The main objective of this study was to determine the prevalence of vitamin D deficiency among black and Hispanic, preschool-aged children from low-income families living in the southern United States. A relatively high prevalence (22%) of vitamin D deficiency was identified among these apparently healthy, minority, preschool-aged children from low-income families in a convenience sample from 2 clinics in Atlanta, Georgia. An even larger proportion (73.6%) had serum 25-hydroxyvitamin D levels below the threshold (<30 ng/mL) that is becoming increasingly accepted as optimal for bone health and protection against malignancies, infections, and other diseases.12,19,20
Higher latitudes are known to reduce serum 25-hydroxyvitamin D levels adversely, and people who live at lower latitudes usually have a low prevalence of suboptimal serum 25-hydroxyvitamin D levels. However, the vitamin D deficiency prevalence in our study was higher than that found among healthy infants and toddlers in Boston, Massachusetts (42° N), which negates the assumption that children in the southern United States may be better off in terms of serum vitamin D status.8
In the Boston study by Gordon et al,8
skin pigmentation was not identified as a factor for deficiency. In our study cohort, however, non-Hispanic black children had an increased prevalence of vitamin D deficiency, compared with their Hispanic counterparts. The prevalence of vitamin D insufficiency was lower among non-Hispanic, nonblack children 1 to 6 years of age in the 2001–2004 National Health and Nutrition Examination Survey (43% for girls and 48% for boys),21
compared with values reported for our cohort of Hispanic and black children. None of the children in our cohort had clinical signs of rickets on physical examination. The skin has an enormous capacity to synthesize vitamin D when exposed to sunlight or ultraviolet B radiation, and any attenuation of ultraviolet radiation penetrating the skin, such as caused by skin pigmentation or sunscreen, reduces dramatically the cutaneous synthesis of vitamin D. Vitamin D deficiency has been reported for individuals with dark skin, but the effects of diet and age have not been assessed systematically in this group.22–25
Exposure to sunlight, measured on the basis of the time spent outdoors, the wearing of ultraviolet radiation-blocking clothing, and the use of sunscreen were not assessed in this study.
A strength of this study was the analysis of dietary vitamin D intake with the use of food diaries. Main sources of dietary vitamin D were milk and breakfast cereals. Hispanic children reported greater intakes of these foods, compared with black children. These food sources are fortified with vitamin D, which makes them good sources for this age group (preschool-aged children); few foods (oily fish such as mackerel, salmon, and sardines, cod liver oil, liver, and egg yolk) naturally contain adequate vitamin D, and these foods are consumed infrequently by children.26
However, serum 25-hydroxyvitamin D levels may be limited by the bioavailability of dietary vitamin D.26–28
This factor was not evaluated but might have affected the prevalence of vitamin D deficiency in this study. Dietary vitamin D intake did not seem to influence vitamin D status in this study, as predicted initially.
Age was an independent predictor of vitamin D deficiency, with older children having significantly reduced risk of vitamin D deficiency. This is in contrast to previous studies that determined vitamin D status in children and noted that older children had increased risk of vitamin D deficiency. 14,15
However, the children in the previous studies were older than those in this study. The observation in our study may be attributable to increased outdoor activities of the older children, compared with the younger children, as well as the use of sunscreens and protective clothing by the younger children. These might have a greater impact on reducing the skin’s ability to synthesize vitamin D than the age-related decline in cutaneous synthesis of vitamin D. Skin pigmentation, indicated by highly pigmented skin (black) and medium/lightly pigmented skin (Hispanic), was not a significant predictor of vitamin D status. These findings are consistent with the results from an earlier study of healthy infants and young children, among whom pigmentation, sunscreen use, and time spent outdoors did not predict vitamin D status.8
Data on behaviors such as time spent outdoors, use of ultraviolet light-protective clothing, and use of sunscreens would be of interest but were not determined in this study. The season of recruitment had a very strong association with vitamin D status. Study participants recruited in the spring and summer were less likely to have vitamin D deficiency, compared with those recruited in the winter, despite year-round sunlight and generally moderate temperatures in Atlanta, Georgia. Similar seasonal observations were recorded among prepubertal girls in the southeastern United States.15
Although there is no consensus regarding the appropriateness of routine vitamin D screening for healthy children after the winter season, daily vitamin D supplementation at the newly recommended dose of 400 IU per day or higher during the winter would be of even greater importance for preschool-aged children with dark skin.
Physical stature and weight did not have significant effects on vitamin D deficiency or status in multivariate models in the current study. In another study in which toddlers were recruited, a significant association was reported between BMI and serum vitamin D levels, indicating that increased BMI is a risk factor for vitamin D deficiency. 8
These findings were not replicated in our study of preschool-aged children. The bioavailability of vitamin D, a fat-soluble vitamin, is decreased in obese individuals, which may result in the increased vitamin D deficiencies observed in such individuals.28
This mechanism also could account for the inverse relationship between fat intake and vitamin D status. The inverse relationship between fat intake and serum calcium levels might be related to the binding of calcium by fatty acids in the small bowel, preventing absorption.29
Our results are limited by the cross-sectional design of the study, the small geographic area used for subject recruitment, and the lack of a cohort of socioeconomically matched white children, which limits the generalizability of our findings. However, the racial mixture of the cohort and the defined geographic location in urban Atlanta may be considered hypothesis-generating strengths of the study. For example, these children are likely to be at highest risk of developing nutrient deficiencies because of social and economic factors.