Interindividual Variation in Metabolism of Soy Isoflavones and Lignans: Influence of Habitual Diet on Equol Production by the Gut Microflora

TLDR

The soy isoflavones daidzein and genistein, along with the lignans matairesinol and secoisolariciresinol, are extensively metabolized by intestinal microflora. The authors investigated the extent of interindividual variation in isoflavone metabolism using a crossover study of low‑ and high‑isoflavone soy foods. The study employed a 17‑day treatment period for each diet, a 25‑day washout, and 24 healthy subjects (19 women, 5 men, mean age 30 yr). The results revealed a 16‑fold and 664‑fold variation in total isoflavonoid and urinary equol excretion, respectively, with 36 % of participants classified as good equol excretors; equol excretion was negatively correlated with dietary fat intake, good excretors consumed less fat and more carbohydrate, O‑DMA varied 76‑fold but was unrelated to equol, and enterolactone showed less variation, suggesting high‑fat diets may reduce gut microbial equol production.

Abstract

The soy isoflavones, daidzein and genistein, and the lignans, matairesinol and secoisolariciresinol, are phytoestrogens metabolized extensively by the intestinal microflora. Considerable important evidence is already available that shows extensive interindividual variation in isoflavone metabolism, and we have investigated the extent of this variation in a crossover study of a soy-containing food low or high in isoflavones (each treatment period lasted for 17 days, and the 2 treatment periods were separated by a 25-day washout period) in 24 healthy subjects [19 women and 5 men, mean age 30 yr, range 19-40, mean body mass index 22.5 +/- 3.5 (SD) kg/m2]. There was a 16-fold variation in total isoflavonoid excretion in urine after the high-isoflavone treatment period. The variation in urinary equol excretion was greatest (664-fold), and subjects fell into two groups: poor equol excretors and good equol excretors (36%). A significant negative correlation was found between the proportion of energy from fat in the habitual diet and urinary equol excretion (r = -0.55; p = 0.012). Good equol excretors consumed less fat as percentage of energy than poor excretors (26 +/- 2.3% compared with 35 +/- 1.6%, p < 0.01) and more carbohydrate as percentage of energy than poor excretors (55 +/- 2.9% compared with 47 +/- 1.7%, p < 0.05). Interindividual variation in the urinary excretion of O-desmethyl-angolensin (O-DMA) was also apparent (76-fold after the high-isoflavone treatment period), but there was no relationship between equol excretion and O-DMA excretion. Enterolactone was the major lignan metabolite in urine and plasma but showed less interindividual variation than equol and O-DMA. It is suggested that the dietary fat intake decreases the capacity of gut microbial flora to synthesize equol.