Isoflavone content of infant formulas and the metabolic fate of these phytoestrogens in early life

Soy-based infant formulas have been in use for >30 y. These formulas are ma nufactured from soy protein isolates and contain significant amounts of phy toestrogens of the isoflavone class. As determined by HPLC, the isoflavone compositions of commercially available formulas are similar qualitatively a nd quantitatively and are consistent with the isoflavone composition of soy protein isolates. Genistein, found predominantly in the form of glycosidic conjugates, accounts for > 65% of the isoflavones in soy-based formulas. T otal isoflavone concentrations of soy-based formulas prepared for infant fe eding range from 32 to 47 mg/L, whereas isoflavone concentrations in human breast milk are only 5.6 +/- 4.4 mu g/L ((x) over bar +/- SD, ia = 9). Infa nts fed soy-based formulas are therefore exposed to 22-45 mg isoflavones/d (6-11 mg.kg body wt(-1).d(-1)), whereas the intake of these phytoestrogens from human milk is negligible (<0.01 mg/d). The metabolic fate of isoflavon es from soy-based infant formula is described. Plasma isoflavone concentrat ions reported previously for 4-mo-old infants fed soy-based formula were 65 4-1775 mu g/L ((x) over bar: 979.7 mu g/L; Lancet 1997:350;23-7), significa ntly higher than plasma concentrations of infants fed either cow-milk formu la ((x) over bar +/- SD: 9.4 +/- 1.2 mu g/L) or human breast milk (4.7 +/- 1.3 mu g/L). The high steady state plasma concentration of isoflavones in i nfants fed soy-based formula is explained by reduced intestinal biotransfor mation, as evidenced by low or undetectable concentrations of equol and oth er metabolites, and is maintained by constant daily exposure from frequent feeding. Isoflavones circulate at concentrations that are 13 000-22000-fold higher than plasma estradiol concentrations in early life. Exposure to the se phytoestrogens early in life may have long-term health benefits for horm one-dependent diseases.