Eating more soy may be tied to better survival odds for many women with breast cancer and may not be harmful for patients treated with hormones, a new study suggests.
Previous nutrition research has linked soy to a longer life, but prior studies have also suggested soy may help tumors spread by making hormone-based cancer treatments, or endocrine therapy, less effective.
For the current study, researchers from Tufts University in Boston examined data on 6,235 U.S. and Canadian women with breast cancer. More than half of the women were followed for at least nine years.
During the course of the study, women who ate the most foods containing soy were 21 percent less likely to die than individuals who consumed the smallest amounts of soy.
The study focused on isoflavones, natural compounds in soybeans that can be found in foods like tofu, miso, edamame and soy milk. Isoflavones belong to a family of plant compounds known as phytoestrogens that are chemically and structurally similar to the female sex hormone estrogen.
Isoflavones most protective for aggressive forms of cancer
Researchers found the strongest link between soy and survival for women with certain aggressive forms of breast cancer that can’t be treated with hormones. They didn’t see a link between soy consumption and longevity for women with tumors that depend on estrogen to grow or women receiving endocrine therapy.
The findings, taken together, indicate that soy food consumption does not have a harmful effect for women treated with hormone therapies. However, the benefit may be limited to women with negative tumor hormone receptors or those who were not treated with hormone therapies.
Soy isoflavones in foods
Overall, women with the highest intake of dietary isoflavones, an average of at least 1.5 milligrams a day, were 21 percent less likely to die than women who consumed less than 0.3 milligrams a day, researchers report in Cancer.
Consuming at least 1.5 mg of soy isoflavones is equivalent to eating one ounce of soy cheese or two tablespoons of edamame. One-half cup of cooked soybeans has 56 mg of isoflavones, ½ cup of edamame has 16 mg, ¼ cup of soy nuts has 41 mg and three ounces of cooked tempeh has 30 mg.
For women with tumors not fueled by hormones, the highest isoflavone intake was tied to 51 percent lower odds of death from all causes.
Among women who didn’t receive hormone therapy, the highest isoflavone intake from diet was associated with 32 percent lower odds of death from all causes.
One limitation: women who ate more soy foods tended to be more affluent and educated, with healthier lifestyles, making it possible that other factors beyond dietary isoflavones influenced longevity.
Still, the findings build on previous research in Asia that linked high dietary soy intake to a lower risk of developing breast cancer.
“Up until now physicians generally discouraged their breast cancer patients from eating soy foods because of potential harm. The researchers stated that women can now be encouraged to eat soy foods because it is safe and it may improve their survival.
Source: Cancer, online March 6, 2017.
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