Observational evidence suggests that vitamin D deficiency may have a role in the progression of OA and there are biologically plausible mechanisms to explain this. However, randomized controlled trials using a sensitive method are required to determine whether intervening with vitamin D supplementation can in fact slow the progression of this disease. In this study, the randomized, placebo-controlled, double-blind design, and the use of MRI to provide sensitive and precise measures of knee structural change will ensure a rigorous evaluation of the impact of vitamin D supplementation on knee OA. It will be the first long term clinical trial to determine comprehensively the effects on knee structural changes (cartilage, bone) utilizing the pioneering MRI techniques and limb muscle strength assessment. This study builds upon previous clinical and epidemiological studies that supports the objectives of the Bone and Joint Decade organization and addresses a National Health Priority Area.