Soy and Breast Cancer: Is There a Connection?

Medically reviewed by Wendy Y. Chen, MD, MPH

Is there a link between soy and an increased risk of breast cancer? “That’s one of the most common questions I get from breast cancer survivors,” says Wendy Chen, MD, MPH, a breast oncologist with the Susan F. Smith Center for Women’s Cancers at Dana-Farber. While laboratory studies on soy compounds in isolation have sparked questions about a possible connection, studies of breast cancer patients in China and Japan have not shown any increased breast cancer risk resulting from soy consumption.

Wendy Chen
Wendy Chen, MD, MPH

There is a biological basis for this line of inquiry. “Soy has what are called phytoestrogens,” Chen says. “These are plant-based estrogens. If you look at certain compounds in isolation, some of them have been shown in lab studies to increase cancer cell growth. However, that has never been shown in people.”

The most extensive population studies on this issue were conducted in China. The Shanghai Women’s Health Study surveyed more than 70,000 Chinese women about their health and food intake, including soy. Soy consumption is exponentially higher in China than in the United States. Many Chinese are lactose intolerant, and so they begin drinking soy milk in childhood. In addition, other forms of soy, such as tofu and edamame, are part of the traditional Chinese diet.

“Studies have not shown any increased risk of breast cancer recurrence or death linked to soy consumption,” Chen says. “In fact, higher amounts of soy were linked to a lower risk of breast cancer recurrence or death. It may be due to the fact that people in China who consume larger amounts of soy tend to lead an overall healthier lifestyle.”

These studies looked at consumption of whole soy food products. In the United States, soy consumption is more likely to come in the form of soy protein isolates, a supplement or additive commonly found as one ingredient among many in nutrition bars and vegetarian “meatless” products.

Research is lacking on soy protein isolates. “The studies (examining the link between soy and cancer) have not included people taking soy supplements,” Chen says. “We just don’t know anything about their effect on humans at this point.

Given the lack of data about soy protein isolates, you might want to stick to whole soy foods in the meanwhile. So, go ahead and order up your favorite spicy tofu dish without concerns that it may put you at risk of anything more than indigestion.