Dairy milk intake may be linked to a greater risk of breast cancer, according to a study which may lead to new clinical dietary recommendations for women to prevent the malignant disease.
The researchers, including those from Loma Linda University in the US, 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.
In their study, published in the International Journal of Epidemiology, they noted that even relatively moderate amounts of dairy milk consumption can increase women's risk of breast cancer by up to 80 per cent depending on the amount consumed.
"Consuming as little as one-fourth to one-third of a cup of dairy milk per day was associated with an increased risk of breast cancer of 30 per cent," said Gary E. Fraser, study co-author from Loma Linda University.
"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," Fraser said.
In the study, the scientists evaluated the dietary intakes of nearly 53,000 North American women, all of whom were initially free of cancer and were followed for nearly eight years.
The researchers said they also asked participants to fill up questionnaires about their demographics, family history of breast cancer, physical activity, alcohol consumption, hormonal and other medication use, breast cancer screening, and reproductive, and gynaecological history.
By the end of the study period, they said, there were 1,057 new breast cancer cases during follow-up.
The scientists said they did not find any clear associations between soy products and breast cancer, independent of dairy.
However, 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, they noted.
According to Fraser, the results had minimal variation when comparing intake of full fat versus reduced or nonfat milks, and there were no important associations noted with cheese and yogurt.
"However, 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," Fraser said.
"This raises the possibility that dairy-alternate milks may be an optimal choice," he added.
Fraser suspects that the association between breast cancer and dairy milk may be due to the sex hormone content of dairy milk, as the cows are lactating, and often about 75 per cent of the dairy herd is pregnant.
He said breast cancer in women is a hormone-responsive cancer, and intake of dairy and other animal proteins is 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. This work suggests the urgent need for further research," Fraser said.
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