Some overweight individuals develop problems with their breathing such that they gradually breathe less and less. This leads to a lack of oxygen and a buildup of carbon dioxide in the blood, called ventilatory failure. As a consequence, if such a person develops a chest infection, they are more likely to become seriously ill and need intensive care. In addition they are much more likely to develop severe complications during and following operations. This problem can be treated with a machine at home used overnight to help breathing. It is interesting that ventilatory failure only happens in some overweight individuals, and the investigators do not understand what factors make this complication develop. There are a number of theories: for example the distribution of the fat, additional lung disease (such as asthma), the addition of obstructive sleep apnoea, a condition when there are periods of cessation of breathing overnight (which is more common in obese individuals), weak muscles of breathing (perhaps due to fatty infiltration of muscles or vitamin D deficiency), and other hormonal changes.