Smith LM, et al. - In African American and Caucasian women, researchers assessed total serum 25 hydroxyvitamin D (T25D), free 25 hydroxyvitamin D (F25D), and serum parathyroid hormone and analyzed the impact of vitamin D dosing to assess any differences by race or age. In two clinical trials, healthy young and older Caucasian and African American females who had inadequate vitamin D levels were randomized to increasing daily doses of vitamin D from 400 to 4800 IU and placebo for 12 months. According to findings, serum F25D and T25D are lower in young (but not older) African American women. Dietary vitamin D is comparable in groups, so the cause of low serum 25OHD in African American women is likely lower UV exposure and lower production of vitamin D in the skin.