Outdoor exposure and vitamin D levels in urban children with asthma

Abstract

Background: The inner-city pediatric population in the United States has a disproportionate burden of asthma. Recent attention has focused on the immunomodulatory role of vitamin D, which may be protective against disease morbidity. As the primary determinant of vitamin D status in humans is exposure to sunlight, we aimed to determine if 25-OH vitamin D levels in urban preschool children with asthma were low, influenced by time spent outdoors, and associated with asthma morbidity.

Methods: Serum 25-OH vitamin D levels were measured at baseline in a cohort of 121 inner-city children ages 2-6 years with asthma in Baltimore, MD. Participants were followed longitudinally at 3 and 6 months to assess time spent outdoors, asthma symptoms through questionnaires and daily diaries, and allergic markers.

Results: In a predominantly black population of preschool children, the median 25-OH vitamin D level was 28 ng/mL (IQR 21.2-36.9), with 54% of the children below the traditionally sufficient level of 30 ng/mL and 7.4% in the range associated with risk of rickets (< 15 ng/mL). The median time spent outdoors was 3 hours/day (IQR 2-4), and greater time spent outdoors was not associated with higher vitamin D levels. 25-OH vitamin D did not show seasonal variation in our cohort (p = 0.66). Lower 25-OH levels were correlated with higher IgE levels.

Conclusions: Urban African-American preschool children with asthma have high rates of vitamin D insufficiency, and increased outdoor exposure is unlikely to correct these low 25-OH vitamin D levels. Repletion in this population may require dietary supplementation.

Figure 1

Distribution of serum 25-OH vitamin D levels. Histogram of 25-OH vitamin D levels. The median 25-OH D level in this cohort was 28 ng/mL (IQR 21–37). The majority (54%) of children had levels below current guidelines of sufficiency (30 ng/mL) (dotted line), with almost a quarter (23%) in the deficient (≤ 20 ng/mL) range (dashed line).

Figure 2

Relationship between daily time spent outdoors and serum 25-OH vitamin D levels. A scatterplot of the average hours per day spent outside vs. serum 25-OH vitamin D shows no correlation between the two. The median time spent outdoors was 3 hours/day (IQR 2–4).

Figure 3

Seasonal variation in 25-OH vitamin D. Box-plots of 25-OH vitamin D by season. Levels of serum 25-OH D did not vary with respect to the season in which they were drawn (p = 0.66).