A study published in the International Journal of Cancer in March has reported that rats consuming milk before puberty had a lower risk of developing mammary tumors than water-fed rats. Milk contains high levels of estrogens, progesterone and insulin-like growth factor 1 (IGF-1), all of which have been reported to be associated with increased risk of breast cancer. In the study, Sprague-Dawley rats were given either whole milk or tap water to drink from day 14 to day 35 after birth; all of the rats received tap water thereafter. Mammary tumors were induced by administering a carcinogen. Milk exposure was found to have increased circulating estradiol levels 10-fold as of day 25. Milk consumption also accelerated vaginal opening, which marks the onset of puberty in rats, by 2.5 days.

However, rats fed milk before puberty were found to have reduced mammary carcinogenesis. Time before the appearance of the first tumor was longer and tumor incidence was lower in the milk-fed rats compared to the water-fed controls. Mammary glands of milk-exposed rats had significantly fewer terminal end buds than controls when sampled on days 25 and 50. Estrogen receptor protein levels were elevated in the terminal end buds and lobules of milk-fed rats, compared to controls, but no changes in cyclin D1 expression, cell proliferation or apoptosis were observed. IGF-1 mRNA levels were found to be reduced on day 50 in the mammary glands of rats exposed to milk at puberty. The authors conclude that consuming milk before puberty reduces later risk of developing mammary cancer in rats. This reduced risk might be mediated by a reduction in the number of terminal end buds and lower expression of IGF-1 mRNA in the mammary glands of milk-exposed animals.

When should women stop drinking milk?

Although milk consumption during adulthood has been found to be associated with higher risk of breast cancer, milk has previously been found to be somewhat protective against breast cancer when consumed in infancy and childhood. A partial explanation for this finding may be the fact that milk is a very good source of calcium and vitamin D, both of which are known to protect against breast cancer. Exposure to estrogen during childhood also appears to be protective against subsequent breast cancer, although the mechanism of action is not understood. Milk consumption is also associated with higher bone mineral density and other measures of bone health.

Based on the available evidence, the point in time during which milk increases breast cancer risk appears to be late adolescence in U.S. girls. We would recommend organic milk (from grass-fed cows, if possible), but not raw milk, for consumption by our children. Much of the milk we drink today is produced from pregnant cows, in which estrogen and progesterone levels are markedly elevated. Non-organic milk contains additional recombinant bovine growth hormone, zeranol and other additives administered to cows; such additives are not likely to be protective against breast cancer at any age.