Effect of Supplemental Vitamin D on Skeletal Muscle Function in Chronic Obstructive Pulmonary Disease (COPD) Patients

Inclusion Criteria: * COPD * Forced expiratory volume in one second (FEV1) \< or = 50% of predicted * Smoking history of at least 10 pack-years * Able to ambulate independently or with the use of an ambulatory assist device (such as a cane or walker) Exclusion Criteria: * Currently taking \> 500 I.U. per day of vitamin D supplements * Primary diagnosis of asthma * Uncompensated heart failure * Heart attack in the previous 6 months * Kidney disease (glomerular filtration rate \< 45 mL/min/1.73m2)