(Reuters Health) - - 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 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.

“All women or all breast cancer survivors can add soy as a component of a healthy diet,” said lead study author Dr. Fang Fang Zhang, a nutrition and science policy researcher at Tufts University in Boston.

The study focused on isoflavone, a compound in soybeans that can be found in foods like tofu, miso, edamame and soy milk. Isoflavone is in a family of plant compounds known as phytoestrogens that are chemically and structurally similar to the female sex hormone estrogen.

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.

“Our findings, taken together, indicate that soy food consumption does not have a harmful effect for women treated with endocrine therapies, but the benefit may be limited to women with negative tumor hormone receptors or those who were not treated with endocrine therapies,” Zhang added by email.

For the study, researchers examined data from dietary questionnaires for women diagnosed with breast cancer from 1996 to 2011. They excluded women who died within one year of completing the first questionnaire.

At the start, women were 52 years old on average and most had at least some education beyond high school.

About 47 percent took hormone therapy for tumors.

On average, women consumed 1.8 milligrams of isoflavone daily, roughly the amount in one ounce of soy cheese and far less than a 3-ounce serving of tofu or a half-cup portion of edamame.

During the study, 1,224 women died.

Overall, women with the highest quartile of dietary isoflavone intake, an average of at least 1.5 milligrams a day, were 21 percent less likely to die than women in the lowest quartile who got less than 0.3 milligrams a day, researchers report in Cancer.

For women with tumors not fueled by hormones, the highest amount of isoflavone intake was tied to 51 percent lower odds of death from all causes.

Among women who didn’t receive hormone therapy, the highest level of dietary isoflavone was associated with 32 percent lower odds of death from all causes.

One limitation of the study is that women who ate more foods with soy tended to be more affluent and educated, with healthier lifestyles, making it possible that other factors beyond dietary isoflavone influences longevity, the authors note.

Researchers also lacked data on the type and length of hormone therapy for women who received this treatment, which could also impact survival odds.

Still, the findings build on previous research in Asia that linked high dietary soy intake to a lower risk of developing breast cancer, said Dr. Omer Kucuk, a researcher at the Winship Cancer Institute of Emory University in Atlanta.

“It is not surprising that this study showed women in North America also benefit from soy intake with reduced mortality from breast cancer while on treatment,” Kucuk, author of an accompanying editorial, said by email.

“Up until now physicians generally discouraged their breast cancer patients from eating soy foods because of potential harm,” Kucuk said. “We can now encourage women with breast cancer to eat soy foods because it is safe and it will likely improve their survival.”