Thanks to Jenni Visuri, Chemistry undergraduate about to start her Graduate Entry Medicine course at Oxford University, for this article. Head to the bottom of the page to find out a little more about Jenni!
Simple, versatile and present in many foods from sausages to ice cream, soy appears to be the perfect food. Famed for its high protein content, polyunsaturated fats, fibre, and vitamins, it’s a strong favourite amongst vegans and body builders. Yet a quick Google search will tell a different story. Articles touting the dangers of soy seem to be present on every health website, warning readers of it’s frightening link to breast cancer. So, what’s this all about?
Soya beans, like other plant foods, contain the natural plant compounds: phytoestrogens.
Soya is a rich source of the phytoestrogens called isoflavones. Isoflavones have a chemical structure similar but not identical to the human hormone oestrogen, and as breast cancer is strongly associated with oestrogens we can see why there might be some concern over this. However, phytoestrogens are NOT the same thing as oestrogen, and can behave differently in the human body. Most importantly, isoflavones act differently in the human body and are
far less potent. In addition to this soy also contains several compounds with the ability to inhibit carcinogenesis including protease inhibitors, phytates and isoflavones.
So, what do the studies say?
Let’s start by comparing two different populations and their risk of breast cancer: Asian women who consume soy regularly and Caucasian women who do not. It was found that the former group have a 3- to 5- fold lower breast cancer risk than the latter. However, other factors may be coming into play here such as genetics and lifestyle factors: Asian women tend to eat more vegetables, be closer to their ideal body weight and drink less alcohol than Caucasian women. When studying Asian women who immigrate to the West, it was found that their daughters who were born in the West had a higher risk of breast cancer, a number similar to that of Caucasian women. As Asian-American women consume fewer soy products than Asian women, it would not be unreasonable to assume that soy protects them from developing breast cancer.
In human studies, the oestrogenic effects of genistein, the phytoestrogen present in soy, have been investigated numerous times with conflicting outcomes. Some of these studies indicate that soy protein isolate increases the volume of breast fluid, a marker of increased breast cancer risk. Other studies that have explored the effect of consumption of phytoestrogens alone or in soy foods on high mammographic density, another strong marker of increased breast cancer risk, have found no effect. When investigating consumption of whole soy foods such as tofu, soy milk, soybeans etc, women who consumed high quantities of soy were found to be 11% less likely to get breast cancer than women who consumed less. The reduction in incidence of breast cancer was found to be much greater in Asian populations than in western populations, however. In addition to this, post-menopausal women benefited more from a high soy consumption than pre-menopausal women.
In the past many women diagnosed with breast cancer were encouraged to steer clear of soy products in order to prevent recurrence. A study followed Chinese and US breast cancer survivors and soy intake was assessed for each of these patients. This included assessing the level and type of soy consumption. The results found that as soy intake increased, the risk of death decreased. The researchers hence concluded that there were no adverse effects of soy on breast cancer prognosis, and that women with a diagnosis no longer needed to be warned against soy consumption.
So in conclusion…
The studies show that there is no increase in risk of getting breast cancer from eating soy, and it may even help to protect you against it. The majority of studies focus on eating whole soy products such as tofu, soy milk etc., and not soy protein isolate, and so more research needs to be done to identify if this is also safe. At the end of the day people should feel confident in their food choices and if you enjoy soy products, the research suggests that they are safe. Now you can keep enjoying the tofu in your stir fry or the soy milk in your coffee and know it’s not doing you any harm and may even be protecting you!
- DOI: 10.3945/jn.110.124230
- DOI: 10.1007/s10549-010- 1270-8
- DOI: 10.1158/1055-9965.EPI- 10-1041