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Objectives: Soy isoflavones have many effects similar to those of estrogen and have become popular among postmenopausal women as an alternative for hormone replacement therapy (HRT). The purpose of this study was to determine the effects of soy isoflavones on glucose, insulin, and lipid profiles in postmenopausal Taiwanese women.

Methods: We conducted a randomized, double-blind, active placebo-controlled clinical trial to compare the effects of isoflavones with estrogen replacement therapy (ERT) on blood glucose, insulin, and lipid profiles in postmenopausal Taiwanese women. Thirty postmenopausal Taiwanese women were randomly assigned to two groups, and each received treatment for 6 months. The isoflavone group received 100 mg isoflavone soft capsules, 300 mg calcium, and a blank vitamin capsule per day. The estrogen active control group received 0.625 mg conjugated estrogen, 300 mg calcium, and blank isoflavone soft capsules per day. We measured baseline levels of fasting blood sugar, insulin, lipid profiles, and isoflavone concentrations prior to the study and repeated the same measurements every 3 months for a total duration of 6 months.

Results: Two-way ANOVA revealed that fasting glucose (p < 0.001) and insulin (p < 0.005) levels were significantly affected by estrogen and isoflavone treatments after 6 months. For the isoflavone group, the average blood genistein concentration was 6–10 times higher than those of the estrogen group. Within the same periods, the fasting blood glucose was reduced to 83% and 85% of the baseline levels, and insulin was reduced to 56% and 67% of the baseline levels, respectively, in the estrogen and isoflavone groups.

Conclusions: Soy isoflavones (100 mg) and 0.625 mg conjugated estrogen equally lower fasting blood glucose and insulin levels in postmenopausal women.