There is a 30% lower risk of developing breast cancer among women in Asian countries who are known for their high soya intake (compared to American women). Also, there is a 21% reduction in mortality among women with breast cancer who consumed more soya.

It is not certain why soya protects against cancer risk. It could be because its isoflavones can increase apoptosis (a genetically programmed mechanism that tells cells to self-destruct when they get DNA damage they’re not able to repair). Without this process, damaged cells can turn into cancer.

It’s difficult to isolate soya’s benefits conclusively:

• No one has given people soya foods, then looked at whether they’re more or less likely to get breast cancer over time than those not given soya.
• One review of evidence into soya's effect on breast cancer risk found studies were adjusted for body mass index (BMI). It means the lower risk of breast cancer could be due to lower BMI, not soya intake.

Isoflavone can act either like estrogen in the body, or it's opposite.

Isoflavone in soya either binds to the alpha estrogen receptor in the body, which stimulates a tumor's growth rate, or it binds to the beta receptor, which decreases growth rate and induces apoptosis.

The impact of soya on breast cancer risk may depend on when we start eating it. Introducing soya after middle-age doesn't seem to reduce the risk or growth rate of tumors, but introducing soya before puberty appears to have a greater effect on reducing tumors.

Some research has found that the benefits that soya provides are mostly determined by a woman's ability to produce equol, a bacteria that around 30% - 50% of adults produce in their intestines after eating soy.

It is possible that a person's ability to produce equol, rather than equol itself, is responsible for the benefits of soya.

Soya doesn't affect testosterone levels in men. A review of evidence found that regular soya food intake was associated with an almost 30% reduction in the risk of developing prostate cancer.

Soya’s benefits depend on the type of soya we eat. The isoflavone content is higher in unprocessed soybeans, compared to processed soya foods. 

Women should be safe to consume soya foods in amounts consistent with the Asian diet. However, the more soya is processed, the lower the level of isoflavones, which are probably the protective element.