BIOLOGICAL EFFECTS OF ISOFLAVONES IN YOUNG-WOMEN - IMPORTANCE OF THE CHEMICAL-COMPOSITION OF SOYBEAN PRODUCTS

To examine the hormonal effects of isoflavones, of which soyabean is a rich source, fifteen healthy nonvegetarian premenopausal women were s tudied over 9 months. They lived in a metabolic suite for between 4 an d 6 months where their diet and activity levels were kept constant and their hormonal status was measured over two or three menstrual cycles . During one (control) menstrual cycle a normal but constant diet cont aining no soyabean products was fed. Then, over a second complete cycl e six subjects consumed a similar diet into which 60 g textured vegeta ble protein (TVP)/d, containing 45 mg conjugated isoflavones, had been incorporated. Three participants had 50 g mise (a fermented soyabean paste), containing 25 mg unconjugated isoflavones, added daily to thei r diet over a menstrual cycle, and six others consumed 28 g TVP/d, con taining 23 mg conjugated isoflavones. Five participants completed a th ird diet period where they were randomly assigned to consume either th e control diet over a cycle, or a similar diet incorporating 60 g of a soyabean product which had had the isoflavones chemically extracted ( Arcon F). Follicular phase length was significantly (P < 0.01) increas ed and peak progesterone concentrations were delayed with 60 g TVP but no effects were observed with Arcon F. The increase in menstrual cycl e length did not reach statistical significance in the three subjects who ate 50 g miso/d, but peak progesterone levels were significantly ( P < 0.05) delayed. Mid-cycle peaks of luteinizing hormone (LH) and fol licle stimulating hormone (FSH) were suppressed with 45 mg conjugated isoflavones as 60 g TVP (P < 0.05 and P < 0.01 respectively). No other changes in sex-steroid hormone levels were observed on any of the oth er diets. A significant reduction in total cholesterol was found with 45 mg conjugated isoflavones (P < 0.05), but not with 23 mg conjugated isoflavone-free Arcon F. There was no effect of menstrual cycle phase on transit time.