This study demonstrated that 25-OH vitamin D insufficiency is present in the majority of a cohort of urban, preschool-aged inner-city children with asthma. These findings extend those seen in heterogeneous populations of older children [12, 13] to a population of predominantly African-American children 2–6 years of age. Even at this early preschool age, 25-OH D levels were already strikingly below the recommended range, as defined by current guidelines for bone health. Over half (55%) were either insufficient or deficient, and a disturbing proportion of children (7%) in our cohort were at the extreme end of deficiency, with levels as low as those seen with rickets (< 15 ng/mL). The implications of these findings may be even greater in younger cohorts, as it is during early childhood when considerable lung growth is still occurring, and when the respiratory system may be particularly sensitive to nutritional and environmental signals that induce changes in lung function .
Our findings are comparable to the NHANES 2001–2004 study of US children (age 1–21 years), where nine percent of all children had levels less than 15 ng/mL, and sixty-one percent had levels in the range between 15 and 29 ng/mL . In fact, within our defined geographical area of inner city Baltimore, 25-OH D levels are much lower among African-Americans than whites, identifying black preschool children as preferentially at risk for 25-OH D deficiency. Recent recommendations from the Institute of Medicine have challenged the traditional cut-offs for 25-OH D, suggesting that levels above 20 ng/mL instead of 30 ng/mL should be considered sufficient . If we consider that suggested threshold, almost a quarter of subjects in our cohort would still be considered deficient. It is still uncertain what levels constitute sufficiency in asthma and other conditions apart from bone health; nevertheless, regardless of which guidelines are used to determine the optimal 25-OH vitamin D level, our study demonstrates that Baltimore, with one of the highest rates of asthma, is representative of a greater national epidemic of 25-OH D insufficiency in black children.
The findings from this study suggest that, although UV light is necessary for dermal vitamin D production, black urban children with asthma appear to be unresponsive to what little daily sunlight they do receive, as higher daily levels of outdoor exposure were not linked to higher 25-OH D. A limited number of other studies have also examined the relationship between sun exposure and 25-OH D, but little attention has been paid to this relationship in children, particularly African-Americans with asthma. Our study is unique in that it investigates this relationship between outdoor exposure and vitamin D status in a predominantly black population of preschool children, a group that includes children with the nation’s highest levels of asthma. In prior studies of healthy people, controlled UVB exposure increased serum 25-OH D in a dose-related manner [26, 27]; however, African-Americans and Caucasians have a notable difference in this dose response. Scragg and Camargo demonstrated that in a racially-mixed population, non-Hispanic blacks had only modest increases in 25-OH D with increasing outdoor activity when compared to the dramatic changes observed in whites, with changes in serum 25-OH D observed only in response to extreme frequencies of outdoor activity (> 31 times in the past month) . Taken with our findings, we conclude that cutaneous synthesis alone during limited outdoor exposure is not sufficient to raise serum levels effectively in black urban children. Exposure to sunlight beyond this range may cross a threshold where 25-OH D production begins to increase, as previous studies have shown that in healthy adults, higher doses of artificial UV are needed in darker-skinned individuals to measurably raise 25-OH D levels, compared those with lighter-skin . However, a true prescription for natural UV exposure to achieve sufficient serum levels in black asthmatic children is yet to be determined.
Similarly, the predominantly African-American children in our study did not show any variation in 25-OH D levels with respect to season, in contrast to prior studies of predominantly-Caucasian populations whose levels are lowest in winter and highest in summer . The observed differences in African-Africans may reflect a blunted response to the varying sunlight availability across seasons due to the diminished capacity of darker skin to synthesize vitamin D. We hypothesize that the lack of improvement in vitamin D status with increasing sun exposure—whether over a period of a day or across seasons—is a feature of increased skin pigmentation compounded by sub-optimal sun exposure. Spending a modest amount of additional time outside in regions with similar UV light availability is not likely an effective strategy to improve vitamin D status. On the other hand, promoting extreme doses of exposure, which may eventually lead to an improvement in levels, may not be practical or desired due to other risks of UV exposure. Alternative strategies to ensure adequacy through an enriched diet or dietary supplementation [8, 30] may be necessary to compensate for such deficiencies.
Finally, our study supplements the small number of observational studies that show an association between personal 25-OH D and allergy and asthma morbidity [12–15, 31]. Within our population, those children with lower 25-OH D levels had higher IgE levels. This inverse association has been shown previously , adding support to an allergy-mediated mechanism behind vitamin D’s effects. Furthermore, higher 25-OH D levels tended to be protective against asthma symptoms, including wheezing/coughing/chest tightness, symptoms while exercising, and cough without a cold or flu, although associations were not statistically significant. More studies, including those that are designed to address causal links, are needed to solidify a link between asthma outcomes and low 25-OH D levels. The 2011 Institute of Medicine review of the literature regarding dietary requirements of calcium and vitamin D concluded that there was insufficient evidence to inform recommendations on intake for extra-skeletal outcomes, and that further trials were needed to assess the benefit of supplementation on extra-skeletal health . If found to be significant, improving individual vitamin D status could help protect against common urban asthma triggers in vulnerable children and result in a sustainable decrease in asthma morbidity.
One limitation of our study is that it was conducted at a single site in an urban area at a specific solar zenith angle, which typically receives less UV radiation than more equatorial regions of the world. However, it appears that astronomical factors may be less influential in determining vitamin D status, given that studies conducted in latitudes with higher UV radiation (e.g., Costa Rica, Georgia, Arizona) [12, 32, 33] show surprisingly similar rates of deficiency. Rather, lifestyle and behavioral factors may be more significant in an increasingly urbanized world. While our study did not record sunscreen use or protective clothing, or allow for direct measures of UV exposure which could vary with time of day and weather conditions, more generalizable features such as time spent outdoors and skin pigmentation may instead be the key determinants of vitamin D status and, in turn, asthma control. Additionally, rates of vitamin D supplementation and dietary intake of D-rich foods were not specifically assessed here. Finally, as with most other studies investigating vitamin D’s role in asthma, our design was cross-sectional, thus limiting our ability to establish a causal link between vitamin D and asthma morbidity. Future clinical trials are necessary to determine if vitamin D truly has the effects upon asthma health suggested by this observational literature.