Dairy, soy, and risk of breast cancer: those confounded milks

https://academic.oup.com/ije/advance-article/doi/10.1093/ije/dyaa007/5743492#199259276

Dairy, soy, and risk of breast cancer: those confounded milks Gary E Fraser, Karen Jaceldo-Siegl, Michael Orlich, Andrew Mashchak, Rawiwan Sirirat, Synnove Knutsen International Journal of Epidemiology, 25 February 2020

Recent media headlines point to a study suggesting that women who drink as little as one cup /250 ml of cow’s milk per day could be increasing their risk of developing breast cancer by up to 50%.  This study is part of the Adventist Health Study, by Synnove Knutsen et al, from Loma Linda University   and published in the International Journal of Epidemiology.  https://doi.org/10.1093/ije/dyaa007 . The study evaluated associations between intakes of soy milk, other soy products, dairy milk and other dairy foods with risk of breast cancer.

This study followed nearly 53,000 Adventist women for 7.9 years focusing on the relative risk of a rare outcome. The authors concluded that as cow’s milk intake increased, regardless of fat level, so did the risk of breast cancer. No clear associations were found between consumption of soy products and breast cancer.

Weaknesses of this study are its observational design (cause and effect cannot be established) and possible residual confounding between dairy and unmeasured factors, despite extensive covariate adjustment. (1) Diet was measured only once at study baseline leaving room for error and omissions. Whether these results can be applied to other populations is unknown and therefore difficult to draw conclusions.  Adventists lifestyle differ considerably from the general population as many follow a plant based diet and exclude processed foods, alcohol and caffeine.

The authors do note that cow’s milk has many positive nutritional qualities and suggest more research is needed to understand if there is a link between dairy intake or other closely-related unidentified factors and breast cancer risk.  Until then, a balanced and a varied diet including sources of calcium, regular physical activity and avoiding smoking and excess alcohol make for a healthy lifestyle.

1. Dairy, soy, and risk of breast cancer: those confounded milks Gary E Fraser, Karen Jaceldo-Siegl, Michael Orlich, Andrew Mashchak, Rawiwan Sirirat, Synnove Knutsen International Journal of Epidemiology, 25 February 2020