Milk is often touted as an elixir for bone strength, but new research suggests that those superpowers may be true only to a certain extent. 

A study published in the Oct. 28 issue of The British Medical Journal suggests that consuming three glasses of milk per day may double women’s risk of dying in 20 years, compared to drinking less than one glass daily, Medical News Today reported.

Researchers in Sweden found that the sugar D-galactose— which comprises half of lactose— increases oxidative stress and inflammation of the body. Experimental evidence in various animal species shows that chronic exposure to galactose can expedite aging and decrease lifespan.

The U.S. Department of Agriculture (USDA) recommends three daily cups of milk for maintaining healthy blood pressure, reducing the risk of cardiovascular disease, protecting from colorectal cancer, and boosting high-quality protein for muscle mass.

“Our results may question the validity of recommendations to consume high amounts of milk to prevent fragility fractures,” the researchers write.

The research focused only on the association of fractures and overall mortality with milk consumption, not on links to cardiovascular disease. There is no definitive evidence that indicates whether milk is good or bad for heart health, but a 2005 study published in BMJ links milk consumption with a decreased risk of heart disease and stroke.

According to the U.S. Centers for Disease Control and Prevention (CDC), calcium-rich low-fat and nonfat milk can reduce the risk of osteoporosis in women, who are more at risk for the disease.

To learn whether the bone health effects of galactose in animals matched that of humans, researchers studied two groups— one comprised of men, the other of women— to analyze whether milk was linked to higher mortality and fracture rates. They questioned more than 61,000 women ages 39 to 74 from 1987 to 1990, and more than 45,000 men ages 45 to 79 in 1997.

In a questionnaire, participants reported their average consumption of up to 96 common foods and beverages, including dairy products. Researchers took the participants’ lifestyles, weight, height, education levels and marital statuses into account, and they analyzed their fracture and mortality rates through national registers. After the initial survey, study authors monitored the women for 20 years and the men for 11 years.

Researchers noted that drinking three glasses of milk was linked to a 50 percent increased risk of hip fracture and a higher risk of mortality among women. They saw a similar trend among men, but the effects were less pronounced than those among the women.

The study authors pointed out that the results should be interpreted cautiously due to the observational design of the study. The study shows only association rather than cause and effect and  further studies are needed before dietary recommendations are altered.

“As milk features in many dietary guidelines and both hip fractures and cardiovascular disease are relatively common among older people, improving the evidence base for dietary recommendations could have substantial benefits for everyone,” Mary Schooling, a professor at City University of New York, wrote in an accompanying editorial.

“As milk consumption may rise globally with economic development and increasing consumption of animal source foods, the role of milk in mortality needs to be established definitively now,” Schooling said.

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