Observational evidence suggests that vitamin D deficiency may have a role in the causes of osteoarthritis (OA) and there are biologically plausible mechanisms to explain this. There is, however, no evidence which shows that intervening with vitamin D supplementation can slow the progression of OA. This study is to determine if vitamin D supplementation can reduce knee pain and slow knee cartilage loss in OA patients comparing with a placebo. Use of MRI will provide sensitive measures of knee OA changes.