NEW YORK - The more milk women drank in a new US study, the less deterioration they experienced in their arthritic knees.
It's not certain that milk was the reason for the slower arthritis progression, researchers said, and the effect was not seen in men. But the results add to evidence that nutrition may help to stave off arthritis.
"Milk is an excellent source of vitamins and minerals, dairy calcium and protein and has long been recognised for its important role in bone health," Dr. Bing Lu told Reuters Health in an email.
Lu, of Brigham and Women's Hospital in Boston, led the study, which was published in Arthritis Care and Research.
"We therefore hypothesized that milk consumption may prevent osteoarthritis progression," Lu said.
Arthritis causes pain and swelling of joints in the hand, hips or knees. According to the Centers for Disease Control and Prevention, it affects more than one third of Americans over age 65.
For the new study, Lu and colleagues analysed data from a study launched in 2002 by the National Institutes of Health and focused specifically on osteoarthritis of the knee.
A total of 2,148 participants with 3,064 arthritic knees were involved in the analysis by Lu's team. They included 888 men and 1,260 women, who were all, on average, in their early 60s at the beginning of the study.
At that point, the participants answered questions about their diets, including how often they drank any kind of milk during the previous 12 months and how much they drank.
Because the space between bones in the knee narrows with deteriorating arthritis, X-rays were used to measure changes in joint space width in the affected knees. The researchers followed up with the participants at 12, 24, 36 and 48 months after the initial examinations and surveys.
Lu's team grouped the participants based on the number of eight-ounce glasses of milk they drank per week: none, fewer than three glasses, four to six glasses, and more than seven glasses.
When they compared the changes in joint space width among the participants over time, they found the width had decreased by 0.38mm in women who drank no milk at all, by 0.29mm in those who had up to six glasses of milk a week and by just 0.26mm in those who had more than seven glasses.
There was no such link between milk consumption and joint space width changes seen among the men.
"The gender differences in the relationship of milk consumption with osteoarthritis progression are not completely understood," Lu said, adding that sex differences have been noted in the prevalence, incidence and severity of arthritis for many years.
Previous studies have shown that the thickness of cartilage at the end of the distal femur, or thigh bone, is already smaller in women compared to men, Lu noted.
Other evidence also suggests that estrogens from outside sources may protect cartilage and affect bone turnover, he said.
"If dietary calcium is a possible (factor linking) milk consumption and knee osteoarthritis progression, women may be more sensitive for the effect of calcium intake through milk than men," Lu said.
Not all dairy was associated with benefits, however. Yogurt consumption appeared to have no effect on arthritis progression, and the women who ate more cheese had greater deterioration in their joint space width.
It's possible that the high levels of saturated fat in cheese may worsen arthritis progression, Lu said, or it could contribute to obesity, which would also worsen arthritis.
It's also possible that high milk consumption is merely a sign of an overall healthy lifestyle, Lu and his colleagues acknowledge out in their report. When they adjusted for total calcium in the diet, the effect of milk weakened somewhat.
Although the exact mechanism linking milk consumption and arthritis is not yet clear, Dr. Cathy Alessi called the study an interesting and careful analysis.
"Osteoarthritis is extremely common in older people and can cause significant impairments in their quality-of-life and in their wellbeing," said Alessi, the acting director of the Geriatric Research, Education and Clinical Center in the VA Greater Los Angeles Healthcare System.
The two most common things doctors generally recommend for patients are to maintain a healthy weight and to remain as physically active as possible, she said.
But there's also growing evidence that nutritional factors beyond dieting to keep a healthy weight may affect the progression of arthritis, Alessi added.
"So this study is, I think, more evidence of a possible link between nutrition and arthritis," she said.