We've known that regular soy consumption appears to both prevent breast cancer--the number one cancer killer of young women--and prolong survival in women battling the dreaded disease, but we haven't understood why.
Soybeans naturally contain weakly estrogenic compounds called phytoestrogens (derived from phyton, the Greek word for "plant"). So the original theory was that the regular presence of the estrogen look-alikes in our bloodstream might trick our body into ramping down actual estrogen production as part of a negative feedback loop, like a thermostat that shuts off the heat when it senses it's getting too warm.
This theory gained empirical support in 2006 when a group of British researchers showed that indeed the presence of phytoestrogens could effectively down-regulate the enzyme that human cells use to make estrogen (at least in a test tube). The theory fell into disfavor, however, when researchers subsequently failed to show that women who consumed soy ended up with less estrogen circulating in their bloodstream. From a breast cancer standpoint, though, we don't care how much is in the blood, but how much is in the breast.
A new study just published this month measured estrogen levels inside the breasts themselves of women placed on a high versus low soy diet. This was accomplished by aspirating ductal fluid from the nipple, which is what bathes the very cells most likely to turn cancerous. After 6 months, the researchers found a trend towards lower estrogen levels inside the breasts of women eating two servings of soy foods a day. Not only does this aid our understanding of why soy may shield us from cancer, but can help explain the findings presented in NutritionFacts.org's new video-of-the-day today, which documents new research suggesting young girls drinking soymilk just twice a week may be protected against premature puberty.