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Weight regulation depends on the difference between weights of absorbed and metabolised carbon and excretion of the end products of metabolism, calories and carbon dioxide, which can be independently and variably excreted. Calories can be variably excreted as heat by vasoconstriction or vasodilation and carbon dioxide can be excreted variably as exhaled carbon dioxide (the major route of carbon excretion). Unless there are changes in ventilatory carbon excretion, ‘metabolism,’ ‘genetic factors,’ ‘hormones’ or ‘exercise’ do not provide complete explanatory mechanisms for weight changes, obesity and failure of diets. Low sensitivity of respiratory centres to carbon dioxide may cause overweight and dietary failures after initial weight loss.