Potential Benefit of Soy
Breast cancer rates vary depending on geographical location with the incidence of breast cancer being relatively high in North America and Western Europe compared to Asian countries such as Japan and China. One factor implicated in contributing to the lower breast cancer rates observed in Asian countries is the high consumption of soy products. However, it has been difficult to dissociate the benefits of soy from other variables including environmental and lifestyle factors.
Our Research
To evaluate the impact of dietary on mammary tumor development, MTB-IGFIR transgenic mice were fed diets where 100% of their dietary protein was derived from either casein or soy protein isolate. These diets were administered throughout embryonic and postnatal development. Somewhat surprisingly, we found that dietary soy promoted, rather than inhibted mammary tumor development in our model.
Since soy metabolites have estrogenic activity, it is possible that the very high levels of soy administered in the above study promoted mammary tumor development. Therefore, a second study was performed where lower levels of soy protein isolate (1% or 5% of the dietary protein) where administered. These mice still developed mammary tumors more rapidly than the mice fed a casein diet. Interestingly, mice fed a standard rodent diet, Teklad 2018, were protected from developing mammary tumors. The Teklad 2018 diet differs in the type of soy (soybean meal), complexity of the carbohydrates (compex carbohydrates) and the levels of a number of vitamins. Our work shows that soy likely interacts with other dietary components to modify breast cancer risk.