Last week I posted a recipe for making homemade soy yogurt.  Delicious, with no added sugar and it takes only two minutes of your time to make.  But many women have read that they should, or have been instructed to, avoid soy or soya if they have a history of estrogen receptor positive (ER+ve) breast cancer. So why did I post that recipe?  Let’s look at what the latest research shows…

Soy foods and soy isoflavones have been shown in many studies to have anti-cancer activity.  In Asian populations where soy is a large part of the diet, throughout their lifetime, the incidence of most cancers is lower than in populations that don’t consume soy.  When individuals from these populations move to another country and no longer consume the high-soy diet they were used to, their rates of cancer increase to the same rate as the population in their new home country.

But getting into the science here,  soy and isoflavones contain compounds called phytoestrogens (plant estrogens) that can bind to estrogen receptors. This was potentially thought to be bad news as it was theorized that eating these foods could stimulate the estrogen receptors (the “estrogenic effect”), just like our own estrogen itself does, and thus accelerate the growth of ER+ve breast cancers.

However, there are two types of estrogen receptors – alpha (α) and beta (β). Our own estrogen favors ER-α, whereas soy phytoestrogens preferentially bind to ER-β. ER-β receptors are activated when something like soy binds to them, at which point they can actually inhibit the cancer growth-promoting ER-α, and thus have a protective effect against breast cancer. This protective effect of soy has been seen in both premenopausal and postmenopausal women.

In the largest study to date on the influence of soy on breast cancer outcome [1], soy intake was monitored on 9514 Chinese and US women breast cancer survivors.  The individuals were followed for, on average, seven years after their breast cancer diagnosis. The soy intake varied considerably between the Chinese and US populations with the Chinese women having an average of 46mg/day of isoflavones compared to the American women who had only 3mg/day.

The study showed that the consumption of the equivalent of one cup of soy foods per day (~10mg isoflavones/day) resulted in a statistically significant 25% reduction in the risk of recurrence of breast cancer. This level of intake also led to a reduced risk of mortality, however this risk wasn’t statistically significant. Confounding factors such as the links between high levels of soy intake and regular exercise, higher consumption of cruciferous vegetables, lower BMI (body mass intake), and not smoking were all taken into consideration.  These results held for both Chinese and US populations.

The anti-cancer mechanisms of soy are not fully understood.  In contrast to its estrogenic effects, there is evidence of soy slowing the rate of growth of cancer cells, having anti-oxidant activity, and reducing inflammation, which are all good things. 

Whole organic soy foods – such as the soy yogurt recipe in last week’s blog post along with soy milk, tempeh, and edamame – are associated with this preventive benefit. On the other hand, isolated soy powder, which is often found in drinks powders and snack bars, lacks the isoflavones and so is not as beneficial. 

[1] Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women






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