The Association Between Serum 25-hydroxy Vitamin D Level and Upper Leg Strength in Patients with Knee Osteoarthritis: Results of the Amsterdam Osteoarthritis Cohort

Objective. Vitamin D deficiency, which is common among elderly people, has been linked to muscle weakness. In patients with knee osteoarthritis (OA), the association between muscle strength and serum 25-hydroxy Vitamin D [25(OH) D] level has not been studied comprehensively. The aim of our study was to examine the association between serum 25(OH) D level and muscle strength in patients with knee OA. Methods. Data of the Amsterdam Osteoarthritis cohort from 319 participants with knee OA were used in a cross-sectional study. Serum 25(OH) D level (nmol/l) was measured by a competitive electrochemiluminescence method. Muscle strength (nm/kg) of the upper leg was measured isokinetically. Univariable and multivariable linear regression analyses were used to calculate the association between serum 25(OH) D level and muscle strength. Results. Serum 25(OH) D level was significantly associated with muscle strength (B = 0.036, 95% CI 0.017-0.054, p < 0.001), adjusted for season of blood collection. After adding body mass index (BMI) to the model, this association was no longer significant (B = 0.011, 95% CI -0.007 to 0.029, p = 0.214). Alcohol consumption, number of comorbidities, and sex were subsequently added and changed the model slightly. Without BMI, this model showed a significant association between serum 25(OH) D level and muscle strength (B = 0.029, 95% CI 0.014-0.043, p < 0.001). Conclusion. The observed association between a low serum 25(OH) D level and muscle weakness in patients with knee OA is attenuated by BMI. Further studies are needed to explain the associations among Vitamin D level, muscle strength, and adiposity in patients with knee OA.