Women with knee osteoarthritis (OA) who drank milk regularly had less progression of their OA – as measured by declines in joint space width – according to a study published recently in Arthritis Care & Research.

Experts have long known that consuming milk and other dairy foods is good for the bones, but its effects on joints haven’t been well studied.

For this study, researchers looked at more than 2,000 people with knee osteoarthritis using data from the Osteoarthritis Initiative, launched in 2002 by the National Institutes of Health. The people surveyed as part of that initiative were asked to recall, among other diet details, how often they drank milk in the past year, and how much they drank. The researchers analyzed this information along with X-rays taken over the next two years that showed how much the participants’ joint space width – one measure used to chart OA progression – decreased over time. (Less space between the bones that make up a joint indicates less cartilage and worse OA.)

Milk consumption was categorized in four groups: none, three or fewer glasses per week, four to six glasses per week, and seven or more glasses per week. At the end of two years, the average decrease in joint space width was 0.38 millimeters, 0.29 millimeters, 0.29 millimeters, and 0.26 millimeters, in the different milk-consumption groups respectively – a difference of 0.12 millimeters between the people who drank the least and the most milk. (The average decrease in joint space a person with knee OA experiences in a year is estimated to be between 0.1 and 0.15 millimeters.) Only in women was milk consumption associated with a smaller decrease in joint space.

High cheese consumption appeared to make osteoarthritis worse. The researchers speculate that the high fat content of cheese might be responsible. Yogurt consumption was not associated with any changes to the joint space width.

“This is the first large cohort study to show this association,” says Bing Lu, MD, assistant professor of medicine at Brigham and Women’s Hospital and the study’s lead author.

Why did milk help? “The reason is not clear,” says Dr. Lu.

In fact, because of the way the study was designed, it’s impossible to tell for sure whether the milk consumption itself was responsible for slowing the joint space narrowing. There could be something about the people who drank milk – other than that dietary habit – that made their OA progress more slowly, says Richard F. Loeser, Jr., MD, director of Basic and Translational Research at the University of North Carolina’s Thurston Arthritis Research Center in Chapel Hill. The researchers did adjust for factors such as weight, smoking and physical activity to make sure these weren’t the cause.

It’s also possible that one of the nutrients in milk, or a combination of nutrients, has an effect on OA. “Knee OA progression has been thought to involve multiple mechanisms besides cartilage loss, including changes in bone composition and shape, … which might be subject to the influences of macro- and micronutrients in the diet,” the authors write. But no one knows which nutrients, if any, help.