It was not possible to enroll the planned amount of subjects. Many patients with chronic obstructive pulmonary disease already took large doses of vitamin D.

The purpose of this study is to examine the effect of vitamin D intervention in patients with chronic obstructive pulmonary disease (COPD) undergoing rehabilitation.

Inclusion Criteria:

• Age ≥ 50 years
• Diagnosed COPD with forced expiratory volume in 1 second (FEV1) < 50 % of predicted
• Breathlessness relative to MRC dyspnoea grade 3 or more

Exclusion Criteria:

• Patients with known diseases of bone metabolism apart from osteoporosis and patients with granulomatous disease
• Patients with hyper- or hypocalcemia at inclusion
• Patients with cardiovascular or musculoskeletal disease that impacts significantly on walking based on the investigator's judgement
• Patients with dementia or other mental conditions rendering them unable to understand the study information and thus provide informed consent
• Patients with terminal disease or known cancer with bone metastases, increasing their risk of hypercalcemia
• Patients taking more than 20 mcg vitamin D on a daily basis. If a patient takes vitamin D below this limit, dose should have been stable 6 months prior to inclusion in order to achieve steady-state.
• Pregnancy