Dairy consumption is a significant correlate of urinary equol concentration in a representative sample of US adults

Equol and O-desmethylangolensin (ODMA) are products of gut bacterial metabolism of daidzein, a phytochemical found predominantly in soy. Dietary sources of equol from animal products have been identified, which has raised the question of the relative contributions of daidzein intake and gut metabolism to equol and of equol intake from animal products in low-soy-consuming populations.

The objective was to evaluate the contribution of dietary food groups to urinary isoflavone and daidzein metabolite concentrations in a representative sample of US adults.

A cross-sectional analysis of dietary and urinary isoflavonoid data from 3115 individuals in the 2001-2002 and 2003-2004 data cycles of the National Nutrition and Health Examination Survey (NHANES) was conducted.

Daidzein intake and consumption frequency of grain products and legumes, nuts, and seeds were significant correlates of daidzein, genistein, and ODMA concentrations; and soy legumes were a stronger correlate than were nonsoy legumes. Milk and milk product consumption and daidzein intake, but not legumes, were significant correlates of urinary equol concentrations; milk products were more strongly correlated (P for trend < 0.001) than was daidzein intake (P = 0.011).

These results suggest that dietary daidzein and legumes may contribute to urinary daidzein, genistein, and ODMA concentrations in this low-soy-consuming population. These results also suggest that equol concentrations in low-soy-consuming populations may reflect equol intakes from mammalian milk sources and may not reflect the endogenous production of equol from the microbial metabolism of daidzein-an observation not yet documented in the US population. These results support the careful design and interpretation of urinary isoflavonoid excretion studies, particularly bacterial metabolites, in low-soy-consuming populations.