Vitamin D and COPD, conceptual image. Computer illustration of molecules of vitamin D (black and white) with healthy (pink) and diseased (grey) lungs, representing the use of vitamin D in patients with COPD (chronic obstructive pulmonary disease). COPD is a form of obstructive lung disease that causes difficulty breathing. It is most commonly caused by smoking. Vitamin D deficiency occurs frequently in COPD. Some studies have shown a strong relationship between vitamin D levels and forced air capacity. It is thought, therefore, that supplementary vitamin D may help protect against COPD flare-ups by protecting against infections, which can trigger COPD attacks.