O-210: Effect of a daily supplement of soy phytoestrogens on body composition and insulin secretion in postmenopausal women

Abstract

We sought to determine if a supplement of soy phytoestrogens would improve parameters of glucose and insulin metabolism and body composition in non-diabetic postmenopausal women on a weight-maintenance diet compared to a placebo supplement. Randomized, double-blind, placebo-controlled trial. Fifteen postmenopausal women, aged 55.6 ± 1.4 years with BMI of 30.5 ± 0.8 kg/m2 (means ± SEM), were randomized either to a daily shake containing soy phytoestrogens (160 mg isoflavones +20 g soy protein, N=9), versus a daily isocaloric casein placebo shake (N=6). At baseline and at 3 months, insulin secretion was measured by a hyperglycemic clamp, total body fat and lean mass were measured by dual energy x-ray absorptiometry (DXA), and visceral fat, subcutaneous abdominal fat, and total abdominal fat were measured by CT scan at the L4/L5 vertebral disc space. Plasma glucose was measured by the glucose oxidase method using an automated glucose analyzer, and serum insulin and C-peptide were measured by RIA. Insulin secretion was estimated from peripheral plasma C-peptide levels by deconvolution analysis and linear regression using a 2-compartment model. Serum isoflavones were measured by LC-multiple reaction ion monitoring-mass spectrometry. Means and SEM’s at baseline and at 3 months, and the differences between means, were compared between soy and placebo groups using the paired and unpaired Student’s t-test for comparisons within and between groups, with P<0.05 considered significant. At baseline, soy and placebo groups were equal with regard to age, BMI, visceral fat, subcutaneous abdominal fat, total fat, lean mass, fasting insulin, and insulin secretion (1st phase, 2nd phase, and total). During the study, weight as measured by DXA did not change significantly within or between groups (+1.38 ± 0.823 kg for placebo vs. +0.756 ± 0.438 kg for soy, p=0.15 & p=0.12 for within group differences respectively, and p= 0.48 for between group differences). However, total abdominal fat and subcutaneous abdominal fat as measured by CT increased more in the placebo group compared to the soy group during the study (for total abdominal fat: +38.62 ± 9.33 cm2 for placebo vs. -11.86 ± 10.49 cm2 for soy, p=0.005 for differences between groups; for subcutaneous abdominal fat: +22.91± 11.67 cm2 for placebo vs. -14.73 ± 7.42 cm2 for soy, p=0.013 for differences between groups). Insulin secretion did not change in the soy group during the study; in the placebo group, total insulin secretion increased by 12% due to increased second phase secretion, but differences between soy and placebo groups were not significant (p= 0.24). Changes in lean mass and fasting insulin did not differ between groups. Isoflavone levels increased significantly more in the soy phytoestrogen group compared to the placebo group (p=0.01 for genistein, p=0.004 for daidzein, p=0.005 for glycitein, p=0.002 for dihydrodaidzein, p=0.03 for O-desmethylangiolensin; all differences between groups. There were no differences between groups in equol levels; p=0.17). We conclude that a daily supplement of soy phytoestrogens in postmenopausal women reduces total abdominal fat and subcutaneous abdominal fat compared to an isocaloric casein placebo. These findings may be relevant for preventing increases in abdominal fat that occur commonly after menopause.