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.

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. One study showed uterine stimulation occurring in 3.37% of women taking isoflavones compared to 0% of those on placebo. However, another study concluded there was no effect on endometrial tissue growth or its associated risks. Also, 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.

How Many Isoflavones?

Research shows that 25 grams of soy protein a day has a modest cholesterol-lowering effect. It is not known whether consuming more than 25 grams of soy protein a day could be dangerous.

Because of this, women may want to be careful with soy pills and powders. Some products contain high levels of isoflavones. In many cases, the amounts are greater than what is consumed in a typical Asian diet, but again there is no evidence that this is harmful.

Potential Interaction With Tamoxifen

The drug tamoxifen is an also estrogen-like compound that keeps human estrogen out of the breasts. Women on tamoxifen should talk to their doctors about their soy consumption, because the isoflavones in soy could compete with the drug and lessen its effect. The amount of isoflavones required to do this is also not known.

Not a Panacea

The bottom line is that soy should not be thought of as a medication. It is wise to include it as part of your diet, but it is not a miracle treatment. Other factors, such as maintaining a healthy weight, are better at reducing the risk of cancer.


Academy of Nutrition and Dietetics
Choose My Plate—US Department of Agriculture


Dietitians of Canada


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Last reviewed October 2017 by EBSCO Medical Review Board Michael Woods, MD, FAAP
Last Updated: 1/23/2014

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