Sandra Yin

November 07, 2011

November 7, 2011 (Washington, DC) — It is all right for breast cancer survivors to consume moderate amounts of soy foods, Bette Caan, PhD, a senior research scientist who specializes in nutritional epidemiology at the Division of Research at Kaiser Permanente Northern California in Oakland told attendees here at the American Institute for Cancer Research (AICR) Annual Research Conference 2011 on Food, Nutrition, Physical Activity and Cancer in a session on cancer treatment and survivorship.

Dr. Caan sought to clarify the highly charged topic of whether soy is safe for breast cancer survivors by reviewing the epidemiologic literature.

Soy has been touted in the mainstream media as "the miracle bean" and laypeople read about everything from the "joy of soy" to the "dark side of soy." It is not surprising that the public and experts are confused, Dr. Caan acknowledged. Part of the problem is conflicting data.

Soy foods can either increase the risk for breast cancer progression or decrease the risk, she observed. Evidence suggests that lifetime endogenous estrogen exposure increases the risk for breast cancer. So people were afraid to tell breast cancer survivors that it was okay to eat soy.

Clinicians have several options based on the epidemiologic literature, and some routinely advise against soy in the diet of patients with breast cancer, Dr. Caan noted, "and I don't think the current science supports that at all."

No Evidence of Harm

After reviewing the 7 epidemiologic studies, Dr. Caan said there was no evidence that soy is harmful for women with breast cancer, and pointed out that 6 of the 7 studies demonstrate some type of benefit.

On the basis of the lack of harm and the benefits reported in the studies she cited, clinicians might want to recommend that patients with breast cancer begin eating whole soy foods to treat breast cancer, but she cautioned that she doesn't think existing data were strong enough to justify that.

"What I do think is that they could adopt a stance of permitting use in patients who want to begin eating reasonable amounts of soy foods or for whom soy foods already represent a normal part of their diet," Dr. Caan said.

The data support this option, and she said they were consistent with the American Cancer Society (ACS) position.

The ACS is in the process of rewording its guidelines. The old ones said that up to 3 servings of traditional soy foods per day are unlikely to be harmful. According to Dr. Caan, those guidelines may be revised to state that women with breast cancer can eat moderate amounts of soy foods.

She added 1 caveat: to avoid concentrated sources of soy, such as pills, powders, or supplements containing high amounts of isoflavones, because data are lacking on the risks and benefits. "We still need to proceed with caution," she said.

The discussion heated up during the session's Q&A when another presenter, Leena Hilakivi-Clarke, PhD, professor of oncology at Georgetown School of Medicine, Washington, DC, whose talk focused primarily on animal research, shared her advice on soy consumption. If a woman had been consuming soy before diagnosis, it would be perfectly safe to eat the same amount or maybe increase it a little bit, she said. But her recommendation was more cautious for another subgroup. "If she had not eaten any soy beforehand, my opinion is that she shouldn't start before we know whether it's safe or not," she said.

A member of the audience took issue with the latter advice. "First, when human data exists, it should surpass animal data," said Mary L. Hardy, MD, medical director at the Simms/Mann University of California at Los Angeles Center for Integrative Oncology. Nor was she convinced that models using a vectorized mouse or rat were an appropriate model for humans. "Third, If we're moving people toward a plant-based diet, and we take out a really good source of plant-based nutrition, how are we taking with one hand and giving with another?"

Because her patients have heard from the media or uninformed physicians that soy is bad, Dr. Hardy said they "freak out" because they see soy in everything, as it is used as an emulsifier and a flow agent. "They'll think they can't eat anything, which can be confusing and demoralizing," she said. "I'm very hesitant for that advice to be promulgated, when I think it's going to cause at many levels more harm than good."

Later Dr. Hardy told Medscape Medical News that she agreed with Dr. Caan's recommendations. But she had another concern about Dr. Hilakivi-Clarke's advice. In her practice, she actively encourages people to eat a plant-based diet and reduce their dependence on red meats. Taking away meat and then soy would confuse patients who want to identify a good-quality protein source, she said. What's more, the average breast cancer patient whose tumor was caught early and is responsive is probably many times more likely to face risk for heart disease than breast cancer, so it wouldn't make sense to take away a heart-healthy food that lowers cholesterol, she said. "I think inappropriately discouraging soy use is going to cause problems."

Dr. Caan told Medscape Medical News that either you eat soy or you don't. If you enjoy soy as part of your diet, you can continue to eat it because more evidence suggests that it's beneficial. Referring to the other speaker's advice that patients with breast cancer who have never eaten soy should not start to eat it to treat their cancer, she said, "Who are those people? You're giving a warning to people who don't exist."

Dr. Caan and Dr. Hilakivi-Clarke have disclosed no relevant financial relationships. Dr. Hardy serves on the scientific advisory board of Dean Foods, which makes soy milk.

American Institute for Cancer Research (AICR) 2011Annual Research Conference on Food, Nutrition, Physical Activity and Cancer. Presented November 3, 2011.

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