Effect of vitamins D2 and D3 supplement use on serum 25OHD concentration in elderly women in summer and winter.

Vitamin D2 and D3 are generally considered equipotent in humans. A few studies have reported that serum 25OHD levels are higher in vitamin D3- compared with vitamin D2-supplemented subjects. As both vitamin D2 and D3 supplements are commonly used by elderly in United States, in the present study we determined the effect of self-reported vitamin D2 and vitamin D3 supplement use on serum total 25OHD levels according to season in elderly women aged 65-77 years. Serum total 25OHD levels were determined in winter and summer in unsupplemented women ( N = 307) and in women who reported taking vitamin D2 ( N = 56) and vitamin D3 ( N = 55) supplements by competitive protein binding assay. In vitamin D2-supplemented women, the contribution of vitamin D2 and D3 to the mean serum total 25OHD level was assessed by HPLC. In summer, there were no significant differences in the mean total serum 25OHD levels (ng/ml) among the vitamin D2 (32 +/- 2.1), vitamin D3 (36.7 +/- 1.95), and unsupplemented (32.2 +/- 0.95) groups. In winter, the mean serum total 25OHD levels were higher in women on vitamin D2 (33.6 +/- 2.34, P < 0.05) and vitamin D3 (29.7 +/- 1.76, NS) supplements compared with unsupplemented women (27.3 +/- 0.72). In vitamin D2-supplemented women, about 25% of the mean serum total 25OHD was 25OHD2, in both summer and winter. Twelve percent of unsupplemented women and 3.6% of vitamin D-supplemented women had a mean serum total 25OHD level below 15 ng/ml in winter. In elderly subjects, both vitamin D2 and Vitamin D3 supplements may contribute equally to circulating 25OHD levels, with the role of vitamin D supplement use being more predominant during winter.