S. intermedius strain can make Equol in aerobic environment (outside of the body)

N. Kikuchi. Saga Nutraceuticals Research Institute, Otsuka Pharmaceutical Co., Saga, Japan.

Intestinal bacteria play a key role in the metabolism and bioavailability of soy isoflavones. Soy isoflavones are absorbed as aglycone forms, are hydrolyzed by intestinal microflora, and are further metabolized to specific metabolites including equol and p-ethyl phenol. Equol is expected to have greater biological effects than other soy isoflavones on health benefits. However, the extent of metabolism to equol appears to vary highly among individuals. These individual differences may influence the biological effects of soy isoflavone consumption. We hypothesized that individuals producing equol from daidzein by intestinal bacteria receive more health benefits. First, we investigated the relationship between equol and menopausal symptoms in 108 dietitians (31–73 y) living in southwestern Japan. The equol detection rate in urine was approximately 50%. Of the 108 participants, 37 were assigned to the higher and lower groups on the basis of menopausal symptoms. Urinary excretion of genistein and daidzein were not different between both groups. However, equol excretion was significantly lower in the higher group than in the lower group. The results suggest that equol plays an important role in the reduction of menopausal symptoms. Next, we tried to identify the human intestinal bacteria that metabolize daidzein to equol by using the feces of subjects who excreted high amounts of equol in urine. The feces were cultivated anaerobically for the isolation or organisms. Specific intestinal bacteria were identified by using morphological and biochemical characteristics, a sugar fermentation test, and the organic acid production spectrum. We identified three strains of bacteria: Bacteroides ovatus ssp., Ruminococcus productus ssp., and Streptococcus intermedius ssp. Only S. intermedius ssp. can be cultivated aerobically and is classified as a lactic fermenter. Therefore, S. intermedius ssp., may be used as a food supplement. Our findings confirmed the importance of equol-producing intestinal bacteria on the biological effects of soy isoflavones.