Objective: Probiotics and prebiotics that affect gut microflora balance and its associated enzyme
activity may contribute to interindividual variation in isoflavone absorption after soy intake, possibly
enhancing isoflavone bioavailability. This study examined the effects of the consumption of
bioactive yogurt (a probiotic) or resistant starch (a known prebiotic) in combination with high soy
intake on soy isoflavone bioavailability.
Methods: Using a crossover design, chronic soy consumption was compared with soy plus
probiotic yogurt or resistant starch in older male and postmenopausal females (n 31). Isoflavone
bioavailability was assessed at the beginning and end of each 5-wk dietary period by sampling
plasma and urine after a standardized soy meal.
Results: Chronic soy intake did not significantly affect plasma or urinary isoflavones after the soy
meal and there were no significant effects of probiotic or resistant starch treatment. However, there
were trends for increased circulating plasma daidzein and genistein after the probiotic treatment and
for 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 increased
plasma equol in equol-positive subjects (n 12) after probiotic treatment.
Conclusion: The weak or absence of effects of probiotic yogurt or resistant starch supplement to
a chronic soy diet suggests that gut microflora were not modified in a manner that significantly
affected isoflavone bioavailability or metabolism.