Although the existence of facultative DIT has been questioned (49, 52), several studies have reported that the recruitment state of BAT is increased in animals fed energy-rich diets (3, 20, 33, 48, 61, 63, 78, 85), and others have found that UCP1 deficiency increases susceptibility for the development of obesity (20, 48, 63, 89) and is associated with reduced obesity- and meal-induced increases in energy expenditure (85). Here, UCP1 is linked to obesity due to melanocortin 4 receptor deficiency.