Soy protein can contain plant estrogens (phytoestrogens), known as isoflavones. Whey protein (which is made from cow's milk) can also contain these isoflavones (due to soy in cow feed) as well the hormone estradiol (an estrogen) that is naturally found in cow milk. However
, as explained in our Protein Powders Review
, amounts of estrogens or phytoestrogens in whey protein powders
are very small and unlikely to be of any significance. Higher concentrations of phytoestrogens are found in soy protein powders
but, as also explained in the Review, these are lower than the amounts in soy flour and levels vary considerably based on how the soy protein was extracted.