Daily intake and urinary excretion of genistein and daidzein by infants fed soy- or dairy-based infant formulas.

Our aims were to measure isoflavone intake from soy- and dairy-based infant formulas and breast milk and to assess the ability of infants to digest and absorb soy isoflavones by measuring daily urinary excretion rates. We recruited 29 infants: 4 received soy-based formula and 25 received dairy-based formula. We collected pooled urine samples from 3-5 disposable diapers worn during a 24-h period and developed and validated methods for extracting isoflavones from the diapers. Infants were studied every 1 or 2 wk, starting at 2-6 wk of age and continuing until 16 wk. Only soy-based formulas contained isoflavones in concentrations detectable by HPLC (limits: 0.05 mg/L for liquids and 0.1 mg/kg for solids). Soy-based formulas provided a mean (+/-SEM) daily dose of isoflavones (genistein plus daidzein) of 3.2 +/- 0.2 mg/kg body wt, which remained fairly constant (CV: 12%) regardless of age < or = 16 wk. Isoflavones were measurable in all samples from soy-fed infants, but not in urine from dairy-fed infants. Daily isoflavone excretion rates varied little among infants [range of mean individual values (mg x kg(-1) d(-1)): daidzein, 0.37 +/- 0.03 to 0.58 +/- 0.06; genistein, 0.15 +/- 0.03 to 0.32 +/- 0.04] and did not change with age < or = 16 wk. The mean percentage of the daily intake recovered in the urine of soy-fed infants was 38 +/- 4% for daidzein and 13 +/- 3% for genistein, and remained constant with age. These values are similar to those for adults and indicate that young infants are able to digest, absorb, and excrete genistein and daidzein from soy-based formulas as efficiently as do adults consuming soy products.