Milk and Breast Cancer: Looking Behind the Headlines
You may have seen headlines: higher intake of cows’ milk is associated with increased risk of breast cancer. As a dietitian I am often asked whether cows' milk is associated with an increased risk of breast cancer. The findings come from an observational study of Seventh Day Adventists published recently in the International Journal of Epidemiology1. The key word to focus on is this study is observational. An observational study is one in which a researcher observes individuals. Observational studies cannot be used to make definitive statements of fact. They cannot prove that milk or anything else causes cancer. They are just observations of a possible relationship.
When looking at this study, it is important to consider if there are other factors that could have contributed to cancer risk - other than just milk intake. There may be something else about women who drink milk or do not drink milk. Women in the study are Seventh Day Adventists and typically have unique eating habits and lifestyles, so the finds may not apply to all women. Many Seventh Day Adventists are vegans (eat no animal foods) or lacto-ovo vegetarians (eat a vegetarian diet with dairy and eggs). Their intake of soy foods is much higher than most other American women (68 grams per day vs 2 grams/day). Soy has a protective affect against breast cancer and vegans tend to eat more soy. A previous study with Seventh Day Adventists showed that vegans have a lower BMI than lacto-ovo vegetarians or non-vegetarians. Weight might be the culprit. Being overweight or obese are proven risk factors for breast cancer. Another flaw in the study is that women were only questioned about their dairy intake at the beginning of the study, and this could have changed over the 8-year study.
Whether or not milk has a negative or positive effect on cancer development has been the subject of research for many decades. The American Institute for Cancer Research, the largest collection of evidence on milk and breast cancer, has found no connection. Some studies even showed a reduced risk of cancer in milk drinkers.
What should you do now? Although this study is concerning and will likely result in further research on the potential link between milk and cancer to find out if there really is a connection, it is not enough to justify stopping you from drinking milk. Never make a decision on the basis of one study, especially an observational study. Milk is one of our richest sources of calcium and vitamin D, which are necessary for healthy bones, and is also an excellent source of protein. Calcium and vitamin D are especially important for breast cancer survivors, who are at higher risk of osteoporosis. Yogurt and cheese are also excellent sources of calcium and were not associated with increased breast cancer risk in the Seventh Day Adventist study. If you unable or do not want to consume dairy product, non-dairy sources of calcium include greens (collards, broccoli, kale and bok choy), soy foods (edamame, soybeans, tofu), other legumes, nuts and seeds (almonds and sesame seeds), and fortified nondairy milks. Put your efforts on proven measures you can take to reduce your chances of getting breast cancer: maintain a healthy weight, limit or avoid alcohol and be physically active.
What are your favorite food sources of calcium and vitamin D?
- Gary E Fraser, Karen Jaceldo-Siegl, Michael Orlich, Andrew Mashchak, Rawiwan Sirirat, Synnove Knutsen, Dairy, soy, and risk of breast cancer: those confounded milks, International Journal of Epidemiology, Volume 49, Issue 5, October 2020, Pages 1526–1537,