Based on previous research, "I was optimistic," says Timothy McAlindon, MD, MPH, associate professor of medicine in the division of rheumatology at Tufts New England Medical Center in Boston.
"But despite increasing vitamin D in the blood, and even though vitamin D is required for good bone health, the supplementation didn't translate into better outcomes for the people with osteoarthritis," he tells WebMD. Osteoarthritis, or OA, causes progressive damage to the joint cartilage, the cushioning material at the ends of bones.
Since vitamin D promotes the absorption of calcium and phosphorus needed for building bone, it was theorized that supplements might help people with OA, McAlindon says.
Anecdotal reports and observational studies have had conflicting results, he says.
In the new study, 146 people, mainly white women with an average age of just over 62, were assigned to take either 2,000 international units (IU) of vitamin D a day or a placebo. Over the course of the two-year study, those in the vitamin group increased their intake in 2,000 IU increments, with the goal of increasing vitamin D levels to above 30 nanograms per milliliter (ng/mL) of blood.
There's no consensus on what level is optimal, but 15 to 80 nanograms per milliliter of blood is generally considered normal, according to McAlindon.
He presented the findings of the National Institutes of Health-funded study here at the American College of Rheumatology Annual Scientific Meeting.
No Difference in Pain, Knee Cartilage Between Groups
After two years, vitamin D levels had increased by an average of 15 ng/mL in the group taking vitamin D, while the group taking the placebo only saw an increase of 1.8 ng/mL. At the start of the study, the average level in both groups was 22.3 ng/mL.
Pain scores dropped by about 2.14 points in the vitamin D group, compared with 1.2 points among patients who were taking the placebo -- a difference so small it could be due to chance. Pain was assessed using a 20-point scale in which higher scores mean worse pain. Similarly, there were no substantial differences in knee cartilage volume and thickness between those people taking vitamin D and those taking placebo, the researchers say.