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In Reply Dr George highlights some limitations of our study. There were 2 primary outcomes, change in tibial cartilage volume and change in the WOMAC pain score over 24 months. For change in tibial cartilage volume, previous studies have shown that a 2-year period is sufficient to observe the progression of knee osteoarthritis assessed by quantitative measurement of cartilage volume on magnetic resonance imaging.1,2 For change in knee pain, as George indicates, continued improvement in WOMAC pain score beyond 12 months was observed in the vitamin D group but not in the placebo group. Vitamin D may have a slow-acting effect on disease progression starting after 1 year of treatment, so it is possible that vitamin D might be effective in improving knee pain if the follow-up were longer than 24 months. Therefore, we agree with George on the need for further investigation with longer-term follow-up.
Jin X, Ding C. Vitamin D Supplementation and Progression of Knee Osteoarthritis—Reply. JAMA. 2016;316(3):348–349. doi:10.1001/jama.2016.6317
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