Researchers have 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.
"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," said study first author Gary E. Fraser from Loma Linda University in the US.
"Consuming as little as 1/4 to 1/3 cup of dairy milk per day was associated with an increased risk of breast cancer of 30 per cent," Fraser said.
By drinking up to one cup per day, the associated risk went up to 50 per cent, and for those drinking two to three cups per day, the risk increased further to 70 per cent to 80 per cent, the researchers said.
For the findings, published in the International Journal of Epidemiology, dietary intakes of nearly 53,000 North American women were evaluated for the study, all of whom 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.
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, the study said.
The researchers noted that the results had minimal variation when comparing intake of full fat versus reduced or nonfat milks; there were no important associations noted 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."
According to the researchers, 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 per cent of the dairy herd is pregnant. Breast cancer in women is a 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, but these need to be balanced against other possible, less helpful effects," Fraser concluded.