Does Dairy Milk Promote Breast Cancer?
One in eight U.S. women will develop invasive breast cancer during her lifetime. To reduce this risk, there are several things women can control. Alcohol consumption, physical activity, body mass index, oral contraceptive use, and breastfeeding are probably the most significant. But what role diet plays, if any, is far from settled. In particular, the effect of dairy and soy consumption on breast cancer risk is unclear. Prior research has suggested that they have opposite effects. This study sought to clarify this issue by combining the data on both into a single analysis.
Researchers surveyed 52,795 North American women (mean age 57.1 years) who are part of the Adventist Health Study-2. The baseline survey questionnaire gathered demographic data along with information about family history of breast cancer, physical activity, alcohol consumption, physical measurements, medication and hormone use, breast cancer screening, and gynecological history. Dietary intakes were estimated from food frequency questionnaires. A subset of 1,011 participants also performed six structured dietary recalls. About half the cohort consumed dairy at rates typical of other U.S. consumers while the remainder ate little or no dairy. Subjects were free of breast cancer at the start of the project, which followed them for 7.9 years. A total of 1,057 new cancer cases were reported after follow-up and confirmed through cancer registries.
A “hazard ratio” measured the risk associated with dairy or soy intake and breast cancer. This ratio has a value of one when there is no difference between groups exposed to two different conditions. A result other than one suggests that different conditions produced different results, which may or may not be statistically significant. In this research, soy, in the absence of dairy, was not associated with a greater cancer incidence. However, consumption of dairy calories and dairy milk, when adjusted for soy intake, were associated with higher rates of breast cancer. This association was not linear but appeared with the consumption of as little as one eight-ounce cup per day. However, substituting soy for dairy produced a marked reduction in breast cancer risk. Pre- or post-menopausal status didn’t affect these outcomes. Whether the dairy was full-fat or low-fat had no effect. Interestingly, cheese and yogurt didn’t produce similar associations.
Prior studies on the risks and benefits of dairy and soy have produced confusing and sometimes contradictory results. This study tried to sort out the confounding variables with a novel approach that used a unique cohort of subjects. These results should give us pause. About 75% of U.S. dairy cows are pregnant while they are producing milk, providing a possible link between hormones in their milk and cancer risk. And yet, U.S. dietary recommendations still suggest that adults consume about three cups (24 ounces) of milk daily. As advocates, we know that health concerns are a primary driver of dietary change that benefits animals. Encouraging more research into this potential link could reduce suffering for millions of dairy cows while improving human health.