Results from the Adventist Health Study-2 (AHS-2; NCT03615599) a long-term health study exploring the links between lifestyle, diet, and disease among members of the Seventh-day Adventist church funded by the Ardmore Institute of Health shows that the intake of dairy milk is associated with a greater risk of breast cancer in women.

The study, which took place between 2001 and 2007, enrolled 96,000 participants (ages 30 to 112) from all 50 U.S. states and Canada. Among the participants 64% were members of the Adventist church; 65.1% were female, with a mean age of 60.2 at enrollment; 65.3% were white (non-Hispanic) 26.9% are Black/African American; 1.1% reported being smokers and 6.6% report current alcohol use.

The study was conducted by researchers at Loma Linda University Health.

The researchers, published in the International Journal of Epidemiology, found that even relatively moderate amounts of dairy milk consumption can increase women’s risk of breast cancer; up to 80% depending on the amount consumed.

Photo 1.0: Gary Fraser, MBChB, Ph.D., Professor, School of Public Health; Professor, Medicine School of Medicine, and Professor, Preventive Medicine School of Medicine. Photo Courtesy: © 2020 Loma Linda University.

Gary E. Fraser, MBChB, Ph.D., the first author of the paper, said the observational study gives “fairly strong evidence that either dairy milk or some other factor closely related to drinking dairy milk is a cause of breast cancer in women.

“Consuming as little as 2 fl oz (1/4 cup/59 ml) to 1/3 cup (2.7 fl oz (1/3 cup/79 ml) of dairy milk per day was associated with an increased risk of breast cancer of 30%,” explained Fraser.

“By drinking up to one cup per day, the associated risk went up to 50%, and for those drinking two to three cups per day, the risk increased further to 70% to 80%,” he added.

Current U.S. dietary guidelines recommend three cups (24 fl oz/ 0.7 liter) of milk per day. “Evidence from this study suggests that people should view that recommendation with caution,” Fraser noted.

As part of the study, dietary intakes of nearly 53,000 North American women were evaluated. These women were initially free of cancer and were followed for nearly eight years.

Dietary intakes were estimated from food frequency questionnaires (FFQ), also repeated 24 hour recalls, and a baseline questionnaire had questions about demographics, family history of breast cancer, physical activity, alcohol consumption, hormonal and other medication use, breast cancer screening, and reproductive and gynecological history.

New cases
By the end of the study period, there were 1,057 new breast cancer cases during follow-up. No clear associations were found between soy products and breast cancer, independent of dairy. But, when compared to low or no milk consumption, higher intakes of dairy calories and dairy milk were associated with greater risk of breast cancer, independent of soy intake, with hazard ratios (HRs) of 1.22 [95% confidence interval (CI): 1.05–1.40] and 1.50 (95% CI 1.22–1.84), respectively, comparing 90th to 10th percentiles of intakes.

Fraser noted that the results had minimal variation when comparing the intake of full fat versus reduced or nonfat kinds of milk.  Interestingly, the researchers did not observe important associations with cheese and yogurt.

“However,” he said, “dairy foods, especially milk, were associated with increased risk, and the data predicted a marked reduction in risk associated with substituting soymilk for dairy milk.”

The researchers noted that substituting median intakes of dairy milk users by those of soy milk consumers was associated with HR of 0.68 (95% CI: 0.55–0.85).

“This raises the possibility that dairy-alternate milk may be an optimal choice,” Fraser explained.

A hazardous effect of dairy is consistent with the recent AHS-2 report suggesting that vegans but not lacto-ovo-vegetarians experienced less breast cancer than non-vegetarians.

Possible reasons
Fraser said the possible reasons for these associations between breast cancer and dairy milk may be the sex hormone content of dairy milk, as the cows are of course lactating, and often about 75% of the dairy herd is pregnant. Breast cancer in women is hormone-responsive cancer. Further, intake of dairy and other animal proteins in some reports is also associated with higher blood levels of a hormone, insulin-like growth factor-1 (IGF-1), which is thought to promote certain cancers.

“Dairy milk does have some positive nutritional qualities,” Fraser said, “but these need to be balanced against other possible, less helpful effects. This work suggests the urgent need for further research.”

Clinical trials
Plant-based Diets and Risk of Cancer in the Adventist Health Study-2 (AHS-2) – NCT03615599.

Reference
Fraser GE, Jaceldo-Siegl K, Orlich M, Mashchak A, Sirirat R, Knutsen S, Dairy, soy, and risk of breast cancer: those confounded milks, International Journal of Epidemiology, dyaa007, https://doi.org/10.1093/ije/dyaa007 [Article]