The isoflavones genistein, daidzein, and their glycosides, found in high concentrations in soybeans and soy-protein foods, may have beneficial effects in the prevention or treatment of many hormone-dependent diseases. Because these bioactive phyto-oestrogens possess a wide range of hormonal and non-hormonal activities, it has been suggested that adverse effects may occur in infants fed soy-based formulas.
To evaluate the extent of infant exposure to phyto-oestrogens from soy formula, the isoflavone composition of 25 randomly selected samples from five major brands of commercially available soy-based infant formulas were analysed, and the plasma concentrations of genistein and daidzein, and the intestinally derived metabolite, equol, were compared in 4-month-old infants fed exclusively soy-based infant formula (n = 7), cow-milk formula (n = 7), or human breast-milk (n = 7).
All of the soy formulas contained mainly glycosides of genistein and daidzein, and the total isoflavone content was similar among the five formulas analysed and was related to the proportion of soy isolate used in their manufacture. From the concentrations of isoflavones in these formulas (means 32-47 micrograms/mL), the typical daily volume of milk consumed, and average bodyweight, a 4-month-old infant fed soy formula would be exposed to 28-47 per day, or about 4.5-8.0 mg/kg bodyweight per day, of total isoflavones. Mean (SD) plasma concentrations of genistein and daidzein in the seven infants fed soy-based formulas were 684 (443) ng/mL and 295 (60) ng/mL, respectively, which was significantly greater (p < 0.05) than in the infants fed either cow-milk formulas (3.2 [0.7] and 2.1 [0.3] ng/mL), or human breast-milk (2.8 [0.7] and 1.4 [0.1] ng/mL), and an order of magnitude higher per bodyweight than typical plasma concentrations of adults consuming soy foods.
The daily exposure of infants to isoflavones in soy infant-formulas is 6-11 fold higher on a bodyweight basis than the dose that has hormonal effects in adults consuming soy foods. Circulating concentrations of isoflavones in the seven infants fed soy-based formula were 13000-22000 times higher than plasma oestradiol concentrations in early life, and may be sufficient to exert biological effects, whereas the contribution of isoflavones from breast-milk and cow-milk is negligible.