Biological effects of isoflavones in young women: importance of the chemical composition of soyabean products

To examine the hormonal effects of isoflavones, of which soyabean is a rich source, fifteen healthy non-vegetarian premenopausal women were studied over 9 months. They lived in a metabolic suite for between 4 and 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 containing no soyabean products was fed. Then, over a second complete cycle six subjects consumed a similar diet into which 60 g textured vegetable protein (TVP)/d, containing 45 mg conjugated isoflavones, had been incorporated. Three participants had 50 g miso (a fermented soyabean paste), containing 25 mg unconjugated isoflavones, added daily to their diet over a menstrual cycle, and six others consumed 28 g TVP/d, containing 23 mg conjugated isoflavones. Five participants completed a third diet period where they were randomly assigned to consume either the 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) increased and peak progesterone concentrations were delayed with 60 g TVP but no effects were observed with Arcon F. The increase in menstrual cycle 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 follicle 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 other 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.