The effect of soy protein isolate on bone metabolism

Objective: This double-blind, 15-month pilot study was designed to investigate the effect of soy protein isolate with varying concentrations of isoflavones on early postmenopausal bone loss and lipids. Design: A total of 65 women, with a mean age of 55 years and 7.5 years since menopause, were randomized to one of three groups; soy protein with 96 mg isoflavones/day, soy with 52 mg isoflavones/day, or soy without isoflavones (<4 mg isoflavones/day). Soy was given for 9 months and then discontinued; participants were followed for an additional 6 months. Bone mineral density (BMD) and blood lipids were measured during this time. Results: Measurement of serum isoflavones at 3 months showed dose-related increases in the three groups. There was no significant effect of the soy supplements on BMD of the spine or femoral neck in any of the three groups. BMD increased significantly in the trochanter at 9 months (P = 0.0219) and at 15 months (P <0.05) in the group given isoflavone-free soy compared with the other two groups. There was no significant effect of soy on lipid metabolism at the end of the intervention. Conclusion: The present study did not find a significant positive effect of soy protein isolate supplemented with isoflavones on BMD and the serum lipid profile in early postmenopausal women.