Taking hormone replacement therapy (HRT), which contains estrogen, increases breast tissue density and denser breast tissue is linked to higher breast cancer risk. Because one compound in soy products -- isoflavone -- is similar to estrogen, there have been concerns that eating a lot of soy products might increase breast cancer risk by increasing breast density as HRT does. The study reviewed here found that postmenopausal women who took large amounts of isoflavone supplements had no change in breast density. The results suggest that a diet rich in soy doesn't affect breast cancer risk.
Breast density is the proportion of fatty tissue compared to non-fatty tissue in the breast. A denser breast has more non-fatty tissue in it than a less-dense breast. Breasts usually get a little less dense over time.
Soybeans are the most widely used, least expensive, and least caloric way to get large amounts of protein with very little fat and no cholesterol. You can eat soybeans in many forms, including tofu, the beans themselves (also known as edamame), soy milk, miso, and soy powder. Asian women typically eat much more soy than women in Western countries. Yet Asian women have lower rates of hormone-receptor-positive breast cancer than women in the United States.
In the study reviewed here, researchers looked at the records of more than 350 U.S. postmenopausal women participating in the OPUS (Osteoporosis Prevention Using Soy) study, which studied the effect of soy on bone health. In the OPUS study, women received either:
The amount of isoflavone supplements taken in the OPUS study is quite a bit higher than what you would typically get from regularly eating soy foods.
After 2 years, the women in the OPUS study had mammograms. The researchers compared the breast density of all the women and found that average breast density was the same for each group: the breast density of the women who took either dose of isoflavone supplements was no different than the breast density of the women who took the placebo.
So while this study suggests that soy doesn't increase breast cancer risk in postmenopausal women, it also doesn't show that soy offers any protection against breast cancer. More research, especially research that includes premenopausal women, is needed to see if eating a diet high in soy foods helps reduce breast cancer risk.
The relationship between soy and breast cancer is complicated by other factors. Most women living in Asia depend on soy as their main source of protein. They consume only small amounts of beef, chicken, and pork -- which means less animal fat and other possibly unhealthy substances (such as growth hormones and antibiotics) in these animal protein sources. Also, compared to the average woman in the United States, the average Asian woman:
All of these other factors add up to produce a healthier lifestyle and a lower overall risk of breast cancer in Asian women living in Asia.
Visit the Breastcancer.org Lower Your Risk section to learn much more about changes you can make in your own diet and lifestyle to lower your breast cancer risk. You can learn more about foods that may keep you as healthy as you can be on the Foods to Consider page.
TORONTO, April 15 (MedPage Today) -- Soy supplements have no effect on the density of breast tissue in postmenopausal women, one study showed.
The finding should reassure women and their doctors who feared that soy supplements would increase breast density -- a marker for increased cancer risk -- in the same way hormone replacement does, according to Gertraud Maskarinec, M.D., Ph.D., of the Cancer Research Center of Hawaii in Honolulu, and colleagues.
On the other hand, there was also no evidence that the supplements had a beneficial effect, the researchers wrote in the May 2009 issue of the Journal of Nutrition.
But, they added, the study considered only women just after menopause and doesn't rule out a possible benefit if the supplements were taken earlier in life.
Soy isoflavones have a functional similarity to human estrogens, the researchers noted, and "may protect against breast cancer as a result of their antiestrogenic activity or increase risk as a result of their estrogen-like properties."
High breast density on a mammogram has been associated with an increased risk of breast cancer, the researchers noted, and it may be that factors that decrease breast density may also lower the risk of malignancy.
But the evidence is "limited and contradictory" on the relation between soy intake and breast density, they said.
To clarify the issue, they turned to the Osteoporosis Prevention Using Soy (OPUS) study, a two-year, placebo-controlled, double-blind clinical trial in three U.S. centers that aimed to investigate the effect of soy supplements on bone density.
Participants, all in the early years of menopause, were randomized to placebo or one of two doses -- 80 or 120 milligrams a day -- of isoflavone supplements.
The doses are equivalent to between 300 and 500 grams of tofu a day or from 500 to 1,000 milliliters of soy milk, the researchers said. They noted that a typical isoflavone intake in Japan, where soy is widely consumed, is estimated to be 25 to 50 milligrams a day.
As part of the study, 88% of the participants had mammograms taken and at the end of the two-year intervention, films from 358 women were available for analysis.
On four measures -- breast area, breast density percentage, dense area, and nondense area -- there was no significant difference between the three groups.
On the other hand, both measures of mammographic density -- dense area and percent density -- decreased significantly over time, at P<0.0001 for both soy and placebo recipients. The average annual decreases were 2.0 square centimeters and 1.6%, respectively, resulting in about a 3% decrease in percent density at the end of the study.
The annual average decrease in densities was 1.4% for placebo, 1.6% for the 80-milligram dose, and 1.3% for the 120-milligram dose.
The total breast area and the nondense area both increased significantly over time, at P=0.006 and P<0.001, respectively.
The finding "offers some reassurance to those who have been concerned about adverse effects of soy supplementation on breast cell proliferation," Dr. Maskarinec and colleagues said.
They added that they haven't ruled out the protective effect that has been suggested by epidemiological evidence, including the lower incidence of breast cancer in Asian women, compared with those in the West. The authors speculated that either earlier exposure to soy or another protective mechanism besides an influence on breast density might apply.
One limitation of the study was the 12% drop out rate related to missing data.
The study was supported by the U.S. Department of Agriculture and the National Cancer Institute.
The researchers said they had no conflicts.
Primary source: Journal of Nutrition Source reference: Maskarinec G et al. "Various doses of soy isoflavones do not modify mammographic density in postmenopausal women" Journal of Nutrition 2009; 139: 981-986.
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