United States Dietary Guidelines recommend that adults and children aged 9 or older consume three cups per day of low-fat or fat-free milk and dairy foods, or alternatively, three cups of calcium-fortified soymilk. However, a large observational study recently published in the International Journal of Epidemiology reports that consuming three cups of dairy milk per day is associated with a risk of breast cancer that is increased by as much as 80%, regardless of the fat content of the milk, with a dose-dependent relationship for lesser quantities of milk. In this interview with i3 Health, Gary E. Fraser, MBChB, PhD, lead author of the study, discusses the potential causes and practical implications of these findings.
Can you comment on the significance of your results?
Gary E. Fraser, MBChB, PhD: I think the main significance is that dairy milk is a complex food, and it is quite possible that there are important downsides to its consumption that should be balanced against its nutritional strengths. Our data certainly raise the possibility that there is an association between risk of breast cancer and consistent intake of relatively small quantities of dairy milk. Whether the association is causal requires more work, but at the least, it seems that either the milk or something very closely associated with it is a causal factor.
What are the most probable explanations behind your findings regarding the association of dairy milk intake with breast cancer risk?
Dr. Fraser: We find that a consistent intake of about two-thirds cup per day of dairy milk is associated with about a 50% increased risk of breast cancer. Expressed differently, what may be a 9% to 10% lifetime breast cancer risk in nondairy milk consumers becomes a 14% to 15% lifetime risk in consumers of relatively low levels of dairy milk.
The biological mechanisms underlying a hypothetical cause are speculative at present, but the idea that the bovine sex hormones present in dairy milk may have adverse effects was published several years back. The cows by definition are lactating, and about 75% of the US dairy herds are pregnant. Another possibility is that a tissue hormone called IGF-1 is increased by dairy milk consumption, and there is some evidence that this increases cancer risk.
What further research needs to be done on this topic?
Dr. Fraser: We need further study of this association between dairy milk and breast cancer, comparing risk in those who avoid milk (not only vegans by any means) to those who consume small and also moderate amounts. We also need a much better understanding of the possible biologic effects of the bovine sex hormones on human adults. Several small studies suggest that there are discernible biological effects.
Based on the results of your study, should physicians begin cautioning patients to reduce their dairy milk intake?
Dr. Fraser: As yet, this is only one study, although a methodologically strong one. My own opinion is that women at higher risk of breast cancer—those with a family history or with reproductive factors increasing risk—should seriously consider changing to plant-based milks if that is easy, until this question has a clearer answer. Perhaps most people could accept the current US Dietary Guideline of three cups per day with some caution.
About Dr. Fraser
Gary E. Fraser, MBChB, PhD, is a Professor of Cardiology and Preventive Medicine in the School of Medicine and a Professor of Epidemiology in the School of Public Health at Loma Linda University in Loma Linda, California. Dr. Fraser's research has focused on diet, exercise, lipids, heart disease, and cancer. He has published extensively in peer-reviewed journals and has authored two books, Preventive Cardiology and Diet, Life Expectancy, and Chronic Disease, both published by Oxford University Press. He is the former principal investigator and a current co-investigator of two studies: the Adventist Health Study–2, which is tracking the incidence of breast, colon, and prostate cancers and mortality among 96,000 black and white Adventists over a 10-year follow-up period, and a study examining cognitive, behavioral, affective, and social manifestations of religious experience and their associations with quality of life and cause-specific mortality.
For More Information
Transcript edited for clarity. Any views expressed above are the speaker's own and do not necessarily reflect those of i3 Health.