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The newspaper of The Johns Hopkins University April 10, 2006 | Vol. 35 No. 29
Soy's cancer prevention properties are in doubt

By Vanessa Wasta
Johns Hopkins Medicine

Johns Hopkins and Georgetown University researchers conducted a meta-analysis of 18 epidemiologic studies revealing that women who eat soy products may have a slightly lower risk of developing breast cancer. But the researchers quickly add that inconsistencies and limitations in the studies raise doubt about the potential benefit of soy, and they warn women that high-dose supplements could do more harm than good.

"At this point, women should not be taking high-dose soy supplements, especially those who are breast cancer survivors and women at increased risk for the disease," said Bruce Trock, associate professor of urology, epidemiology, oncology and environmental health sciences at Johns Hopkins' Brady Urological Institute and Kimmel Cancer Center. "We don't have long-term data on the effects of these supplements, and there is some evidence that they could be harmful."

Tests of refined soy products in animals reveal increased tumor growth, and shortterm studies of women taking the supplements show changes in breast cell growth that might actually increase risk for breast cancer.

Results of the review by Trock and his colleagues at Georgetown, published in the April 5 issue of the Journal of the National Cancer Institute, found inconsistencies among the studies. Many differed in whether or not—or how—they accounted for the many factors that may impact development of breast cancer, such as body mass index. Studies that took BMI into account showed that soy had less of a protective effect than those that ignored BMI. "This is consistent with the idea that people who eat a lot of soy most likely eat fewer calorie-heavy foods and are less likely to have high BMI," Trock said. "This means that breast cancer risk may not be due to eating soy products but to other dietary or lifestyle factors."

After averaging results from the studies, which span a quarter century, the overall relative reduction in breast cancer risk for soy eaters was a modest 14 percent in Caucasian women. The effect was not statistically significant in Asian women. Differences in pre- and postmenopausal women were slight and potentially biased due to the small number of studies that accounted for menopausal status.

Soy became a popular cancer prevention topic in the 1980s after early laboratory data showed that high doses of compounds within soy, called isoflavones, may block estrogens, promote cell death and have anti-inflammatory qualities. Soy products like tofu and bean curd are eaten widely by Asian women, who have low breast cancer rates, at least until they move to the United States, where their daughters' rates begin to approach those of Caucasian women. Some believe this may be due to changes in diet, and the observation triggered research to find which Asian foods, like soy, might provide cancer protection.

"We need to take into account that Asian women are more physically active, drink less alcohol, have children earlier, and their entire diet is different from Western women's, all of which decrease their breast cancer risk," Trock said.

Many of the studies reviewed also failed to gauge the amount of soy women actually consumed because it is also found in products where it is not expected, such as doughnuts, white bread, instant coffee and canned tuna. "At the time most of these studies were done, few Western women were eating tofu, but many were getting soy without noticing it," said Trock, who believes that this confounds self-reporting of soy intake and may affect the ability to measure potential health benefits. Future studies, he said, should include urine samples taken at different times to improve measurements of total soy exposure.

Trock and his colleagues also found inconsistencies between the soy dosage and biological response. If soy were beneficial, researchers would expect a "some is good, more is better" result. It would also mean that at some point the dose-response "curve" would flatten, the effect would plateau, and no additional amount of soy above the threshold would improve protection.

But instead, the researchers said their review found that Western women eating low doses of soy appeared to have protection from breast cancer equal to or greater than Asian women with very high intake of soy. This would suggest that the benefits of soy reach their peak at a very low dose, but this effect is not supported by prior animal or tissue culture data.

Trock also points to evidence that soy exposure early in life may be most important. One of the studies he reviewed showed that Asian-American soy eaters born in Asia had more protection from breast cancer than those born in the United States, possibly reflecting soy exposure in utero and before puberty. "But there are no studies that have followed women long enough to solidify this," he added.

Trock says that definitive studies, which track participants over time and before they get cancer, may take decades before the real impact of soy is known.

For now, Trock says that there is no problem with adding soy to the diet since it has health benefits regardless of whether it protects against breast cancer. "The important aspect is eating actual soy-based foods like tofu, not highly purified isoflavone supplements," he said. "Highly refined components of soy can have very different biological effects than eating tofu or drinking soy milk."

Instead of pill supplements for people who want to add soy to their diet, Trock suggests replacing some meats with soy foods such as tofu, soy milk or soy nuts.

Additional study authors are Leena Hilakivi-Clarke and Robert Clarke, both of Georgetown University.

Funding was provided by the National Cancer Institute and the American Institute of Cancer Research.


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