Association of Race, Body Fat and Season With Vitamin D
Status Among Young Women: A Cross-sectional Study
Kevin McKinney; Carmen Radecki Breitkopf; Abbey B. Berenson
Clin Endocrinol. 2008;69(4):535-541. ©2008 Blackwell Publishing
Posted 01/02/2009
Summary and Introduction
Summary
Objective: The purpose of this study was to provide an estimate of vitamin D status in young women residing in southeast Texas and to determine factors that predict 25-hydroxyvitamin D (25-OHD) concentration.
Design: A cross-sectional study was conducted on 800 non-Hispanic white, non-Hispanic black, and Hispanic women 1633 years of age, who were seen in an outpatient clinic.
Measurements: Information was obtained on race, smoking, exercise and dietary intake of vitamin D. Percentage total
body fat (%TBF) was assessed using dual-energy X-ray absorptiometry (DXA). Exposure to sunlight was estimated by
examining national records of temperature, hours of daylight and UV index for the latitude of the study site. To determine
the relationship between 25-OHD and %TBF, season, race, body mass index (BMI), dietary vitamin D, age and smoking
in a multivariate context, stepwise linear regression analysis was performed.
Results: Serum 25-OHD levels differed among the racial groups (all pairwise differences P < 0·001), with the lowest
value among non-Hispanic blacks (37·7 nmol/l) and the highest value among non-Hispanic whites (71·8 nmol/l). Among
Hispanics, mean serum 25-OHD was 47·9 nmol/l. Serum 25-OHD was negatively associated with %TBF (r = -0·28), BMF
(r = -0·36) and TBF (r = -0·33), all P < 0·001, and positively associated with dietary vitamin D (r = 0·10) and pack years of
smoking (r = 0·11), both P < 0·01. In the summer months, serum 25-OHD values were higher (55·4 nmol/l) than in the
winter months (48·1 nmol/l), P < 0·001. The final regression model predicting serum 25-OHD levels included race, %TBF
and season (all P < 0·05) and explained 36% of the variance in 25-OHD.
Conclusions: Favourable environmental conditions do not result in sufficient vitamin D status for young women,
especially non-Hispanic blacks, Hispanics and the obese.
Introduction
It has been suggested that the prevalence of vitamin D deficiency ranges from 5% in white women and up to 45% in black
women in the USA.[1] Data are inadequate, however, on the prevalence of vitamin D deficiency in Hispanic women as few
studies have included this racial group. The largest study of national estimates of health and nutritional status that has
included Hispanic women to date, the Third National Health and Nutrition Examination Survey (NHANES III), found
vitamin D levels for this group to be between those of non-Hispanic blacks and non-Hispanic whites.[2-4] Racial differences
are difficult to interpret from these data, however, because of the sampling methods used. For example, serum was
collected during the summer from participants residing in the north and during the winter from those living in the south.[4]
The population of southern states includes the majority of non-Hispanic blacks in the USA and the second largest number
of Hispanics. This could have influenced the results because ultraviolet radiation type B (UV-B) from sunlight is
responsible for the formation of vitamin D. Furthermore, lower average temperatures in portions of the northern USA may
lead to more coverage with UV-blocking clothing and less formation of vitamin D.
In addition to race, obesity has also been suggested as a risk factor for vitamin D deficiency. In a study of 410 nonHispanic black and white women, Arunabh et al. demonstrated that the percentage of body fat had a modest effect on
levels of serum 25-hydroxyvitamin D (25-OHD).[5] Using NHANES III data, Looker also observed a modest association
between percentage body fat and serum 25-OHD, which was stronger in non-Hispanic whites than in blacks.[6] Similarly,
Parikh et al. found that both serum 25-OHD and 1,25-dihydroxy vitamin D were negatively correlated with body mass
index (BMI).[7] No Hispanics were included in any of these studies, however, so the strength of the association between
obesity and vitamin D levels in this population remains uncertain.
Moreover, it has been suggested that living in a climate with little seasonal variation may not provide adequate protection
against vitamin D deficiency. For example, a study conducted in south Florida demonstrated that seasonal variation in 25OHD levels may exist even in sunny climates.[8] This question requires further study, however, because the subjects had a
mean age of 54 years and 55% of the sample reported mild sun exposure; thus it is not known whether the results would
be the same among younger individuals who are more likely to spend time outdoors. In addition, the effect of the ageing
process may have contributed to the findings, as ageing decreases the capacity of the skin to produce vitamin D. [9-11]
A recent meta-analysis emphasized the need for additional investigations on vitamin D in diverse racial and ethnic groups
of premenopausal women.[12] The purpose of the current study was to provide an estimate of vitamin D status in young
women residing in south-east Texas and to determine factors that predict 25-OHD concentration.
Methods
A total of 805 non-Hispanic white, non-Hispanic black and Hispanic women between 16 and 33 years of age were
recruited between 9 October 2001 and 14 September 2004, as part of a larger study of the effect of hormonal
contraception on bone mineral density (BMD). Determination of race was based on self-identification. Criteria for
exclusion for this investigation were as follows: currently pregnant or breastfeeding; planning a pregnancy within 3 years;
use of depot medroxyprogesterone acetate (DMPA) within the past 6 months, oral contraceptive pills within the past 3
months, or current use of a hormonal intrauterine device; use of over-the-counter phytoestrogens (e.g. Estroven,
Promensil); dietary isoflavone intake exceeding 84 mg per day; medical contraindication to the use of hormonal
contraception (e.g. thromboembolic disease, acute liver disease); illness or use of medications known to affect BMD;
current eating disorder or strict vegetarian diet; amenorrhoea for more than 3 months within the past year; and bilateral
oopherectomy. Of the 805 women, 800 (99%) completed a baseline visit and provided data for the present analysis. All
data reported in this paper were collected at the baseline visit.
This study complied with the recommendations of the Declaration of Helsinki and was approved by the Institutional
Review Board of the University of Texas Medical Branch, Galveston. All participants provided written informed consent,
received free well-woman care over the course of the study, and were compensated for their time and travel to the clinic.
Participants resided within a 40-mile radius of Galveston, Texas, at elevations from sea level to 24 feet, and with a daily
UV index of 6-7 (high) to 11 (extreme) during April to October.[13]
Based on a review of nationally recorded monthly temperatures, UV indices and hours of daylight for the site, baseline
visits that occurred during April-October were classified as 'vitamin D summer'. Visits occurring during November-March
were classified as 'vitamin D winter'. Designation of a 'vitamin D season' has been described in the literature. [10] These
classifications comprise a 'season' variable, based on regional conditions, that is included in the multivariate analyses.
We examined prevailing temperatures by month during 2001-04 (the years in which we collected blood) to verify months
for inclusion in our 'vitamin D summer'. The Texas upper coast experiences a mild winter during December and January,
followed by a short spring in late February and March. Average temperatures consistently range from 70 °F to 80 °F (2127 °C) in April and May, increasing to and sustaining a range of 80-95+ °F (27-35 °C) from late May to October.[14] During
the months of 'vitamin D summer', the mean temperatures for the region consistently range from 70 °F to 95 °F (21-35
°C), indicating potentially favourable conditions for outdoor activities and opportunities for exposure to UV radiation. In
addition, during 2001-04, the months April-October coincided with daylight saving time, increasing the potential for
participants' exposure to sunlight.
As temperature and hours of daylight do not indicate UV exposure levels, we also examined the daily UV index for the
Galveston area during these months.[13] We found that during the 214-day period (April-October) in all years, the recorded
UV index on clear days consistently fell within the high (7) to extreme (11) solar UV radiation exposure categories as
defined by the World Health Organization (WHO) Global Solar UV Index. [15] Clear days were recorded in all years for over
75% of our 214-day seasonal period.[13] Thus, based on prevailing temperatures and UV index, these 214 days of 'vitamin
D summer' constitute a stable annual period for a seasonal designation.
It has been estimated that 1000 IU of vitamin D3 (cholecalciferol) per day is necessary to sustain an adequate level of
vitamin D (above 75-80 nmol/l). Certain at-risk populations, such as postmenopausal black women, may require a
significantly higher dose (2800-4000 IU/day).[16-18] At 29°N, during April-October, the exposure time required to synthesize
the equivalent of 1000 IU vitamin D occurs in minutes, not hours, regardless of skin type. [19,20] In comparison, in more
northern latitudes, adequate exposure time may be 30 min or more, several times a week.[16,21-23] Thus, the environmental
conditions as reflected in latitude, climate and UV index indicate sufficient solar UV radiation for the young women in our
study to potentially achieve and maintain adequate vitamin D status throughout the year, with minimum daily exposure.
Participants completed demographic measures including age, race, marital status, education, income and parity. Smoking
was assessed using questions derived from the MONICA Smoking Assessment proposed by the WHO, [24] and is reported
as pack years. BMI was calculated as weight (kg)/height (m)2. Weight was measured when women were wearing light
indoor clothing with a digital scale accurate to the nearest 0·1 kg. Height was measured using a wall-mounted stadiometer
(Heightronic, Snoqualmie, WA), accurate to the nearest 0·001 m. To obtain estimates of dietary intake of vitamin D, a
registered dietician conducted a 24-h dietary recall interview with each participant. Nutrient calculations were performed
using the Nutrition Data System for Research (NDS-R) software (Nutrition Coordinating Center, University of Minnesota,
Minneapolis, MN, Food and Nutrient Database, Version 33, released July 2002). Physical activity was assessed using the
Exercise Module of the Behavioral Risk Factor Surveillance System Questionnaire.[25] This measure lists 56 common
activities for which women were asked to report the frequency and duration of up to two activities performed during the
past month. Data are reported as the total number of minutes per week spent participating in these activities.
To determine levels of serum calcium, phosphorus and 25-OHD, a 12-h fasting blood draw was performed on each
participant between 0700 and 1000 h during the follicular phase of the menstrual cycle. Serum phosphorus and calcium
were measured calorimetrically. Levels of 25-OHD were measured by chemiluminescence protein-binding assay (CLPBA)
using an automated immunoassay method that, according to the manufacturer, accurately quantifies the sum of vitamin
D3and vitamin D2 [Nichols Institute Diagnostic (NID) Advantage, performed at ARUP Laboratories, a national medical
reference laboratory, Salt Lake City, UT]. The reported range of detection on this assay was 17·5-300 nmol/l; inter- and
intra-assay coefficients of variation (CVs) were 4·5% and 6·0%, respectively. In 2004, NID found that in some cases the
assay may under-recover vitamin D2, and ARUP began reflex testing with the DiaSorin radioimmunoassay (DiaSorin, Inc.,
Stillwater, MN) if test results were between 25 and 130 nmol/l, to ensure that vitamin D2 levels were not significantly
underestimated. Sixteen test results (nine non-Hispanic black, five Hispanic, two non-Hispanic white) from this study were
reflexed; no significant differences were found in total vitamin D. We determined from Heaney[26] that the analytic defect
would not be likely to heavily impact our preliminary baseline findings in these women, who were not being treated for
osteomalacia or other diseases requiring prescribed supplementation. Nevertheless, we used the equation DiaSorin =
Nichols × 0·75 + 5·6 to compute serum 25-OHD values for use in all analyses reported here.[27] We used the calculated
DiaSorin values in our analyses and interpretation because the assay has been used extensively in epidemiological
surveys including NHANES.[28]
Percentage total body fat (%TBF) was determined by performing a whole-body scan using dual-energy X-ray
absorptiometry (DXA; Hologic QDR 4500 W Elite fan-beam bone densitometer, Waltham, MA). The in vitro CV of the
instrument is 0·27 g/cm2 ± 0·003, based upon daily calibration using a spine phantom supplied by the manufacturer.
%TBF by DXA was calculated using the following formula: [fat mass (g)/fat mass (g) + lean mass (g) + total bone mineral
content (g)] × 100 (Hologic QDR for Windows, Version 11·2).
Statistical Analysis
Descriptive statistics are presented as mean (M) ± standard deviation (SD). Bivariate relationships were evaluated with
independent group t-tests, one-way analysis of variance (ANOVA), Pearson product moment correlation coefficients and
partial correlation analysis as appropriate. To determine the relationship between body composition and 25-OHD in a
multivariate context, stepwise linear regression analysis was performed, regressing 25-OHD onto %TBF, BMI, age, race,
season, dietary intake of vitamin D and smoking. In this analysis, entry and removal criteria were set at P = 0·05 and P ≥
0·10, respectively. All analyses were performed using SPSS version 15·0 (SPSS Inc., Chicago, IL). An alpha level of 0·05
was used to determine statistical significance.
Results
The characteristics of the study cohort are shown in Table 1 . Almost all Hispanic women were of Mexican descent. All
women were premenopausal and lived in areas of moderate to light population density; no participants resided in large
inner-city environments. On average, the sample was overweight, with a mean BMI equal to 28·2 ± 7·0 kg/m 2 (range 16·449·3 kg/m2). Mean dietary intake of vitamin D was 2·7 ± 2·9 μg (range 0-24·1 μg). On average, women reported
participating in physical activity for 387 ± 502 min, or approximately 6·5 h/week. Physical activity ranged from 0 to 6540
min/week, with a median value of 230 min/week. Body composition data and serum laboratory values for 25-OHD,
calcium and phosphorus are shown in Table 2 for the overall sample and stratified by race.
Table 3 shows the Pearson product moment correlations between 25-OHD and patient characteristics including TBF,
%TBF, BMI, age and dietary intake of vitamin D. Physical activity was not associated with 25-OHD (P = 0·17). Because of
its lack of association with the criterion and the reported nonparticipation in specified physical activities during the past 30
days, physical activity was excluded from further analysis. Mean differences in serum 25-OHD were observed by race,
F2,796 = 151·80, P < 0·0001 ( Table 2 ). Additionally, mean differences in serum 25-OHD were observed by season, with
higher mean values observed in the 'summer' months (55·42 ± 27·48 nmol/l) relative to the 'winter' months
(48·15 ± 25·31 nmol/l), t = 3·84, P < 0·0001. Mean serum 25-OHD levels during the summer for non-Hispanic white, nonHispanic black and Hispanic women were 76·90 ± 29·95 nmol/l, 40·77 ± 17·40 nmol/l and 48·35 ± 18·42 nmol/l,
respectively (P < 0·0001, all pairwise comparisons significant). During the winter, mean serum 25-OHD levels were
64·48 ± 29·09 nmol/l for non-Hispanic whites, 33·50 ± 17·22 nmol/l for non-Hispanic blacks and 47·51 ± 19·71 nmol/l for
Hispanic women (P < 0·0001, all pairwise comparisons significant). Finally, serum 25-OHD was higher, on average,
among smokers (those with pack years > 0) (58·43 ± 27·52 nmol/l) relative to nonsmokers (pack years = 0)
(48·99 ± 25·34 nmol/l), t = -4·82, P < 0·0001.
The partial correlation coefficient between serum 25-OHD and %TBF (partialling out race, age, season, smoking and
dietary vitamin D) was statistically significant, r = -0·29 (P < 0·0001). Multiple stepwise linear regression analysis was
performed, with serum 25-OHD as the criterion variable and %TBF, BMI, age, race (two dummy codes, with Hispanic as
the reference group), dietary intake of vitamin D, season and smoking (pack years) entered as predictors. Because
stepwise regression can overfit the data and capitalize on chance, forward selection was used on 80% of the sample
followed by cross-validation with the remaining 20% of cases.[29] Stepwise multiple regression analysis was performed on
a randomly selected subsample of approximately 80% of the total sample (n = 600), allowing the computer to generate
the 80% subsample by randomly selecting subject ID numbers from the total sample of 800. Smoking and age failed the
criteria for entry into the model at any step. White race entered the model at the first step, explaining 25% of the variance
in 25-OHD. In step 2, BMI entered the model and explained an additional 8% of the variance, which was significant. In
steps 3 and 4, black race and %TBF entered the model, explaining an additional 1·3% and 1·4%, respectively, of the
variance in 25-OHD levels. BMI was removed in step 5 of the analysis and 'season' entered at step 6. The final model
predicting 25-OHD included race, %TBF and season as statistically significant parameters. Together these variables
explained approximately 36% of the variance in serum 25-OHD levels. Following the regression analysis, a predicted
score for 25-OHD was calculated for each individual in the 20% cross-validation sample (n = 156) based upon the step 6
regression model, where predicted score = 87·873 (constant) + 21·318 (white race) - 10·929 (black race) - 0·925 (%TBF)
- 3·879 (season). The Pearson product moment correlation coefficient between the predicted serum 25-OHD value and
the actual (observed) value was 0·559. This value squared (r2 = 0·312) is smaller than the adjusted R2 for the model
(R2 = 0·361), raising the caveat that, although the discrepancy is not considerable, the stepwise analysis may have slightly
overfit the data.
Discussion
We observed, in a diverse sample of young reproductive aged women, that vitamin D deficiency is extremely common.
Using the traditional cutoff value of < 50 nmol/l, 54% of the sample had vitamin D deficiency.[23] Recent publications cite
> 75 nmol/l as the desired 25-OHD concentration. Using this value, 85% of the women were vitamin D deficient. [30] When
stratified by race, the mean serum 25-OHD level for Hispanic women was lower than the level for whites, but higher than
that for blacks.[2-4,31] This finding, which is similar to those of the NHANES III, is consistent with the explanation that
increased skin pigmentation contributes to vitamin D inadequacy.[9,32]
Although we studied a much younger group of women than Levis et al. (mean age of 24 years in our sample compared to
almost 55 years),[8] we also observed that vitamin D deficiency commonly occurs among females residing in a southern
climate. Furthermore, our 'seasonal' serum 25-OHD levels were similar to the mean levels for females reported by Levis
et al. during the 'winter' (56·0 ± 20·5 nmol/l) and 'summer' (62·5 ± 23·5 nmol/l) months. Thus, it appears that residing in a
sunny climate is not adequate protection for younger or older women against vitamin D deficiency and that seasonal
variation in 25-OHD levels is measurable in these climates.
We also demonstrated in this young, diverse population that a negative relationship exists between serum 25-OHD levels
and common measures of obesity and adiposity (i.e. BMI, %TBF). This finding has been observed previously in other age
groups and ethnicities.[5,33-36] It is important to note that in our multivariate analysis, %TBF remained in the final statistical
model, while BMI did not. This is consistent with the findings of both Arunabh et al. and Bolland et al. and further
demonstrates the independent influence of adiposity on serum 25-OHD levels while controlling for race and season of the
year.[5,35] It is clear that the message of reducing body fat through healthy lifestyle choices must be promoted throughout a
person's lifetime. Vitamin D supplementation has been demonstrated to decrease bone loss and thus may be beneficial to
obese individuals.[37]
It has been well established that smoking has a negative association with BMD, and is therefore a risk factor for
osteoporosis. A new question in this area is whether the effect of smoking on BMD is manifested through an altered
metabolism of vitamin D. Our data show that women who smoke have significantly higher levels of serum 25-OHD, on
average, than women who do not smoke. This finding is in contrast to an earlier study of elderly women in Nebraska in
which serum 25-OHD levels were lower in heavy smokers relative to nonsmokers.[38] A recent study on adult women
residing in a similar latitude to that of our subjects showed no relationship between smoking status and vitamin D levels. [39]
It is possible that the young age of our sample, the racial diversity, as well as latitude, altitude and climate of our site may
have contributed to the difference in findings. When analysed in a multivariate context, the effect of smoking on serum 25OHD levels failed to meet statistical entry criteria in the context of other predictors including race, BMI, %TBF and season.
This suggests that smoking may be less important as an indicator of serum 25-OHD, at least with regard to our self-report
measure of smoking. However, smoking has become more regulated in our region, and individuals who want to smoke
must go outside public buildings. It is possible that cumulative differences in sun exposure may result in reliably higher
levels of serum 25-OHD among smokers in our region.
This study has several limitations. First, we used a 24-h dietary recall to assess vitamin D intake. Recall bias may have
led to under-reporting, which could explain in part why the intake was low, at about half of the adequate daily intake (5 μg
or 200 IU) recommended for females up to 50 years of age by the Food and Nutrition Board of the Institute of Medicine. [40]
Calvo and Whiting noted that few natural foods are rich in vitamin D, with the exception of fatty fish. [41] According to
experts, fatty fish would have to be eaten three or four times a week to satisfy the body's vitamin D requirement. [42]
Regional dietary patterns do not include frequent intake of oily fish such as salmon, mackerel or herring, as these fish are
not found in area waters and are expensive when available for market purchase. In addition, intake of fortified foods,
especially milk, may be avoided by non-Hispanic black and Hispanic women because of lactose intolerance. [33,41,43] Thus,
our regional findings mirror the low intakes of vitamin D from food as demonstrated for two decades in the nationally
representative NHANES.[33,41,44,45] In fact, a number of experts in the field have agreed that current dietary intake does not
provide adequate vitamin D and have called for updated recommendations above those made in 1997.[30,42-44] Second, we
did not collect data on use of vitamin D supplements, which would have affected overall intake levels. Third, the women
we studied were also enrolled in a larger clinical study related to contraceptive use and BMD; it is unknown the extent to
which this introduced sampling bias. Fourth, the Hispanic women in our study were predominantly from Mexico or of
Mexican descent. Extension of our interpretation of the data to Hispanic women of other origins should be done with
caution.
Although we have used environmental indices to estimate potential exposure to sunlight in our region, we do not have
information based on individual data, such as ratings of skin pigmentation, hours and time of day spent outdoors, daily
skin exposure or frequency of sunscreen usage. In addition, our data are unable to predict the influence of low elevation
or atmospheric conditions such as cloud cover or ozone level. However, for the purposes of our study, low vitamin D
status is very common among young women in our region despite an extended 'vitamin D summer', with potential for
more than adequate exposure to UV-B solar radiation. Under similar geographical and seasonal conditions as our site
(latitude 28°N, with 7 months of abundant sunshine), a small study of Egyptian women (n = 80; mean age 32 years) found
that hypovitaminosis D (cutpoint 50-100 nmol/l) was prevalent not only in the majority of patients but also in 60% of
healthy controls.[46] Regional clothing factors contributed to their findings. In another small study (n = 93 men and women,
mean age 24 years), carried out in Hawaii, where clothing habits and environmental conditions (latitude 21 °N) are more
similar to those in Galveston, low vitamin D status (cutpoint 75 nmol/l) has been reported.[47]
In summary, serum 25-OHD levels are lower in Hispanic and non-Hispanic black adolescent and young adult women in
comparison to non-Hispanic whites, even among those residing in one of the southernmost parts of the country. Our
sample differs from other studies, as ours is young, of reproductive age and racially diverse. The age group in our study is
of interest because most of the existing literature has examined postmenopausal women, yet it is clear that women should
strive to achieve peak bone density before the expected programmed decline in bone density begins manifesting itself
after age 30. It is thought that low levels of vitamin D may increase the lifetime risk of bone fracture, especially in women.
Combined with very low dietary intake of vitamin D and the high prevalence of obesity in this region of the country, our
preliminary findings indicate that low levels of vitamin D are found frequently in healthy young women who reside in a
region with environmental potential for year-round synthesis of vitamin D, especially during an extended 7-month, 'vitamin
D summer'. With the emerging evidence that vitamin D sufficiency may be linked to prevention of common cancers, heart
disease, autoimmune diseases, pregnancy outcomes and infant health, we conclude that young women need education
regarding the importance of vitamin D, the insufficiency of vitamin D intake in most diets and the benefits of
supplementation.
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Table 1. Characteristics of the Cohort (N = 800)
Table 2. Body Composition Data and Serum Laboratory
Values for the Total Sample and by Race*
Table 3. Pearson Product Moment Correlations Between
25-OHD and Patient Characteristics
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