Vitamin D supplementation and the maintenance of sufficient vitamin D levels may improve foot pain in patients with symptomatic knee osteoarthritis (OA), according to study results published in Arthritis Care & Research.

The objective of the post-hoc analysis from a randomized double-blind placebo-controlled trial (Vitamin D Effect on Osteoarthritis Study; ClinicalTrials.gov Identifier: NCT01176344) was to assess the potential benefits of vitamin D supplementation and the maintenance of sufficient vitamin D levels for foot pain in patients with symptomatic knee OA.

Researchers enrolled 413 patients (mean age, 63.2 years; 49.7% women) with symptomatic knee OA and serum 25-hydroxyvitamin D levels between 12.5 and 60 nmol/L. A total of 340 patients completed the study. The study participants were randomly assigned to monthly vitamin D3 treatment or placebo, over 2 years. Researchers used the Manchester Foot Pain and Disability Index (MFPDI) to assess foot pain.

At baseline, the mean MFPDI score was 22.8±7.3; 214 patients reported foot pain, including 98 (23.7%) who had disabling foot pain according to the MFPDI case definition. In the group of patients who received vitamin D, MFPDI scores did not significantly change over the study period (-0.03; 95% CI, -0.80 to 0.74), but these scores worsened in the placebo group (1.30; 95% CI, 0.51-2.09; P =.013). Furthermore, MFPDI scores decreased in patients who maintained sufficient vitamin D levels and increased in those who were not maintaining sufficient vitamin D levels over 2 years (-0.09 [95% CI, -0.79 to 0.61] vs 2.19 [95% CI, 1.21-3.18], respectively; P =.001). The decrease in MFPDI scores was greater among patients with disabling foot pain at baseline who maintained vs did not maintain sufficient vitamin D levels (-4.63 [95% CI, -6.35 to -2.92] vs -0.14 [95% CI, -2.75 to 2.48], respectively; P =.005).


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Among participants who were men, the MFPDI scores improved significantly with vitamin D supplements compared with placebo. However, there was no significant interaction between sex and vitamin D supplementation or maintaining sufficient vitamin D levels in terms of change in MFPDI scores.

The study had several limitations, including those secondary to a post-hoc analysis, lack of data on the clinical importance of the differences in MFPDI scores, and potential underestimation of the benefits of vitamin D, as >60% of patients in placebo group had sufficient vitamin D levels at the end of follow-up.

“Vitamin D supplementation and maintaining sufficient vitamin D level may improve foot pain in patients [with] knee OA,” concluded the researchers.

Reference

Tu L, Zheng S, Cicuttini F, et al. Effects of vitamin D supplementation on disabling foot pain in patients with symptomatic knee osteoarthritis [published online Jul 5, 2020]. Arthritis Care Res (Hoboken). doi:10.1002/acr.24371