UC Davis

BACKGROUND: UVB light from the sun increases serum 25-hydroxyvitamin D [25(OH)D] concentration, but this relation may depend on skin pigmentation among different racial/ethnic groups. OBJECTIVE: We used quantitative measures of exposed (facultative) and unexposed (constitutive) skin color to examine relations between serum 25(OH)D concentration, tanning, race/ethnicity, and constitutive skin color over the summer, following winter vitamin D supplementation. METHODS: The subjects (n= 426, mean age 11.7 ± 1.4 y, 51% female) were racially/ethnically diverse schoolchildren (57% non-white/Caucasian) enrolled in a 6-mo vitamin D supplementation trial (October-December to April-June). In this secondary analysis, measures of serum 25(OH)D concentration and skin color, with the use of reflectance colorimetry, were taken over a 6-mo period after supplementation, from pre-summer (April-June) to post-summer (September-December). Multiple linear regression was used to evaluate longitudinal relations. RESULTS: Following supplementation, mean serum 25(OH)D concentration was 29.3 ± 9.5 ng/mL but fell to 25.6 ± 7.9 ng/mL (P< 0.0001) by the end of summer. The decrease in white/Caucasian children was less than in black/African American children (P< 0.01) and tended to be less than in Hispanic/Latino, Asian, and multiracial/other children (P= 0.19-0.50) despite similar changes in sun-exposed skin color among all groups. Tanning was significantly associated with post-summer serum 25(OH)D concentration (β = -0.15,P< 0.0001), as was race/ethnicity (P= 0.0002), but the later association disappeared after adjusting for constitutive skin color. CONCLUSIONS: Tanning significantly contributed to serum 25(OH)D concentration over the summer, independent of race/ethnicity, but was not sufficient to maintain serum 25(OH)D concentration attained with supplementation. Much of the variation in serum 25(OH)D concentration between racial/ethnic groups may be explained by skin color. This trial was registered atclinicaltrials.govasNCT01537809.