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Is soy OK for breast cancer patients?

Soy and breast cancer
Soy and breast cancer 01:40

Eating soy-based foods does not appear to be harmful and may even benefit some women with breast cancer, new research shows.

The study may help clear up confusion over whether soy products such as tofu, soy milk, edamame, tempeh and miso – foods rich in estrogen-like chemicals – are OK to consume if you have breast cancer.

Higher estrogen levels have been linked to the growth and spread of breast cancer in the most common form of the disease.

Dr. Fang Fang Zhang, a cancer epidemiologist at the Friedman School of Nutrition Science and Policy at Tufts University, studied more than 6,000 women living with breast cancer in the U.S. and Canada. They were enrolled in an international breast cancer registry and their diet was assessed over an average of 9 1/2 years. During that time, 1,224 deaths occurred.

“We found that women who consumed a higher level of isoflavones – the compound found mainly in soy – had a 21 percent reduction in all-cause mortality compared to women in the lowest quartile of soy intake,” Zhang told CBS News.

“I think it’s safe to say soy doesn’t have harmful effects on breast cancer,” she concluded.

The results weren’t surprising, she said, because previous studies in Asian women have found that a higher intake of soy products is associated with a reduced risk of cancer recurrence and improved survival. Soy consumption is associated with a decreased incidence of many cancers, the author of an accompanying editorial in the journal Cancer said.

But lab studies have suggested that isoflavones can sometimes act like estrogen and potentially promote tumor growth. The results have raised concern about eating soy foods, and some women have avoided them as a precaution. There have also been worries that isoflavones could interfere with some therapies used to treat hormone receptor-positive breast cancers.

“The history is that it’s a controversial issue for women,” Zhang said. “The evidence has been found in two directions so that’s why it’s controversial.”

Marianne Lu, 32, is battling aggressive breast cancer. She’s always had a healthy approach to her diet but told CBS News she’s stayed away from soy products. 

“Whenever the whole soy thing started happening, where people were like it’s bad for you, you shouldn’t eat it. I started to avoid it and I haven’t eaten soy,” said Lu.

Dr. Heather McArthur of Cedars-Sinai Medical Center in Los Angeles said the new findings are promising and appear to show the most benefit for patients whose breast cancers are not driven by estrogen. But it’s also good news for patients with hormone related cancer, she said.

“That the consumption of soy is safe and doesn’t appear to impact survival is really encouraging, particularly for women taking hormone therapy for breast cancer,” said McArthur.

Zhang said women in the U.S. only eat about a half to one serving of soy foods a week – not nearly as much as Asian women who, on average, consume about eight or more servings per week.

Soy has been touted as a healthy food choice for other reasons, too.

“Soy products are a good source of fiber, plant-based protein and healthy fats,” she said.

The study only looked at soy found in foods, not in supplements, the author pointed out.

“I don’t want women to start to buy supplements because of this study. We didn’t study supplements. We recommend consuming soy as part of an overall healthy diet,” Zhang said.

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