Previous studies on animals and humans have linked the consumption of soy and isoflavones-rich soy protein with reduced body weight. These studies did not investigate the individual effects of the pure soy protein and soy isoflavones. Maybe only the higher intake of soy proteins may explain the observed effects. This German study investigated the possible effect of soy isoflavones as appetite suppressant and on body weight. It is known that estrogen (estradiol) and similar components interact with so called satiety hormones, such as peptide YY and ghrelin. The researchers theorized that the intake of soy isoflavones, which have a similar structure than estradiol, may also have this effect. Studies with ovariectomized rats have shown that low estrogen levels increase body weight, an effect that is reversed with oral intake of genistein. Treatment of hysterectomized postmenopausal women with estrogens increased the levels of ghrelin. Peptide YY is a short protein that appears to reduce appetite and is released by cells in the ileum and colon in response to feeding, whereas ghrelin is a hormone produced mainly by cells in of the stomach cells pancreas that stimulates appetite.
The researchers tested the effect of dietary isoflavones (50 mg isoflavones/day) on 34 healthy postmenopausal women in a randomized, double-blind, placebo-controlled, cross-over trial with a washout period of eight weeks. The isoflavones intake corresponds to the upper range of a traditional Asian diet. About halve the women were classified as equol producers. Equol is an isoflavandiol metabolized from isoflavones by bacterial flora. Only about 30-50% of people have intestinal bacteria that make equol (=equol producer). They found that the intake of isoflavones had no significant effect on body weight but significantly increased peptide YY and independent of equol production. Ghrelin production was not influenced by isoflavones treatment but its production was significantly lower in equol producers than in equol non-producers. The duration of this study may have been to short to detect an influence on body weight.
The study concluded that isoflavones had an effect on peptide YY production but that this effect did not translate in reduced energy intake or reduced bodyweight. It showed that peptide YY has no major role in the regulation of body weight. A larger and more rigorous study should be conducted to detect a possible effect of soy isoflavones on body weight.
Source: Weickert MO, Reimann M, Otto B, Hall WL, Vafeiadou K, Hallund J, Ferrari M, Talbot D, Branca F, BŁgel S, Williams CM, Zunft HJ, Koebnick C. Soy isoflavones increase preprandial peptide YY (PYY), but have no effect on ghrelin and body weight in healthy postmenopausal women. J Negat Results Biomed. 2006 Aug 14;5:11