Low levels of vitamin D — measured as serum 25-hydroxyvitamin D, or 25(OH)D — have been associated with many health conditions, including chronic pain. A newly reported study found that, compared to Caucasians, black Americans with painful knee osteoarthritis (OA) typically display lower levels of vitamin D along with greater pain sensitivity.
Writing in an early online edition of the journal Arthritis & Rheumatism from the American College of Rheumatology (ACR), researchers at the University of Florida report that they recruited 94 patients with painful osteoarthritis of the knee for their study [Glover et al. 2012]. Three-quarters (75%) were female, 45 were black and 49 white, average age 56 years (range 45-71 years), and all were living in a sunny southern United States climate.
All subjects completed questionnaires regarding their knee OA symptoms, and underwent quantitative testing that assessed sensitivity to mechanical pressure and heat pain on an affected knee and a forearm on the same side of the body. Testing an unaffected forearm provided an overall measure of pain sensitivity in the individual. Serum 25(OH)D levels were measured via high-performance liquid chromatography.
Results indicated that a significantly greater proportion of black participants had 25(OH)D levels <30 ng/mL compared with white subjects; 84% vs 51%, respectively, P<0.001. The average vitamin D level was significantly lower in black Americans (19.9 ng/mL, indicating deficiency) than in Caucasian subjects (28.2 ng/mL, depicting insufficiency; P<0.001 between groups). Black subjects also reported significantly greater overall knee pain due to OA — P<0.05 on Western Ontario and McMaster Universities Index of Osteoarthritis, or WOMAC — and blacks with lower 25(OH)D levels displayed greater sensitivity to experimental heat and mechanical pain.
Based on their data, the researchers conclude that differences in overall pain sensitivity between the two races may be mediated at least in part by variations in vitamin D levels. Along with that, vitamin D deficiency may be a risk factor for increased pain from knee OA in black Americans.
COMMENTARY: An ongoing series of Pain-Topics UDATES articles [here] has focused on the relationship of inadequate vitamin D levels and musculoskeletal pain of various types. However, other than the fact that dark-skinned persons synthesize less vitamin D than their lighter-skinned counterparts during comparable exposure to sunlight, racial disparities regarding the association of vitamin D and pain have not been addressed in the literature.
According to background information in this report by Glover and colleagues, vitamin D insufficiency (21-29 ng/mL) and more severe levels of deficiency (<20 ng/mL) are more prevalent in black Americans [and probably in any dark-skinned individuals of African descent in other countries]. Separately, ethnic and racial group differences have been reported in both clinical and experimental pain, with blacks reporting greater pain.
An implication of the present study is that special attention should be paid to vitamin D levels in persons of African ancestry complaining of musculoskeletal pain, and supplementation provided as necessary to raise 25(OH)D above 30 ng/mL. However, there were some limitations of this research.
Participants represented a convenience sample of community-dwelling adults ranging widely in age, and the cross-sectional examination of clinical and experimental pain did not reveal the direction of the relationship between vitamin D level and chronic pain. For example, persons with debilitating knee OA may spend less time outside, thus limiting their sun exposure and reducing natural vitamin D synthesis even in the sunniest climates. Another confounding factor is this study might have been differences in the use of analgesic medications, which was not fully taken into account.
Glover et al. state, “Results were shared with participants and, if their vitamin D level was ≤30 ng/mL, they were encouraged to discuss this result with their primary care provider.” Unfortunately, the researchers did not take the extra step themselves of providing vitamin D supplementation and assessing patient response at a followup point. This could be essential because some preliminary research demonstrated disappointing results of vitamin D for knee OA [see earlier UPDATE here].
There is now a preponderance of evidence supporting a relationship between inadequate vitamin D and chronic musculoskeletal pain conditions, but it is generally of lower quality and cause-effect associations need further confirmation. This present study adds another element of complexity — possible differences due to race. As we have stressed previously, there is a need for prospective, longitudinal studies of good quality — whether observational designs or randomized controlled trials — to better understand the relationships and validate clear and convincing benefits of achieving higher levels of vitamin D.
REFERENCE: Glover TL, Goodin BR, Horgas AL, et al. Vitamin D, Race, and Experimental Pain Sensitivity in Older Adults with Knee Osteoarthritis. Arthritis & Rheumatism. 2012(Nov); online ahead of print [abstract here].
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