To the Editor Dr Jin and colleagues1 reported that the use of vitamin D did not prevent progression of knee osteoarthritis. However, the results of this study have limitations that indicate the need for further studies.
First, the follow-up period was 2 years. It is unclear whether this is long enough to demonstrate significant changes in disease progression of a slowly progressing disease such as osteoarthritis.2 The primary outcome was the Western Ontario and McMaster Universities Arthritis Index (WOMAC) pain score, which was not different between the groups at 24 months. However, there was continued improvement in the WOMAC pain score beyond 12 months in the vitamin D group, whereas the scores were unchanged in the placebo group. Although it is impossible to predict what the results would have been if the follow-up was longer, the possibility that vitamin D may be effective at a later date cannot be ruled out. Unfortunately, the authors did not provide the results for the month × treatment interaction, although it is described in their methodology. The reported P values merely indicate the difference between baseline and 24 months, whereas the P values for the interaction could have provided more information about the possibility of time-dependent effects of vitamin D.
George J. Vitamin D Supplementation and Progression of Knee Osteoarthritis. JAMA. 2016;316(3):347-348. doi:10.1001/jama.2016.6314