wintertime when sunlight is of insufficient intensity
to stimulate dermal vitamin D synthesis.
We did not identify any significant physiological or lifestyle
predictors of low vitamin D status during summer, indicating
that UVB exposure, which was not directly assessed in the
current study, was the overriding determinant of vitamin D
status at this time. Despite being by far the most efficacious
method of improving vitamin D status(3), recommendations
for increased UVB exposure are fraught with public health dif-
ficulties in relation to evidence for increased risk of skin
cancer. Thus, a dietary strategy (including supplemental and/
or fortification sources of vitamin D) appears a more
acceptable mode of improving vitamin D status of the popu-
lation, including adolescents. More research is needed to
define the levels of intakes required and the best strategies
to achieve these intakes on a population basis. The lack of
an international consensus on cut-off levels for vitamin D
deficiency/insufficiency is certainly a limitation in terms of
better defining dietary requirements. While there are several
Table 3. Multiple logistic regression analysis of predictors of serum
25(OH) D values ,50 nmol/l in 1015 Northern Ireland adolescents*
OR95% CIP value
Time of year
Vitamin D intake
Greater than median (.1·7mg/d)
Less than median (,1·7mg/d)
*For details of subjects and procedures, see Subjects and methods.
Vitamin D status of adolescent boys and girls1065
British Journal of Nutrition
reasons for this uncertainty(43), it is further complicated by
25(OH)D levels. Different methods of analysis can produce
different serum 25(OH)D levels, as illustrated recently in
round robin analyses of the Vitamin D External Quality
Assessment Scheme samples(44–46). In addition, significant
variation between laboratories using the same analytical tech-
nique can exist due to operator bias. For these reasons, partici-
pation of laboratories in the Vitamin D External Quality
Assessment Scheme is highly recommended, as well as the
future inclusion of standard reference materials for serum
25(OH)D, which are currently being developed in the USA.
In conclusion, there was a high prevalence of low vitamin D
status among adolescents living in Northern Ireland at latitudes
tobeaconsequenceofthe failureofalowdietary intakeofvita-
period. Increased emphasis needs to be given to exploring strat-
egies for improving vitamin D status in adolescents.
This project was financially supported by the Higher Edu-
cation Authority under their Strand 1: North/South Programme
for Collaborative Research and the Northern Ireland Depart-
ment of Health, Social Services and Public Safety. None of
the authors has a conflict of interest.
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