Effects of soy isoflavones and genistein on glucose metabolism in perimenopausal and postmenopausal non-Asian women: a meta-analysis of randomized controlled trials

Objective: Several randomized controlled trials (RCTs) have examined the role of soy isoflavones on cardiovascular risk factors in perimenopausal and postmenopausal women and have yielded inconsistent results. This meta-analysis aimed to assess the overall effect of soy isoflavones on glucose metabolism: fasting blood glucose, insulin, and insulin resistance. Methods: We searched for all articles published in English and indexed in Medline from January 1990 to December 2009. We included RCTs for soy isoflavone supplementation in perimenopausal and postmenopausal women not taking hormone therapy, selecting non-Asian women only. The main outcomes were fasting blood glucose changes from baseline. Results: We identified 10 eligible RCTs containing blood glucose data of 794 women. The main result was that soy isoflavones did not affect fasting blood glucose significantly. Under a random-effects model, the average difference in fasting blood glucose values between women assigned to isoflavones and women assigned to placebo was -2.16 mg/dL (95% CI, -5.21 to 0.89 mg/dL; P = 0.17). In genistein studies, the mean difference was -7.15 mg/dL (95% CI, -11.47 to -2.82). However, the effects on insulin and homeostasis model assessment insulin resistance were significant: -1.37 mu IU/mL (95% CI, -1.92 to -0.81 mu IU/mL) and -0.39 (95% CI, -0.65 to -0.14), respectively. Subgroup analyses did not show a significant effect of isoflavone dose, whereas isoflavone mixtures and genistein had a different effect on fasting blood glucose. Conclusions: This meta-analysis of RCTs showed that isoflavone use was not associated with a significant glycemia reduction in perimenopausal and postmenopausal non-Asian women. However, the few studies that reported insulin and homeostasis model assessment insulin resistance changes suggested that soy isoflavones and genistein alone had a beneficial effect on glucose metabolism.