Soy: Can You Get Too Much of a Good Thing?

Image for soy foods article The US Food and Drug Administration (FDA) allows food labels to display a health claim stating that soy products can lower blood cholesterol. However, while soy may lower blood cholesterol, it is unknown whether it may also increase or decrease the risk of breast cancer.

Why the Concern?

Soy contains estrogen-like compounds called isoflavones. Studies on the effect that isoflavones have on breast cancer have yielded mixed results. However, some women have started consuming large amounts of soy to reduce their risk. Researchers are concerned because the effects of different levels of soy on the risk for breast cancer have not been determined.

Estrogen-like Effects on Breast Cancer

Women who have breast cancer that depends on estrogen to develop and progress often consider increasing their isoflavones intake to help block tumor production. The method by which isoflavones are thought to block tumor production goes as follows: Breast cells contain estrogen receptors, which enable them to recognize estrogen and take it into breast tissue. However, the type of breast cancer that often strikes essentially feeds off estrogen, so the goal is to keep estrogen out. That's where isoflavones come in. These weak plant estrogens are close enough in structure to human estrogen that breast receptors mistake them as such and allow them in, which in effect blocks the entry of harmful human estrogen.Epidemiologic evidence supports the theory. Women throughout Asia, who for centuries have eaten much more tofu and other soy products than Americans, are much less likely to develop breast cancer than American women. However, there is no evidence that soy is responsible for their reduced risk. This only means that soy may offer protective benefits to women who have a history of breast cancer—lowering the risk of death and cancer recurrence.

Can Isoflavones be Harmful

Word about soy estrogens has gotten out—to the tune of hundreds of millions of dollars in sales of soy products annually. Women are buying not just tofu but also soy milk, soy-laced energy bars, soy cheese, soy ice cream, and soy-based meat products meant to resemble turkey, chicken, hamburger, and bologna. Then there are the soy-based powders and pills, many with high concentrations of isoflavones.Science presents a less certain picture of soy's benefits. In one study, women who were given soy supplements experienced increased growth of breast cells, at least at first. That's a potential problem because the more breast cells grow, the greater the chance of a mutation that could give rise to cancerous cells that would quickly grow into a tumor. However, there is no evidence to prove this. Estrogen also causes uterine cells to grow and potentially turn cancerous. Though one study showed uterine stimulation occurring in 3.37% of women taking isoflavones compared to 0% of those on placebo, there is no evidence that isoflavones in soy at high doses increase the risk of uterine cancer. The research thus far by no means identifies the isoflavones in soy as dangerous. But it does give researchers some pause about how much soy might be too much.

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