We found African American and Caucasian children residing in the Northeast to be equally vulnerable for vitamin D insufficiency during summer and winter despite having a mean daily intake of vitamin D above the current AAP recommended intake (400 IU/day). As most children in this study were enrolled from a pediatric clinic, it's likely that dietary advice from pediatric providers may have influenced the parents' to promote greater intakes of vitamin D and calcium. However, our data suggests that the current AAP recommended dietary allowance for vitamin D may be inadequate for optimization of vitamin D status of 6–12 yr old children throughout the year, as 1/3rd of African American and Caucasian children were vitamin D insufficient during winter. The risk of vitamin D insufficiency was higher during winter than summer in both racial groups (AA - 34.1% vs. 17.2%; Caucasian - 32.5% vs. 14.3%).
African American children have a significantly lower concentration of serum 25(OH)D during summer when compared with Caucasian children. In our adjusted multivariable model, race/season and dietary intake of vitamin D stratified by race/season were significant predictors of serum 25(OH)D concentrations. Serum 25(OH)D concentrations are lower in African Americans during summer and winter and in Caucasians during winter when compared with measurements in Caucasians during summer. Race/season was the dominant determinant of vitamin D status. BMI, reported duration of sun exposure, sunscreen use, age, gender, and sunreactive skin type did not influence serum 25(OH)D concentrations.
The influence of dietary vitamin D on serum 25(OH)D varied by race/season – dietary vitamin D positively correlated with serum 25(OH)D only in Caucasian children during winter. As casual sun exposure is the major determinant of vitamin D status – its probable that dietary vitamin D has no significant effect on serum 25(OH)D in either racial group during summer. The observed racial differences in the relationship between dietary vitamin D and serum 25(OH)D during winter suggests that African American children may need higher oral inputs of vitamin D compared to Caucasian children to have a positive impact on serum 25(OH)D.
The magnitude of change in serum 25(OH)D from winter to summer was 45% higher in Caucasian children compared to African American children. This disparity can be explained by the variation in skin pigmentation between the two racial groups. Harris and Dawson-Hughes found similar pattern in the amplitude of seasonal changes in plasma 25(OH)D of 20–40 year old Caucasian and African American women residing in Boston, Massachusetts (latitude: 42°N).7
These findings highlight the relevance of season, skin color, and latitude in the determination of vitamin D status.8–9, 12, 21–22
Residents of higher latitudes (>35°N or S) are at risk for wintertime hypovitaminosis D, as their winter sunlight lacks the UV potency for vitamin D photoproduction.23
Although the capacity for vitamin D photoproduction is similar across all racial groups – dark skinned individuals need 6–10 fold greater duration of sunlight exposure than light-skinned individuals to raise their vitamin D3
to the same concentration.24
Despite the likelihood of higher duration of summertime sunlight exposure and less frequent sunscreen use, African American children had lower 25(OH)D compared to Caucasian children. Although Caucasian children had a higher likelihood of travel to a sunny location below 35° latitude during summer, the effect of such travel on their vitamin D status could have been mitigated by their increased likelihood of sunscreen use.
Many observational and cross-sectional studies of US children have reported higher rates of vitamin D deficiency and insufficiency than what we found.3–4, 25–26
In 2001–2004 National Health and Nutrition Examination Survey (NHANES), among 6–11 yr old children (n=1837) – 18% were vitamin D deficient (serum 25(OH)D <20 ng/mL) and 53% were vitamin D insufficient (20 - <30 ng/mL).4
Furthermore African American children (n=664) were more likely to be classified as vitamin D deficient than Caucasian children (n=510) (51% vs. 9%). Similar to our study, both the racial groups were equally at risk for vitamin D insufficiency (AA [44%] vs. C [51%]). Less consumption of milk and vitamin D supplements and sedentary behavior (surrogate for less sun exposure) were the risk factors for low vitamin D status in the NHANES 2001–2004 survey of 1–21 yr old children.3
Weng et al. found higher prevalence (55%) of hypovitaminosis D (serum 25[OH]D <30 ng/mL) in 6–21 yr old children from Philadelphia, PA, residing in similar latitude as our cohort.27
However, their reported dietary intake of vitamin D was lower. The lower rates of vitamin D deficiency in our cohort may be explained by their relatively higher intakes of dietary vitamin D.
Defining the serum 25(OH)D cutoff values for vitamin D deficiency and insufficiency remains contentious and lacks consensus; and data for defining the threshold levels of serum 25(OH)D for optimal skeletal health in children are limited.28–30
If calcium intake is adequate, a desirable level of serum 25(OH)D will i) suppress PTH to a mid-physiologic range, ii) not decrease PTH during vitamin D supplementation, and iii) maximize calcium absorption, bone accretion, and bone mineral density.13, 31–34
Inverse association between serum 25(OH)D and PTH have been documented in older children and adolescents,35–37
and their threshold levels of 25(OH)D associated with optimal vitamin D status in terms of calcium homeostasis tend to fall between 16–36 ng/mL.35, 38
In adults, calcium absorption is maximal when serum 25(OH)D is ≥32 ng/mL,34
and increasing serum 25(OH)D from 32 to 40 ng/mL is beneficial for bone mineral density.32
However, the recent IOM report, has concluded that in the face of adequate calcium intake, serum 25(OH)D levels ≥20 ng/mL is sufficient for skeletal health, and levels <12 ng/mL and 12-<20 ng/mL are indicative of deficiency and insufficiency respectively.6
The IOM report calls to question the recent reports of high prevalence of vitamin D deficiency and insufficiency based on higher serum 25(OH)D cutoffs for defining vitamin D deficiency (<20 ng/mL) and insufficiency (20-<30 ng/mL).
Serum PTH is a recognized secondary outcome measure in the assessment of vitamin D status.13, 39
Hypovitaminosis D can result in calcium malabsorption and compensatory hyperparathyroidism.10, 3
PTH was inversely associated with 25(OH)D in our entire cohort during summer and winter, suggesting that hypovitaminosis D may have a negative impact on skeletal health of children. Furthermore, the serum PTH was significantly higher in our cohort of African American children when compared with Caucasian children during winter. The observed differences in serum calcium and serum albumin between the two racial groups were clinically non-relevant.
Our study has several limitations. Sampling timeframe would influence the seasonal differences in vitamin D status. Therefore, our findings would have been exaggerated if we had sampled only during the peak (end of August) and nadir (end of March) time-points for seasonal change in serum 25(OH)D status, instead of throughout summer and winter. Assessment of degree of melanization using a skin spectrophotometer would have been more objective than the parent reported sunreactive skin typing. Although our vitamin D intake questionnaire was detailed and thorough, we would like to caution the potential limitation in the reliability of questionnaires for assessment of dietary intake of vitamin D. Our observed effect size for differences between the racial groups could be limited by the smaller number of Caucasian children. Collection of blood samples for PTH assessment throughout the day instead of a specified time could potentially confound those results, as PTH has an endogenous circadian rhythm with a high peak in the early hours of morning and a nadir in the late morning and a small peak in the late afternoon.40–41
However, our study has several strengths. We enrolled significant proportion of African American children (67%), and had good retention rate (87%). We assessed serum 25(OH)D with the state of the art LC-MS/MS assay which reduces the potential confounding associated with variability in measurements of serum 25(OH)D among various assays and laboratories.20, 42
Moreover, our cohort had an adequate dietary intake of vitamin D, which enabled us to discern the effect of season, latitude, and skin color on vitamin D status of young school age children.