Objective: As the intakes of soy foods rise in the American diet, the weak estrogenic activity of plant phytoestrogens may reach biologically effective levels. We determined the content of phytoestrogens (plant compounds with weak estrogenic activity in human beings) in tofu, a commercially produced soy drink, and three soy-based formulas.
Design: A modified isotope dilution gas chromatography-mass spectrometry method was used to analyze samples obtained from supermarkets or manufacturers.
Samples: Two or three lots of duplicate samples of four brands of tofu, one commercially produced soy drink, and three soy-based specialty formulas were analyzed. Means and standard deviations were calculated for the isoflavones daidzein, Biochanin A, genistein, coumestrol, and formononetin.
Results: Tofu soy products were highest in isoflavone content; means of brands ranged from 73.0 to 97.5 micrograms/g daidzein per g wet weight and 187.4 to 215.9 micrograms genistein per g wet weight. The commercial soy drink followed with 7.0 micrograms daidzein per g wet weight and 21.0 micrograms genistein per g wet weight; the soy-based formulas were nearly devoid of these two isoflavones. Mean levels of Biochanin A, formononetin, and coumestrol were very low or nil (eg, 0 to 1.0 microgram/g) in all products.
Conclusions: Tofu contained the highest amounts of isoflavones among the products tested, and there was some variability from brand to brand. The soy drink contained lesser amounts, and soy-based formulas were devoid of isoflavones.