Early cow’s milk consumption may cut breast-cancer risk

Research studies paint a muddy picture of milk’s cancer threat. Some have linked consumption of cow’s milk with a heightened risk of breast-cancer malignancies. Others have suggested milk drinking might be protective. A new animal study suggests these data may not be quite as contradictory as they at first seemed.

Leena Hilakivi-Clarke of Georgetown University’s Lombardi Comprehensive Cancer Center and her colleagues find that — at least in rats — youthful exposures to cow’s milk alters one of the key structures in developing mammary tissue. Called terminal end buds, these are the sites at which cancers normally start. Milk drinking’s ability to transform them into a more mature state just might render this tissue resistant to carcinogen exposures, the researchers argue in the January 1, 2011, International Journal of Cancer.

In their study, pregnant rats were randomly assigned to slake any thirst with either milk or tap water. Each mom continued to get the same beverage after delivering her pups. And at weaning, female pups got mom’s designated beverage as well — until they reached sexual maturity at around 33 days old.

Offspring exposed to cow’s milk constituents — beginning in the womb and on through to puberty — attained sexual maturity about 2.5 days earlier than rats that had been drinking water. Moreover, milk-reared animals also had substantially fewer terminal end buds at 25 days old (40 versus around 55) and as a 50-day-old adult (around 18 versus 22).

This difference in bud counts may help explain why the milk-reared rats proved relatively resistant to the impacts of a chemical carcinogen administered on day 50: DMBA (for 7,12-dimethylbenz[a]anthracene).

Over the next 18 weeks, only 46 percent of the milk-reared rodents went on to develop mammary cancers, compared to more than 63 percent of the rats that had drunk only water. Moreover, when tumors developed, they tended to emerge two weeks later in the milk-reared offspring — and the average number of tumors per animal was just half as many as in the water-quaffing rats.

One obvious suspicion: the milieu of hormones that milk brings to the drinking bottle.

Cows rev up their estrogen production during pregnancy and lactation. Studies have shown that relatively low levels of estrogens — 60 nanograms per liter of blood or less — circulate in nonpregnant cows and bovine moms through their first trimester. But from the second trimester until their calves’ delivery, estrogen levels can soar to 1,600 nanograms per liter of blood. And cows will shed large concentrations of these estrogens into their milk for quite some time.

Bottom line, Hilakivi-Clarke and her colleagues observe: “Estrogen levels in commercial milk are high.” And not surprisingly, they report, high concentrations of estrogens were measured in the blood of pups who had drunk milk — a stark contrast to what was evident in pups whose only beverage had been water.

A woman’s breast cancer risk tends to rise with her lifetime exposure to estrogens: those produced by her body, encountered as drugs or polluting her diet. If the new rodent data mimic what occurs in people — always a big if — milk-borne estrogens might be beneficial, at least during some windows of development.

Since more than 90 percent of human breast cancers originate in tissue analogous to the rats’ terminal end buds, the researchers conclude, girls who drink milk prior to puberty may face a later diminished risk “by eliminating structures that give rise to cancer.”

I come from a long line of serious milk drinkers. Heck, when I was growing up, my family would go through a couple gallons during a Thanksgiving dinner — and that with just six little kids at the dinner tables (we usually had three), meaning adults accounted for most of the consumption. It’s therefore reassuring for me to learn that the milk habit I instilled in my daughter might ultimately pay off with important benefits beyond her strong bones and teeth.