Energy Expenditure

Energy Expenditure

Energy expenditure refers to the total amount of energy utilized by the body over a given period, typically expressed in kilocalories per day. It is conventionally divided into three major components: basal metabolic rate (BMR), which accounts for approximately 60-70% of total energy expenditure and represents the energy required to maintain vital organ function at rest; the thermic effect of food (TEF), which constitutes roughly 10% and reflects the metabolic cost of digesting, absorbing, and processing nutrients; and physical activity energy expenditure (PAEE), which is the most variable component and includes both structured exercise and non-exercise activity thermogenesis (NEAT). Total energy expenditure can be measured using indirect calorimetry (which estimates energy use from oxygen consumption and carbon dioxide production) or doubly labeled water methodology in free-living conditions.

A fundamental challenge in obesity treatment is that weight loss triggers adaptive reductions in energy expenditure, a phenomenon sometimes called metabolic adaptation or adaptive thermogenesis. As body mass decreases, BMR declines proportionally, and hormonal changes (such as decreased leptin and thyroid hormone levels) can reduce energy expenditure beyond what would be predicted from the loss of metabolically active tissue alone. This metabolic adaptation is a key driver of weight-loss plateaus and weight regain, making therapies that can sustain or increase energy expenditure particularly valuable.

Clinical Relevance to Retatrutide

Retatrutide’s inclusion of glucagon receptor agonism is of particular interest in the context of energy expenditure. Glucagon is a well-established stimulator of hepatic energy expenditure and thermogenesis, and preclinical data suggest that glucagon receptor activation can increase whole-body energy expenditure through enhanced fatty-acid oxidation and futile cycling in the liver. This mechanism may partially counteract the adaptive reduction in energy expenditure that typically accompanies weight loss, thereby contributing to the exceptional magnitude of weight reduction observed with retatrutide in clinical trials. The combination of reduced energy intake (via GLP-1 and GIP receptor-mediated appetite suppression) and potentially increased energy expenditure (via glucagon receptor activation) represents a dual mechanism that distinguishes retatrutide from single- or dual-agonist therapies.