Effects of soy protein and isoflavones on glycemic control and insulin sensitivity: a 6-mo double-blind, randomized, placebo-controlled trial in postmenopausal Chinese women with prediabetes or untreated early diabetes

BACKGROUND: In vitro and animal studies have suggested that soy protein and isoflavones have favorable effects on glucose and insulin regulation, but intervention studies in humans are limited, and the results are controversial. OBJECTIVE: We investigated whether soy protein with isoflavones and soy isoflavone extracts could improve glycemic control and insulin sensitivity in postmenopausal women with early hyperglycemia. DESIGN: This was a randomized, double-blind, placebo-controlled trial that included 180 postmenopausal Hong Kong Chinese women with prediabetes or early untreated diabetes. After a 2-wk adaptation period, participants were randomly assigned to 1 of 3 arms to receive 15 g soy protein and 100 mg isoflavones, 15 g milk protein and 100 mg isoflavones, or 15 g milk protein on a daily basis for 6 mo. RESULTS: Three- or 6-mo treatments with soy protein with or without isoflavone supplementation did not result in favorable changes in the descriptors for glycemic control and insulin resistance, namely fasting and 2-h postload glucose, fasting and postload insulin, glycated serum protein, and homeostasis model assessment for insulin resistance and beta-cell function. CONCLUSIONS: This 6-mo randomized controlled trial did not support the hypothesis that soy protein with or without isoflavone supplementation had favorable effects on glycemic control and insulin sensitivity among postmenopausal Chinese women. The favorable change in postload glucose needs to be further confirmed.