Strength of signal is not the same as the strength of binding. That is, just because the phytoestrogen sends a weaker signal (once bound to the estrogen receptor), that doesn’t mean that it doesn’t bind at the same (or more, or less) level.
So the idea is that phytoestrogens act as competitive inhibitors to estrogens in the body; they tie up the receptor but send much less of a signal.
In premise, this means that they might reduce overall estrogenic signalling (b/c the receptor is tied up by the phytoestrogen but the signal being sent is weaker).
When I looked into soy for my protein book, what I basically found was that, like one of the links in this thread says,it has pros and cons.
On the pro side, it has a pretty decent amino acid profile (especially some of the isolates); on the con side, it doesn’t appear to support protein synthesis or growth as well as milk proteins when consumed after resistance training.
On the pro side, it may improve endogenous anti-oxidant status. On the con side, there is the issue of hormonal effects.
Some work has found a negative impact on hormones (e.g. lowered testosterone) above a certain amount of phytoestrogens (roughly 50-60 mg/day from memory). That’s maybe one serving (20-25 grams protein) of soy per day since a typica soy protein has about 2-3 mg phytoestrogens per gram protein.
Even this is variable. In one recent study, while one guy saw a massive drop in testosterone (he was starting off from a super high level), a few subjects showed a slight increase. So it gets complicated and there seems to be a lot of individual variance.
I think a lot of the concern over soy comes from the way we do things in the US. People hear that something is ‘healthy’ and that means ‘eat as much of it as you can put down’.
If something is good in small amounts, it must be great in large amounts, right?
Usually not.
If you look at actual soy intake among Asian cultures on a day to day basis, it isn’t massive (a point made in the second article linked). But Americans took the ‘some is good so more is better thing’ and soy is being included in a lot of foods. Many people are already getting soy in their daily diet without adding more via a protein powder.
And excessive amounts can cause problems (including impaired thyroid conversion, esp. in the context of low sodium diets).
As well, the increased conversion of T4 to T3 was only shown in animal models, not humans. So that claim in the elitefts article is incorrect.
Basically, a small amount is probably ok but large amounts can be problematic.
Lyle