Antibiotics may decrease the effects of soy isoflavones such as genistein and daidzein. Soy isoflavones are converted into their active forms in part by intestinal bacteria, thus alteration of the intestinal microflora by antibiotics may decrease availability of activated isoflavones. Data are limited. In a study consisting of 11 children ages 4 to 17 years who were fed a body weight-adjusted dose of soy nuts, the urinary excretion rates for genistein and all native isoflavones combined (i.e., genistein, daidzein, and glycitein) were reduced by 37% and 24%, respectively, during oral antibiotic treatment for infection compared to the healthy state when not on antibiotics. The urinary excretion rate for total isoflavones (i.e., native and activated isoflavones combined) was also reduced by 24% during antibiotic administration. The clinical significance of these changes is unknown. There was some indication that the infectious process itself may have played a role, either by altering the colonic flora or some other mechanism, although the extent is unclear.