Soy May Reduce Breast Cancer Recurrence: Study
MONDAY, Oct. 18 -- For women past menopause who have had breast cancer, a higher intake of soy may help reduce the risk of the disease's recurrence, a new study of Chinese women suggests.
The same link was not found in premenopausal women with breast cancer, whatever their soy intake, the study authors said.
The study, while called intriguing by U.S. experts, was not large and included only women with breast cancer receiving care in China. It's not known if the results would apply to other groups of women, said Marji McCullough, a spokeswoman for the American Cancer Society.
"Chinese women may have been likely to have a lifelong high consumption of soy," she said. "We don't know whether starting on a diet high in soy after a breast cancer diagnosis would have the same effect as eating a lifelong diet high in soy."
For the study, Dr. Qingyuan Zhang of the Cancer Hospital of Harbin Medical University in Harbin, China, evaluated 524 women who had undergone breast cancer surgery between August 2002 and July 2003. They were receiving endocrine therapy such as anastrozole (Arimidex) or tamoxifen to reduce cancer recurrence risk.
The researchers measured the women's dietary intake of soy isoflavones at the start of the study, and then followed them for about five years to see if breast cancer recurred.
For the premenopausal women, soy had no apparent effect on the risk of subsequent breast cancer.
But postmenopausal women with the highest intake of soy -- more than 42.3 milligrams of soy isoflavones a day -- had a 33 percent reduced risk of cancer recurrence. For patients receiving anastrozole and whose breast cancer was estrogen receptor-positive and progesterone receptor-positive, the risk reduction link was even stronger.
The study findings were published Oct. 18 in CMAJ (the Canadian Medical Association Journal).
A serving of soy milk has about 30 or 40 milligrams of isoflavones.
But Zhang cautioned that larger studies with women from many medical centers are needed before scientists can say that soy may help reduce the risk of breast cancer's return.
McCullough agreed. "It's still possible that other lifestyle differences in women in China who were eating less soy" might explain their higher likelihood of getting breast cancer again, she said.
There have been concerns about the effect of soy consumption on women with estrogen receptor-positive and progesterone receptor-positive breast cancer because soy isoflavones are similar to estrogen in chemical structure, and because tumor growth is dependent on estrogen, the study authors said in a news release.
Dr. Joanne Mortimer, director of the women's cancers programs at the City of Hope Comprehensive Cancer Center in Duarte, Calif., said some doctors tell women to avoid soy, concerned that too much might be hazardous. "I think this is yet another study that confirms that soy products do not cause an increased risk of breast cancer or even recurrence," she said.
But, it's not a reason to go overboard on soy, Mortimer said. "I wouldn't tell people they should eat a lot of soy," she said. Rather, "they should eat a healthy diet and if they eat soy, this study suggests there is not harm in it."
According to current American Cancer Society guidelines, which are under review, up to three servings a day of soy foods is considered safe, McCullough said. But women are advised to avoid the high soy dose found in more concentrated sources such as soy powders and isoflavone supplements, she said.
To learn more about soy, visit the American Cancer Society.
Posted: October 2010