Soy: Can You Get Too Much of a Good Thing?
While the United States Food and Drug Administration (FDA) allows food labels to display a health claim stating that soy products can lower blood cholesterol, the amount of soy necessary—as many as four servings a day—is awfully close to the safe upper limit for protection from breast cancer, according to some researchers. That's right. There are concerns among scientists that too much soy could harm rather than protect the breasts from malignant tumors.
Why the Concern?
"Most of the women who are taking large amounts of soy…my impression is that they're doing it to avoid breast cancer," says Regina Ziegler, PhD, an epidemiologist with the National Cancer Institute. "But there's a dearth of information about [the effects of] different levels of soy on the risk for breast cancer. I don't want to scare people away from it. But as a scientist, I can't say what levels are safe or unsafe."
Mark Messina, PhD, a soy expert who serves as an adjunct associate professor of nutrition at Loma Linda University, agrees. "It's basically an unresolved issue," he comments. Certainly, he points out, soy has estrogen-like compounds, so if you have the type of breast cancer that depends on estrogen to develop and progress, "you have to think about whether you want to run out and start eating three servings of soy a day."
Adds Barry Goldin, PhD, who has researched soy at Tufts University Medical School in Boston, "Whenever you mention estrogen, you're talking about concern. Estrogen is a double-edged sword."
The plant estrogens present in soy are known as isoflavones. In the 1980s, investigators found that feeding soy to rats reduced their incidence of mammary tumors. More important, when the scientists removed the isoflavones from soy and again fed it to rats, the soy no longer suppressed tumor growth, further implicating the plant estrogens as anti-cancer agents.
The mechanism 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. But 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. Soy may even offer protective benefits to women who have a history of breast cancer—lowering the risk of death and cancer recurrence.
Word about soy estrogens has certainly 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.
But emerging science presents a less certain picture of soy's benefits. In one study, women who were given soy supplements experienced increased proliferation of breast cells, at least at first. That's a potential problem because the more breast cells proliferate, the greater the chance of a mutation that could give rise to cancerous cells that would quickly grow into a tumor.
Estrogen also causes uterine cells to grow and potentially turn cancerous. Although soy isoflavones do not seem to usually cause this problem, in one fairly large long-term, double-blind, placebo-controlled study (376 participants followed for five years ) uterine stimulation occurred in 3.37% of women taking isoflavones compared to 0% of those on placebo. This could indicate that some women have an increased risk of uterine cancer if they take isoflavone at high doses.
The research thus far by no means identifies the isoflavones in soy as dangerous.
"I don't know of any solid evidence that in humans soy has actually caused a disease," says Dr. Goldin. But it does give researchers some pause about how much soy might be too much.
How Many Isoflavones?
Scientists are not concerned about soy protein. That's the component of soy that helps lower cholesterol levels, and research shows that 25 grams a day has a modest cholesterol-lowering effect. (A 4-ounce serving of tofu has about 10 grams; a cup of soy milk, 6 to 8 grams.) But they are concerned about isoflavone intake. In the study noted above where uterine stimulation occurred, women were taking 150 milligrams of isoflavones daily.
"I think healthy people should not consume more than 100 milligrams of isoflavones per day," says Dr. Messina. "In my opinion, the weight of the evidence suggests that even somewhat more than that is perfectly safe, whether or not you have breast cancer. But there's no historical precedent for consuming more," he points out. "A hundred milligrams is like three servings a day, and maybe 5% to 10% of the Japanese population consumes that much." Most have one serving of tofu or another soy food daily.
"I can't point to data showing that 100 milligrams is dangerous," Dr. Messina stresses. "It's not like if you take 100 milligrams of isoflavones and then you take 101, you're risking your life. But the 100 milligram limit is consistent with a healthy, plant-based diet."
Be Careful With Pills and Powders
Dr. Goldin has independently come up with the same limit. "If a person wanted tofu mixed into her stir-fried vegetables once or twice a day, I would not see any problems with it at all."
Dr. Goldin is also comfortable with a woman at high risk for breast cancer having three to four ounces of tofu two to three times a week. "But I would not take 100 milligrams of isoflavones a day," he says, meaning he would not eat more than a couple of servings of soy-containing foods daily and would be very careful about choosing a soy powder or supplement, some of which contain quite high levels of isoflavones. "The average Japanese person consumes 40 to 60 milligrams a day," he adds.
Dr. Ziegler, like the others, is fine with modest servings of soy-based foods but also throws up a red flag for powders and pills. "I'm not concerned about soy intake comparable to what we see in Asian communities," she says. "Even if a woman already has breast cancer, I don't think she needs to worry about going into a Chinese restaurant and having a tofu dish."
"On the other hand," she says, "given the supplements, we're going beyond what one would see in a typical Asian diet. We don't know how that plays into a woman's health. I don't think we can say that pharmacologic levels of soy are useful in avoiding breast cancer."
Potential Interaction With Tamoxifen
Margo Woods, DSc, a breast cancer researcher in the Department of Family Medicine and Community Health at Tufts University Medical School, is particularly concerned about high levels of isoflavones for women who are taking the drug tamoxifen, which, like the isoflavones in soy, is an estrogen-like compound that keeps human estrogen out of the breasts. And some women who already have breast cancer or are at very high risk are given the drug to slow down cancer progression or development. But the isoflavones in soy could compete with tamoxifen for uptake by breast tissue, Woods says, adding, "once a woman is on tamoxifen, my position is that it's not wise to interfere." That is, isoflavones in soy could potentially lessen the drug's effect.
Dr. Woods also is not comfortable with an upper limit of 100 milligrams of isoflavones a day for everyone else. "I feel most comfortable with a range of 35 to 55 milligrams" daily, she notes. In other words, she's most comfortable with a maximum that's the equivalent of two to three ounces of tofu a day.
Not a Panacea
"People shouldn't be thinking of soy as a medicine," cautions Dr. Woods. Including it as one "part of a fruits, vegetables, whole-grains package is a wise thing to do. The population that has been eating that way for years is at lower risk. But soy is not a miracle treatment. People think a food is going to be like penicillin. It's not."
Dr. Ziegler sees it the same way. "We should get away from soy as a cancer preventer," she says. "Maintaining an ideal weight— that will definitely reduce the risk. And it can be achieved by a diet that's high in fruits, vegetables, grains, and legumes. Soy is one type of legume, but it's not the answer."
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Last reviewed March 2010 by Brian Randall, MD
Last updated Updated: 6/27/2010
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