Vitamin D and its association with symptom severity in knee osteoarthritis: a cross sectional study at a national referral hospital in Uganda

Background Vitamin D deficiency is highly prevalent among patients with osteoarthritis. It is associated with joint pain, stiffness and worse physical function. Whether vitamin D deficiency is associated with osteoarthritis is controversial. We investigated serum vitamin D levels and its association with symptom severity in patients with knee osteoarthritis. Methods Between January 2020 to March and May 2020, we conducted a cross sectional study at a national referral hospital in Uganda. Using the American College of Rheumatology clinical criteria, 107 consenting adults were diagnosed with knee osteoarthritis. A questionnaire captured patient demographics and clinical characteristics. Joint pain, stiffness and physical function severity were assessed and graded based on the Western Ontario and McMaster Universities Arthritis Index (WOMAC). We determined serum vitamin D levels by electrochemilumniscence immunoassay. The data were analysed and adjusted for age, sex, education, occupation, family history, body mass index (BMI) and calcium supplementation. Results Of the 107 patients, 92 (86%) patients were females, mean (SD) age was 58.1 (12.6) years. Nearly 65% of the patients had suboptimal serum vitamin D levels < 30 ng/ml. The median (Q1, Q3) WOMAC joint scores were as follows: pain 8.0 (5, 11), stiffness 1 (0, 2), physical function 29.0 (16, 41) and total WOMAC 39.0 (21, 54). Spearman correlations between serum vitamin D levels with symptom severity were as follows: joint pain (r = 0.18, p = 0.06), stiffness (r = 0.13, p = 0.17), physical function (r = 0.09, p = 0.36) and total WOMAC (r = 0.13, p = 0.19). Conclusion Serum vitamin D levels are not associated with joint pain, stiffness and physical function severity. Older age and higher BMI are associated with vitamin D deficiency in patients with knee osteoarthritis attending a national referral hospital rheumatology clinic in Uganda. Suboptimal vitamin D is an independent risk factor for total mortality in the general population. Clinical guidelines and further studies to determine age and BMI ranges required for vitamin D screening are needed in patients with osteoarthritis in Uganda. Patients are advised to keep a normal BMI.