Leptin and energy expenditure.
PURPOSE OF REVIEW: A fundamental advance in our understanding of endocrine control of energy balance and body weight came with the discovery of the adipocyte-derived hormone leptin. The leptin pathway appeared to be the long-sought peripheral signal pathway from the adipose tissue to the brain involved in the regulation of feeding and energy balance. RECENT FINDINGS: Initially, leptin was considered to function as the long-sought antiobesity hormone. According to this hypothesis, rising concentrations of leptin with increasing adiposity would generate a signal to reduce food intake and increase energy expenditure in order to limit further weight gain. However, widespread resistance to the proposed antiobesity action of leptin is observed in humans, which might reflect the fact that the inability to store energy efficiently at times of abundance is evolutionarily disadvantageous. According to this alternative view, falling leptin concentrations observed during fasting act as a peripheral signal of starvation, which serves to conserve energy in the face of limited reserves. However, leptin administration failed to blunt the changes in energy expenditure during severe energy restrictions in several clinical studies. In addition, leptin therapy in several different human low-leptin states failed to affect energy expenditure in recent studies. SUMMARY: Increasing evidence from human studies suggests that leptin predominantly influences the human energy balance through appetite but appears not to be involved in regulating energy expenditure. None of the expected factors such as resting metabolic rate, total diurnal energy expenditure or dietary induced thermogenesis was related to blood leptin concentrations.