Stinky tofu as a rich source of bioavailable S-equol in Asian diets

The compound S-equol (4',7-dihydroxy-isoflavandiol), a gut bacterial metabolite of isoflavone daidzein, benefits health, but only 20-60% of humans can produce equol after ingesting isoflavones, and it exists only in foods of animal origin in trace amounts. A recent study found a source of stinky tofu contained S-equol. As stinky tofu is a popular traditional fermented soy food in Taiwan, we analyzed S-equol contents of commercial samples, surveyed the intake frequency, and investigated the bioavailability of S-equol by monitoring urinary kinetics following ingestion. Our results showed 91% of the 138 stinky tofu dishes contained S-equol. The mean content per serving (average 198. g) was 2.3±2.5 mg, the highest being 16.3. mg. Stinky tofu eaters on average ingested this food 3.3 times per month. S-equol from ingested stinky tofu appeared in urine within 1. h, reaching maximum excretion at 3.4. h, and 67% of the ingested S-equol was recovered in urine, indicating a rapid and high absorption. Our studies suggest stinky tofu can be a promising dietary source of S-equol for its high content and bioavailability. Further study on the S-equol producing bacteria in stinky tofu is merited for the development of other S-equol rich soy products.