The greater degree of pain reported by blacks with knee osteoarthritis may relate to their low levels of vitamin D, a cross-sectional study suggested.

Black patients, whose vitamin D levels averaged 19.9 ng/mL, experienced lower thresholds on sensory testing for heat pain in the osteoarthritic knee (β = 0.23, P=0.03) compared with whites, whose level of the vitamin was 28.2 ng/mL, according to Roger B. Fillingim, PhD, of the University of Florida in Gainesville, and colleagues.

In a statistical analysis exploring the indirect and direct effects between race, experimental pain, and vitamin D levels, the low level of vitamin D was a significant mediator of the difference between whites and blacks on the threshold for heat pain (95% CI 0.01 to 1.11), the researchers reported online in Arthritis & Rheumatism.

Considerable interest has arisen during the past decade in the hormonal actions of vitamin D, and the association of deficiency of the vitamin with various diseases such as cancer and diabetes, as well as with chronic pain from conditions such as osteoarthritis.

“Research suggests that the brain reorganizes in the presence of chronic pain, which may reflect fundamental changes in how the brain processes pain-related information,” the researchers wrote.

It also has become clear that much of the population has inadequate circulating serum levels of 25(OH) vitamin D, particularly during the winter months and at northern latitudes.

About 70% of white residents of the United States are thought to have inadequate levels of circulating vitamin D, whereas the number is estimated to be 95% for black Americans, who require more sun exposure to synthesize the vitamin.

Fillingim and colleagues had previously noted ethnic disparities in pain on sensory testing and sought to determine if low levels of the vitamin could be implicated in this difference.

Accordingly, they enrolled 94 patients with symptomatic knee osteoarthritis, testing them for thermal and pressure sensitivity thresholds and their ability to tolerate increasing levels of pain on the arthritic knee.

They also tested sensitivity and tolerance at the ipsilateral, nonpainful wrist.

Among the 45 black patients, 31 were women, while among the 49 whites, 39 were women.

Mean age was 55, and most were overweight or obese.

On the Western Ontario and McMaster Universities Index of Osteoarthritis (WOMAC), which rates pain, physical function, and stiffness, the mean score among blacks was 41.3, compared with 29.4 in whites.

In an analysis simply assessing racial differences in pain, blacks reported having significantly lower thresholds in the affected knee for heat pain (P=0.02) and less tolerance for increasing heat pain (P

<0.001).

In addition, they had lower heat pain thresholds and tolerance in the unaffected wrist.

Similar results were seen for mechanical pain, with blacks having lower thresholds for pressure in the knee (P=0.002) and wrist (P=0.05).

Then, when the analysis also included level of vitamin D, blacks again had greater sensitivity in the knee for pressure (β = 0.23, P=0.02).

Finally, in the mediator analysis, vitamin D level was found to explain the differences between whites and blacks on threshold for pressure pain at the knee (95% CI 3.43 to 60.86) and at the unaffected wrist (95% CI 12.44 to 67.55).

However, an exception was in the relation between vitamin D level and tolerance for heat pain, which was not significant for either the knee (β = 0.10, P=0.37) or wrist (β = 0.13, P=0.22).

"This outcome suggests that vitamin D level may be related to pain pathways involved in initial perception of pain, but not how much pain an individual can tolerate," the researchers observed.

"Pain tolerance is dependent, in part, on individuals' willingness to endure noxious stimulation, which is not necessarily related to vitamin D levels," they explained.

While vitamin D levels were found to mediate the differences between blacks and whites in the experimentally induced heat and pressure pain, no such relation was seen with clinical WOMAC scores.

This could reflect recall bias when patients were asked to rate their pain over the previous 2 days, according to the researchers.

"Alternatively, vitamin D may influence certain aspects of pain processing reflected by pain thresholds, while osteoarthritis-related symptoms assessed by the WOMAC are likely driven by multiple factors over and above nociceptive processes," they suggested.

If further research confirms these findings, treating vitamin D insufficiency could be a safe and inexpensive way of easing the chronic pain of osteoarthritis, and help eliminate the health disparities experienced by many U.S. blacks, the researchers concluded.

Limitations of the study included its cross-sectional design, and a lack of information on factors such as how much time participants spent outdoors and whether they used non-opioid analgesics.

The study was supported by the National Institutes of Health and the University of Florida Clinical and Translational Science Institute.

The authors reported no financial conflicts.

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Primary source: Arthritis & Rheumatism
Source reference: Goodin B, et al “Vitamin D, race, and experimental pain sensitivity in older adults with knee osteoarthritis” Arthritis Rheum 2012; DOI: 10.1002/art.37687.