Inclusion Criteria COPD patients:  Written informed consent. Age 60 or older. A body mass index (BMI) between 18.5-40.0 kg·m-2 and a weight ≥ 40 kg at enrolment. A diagnosis of chronic obstructive pulmonary disease (FEV1/FVC < 0.7). Smoking history of at least 10 pack years, but currently ex-smoker. Not using any inhalation steroids. A worsening of symptoms in relation to respiratory infections. Eosinophil count < 0.3 at inclusion or within 3 months.  Exclusion Criteria COPD patients:  Exacerbation of COPD or severe airway infection within the last two months. Chronic use of supplements containing vitamin B or NR. Planned surgery during the course of the trial. Dementia/cognitive impairment or symptomatic psychiatric illness. Cancer diagnosis within last 5 years. Inability to speak and read Danish. Unwillingness or inability to follow the procedures outlined in the protocol. Concurrent enrollment in another clinical study involving an investigational treatment.  Inclusion Criteria Controls:  Written informed consent. Age-, sex- and BMI-matched with COPD patients. No history of lung disease. Never-smoker.  Exclusion Criteria Controls:  Chronic use of supplements containing vitamin B or NR. Planned surgery during the course of the trial. Dementia/cognitive impairment or symptomatic psychiatric illness. Cancer diagnosis within last 5 years. Inability to speak and read Danish. Unwillingness or inability to follow the procedures outlined in the protocol. Concurrent enrollment in another clinical study involving an investigational treatment