Soy contains substances called isoflavones (specifically genistein and daidzein) that are similar to the hormone called estrogen. These “phyto-estrogens” (meaning “plant-based estrogens”) are much weaker than the actual hormone – in fact, they have only about 0.1% the activity. Soy has been used for years as an alternative therapy for menopausal hot flashes – so there does seem to be a mild hormonal effect in people that already have an estrogen imbalance. But in babies taking soy, I rarely ever see any signs of hormonal imbalance. For older children and adults, I believe the health benefits (decreased risk of certain cancers, heart disease, etc.) of soy far outweigh the potential risks. I use soy myself and for my kids. Populations that eat a lot of soy seem to be healthier than Americans. The only time I don’t like to use soy is in young babies as their only source of protein, unless they have a documented milk allergy. Many babies are on soy formulas that probably don’t need to be. There are several reasons for this, click here for full discussion of soy formula.