Taken at first blush, it sounds like a perfidious piece of pseudo-science: soy can treat breast cancer.
While the science still far from settled, however, there is a rapidly accumulating body of evidence that suggests a correlation between soy intake and breast cancer survival. The evidence is all the more remarkable because until lately the oestrogen-mimicking isoflavones present in soy products were considered possible risk factors for breast and prostate cancer.
The most recent findings pointing to an anti-carcinogenic potential of the popular foodstuff comes from a team led by Fang Fang Zhang of the Friedman School of Nutrition Science and Policy at Tufts University in Massachusetts. Published in the journal Cancer, Zhang’s team set out to resolve the apparent contradiction between laboratory and epidemiological studies that found a link between higher isoflavone intake and reduced mortality, and others that suggested the same compounds might reduce the effectiveness of hormone therapies used to treat breast cancer.
Although they are found in other plant species, soy represents by far the largest source of isoflavones in the human diet.
To resolve the issue, the researchers studied the dietary intake of 6235 American and Canadian women diagnosed with breast cancer. Over a nine-year period, the results indicated that women who consumed large amounts of isoflavone had a 21% lower risk of dying than those who didn’t.
Zhang and her colleagues report that the outcome was largely confined to women whose cancers were hormone receptor-negative – that is, they did not contain a protein to which oestrogen will bind. Cancer cells that are hormone receptor-negative do not need oestrogen to grow, and usually don’t stop growing when treated with oestrogen-blocking hormones.
How isoflavones interact with cancer cells remains unclear. However, says Zhang, “For women with hormone receptor-negative breast cancer, soy food products may potentially have a protective effect.”
The team’s work comes on the back of at least two other 2017 studies into the relationship between soy products and cancer.
In February, researchers from the Universidade de Lisboa in Portugal reported on the abilities of soy products to inhibit the growth of matrix metallopeptidase 9, a class of enzymes strongly associated with cell degradation in cancer growth. The team, led by Ana Lima and published in journal Nutrients, acknowledged the previously discovered ability of isoflavones to inhibit the enzymes, but went on to show that soy protein extracts were also protective.
Such extracts, they suggested, “can be of significant importance for cancer preventive diets, particularly considering the increasing use of soy proteins in food products and the controversy around isoflavones amongst consumers.”
A third study, led by biochemist Atif Zafar of the Aligarh Muslim University in Uttar Pradesh, India, looked at the possible cancer-fighting properties of a specific soy phyto-oestrogen called coumestrol. The compound, the researchers noted in the journal Food and Chemical Toxicology, is “recognised as a potential cancer therapeutic agent against breast cancer”, although the mechanism involved remains unknown.
The team focused on the fact that many cancers express elevated copper levels, and discovered that in copper-rich tumours coumestrol inhibited malignant cell growth and caused some of the existing cancer cells to die. When the copper was removed, through a process known as chelation, the soy hormone ceased to work.
If further studies continue to support a link between cancer treatment and soy intake, the US will be in a very good position to take advantage. Although soy-based foods are most prominent in a number of Asian cuisines, America is well and truly the world’s largest soy producer. The plant is added extensively to a range of foodstuffs, including flour, cakes, chocolate, vegetable oils and table spreads.