Serum 25(OH)D response to soy isoflavone supplementation in postmenopausal women

Abstract

Soy isoflavones (ISO) are believed to have anti-osteoporotic properties possibly mediated through vitamin D. The impact of ISO supplementation on serum 25(OH)D was assessed in a multisite, randomized, double-blind, 2-yr bone study in 403 early postmenopausal women (aged 54.0±4.0 yr) with 1/3 on placebo (PL), 1/3 on 80 mg/d ISO and 1/3 on 120 mg/d ISO. All subjects received 1,000 mg/d CaCO3 and 400 IU/d vitamin D3. Serum 25(OH)D was measured at baseline and annually using radioimmunoassay (Diasorin Inc.). Repeated measures ANCOVA adjusting for baseline 25(OH)D was employed in the statistical analysis. Baseline characteristics were similar in all groups, including mean 25(OH)D values (78.9, 74.9 and 82.4 nmol/L for the PL, 80 mg and 120 mg groups, respectively). ISO supplementation did not alter 25(OH)D concentrations, yet there was a significant trend such that 25(OH)D increased over time (p<0.05). The percentage of participants with serum 25(OH)D <50 nmol/L fell from 11.0% at baseline to 5.8% at yr 1 and 4.1% at yr 2. Soy ISO supplementation did not alter circulating 25(OH)D over 2 yr, but 400 IU/d vitamin D was effective at improving serum 25(OH)D in healthy postmenopausal women with high baseline values. Supported by Initiative for Future Agriculture and Food Systems Grant no. 2001-52102-11255 from the USDA CSREES.