Soy linked to less belly fat in postmenopausal women

Women receiving a daily soy shake experienced reductions in their subcutaneous abdominal fat, while women receiving a casein placebo experienced gains in fat around their waist and abdomen.

"To the best of our knowledge, this is the first report of an effect of soy isoflavones on abdominal body fat distribution in humans," wrote lead author Cynthia Sites in the journal Fertility and Sterility .

The randomised, double-blind, placebo-controlled trial suggests significant health benefits since excess abdominal fat has been associated with increased inflammation and oxidative stress, both of which are linked to risks of cardiovascular disease and diabetes.

Fifteen postmenopausal women (average age 55.6, average BMI 30.5 kg per sq. m) were assigned to receive either a daily soy shake containing 20 grams of soy protein plus 160 mg isoflavones, or a placebo (casein) shake for three months.

Both active and placebos were donated by Revival Soy.

At the end of the trial there were no differences in the weight of women in both groups, as measured by dual energy x-ray absorptiometry.

However, measures of subcutaneous abdominal fat were significantly different between the groups, with women in the soy group experiencing a 14.7 sq. cm reduction compared to a 22.9 sq. cm increase in the placebo group.

On the other hand, no differences were observed in total body fat, lean mass, and insulin secretion.

"Our supplement, which is produced from soy protein isolate and soy germ rather than from whole soybeans, produced a marked increase in five different serum isoflavone levels during the same time period, including a 90-fold increase in daidzein, a 65-fold increase in genistein, and a 42-fold increase in glycitein," reported the researchers.

"Thus, our supplement may not be comparable to other supplements."

Sites and co-workers report that the mechanism behind the apparent benefits of isoflavones on fat cells and tissue is not well understood.

They note that the animal studies have suggested a role of the isoflavone genistein may offer benefits by decreasing lipoprotein lipase messenger RNA levels, which reduces the build-up of fat cells (adipocytes).

Alternatively, isoflavones may affect adipocytes by gene regulation, they added, by repressing the differentiation of adipocytes.

"Whatever the mechanism, our data suggest that soy protein containing isoflavones may prevent the accumulation of fat in the abdominal depot," they stated.

"Our trial suggests a new dietary option for the prevention of abdominal adipose tissue gain that occurs after menopause," concluded the researchers.

"Long-term trials in a population that is more ethnically diverse should be conducted in the future."

Source: Fertility and Sterility (Elsevier) December 2007, Volume 88, Issue 6, Pages 1609-1617 "Effect of a daily supplement of soy protein on body composition and insulin secretion in postmenopausal women" Authors: C.K. Sites, B.C. Cooper, M.J. Toth, A. Gastaldelli, A. Arabshahi, S. Barnes