Heavy milk drinking may double women’s mortality rates

Despite delivering calcium and protein, drinking a lot of milk doesn’t seem to provide a net health benefit for women and may even hinder their long-term survival prospects, Swedish researchers find. Over the course of about 20 years, women who drank three or more glasses of milk per day were almost twice as likely to die as those who drank less than one, other things being equal. Intake of cheese, yogurt or buttermilk might offer a better approach to dairy, the data suggest.

The study raises hard questions about the nutritive value of milk, says epidemiologist C. Mary Schooling of the City University of New York and Hunter College in New York City. “There’s something about milk and purity that got all tangled up. It’s almost a cultural belief.” The widespread assumption that milk is inherently good has arisen despite a lack of randomized trials capable of parsing the true health value of the drink, she says. The authors of the new study deserve credit, she says, “because they were thinking about the biology” of milk consumption.

Specifically, the researchers augmented their mortality findings with evidence suggesting a sugar called D-galactose underlies milk’s downsides. Chronic exposure to the sugar triggers chronic inflammation, oxidative stress and rapid aging in mice, the researchers note. Study coauthor Karl Michaëlsson, a physician and researcher at Uppsala University, points out that milk delivers plenty of lactose, which breaks down into D-galactose.  Cheese and other fermented milk products, though, contain much less of both sugars.

Michaëlsson and colleagues analyzed roughly 20 years of health data from more than 60,000 adult women. The participants had provided information on their diet and other lifestyle factors through questionnaires at the outset of the study and again several years later. The heaviest imbibers were 1.93 times as likely to have died during the study as women who averaged less than a glass a day, the researchers report October 28 in BMJ.

Compared with those drinking little or no milk, the women who drank the most were also slightly more likely to die of cancer or to break a hip. But their overall risk of any fracture was barely higher than the non-milk drinkers’ risk.

The researchers accounted for a host of factors in their analysis, including age, education, diet, smoking, alcohol intake and supplementation with calcium or vitamin D.

In a similar assessment of more than 45,000 adult men in Sweden followed for 11 years, the researchers found little effect from drinking three or more daily glasses of milk. Those who drank that much were 10 percent more apt to die during the study than those who drank little milk, but they weren’t more prone to fractures or lethal cancer.

Michaëlsson says the difference in results between men and women is curious. It might reflect weaker data due to the fact that there were fewer men, they weren’t followed as long and they filled out only one questionnaire, he says. 

The scientists also assessed oxidative stress in the body by analyzing urine samples from hundreds of the men and women in the study. People who consumed lots of milk had higher concentrations of a telltale oxidative stress marker. Men who drank plenty of milk also averaged higher levels of interleukin-6, an inflammatory marker, a blood analysis showed. Inflammation and oxidative stress are linked to many ailments.

High consumption of yogurt and buttermilk produced the opposite effect — less inflammation and oxidative stress. One possibility, Michaëlsson surmises, is that fermented milk products “might render a better bacterial profile in the intestines.”

That’s possible, says Schooling. Even so, she says, the ability to digest milk is a trait that has been selected for evolutionarily as some groups gained tolerance to lactose. So milk must have value, she says. “But there are people in other parts of the world who very rarely drink milk, and they are fine.”

While Michaëlsson hesitates to make broad recommendations based on the findings, he has taken them to heart. “I like milk. In Sweden, we like milk,” he says. “But I’ve changed my diet to yogurt because of these studies.”