Soy isoflavones increase preprandial peptide YY (PYY), but have no effect on ghrelin and body weight in healthy postmenopausal women

Author: Martin O et al
Publication: Journal of Negative Results in BioMedicine 2006, 5:11.

Studies show that estradiol may interact with satiety hormones such as peptide YY and ghrelin. Pancreatic Peptide is a peptide produced in the small intestine and colon that reduces appetite in response to feeding, whereas ghrelin is a hormone produced in stomach that stimulates appetite. Because soy isoflavones have structural and functional similarities to estradiol, the researchers were interested in their possible weight reducing effect. Other studies suggest that the consumption of soy and isoflavone-rich soy protein show an inverse relationship with obesity. It is not clear this effect is caused by the soy protein or the soy isoflavones.

The study design was randomized, double-blind, placebo controlled and cross-over trial, using 34 healthy postmenopausal women. They were divided in two groups: the isoflavones group and the control group, which received respectively isoflavone-enriched cereal bars or control bars without isoflavones. The researchers measured plasma levels of peptide YY and ghrelin, energy intake and weight at the start, after 4 weeks and after 8 weeks.

They found that the body weight for both group increased. The isoflavone group showed a slightly lower increase in body weight (-260g) than the control group, but this difference was not significant. The serum level of the peptide YY but not that of ghrelin increased significantly in the isoflavone group. Changes in the levels of peptide YY where not influenced by the equol status of the participant. This study was not able to detect an influence of the consumption of isoflavones on energy intake or body weight, although the plasma level op peptide YY increased. The researchers suggested to larger experiment over a longer period might be able to detect a smaller influence energy intake. Another limitation of this study is that energy intake was calculated from food diaries and not from weighed food intake.

The study concluded that isoflavone intake had no effect on energy intake or body weight, although peptide YY levels were significantly increased. This study further indicate that peptide YY is not a major factor in the regulation of body weight.