The Great Soy Debate

Accused one minute for causing breast growth in boys and men, celebrated as a healthy alternative for those with lactose intolerance, the next. For breast cancer patients, the question is whether soy cures breast cancer, or causes it.

The science is split: there are just as many pro-soy studies as there are those that seek to demonize it. Nevertheless, soy continues to be a popular area for continued research.

Both sides of the argument focus on an element of soybeans called isoflavones (or phytoestrogens).

In vitro studies (research conducted outside of a living organism, usually in a test tube or culture dish) showed that certain amounts of isoflavones can inhibit the growth of many cancer cells, including breast cancer. Amounts less than this ‘sweet spot’, however, have the opposite effect - the isoflavones begin mimicking estrogen and stimulate cancer cell growth (Source: Messina et al., 2001).

Similarly, soy consumption has been both suggested as capable of reducing hot flushes and vaginal dryness in postmenopausal women, and of interfering with the efficacy of Tamoxifen. (Source: Albertazzi et al., 1998). In vitro studies showed that soy effectively blocked Tamoxifen from working, whereas animal studies showed that combining soy isoflavones with Tamoxifen was 33% more effective than tamoxifen alone. (Source: Messina et al., 2001).

Researchers were first prompted to look deeper into soy, after a study, carried out in 1990, found that soy protein provided a protective effect to rats that had been chemically induced with breast cancer (Source: Barnes et al., 1990). Human-based studies began and from 1991 onwards, results seemed to support the benefits of soy (Source: Lee et al., 1991).

As it turns out, for soy to have protective effects, it needs to be transformed into a state that the body can use, kind of like a currency exchange – the body can only use it once it has been converted into a form that it recognises. This usable form of soy is called ‘equol’ and Asian populations living in high soy consumption areas, in Asian countries, are best able to do this. Caucasians are the least able (Source: Kyung Bin Song et al., 2006).

While seaweed consumption and a vegetarian diet were two factors that appeared to increase soy-to-equol conversion ability, the most likely reason for the varying conversion abilities is time. Asian populations have been eating soy beans for millennia and have adapted, whereas soy is still a relatively recent addition to Western diets, too recent for it to be reflected in the make-up of intestinal bacteria. (Source: Fen-Jin He, et al., 2013).

So, should I eat it, or not?

While there is not enough evidence to label soy a saviour, there is no substantial evidence to declare it a no go either (Source: Hamer et al., 2017). This means that, if you like soy products, consuming them in moderation shouldn’t hurt you. If you are suffering from hot flushes, trying soy is still an option, provided that your oncologist agrees, but if after a few months you haven’t noticed any reduction, there’s no need to continue.

Note: Always consult your oncologist or doctor for guidance.