Association between quantitative measures of skin color and plasma 25-hydroxyvitamin D

Summary: We examined the relationship between vitamin D and skin color measured by reflectance colorimetry at an exposed and un-exposed site in 321 people. Exposed but not unexposed skin color was associated with better vitamin D status. Sun-exposure was more important than natural skin color in determining vitamin D status in our population. 

Introduction: Vitamin D is obtained through UV synthesis in the skin where melanin limits its synthesis. Ethnicity is often used as a proxy for skin color, but skin color varies considerably. The relation between quantitative measures of skin color and plasma 25-hydroxyvitamin D (25OHD) concentration has not been well described. 

Methods: The aim of this study was to determine the association between constitutive (natural) and sun-induced skin color and 25OHD in a group of Pacific People (n=87) and Europeans (n=255) living in NZ (46°S) in summer. Plasma 25OHD was determined and sun-induced (outer fore-arm) and constitutive (upper inner-arm) measured by reflectance colorimetry. 

Results: Mean (SD) 25OHD was significantly higher in Europeans than Pacific People, 88 (31) nmol/L vs. 75 (34) nmol/L, respectively. Based on constitutive skin color, 35% of participants were very light, 45% light, 16% intermediate, 4% tanned, and 0% brown or dark. Skin color at the forearm but not constitutive skin color was a significant predictor of 25OHD. Each 10° lower skin color value at the forearm (more tanning) was associated with a 5 nmol/L higher 25OHD (P<0.001). 

Conclusions: Tanning but not natural skin color was an important determinant of 25OHD. Further study is needed in a population with a higher proportion of darker skin people.