In vitro incubation of human feces with daidzein and antibiotics suggests interindividual differences in the bacteria responsible for equol production.

Daidzein can be metabolized to equol, dihydrodaidzein (DHD), and O-desmethylangolensin (ODMA) by intestinal bacteria. Only one third to one half of individuals produce equol, and evidence exists to support potential cancer-protective effects of equol production. We investigated the in vitro metabolism of daidzein by fecal bacteria and assessed the effect of several antibiotics on metabolism. Fresh or previously frozen feces from 7 equol producers and 6 nonproducers were incubated with daidzein, with or without antibiotics, for 5 d at 37 degrees C. With the exception of one previously frozen sample, fecal inoculates from equol producers converted daidzein to equol. Conversion occurred under anaerobic, but not aerobic conditions. Fecal inoculates from equol nonproducers did not produce equol, but some produced ODMA and DHD. Between-subject differences in the effects of antibiotics on daidzein metabolism were apparent. Some antibiotics inhibited the production of equol but had no effect on DHD production. These results suggest that several bacteria may be involved in daidzein metabolism, and that they may differ among subjects. This simple in vitro system can facilitate the study of factors influencing equol production and minimize the need for animal models or human interventions. Furthermore, these analyses can be conducted on fecal samples that have been frozen and stored.