Beneficial effect of soy isoflavones on bone mineral content was modified by years since menopause, body weight, and calcium intake: a double-blind, randomized, controlled trial.

OBJECTIVE: Many studies have shown that soy isoflavones have an effect in preventing estrogen-related bone loss, but no data reported whether such an effect could be influenced by other important factors affecting bone loss. This study examines whether the associations between isoflavone supplementation and rates of change in bone mineral content (BMC) could be modified by years since menopause (YSM), body weight (BW), and dietary calcium intake in postmenopausal Chinese women aged 48 to 62 years.

DESIGN: A group of 203 eligible women were randomly assigned to three treatment groups: placebo (daily dose of 0 mg isoflavones + 500 mg calcium, n = 67), mid-dose (40 mg isoflavones + 500 mg calcium, n = 68); and high-dose (80 mg isoflavones + 500 mg calcium, n = 68). Bone mineral density (BMD) and BMC at the whole body, spine, and hip were measured by dual-energy x-ray absorptiometry at baseline and posttreatment after 1 year. YSM, BW, and dietary calcium intake stratified analyses were performed to evaluate whether the associations between isoflavones supplementation and BMC change rates were varied with these factors.

RESULTS: Both univariate and multivariate analyses observed significant favorable effect of isoflavone supplementation on rates of change in BMC at the total hip and trochanter among later postmenopausal women (> 4 y), in women with lower BW (< or =median, 55.5 kg), or among women with lower level of calcium intake (< or =median, 1095 mg/d).

CONCLUSIONS: The independent effect of soy on the maintenance of hip BMC is more marked in women in later menopause or those with lower BW or calcium intake.