The investigators will aim to study women, age 65 and older, with 25(OH)D levels below 60 nmol/l in a double-blind, 4-month randomized controlled pilot study. Eligible subjects will be randomized to treatment with 4000 IU per day of vitamin D3 or placebo. Blood and 24 hour urine will be measured at the beginning and end of the study as outlined in the intervention and measurement schedule below. In addition, a safety random spot urine will be analyzed for calcium and creatinine on day 30. Muscle performance measures will be measured and muscle biopsies will be performed at the beginning and end of the study.