By  Elaine Lies
Reuters

Taking daily vitamin D doesn't keep knee pain from getting worse or slow the loss of cartilage for people with osteoarthritis, according to a U.S. study. 

Previous research suggested that among people with the joint disorder, those with higher levels of vitamin D in their blood tended to have a slower progression of symptoms. But whether that meant taking more in supplement form would also have a protective effect was unclear.

"It looked compelling at that point," said lead author Timothy McAlindon, from Tufts Medical Center in Boston.

For the new study, published in the Journal of the American Medical Association, he and his colleagues randomly assigned 156 of their patients with knee osteoarthritis to take a daily dose of vitamin D or a vitamin-free placebo for two years.

None of the participants knew which type of supplement they were assigned to take. The vitamin D doses started at 2,000 international units (IU) per day and were increased to as high as 8,000 IU daily in some patients. For most adults, the recommended daily allowance of vitamin D is 600 to 800 IU.

The vitamin D group started out slightly worse off than their comparisons on measures of knee pain and function, but the vitamin didn't seem to offer clear relief.

On a 0-to-20 point pain scale, people taking vitamin D saw a 2.3-point decrease during the two years, compared to a 1.5-point decrease among those taking placebos - a difference that could have been due to chance.

Changes in knee cartilage volume - a measure of the progression of osteoarthritis - and knee function were also similar among the two groups during and after the study period.

Robert Heaney, who has studied vitamin D at Creighton University School of Medicine in Omaha, Nebraska, said he wasn't surprised the study didn't find a beneficial effect on knee pain across all patients.

"It's almost certain that vitamin D's effects are different from person to person," said Heaney, who wasn't involved in the new research, to Reuters Health. "It's very important for some people, but may not make any difference for others."

That may have to do with genetics or other factors that doctors aren't yet able to test for before the prescribe vitamin D, Heaney said, though he noted that it's still worth trying since it may have other small health benefits.

McAlindon, however, disagreed, saying that while it's possible the higher levels of vitamin D in the blood could help knee pain, so far the results don't support that idea.

"Vitamin D broadly is the vitamin of the moment," McAlindon said. "There are hopes that it will have wide health benefits."

But he noted that this study shows each of those claims needs to be checked out carefully.