While previous studies have associated vitamin D deficiencies with an increased risk for severity of knee osteoarthritis, the new study is thought to be the first look at vitamin levels in relation to pain and disability.
Speaking at the American College of Rheumatology annual meeting in San Antonio this week, the researchers said they measured changes in pain, physical function, muscle strength and serum levels of vitamin D twice or more times across a 15-and 30-month period in a group of 221 patients, aged 67 years old on average.
At the outset, the 48 per cent of patients with low levels of vitamin D (at or below the minimal 20 ng/ml needed to satisfy the body's requirements) experienced more pain and disability than those with levels above 20 ng/ml. Those with deficient serum vitamin D were also weaker, but this was not significant.
The study also found that changes in vitamin D status over time predicted changes in disability. Those with sufficient serum vitamin D that became deficient over time experienced worsening disability scores, while those with deficient serum vitamin D that became sufficient over time improved their disability scores.
Change in serum vitamin D levels also showed a similar pattern in changes in pain but this was not significant.
"Data suggests that many people may be vitamin D deficient, especially those living in the northern hemisphere and darker skinned individuals," said investigator Kristin Baker, from Boston University. "The good news is vitamin D levels are easily modifiable through safe, short-term exposure to sun and/or dietary intake, and may lessen the disability and pain of osteoarthritis."
Vitamin D, which comes primarily from exposure to sunlight, promotes the absorption of calcium and phosphorus in bone mineralization, growth and repair. Sources of vitamin D are available to a lesser extent from dietary sources typically found in fortified margarine, oily fish, liver, fortified breakfast cereals and dairy products. However, the elderly are less efficient at producing vitamin D from sunlight and absorbing it fromfood.
To address their higher risk for D deficiency, the elderly population is often directed to take a vitamin D supplement of 400-600 IU per day.
More than 7 million adults in the UK - 15 per cent of the population - have long-term health problems due to arthritis and related conditions, according to the Arthritis Research Campaign, and 550,000 have moderate to severe osteoarthritis in their knees.
Incidence is rising in ageing populations and those with increasing levels of obesity.