Vitamin D supplements don't appear to ease knee pain in people with kneeosteoarthritis, a new study suggests.

According to the researchers, people with kneeosteoarthritiswho took vitamin D supplements every day for two years did not experience improvements in knee pain compared to people who took a placebo.

Knee osteoarthritis is a form of arthritis in which cartilage in the knee joint breaks down, resulting in bone rubbing on bone. It typically affects men and women over 50 years of age, and occurs most frequently in obese individuals. Symptoms include pain or stiffness in or around the knee, swelling and limited range of motion. Currently, there are no proven treatments to slow the course of the disease, the researchers said.

Vitamin D plays a role in bone health, and previous studies had suggested that knee osteoarthritis progresses more slowly in people with high vitamin D levels.

During the new study, 146 people ages 45 and older who had knee osteoarthritis were randomly assigned to receive either 2000 International Units (IUs) of vitamin D per day or a placebo. Dosages of vitamin D were periodically increased to more than 2000 IUs so that the amount of the vitamin in the blood reached a level that has been associated with benefits. (The recommended daily dose is 600 IUs for people younger than 70, and 800 IUs for people older than that.)

Every two to four months, subjects rated the pain in their knee on a scale of zero to 20, with 20 representing extreme pain. Researchers also took images of each person's knees using magnetic resonance imaging (MRI).

After two years, the two groups showed no difference in knee pain: on average, pain scores in both groups decreased by about two points.

In addition, the image results showed that people who took Vitamin D lost just as much cartilage in the knee joint as those who took a placebo.

Unlike earlier studies, the new study was a randomized, controlled clinical trial, the "gold standard" in medical research.

The study is disappointing news for people looking for a magic bullet for osteoarthritis knee pain, said Dr. Robert Graham, an internist at Lenox Hill Hospital in NY, who was not involved in the study.

However, the new study lasted just two years, and it's possible longer studies might show an effect, Grahamsaid. In addition, the effect may have been too small to be detected in a population of people already taking medication, Grahamsaid. More than half of participants in the study said they were taking nonsteroidal anti-inflammatory drugs, or NSAIDs, a type of pain medication.

The study researchers cannot say whether or not people who take vitamin D before they develop knee osteoarthritis might experience a benefit. But previous studies have found no link between a person's vitamin D levels and the risk of developing knee osteoarthritis, said study researcher Dr. Timothy McAlindon, a rheumatologist at Tufts University Medical Center in Boston.

Pain medication, as well as physical therapy, may help reduce pain from osteoarthritis, according to the Mayo Clinic. Sometimes, doctors may perform knee replacement surgery to treat the condition. Because being overweight or obese stresses the joints, losing weight may reduce the risk of developing the disease, Graham said.

The new study is published Jan. 9 in the Journal of the American Medical Association.