Concentrations of isoflavones in plasma and urine of post-menopausal women chronically ingesting high quantities of soy isoflavones

Soy food or food supplements based on soy containing isoflavones (Isos) are increasingly available in Western countries. However, the variability of Isos levels in urine and plasma in humans during chronic ingestion is poorly documented. Nevertheless, this is the way these compounds will most probably be used in the future, especially if the soy-based supplements market goes on increasing. Here, glycosilated Isos in an enriched extract of Prevastein equal to 100 mg of equivalent Isos aglycone was given daily to 27 post-menopausal women for 30 days and to 12 post-menopausal women for 60 days. Volunteers were given Prevastein in a cereal bar (25 mg Isos) and in a yoghurt (25 mg Isos) both at breakfast and dinner. Plasma samples were collected after overnight fasting. Urine samples were aliquots of a 24 h collection checked on volume and creatinin excretion levels. Genistein, daidzein and equol were measured at day 0 and every 15 days afterwards, using original specific ELISAs. Constant levels were reached from the 15th day. About 59.2% of the volunteers were significant equol producers in the first experiment and 58.3% in the second. A large variability in plasma and urine levels was observed among post-menopausal women consuming 100 mg Isos per day, although remaining relatively stable in each individual subject. This could partly account for the controversial effects of Isos recorded so far in clinical studies. So Isos plasma levels would have to be assayed during chronic exposures, and could help to better understand the large variability of the effects classically observed in clinical studies. ELISA techniques could be easily exported to analytical laboratories to help physicians and nutritionists with their prescriptions.