Metabolic adaptation to weight changes relates to body weight control, obesity and malnutrition. Adaptive thermogenesis (AT) refers to changes in resting and non-resting energy expenditure (REE and nREE) which are independent from changes in fat-free mass (FFM) and FFM composition. AT differs in response to changes in energy balance. With negative energy balance, AT is directed towards energy sparing. It relates to a reset of biological defence of body weight and mainly refers to REE. After weight loss, AT of nREE adds to weight maintenance. During overfeeding, energy dissipation is explained by AT of the nREE component only. As to body weight regulation during weight loss, AT relates to two different set points with a settling between them. During early weight loss, the first set is related to depleted glycogen stores associated with the fall in insulin secretion where AT adds to meet brain's energy needs. During maintenance of reduced weight, the second set is related to low leptin levels keeping energy expenditure low to prevent triglyceride stores getting too low which is a risk for some basic biological functions (e.g., reproduction). Innovative topics of AT in humans are on its definition and assessment, its dynamics related to weight loss and its constitutional and neuro-endocrine determinants.
Keywords: Activity-related energy expenditure (AEE); Adaptive thermogenesis (AT); Body composition; Caloric restriction; Diet-induced thermogenesis (DIT); Energy balance; Energy expenditure; Exercise activity thermogenesis (EAT); Fat mass; Fat-free mass (FFM); Insulin; Leptin; Metabolic efficiency; Non-exercise activity thermogenesis (NEAT); Obesity; Overfeeding; Resting energy expenditure (REE); SNS activity; Thyroid hormones; Total body water (TBW); Total energy expenditure (TEE); Weight loss; Weight maintenance.