Does soy food prevent breast cancer?

Soy contains many phytochemicals that are beneficial for health. Soy also contains isoflavones, which have a weak estrogenic effect. Isoflavone levels vary in different types of tofu and soy milk products.

Research about the effects of soy on cancer risk reduction has been inconclusive. Most research that has suggested soy may prevent breast cancer has come from observational studies that have observed cultures such as the Japanese, who eat more soy in the form of tofu and other fermented products than people in Western countries. However, there are many other factors to considered when trying to understand the connection between soy and breast cancer in Asian women. Most women living in Asia depend on soy as their main source of protein and consume only small amounts of animal protein foods. This also means they eat less fat from animal foods and possibly less unhealthy substances such as growth hormones and antibiotics and chemicals formed during cooking. Also, compared to many Australian diets, traditional Asian diets are:

• higher fresh vegetables
• lower in alcohol
• lower in meat

Many Asian women who follow an eastern style diet are also less likely to be overweight and more physically active. All of these other factors add up to produce a healthier lifestyle and a lower overall risk of breast cancer in Asian women living in Asia. However, Asian women living in western societies, therefore eating a typical western diet, do not have the same low rates of breast cancer.

It is not clear whether soy isoflavones affect breast cancer, especially ER-positive breast cancers. Isoflavones may interfere with the effect of hormonal therapies, such as estrogen inhibitors, as they could potentially compete with the medications ability to bind to estrogen receptors. At this stage, it is recommended that women who take hormonal therapy or who have estrogen-receptor-positive breast cancer avoid soy or phytoestrogen supplements. Although it’s still okay to consume moderate amounts of soy foods (e.g. one to two serves each day) as part of a balanced diet. If you are taking hormonal therapy to fight off a hormone-receptor-positive breast cancer, and you are concerned about any phytoestrogen effects, ask your doctor or dietitian about how much soy you should eat.