A new study suggests intake of dairy milk is associated with a greater risk of breast cancer in women.
Dairy, soy and risk of breast cancer: Those confounded milks, was conducted by researchers at Loma Linda University Health, and published in the International Journal of Epidemiology.
According to a press release, researchers found that 'even relatively moderate amounts of dairy milk consumption can increase women's risk of breast cancer - up to 80 percent depending on the amount consumed'.
The study evaluated the dietary intake of nearly 53,000 North American women over a nearly eight-year period, using food frequency questionnaires.
The women also completed a baseline questionnaire featuring 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.
All the women were cancer-free at the beginning of this period. By the end of the study period, there were 1,057 new breast cancer cases during follow-up.
'Fairly strong evidence'
First author of the paper, Gary E. Fraser, MBChB, Ph.D., said the study provides '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'.
He added: "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 percent. By drinking up to one cup per day, the associated risk went up to 50 percent, and for those drinking two to three cups per day, the risk increased further to 70 percent to 80 percent."
He suggested the study provides evidence to suggest that the current U.S. Dietary guidelines - which recommend three cups of milk per day - should be 'viewed with caution'.
"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," says the release. This led Fraser to suggest that 'dairy-alternate milks may be an optimal choice'.
When compared to low or no milk consumption, higher intakes of dairy calories and dairy milk were associated with greater risk of breast cancer.
There was minimal variation when comparing intake of full fat versus reduced or nonfat milks, and no important associations noted with cheese and yogurt.
Fraser believes the possible reasons for these associations between breast cancer and dairy milk may be the sex hormone content of dairy milk, as 'cows are of course lactating, and often about 75 percent of the dairy herd is pregnant'.
Breast cancer in women is a hormone-responsive cancer.
In addition, intake of dairy and other animal proteins has been associated with higher blood levels of a hormone, insulin-like growth factor-1 (IGF-1), which is thought to promote certain cancers in some reports.
"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."