Here's what I know about soy. It contains a phytoestrogen, meaning naturally occuring hormone, similar to the estrogen women make in their bodies (or take in birth control pills or HRT). An excess of estrogen has been linked with higher rates of breast cancer in a large population over time. How much increase? It's very hard to quantify--probably only a slight rise, on the order of 1.3 relative risk (baSed on a norm of 1.). Does phytoestrogen act like the body's estrogen? Again, it's hard to say. Some sources say that it's far less potent, others that it's a potential risk. The other confusing point is that estrogens appear to act differently on different women, with some able to process relatively high amounts with no problems and others having bad effects (it may depend at least in part on how much estrogen the woman's body is itself producing).

So here's what I recommend, and what I do: I limit my soy intake to one serving a day (tofu, soy granules, soy lecithin, soy butter, etc.), and do not take soy supplements. Until there are more reliable findings, I think it's best to treat soy with some reservations. But that's a good rule of eating anyway--doing everything in moderation.