Relationship between body mass index and serum 25-hydroxyvitamin D stronger among Caucasians than African Americans in NHANES adults 2001-2006

The rapid decline in circulating levels of serum 25-hydroxyvitamin D in both African Americans and Caucasians in the US population remains unexplained, and appears to parallel the obesity epidemic. The cross sectional data on 7349 Caucasian and African American adults between 21 and 69 years of age from the continuous National Health and Nutrition Examination Study (NHANES) 2001 to 2006 were used to model by race, with smoothing functions, the true relationship between body mass index (BMI), waist circumference and serum 25-hydroxyvitamin D in men and women. Weighted regressions of determinants of serum 25-hydroxyvitamin D levels were analyzed. Serum 25-hydroxyvitamin D is negatively associated with BMI linearly above an inflection point at 20 kg/m2 and positively associated below a BMI of 20 kg/m2. The gender- and age-adjusted regression coefficients of BMI on 25-hydroxyvitamin D levels demonstrate a 50% lower coefficient (r = –0.18 ng/ml/ BMI unit) among African Americans than among Caucasians (r = –0.36 ng/ml/BMI unit). These relationships were as great in men as in women and were replicated when waist circumference was used as a surrogate for visceral fat levels. The extent to which BMI is a strong predictor of serum 25-hydroxyvitamin D levels depends upon BMI being greater than 20 kg/m2. The hockey stick shape of the BMI 25-hydroxyvitamin D relationship needs to be taken into account when adjusting serum values for BMI. Both this inflection and reduction in serum levels of 25-hydroxyvitamin D in the population may be due to sequestration in an increasing adipose tissue reservoir. The interpretation of serum 25-hydroxyvitamin D levels as a biomarker of vitamin D adequacy requires appropriate adjustment of body fat mass.