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

Vitamin D-2 and D-3 are generally considered equipotent in humans. A few studies have reported that serum 25OHD levels are higher in vitamin D-3- compared with vitamin D-2-supplemented subjects. As both vitamin D-2 and D-3 supplements are commonly used by elderly in United States, in the present study we determined the effect of self-reported vitamin D-2 and vitamin D-3 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 D-2 (N = 56) and vitamin D-3 (N = 55) supplements by competitive protein binding assay. In vitamin D-2-supplemented women, the contribution of vitamin D-2 and D-3 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 D-2 (32 +/- 2.1), vitamin D-3 (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 D-2 (33.6 +/- 2.34, P < 0.05) and vitamin D-3 (29.7 +/- 1.76, NS) supplements compared with unsupplemented women (27.3 +/- 0.72). In vitamin D-2-supplemented women, about 25% of the mean serum total 25OHD was 25OHD(2), 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 D-2 and Vitamin D-3 supplements may contribute equally to circulating 25OHD levels, with the role of vitamin D supplement use being more predominant during winter.