Increased probiotic yogurt or resistant starch intake does not affect isoflavone bioavailability in subjects consuming a high soy diet

Objective: Probiotics and prebiotics that affect gut microflora balance and its associated enzymeactivity may contribute to interindividual variation in isoflavone absorption after soy intake, possiblyenhancing isoflavone bioavailability. This study examined the effects of the consumption ofbioactive yogurt (a probiotic) or resistant starch (a known prebiotic) in combination with high soyintake on soy isoflavone bioavailability.Methods: Using a crossover design, chronic soy consumption was compared with soy plusprobiotic yogurt or resistant starch in older male and postmenopausal females (n 31). Isoflavonebioavailability was assessed at the beginning and end of each 5-wk dietary period by samplingplasma and urine after a standardized soy meal.Results: Chronic soy intake did not significantly affect plasma or urinary isoflavones after the soymeal and there were no significant effects of probiotic or resistant starch treatment. However, therewere trends for increased circulating plasma daidzein and genistein after the probiotic treatment andfor increased plasma daidzein and genistein 24 h after soy intake with resistant starch treatment.Neither treatment induced or increased equol production, although there was a trend for increasedplasma equol in equol-positive subjects (n 12) after probiotic treatment.Conclusion: The weak or absence of effects of probiotic yogurt or resistant starch supplement toa chronic soy diet suggests that gut microflora were not modified in a manner that significantlyaffected isoflavone bioavailability or metabolism.