However, I've been hesitant about using soybeans as a base. Soy is notorious for its phytoestrogen content (though it's certainly not the only food containing phytoestrogens), and I've always tried to avoid it whenever possible. I know that fermenting soy reduces phytoestrogens dramatically---I often hear folks talking about a 300% reduction, although I don't know how reliable that is---but surely it'd be better to avoid them altogether. Why not just cut out the soy completely and use some other, more benign bean, I thought? Therefore, I started Googling for non-soy natto recipes.
Along the way, I stumbled across this article: The Truth About Soy and Man Boobs
...the phytoestrogens in soy have a very weak effect compared to normal human estrogen or estrogen from animal sources. These phytoestrogens compete for the same hormone receptors and so reduce the effect of normal estrogen which has a much more potent effect on the body. They act as what’s known as ‘competitive inhibitors’ of estrogen...This piqued my interest. Could it be? After searching some more and reading several journal articles, the author's comments seem justified. Apparently there's been some very interesting research over the past few years that seems to indicate the phytoestrogens in soy may actually exert an anti-estrogenic effect, depending on the dose. It doesn't seem to be very well understood yet, but the results are promising (and interesting). We're talking about very large diet surveys here, and also randomized controlled trials. In one randomized study they gave some women a low dose of soy estrogen and measured the estrogen activity in their blood and saw it drop.
This is fascinating to me. It may explain why estrogen-related cancers are rarer in Japan and Asia (e.g., breast cancer is much rarer there). We already know that extra estrogen makes breast cancer worse... oh, and we know the women in Japan who eat soy every day have lower rates of breast cancer. Hmm.
Now, this doesn't mean eating TONS of soy is necessarily good. Despite the 'weaker' nature of phytoestrogens, presumably they could have a big effect if you ate a LOT of them. Nevertheless, it's beginning to become clear that smaller doses of soy---like the sorts of doses normal people eat in Japan---might be beneficial and anti-estrogenic. Bear in mind, too, that fermented soy contains lower levels of phytoestrogen, whereas processed soy contains higher levels.
In conclusion: I'm not going to bother making natto with other beans. I might as well just use soybeans. If the theories I've read are true, then I suppose it might actually LOWER my estrogen levels---or to be more precise, it might lower the estrogenic activity in my body. And I'm perfectly fine with that. :-)