Soy foods do not appear to increase the risk of breast cancer recurrence among survivors of the disease and may even confer some health benefits, new research suggests.
The study, published in today's issue of the Journal of the American Medical Assn., should reassure breast cancer survivors that they need not scrupulously avoid soy foods, which have become increasingly popular in the United States in recent years. Research in animals has indicated that soy might increase the chances of breast cancer recurrence because it can act like the hormone estrogen, which promotes tumor growth.
"Some doctors have advised women not to eat soy foods," said Dr. Xiao Ou Shu, a professor of medicine at Vanderbilt University and lead author of the paper. "But another school of physicians think it's safe. So it has been controversial. Our findings are important because, nowadays, it's very difficult to avoid soy exposure. Soy flour and soy protein has been added to many foods in this country. Women may consume it and not even know it."
Shu and her colleagues analyzed data from the Shanghai Breast Cancer Survival Study of 5,042 women in China. The breast cancer survivors were ages 20 to 75 and were followed for an average of four years.
The study showed that the higher a woman's intake of soy foods, the lower her chances of cancer recurrence and death. Patients with the highest intake had a 29% lower risk of death during the study period and a 32% lower risk of breast cancer recurrence compared with patients with the lowest intake of soy foods. Soy food intake was measured by either soy protein or soy isoflavone intake. Isoflavones are hormones found in plants.
"Isoflavones can act as estrogens and add to the circulating pool of estrogen that is available and promote tumor growth. That is the concern," said Bette J. Caan, a senior nutritional epidemiologist at Kaiser Permanente in Oakland, who was not involved in the current study.
In research released earlier this year, Caan and colleagues at UC Berkeley also found that higher soy intake was linked to lower rates of breast cancer recurrence. That study, published in the journal Breast Cancer Research and Treatment, followed almost 2,000 U.S. breast cancer survivors.
"We do not see a harmful effect of soy. That is the main message out of both studies," Caan said. "Breast cancer survivors shouldn't go out and take soy supplements, but they shouldn't be afraid to drink soy milk in their coffee or eat tofu."
Shu's study found no adverse effects from soy food intake among women whose tumors grow faster because of exposure to estrogen, called estrogen receptor-positive breast cancer, or among those whose cancers are unaffected by estrogen, called estrogen receptor-negative.
Nor were there differences in the findings among women who used the breast-cancer drug tamoxifen and those who did not. However, the study found that the drug was related to improved survival only among women who had low or moderate soy food intake, not higher intakes.
Women who did not take tamoxifen but who had the highest intake of soy food had a lower risk of death and cancer recurrence than women who had the lowest levels of soy food intake and used tamoxifen.
That soy food intake may yield benefits that are comparable to tamoxifen is noteworthy, Caan said, because some experts fear that soy intake could counteract some of the effects of tamoxifen.
"People are afraid soy might actually counteract the effects of tamoxifen because it may be competing for the same receptors," she said. "That is why [Shu's] study is so interesting. She found it did not counteract any of the benefits of tamoxifen and, at high levels, soy is as effective as tamoxifen."
Women should not stop taking tamoxifen or use soy foods to replace the medication, Caan and Shu warned. Further studies will be needed to measure the effect of soy foods with or without tamoxifen.
It's also not clear why soy may lower cancer recurrence and death rates, Shu said.
"We cannot conclude from this study that there are no negative effects" from soy, she said. "We are studying soy as a whole food. We are not studying its components. It could be some components are not good for some people. But overall, we see women who eat a high amount of soy with better outcomes."
However, the quantity and quality of soy foods differ among U.S. and Chinese women, said experts in an editorial that accompanied the study. It is also difficult to compare U.S. and Chinese women because of differences in screening rates and treatments.
"Both this study and the Kaiser Permanente study give us a little more reassurance that soy foods are safe," said the lead author of the editorial, Dr. Rachel Ballard-Barbash of the National Cancer Institute. "But on the basis of just these two studies, we can't tell women to go out and significantly increase the amount of soy they eat."