Breast cancer health centre

Soya and breast cancer

There has been much controversy about the links between soya and breast cancer.

Initially, scientific research was pointing to a link between soya products and an increased risk of breast cancer, but more recent research indicates that eating soya products is not harmful to breast cancer survivors or women at risk of developing breast cancer - and in moderation they might even be beneficial.

What is the link between soya and breast cancer?

The female sex hormone oestrogen triggers the growth of certain types of breast cancers, sometimes referred to as oestrogen-positive breast cancer. Breast cells contain oestrogen receptors, so increased levels of oestrogen in the body can increase the risk of breast cancer. Women who are at risk of having higher oestrogen levels are those who didn't have babies or had them late in life ( pregnancy reduces oestrogen levels) and those who are obese during menopause as oestrogen forms in fatty tissue.

Oestrogen is not only a hormone produced in the human body: it is also found in plants (referred to as phyto-oestrogens - 'phyto' means 'plant') and soya plants produce high levels of oestrogen-like chemicals known as isoflavones.

Scientists have been looking at diet and nutrition to reduce breast cancer. They have been unsure about how plant-based oestrogens affect the human body and there have been different theories. One theory supports the idea that they can boost natural oestrogen levels in the human body, which means there would be a higher risk of breast cancer. An opposite theory, however, explains that they compete with human oestrogen and can lead to a reduction of oestrogen levels in the body. Isoflavones may have properties that block oestrogens from binding with oestrogen receptors.

Some research in the past has been done on rodents using isolated soy compounds rather than whole soya products. In these studies, plant-based oestrogen led to the growth of cancer breast cells. However, the effects plant-based oestrogens have on the rodents may not be the same as in humans. Rodents metabolise genistein, a compound in isoflavones, in a different way than humans do, for example. In more recent studies in women, the results suggest plant-based oestrogens can help prevent oestrogen-positive breast cancers recurring in women who have already had breast cancer. This might be because they bind to different types of oestrogen receptors. In rodents they bind to receptors that are thought to promote cell growth, but in humans the receptors they bind to may block cell growth in tumours.

One large study looked at around 5,000 women in Shanghai, China who had breast cancer. After almost 4 years, the study found that the highest survival rates and the lowest risk of breast cancer recurrence occurred in those who consumed the most soya foods. However, it is not clear if the isoflavones in soy foods had an impact or if another nutrient was responsible. The study also found that consuming a moderate amount of soya-based foods, at 11g a day, provided a similar level of protection when compared to those that had larger amounts of soya-based foods.