Influence of soybean processing, habitual diet, and soy dose on urinary isoflavonoid excretion

In an attempt to explain the wide individual variation seen in urinary isof lavonoid phytoestrogen excretion, we conducted a series of 3 human feeding studies: a large cross-sectional study of equol production in humans with a soy challenge, a comparison of phytoestrogen metabolism when subjects cons umed fermented and unfermented soy products, and a dose-response study of u rinary isoflavonoid excretion at the low end of soy consumption. All studie s were conducted in young, healthy humans. Urinary isoflavonoids were measu red by isotope-dilution gas chromatography-mass spectrometry. Similar to re sults from other studies, 35% of screened subjects (30 men and 30 women) ex creted equol (>2000 nmol/d). In women, equol excretion was associated with higher intake of dietary fiber and carbohydrate. Fermentation of soy decrea sed the isoflavone content of the product fed but increased the urinary iso flavonoid recovery, suggesting that fermentation increases availability of isoflavones in soy. When soy-protein powder was fed at 0, 5, 10, and 20 g/d (0-36 mg isoflavones), there was a linear dose response of urinary isoflav onoid excretion to soy consumption that did not differ between subjects wit h high and low equol excretion. These results suggest that equol excretion may be related to the fermentable carbohydrate content of the diet; additio nal study is needed. Processing of soy affects isoflavone metabolism and mu st be considered in recommending exposure to isoflavones from soyfoods. Alt hough optimal isoflavone exposure for disease protection has not been deter mined, urinary isoflavonoid excretion appears linear at low-to-moderate soy consumption.