As a health psychologist, I strive to learn all that I can about a lifestyle that promotes well being, including healthy eating. Of late, I have become further interested in the ethical and environmental aspects of food choices. Books like The Omnivore’s Dilemma and Cooked, by Michael Pollan, and Eating Animals by Jonathan Safran Foer offer much food for thought along these lines.
Recently, I viewed a film, What the Health, an investigative documentary that follows Kip Anderson on a quest to understand the links between agribusiness and government and how these impact Americans’ health. In Michael Moore style, Anderson confronts officials from national health organizations, when they will grant him an interview, with pointed, but sincerely put queries. One he posed to the Susan G. Komen Foundation was “we’re wondering why you don’t have a huge warning about the dangers of consuming dairy on the website when there is a direct link to breast cancer.” The impetus for this question was a study that, according to the film, showed “for women who have had breast cancer, just one serving of whole dairy a day increases the chance of dying from the disease 49 percent and dying from anything 64 percent.” If this were true, like Anderson, I wondered “Why weren’t breast cancer sites like Susan G. Komen warning everyone about this?”
This sent me to do some investigating in the scientific literature. I was able to locate the study that Anderson featured1 and found that the information he presented was accurate: in a sample of 1,893 women diagnosed with early-stage invasive breast cancer followed for 11.8 years, compared to those who consumed less than half a serving a day of high-fat dairy products, such as milk, cheese, dairy desserts, and yogurt, those who consumed higher amounts had significantly higher rates of breast cancer mortality, all-cause mortality, and non-breast cancer mortality. However, other findings from the study showed that low-fat dairy intake was inversely related to these mortality outcomes in minimally adjusted analyses (where just age and time between breast cancer diagnosis and the assessment of dairy intake was controlled for) and not related to these outcomes in analyses that adjusted for additional important factors (such as disease severity; type of cancer treatment; level of education; ethnicity; intake of calories, red meat, alcohol, fiber, and fruit; body mass index; physical activity levels; and smoking status). Similarly, overall dairy consumption was related only to overall mortality only in adjusted analyses. Breast cancer recurrence was not related to dairy intake (low-fat, high-fat, or overall) in adjusted or unadjusted analyses. Thus, the picture for me became somewhat cloudier.
The authors did offer a compelling rationale for the connection between dairy fat intake, estrogen levels, and the incidence and progression of hormone-related cancers such as breast, ovarian, postmenopausal endometrial, and prostate, but also noted that another study found that low-fat dairy intake was inversely associated with prostate cancer. Other researchers have posited that female sex hormones may be the link between dairy consumption and hormone-related cancers particularly because the milk we consume today, distinct from 100 year ago, is from pregnant cows that have elevated hormone levels.2
To gain some clarity, rather than focus on single studies regarding the link between consumption of dairy products and breast cancer, I consulted overviews of the research literature, particularly systematic reviews and meta-analyses. One, described as an assessment of the totality of the scientific evidence, reported that the link from consumption of dairy products and risk of breast cancer was inconclusive or inverse, perhaps due to the protective effects calcium and vitamin D.3 The authors concluded that “intake of milk and dairy products contributes to meeting nutrient recommendations and may protect against the most prevalent, chronic non-communicable diseases, whereas very few adverse effects have been reported.” The disclosures of the authors, however, listed support from a number of diary organizations, such as the Dairy Research Institute, the Danish Dairy Research Foundation, and the Global Dairy Platform, among others. These were followed with the disclaimer, for just two of the five authors who received this support, that the sponsors had no role in design and conduct of their prior work. A meta-analysis of prospective studies also found no linear association between total milk, whole milk, and yogurt consumption and risk of breast cancer and found an association between skim milk consumption and decreased risk of breast cancer. The authors of this review, however, did not report any dairy industry support. 4
Mixed findings and industry involvement reflect the difficulty distilling firm conclusions about healthy eating, even from authoritative scientific sources. While I continue to try to minimize my consumption of animal products for ethical reasons, my review of the scientific literature on this issue brought more questions than answers.
1 Kroenke, C. H., Kwan, M. L., Sweeney, C., Castillo, A, & Caan, B. J. (2013). High- and low-fat dairy intake, recurrence, and mortality after breast cancer diagnosis. Journal of the National Cancer Institute, 105, 616-623. doi: 10.1093/jnci/djt027.
2 Ganmaa, D., & Sato A. (2005). The possible role of female sex hormones in milk from pregnant cows in the development of breast, ovarian and corpus uteri cancers. Medical Hypotheses, 65, 1028-1037.
3 Thorning, T. K., Raben, A., Tholstrup, T., Soedamah-Muthu, S. S., Givens, I., & Astrup, A. (2016). Milk and dairy products: good or bad for human health? An assessment of the totality of scientific evidence. Food & Nutrition Research, 60, 32527. doi: 10.3402/fnr.v60.32527.
4 Wu, J., Zeng, R., Huang, J., Li, X., Zhang, J., Ho, J. C.-M., & Zheng, Y. (2016). Dietary protein sources and incidence of breast cancer: A dose-response meta-analysis of prospective studies. Nutrients, 8, 730. doi: 10.3390/nu8110730