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Leptin is a hormone secreted primarily by adipose tissue that functions as the body's primary long-term energy status signal, communicating fat mass levels and energy availability to the hypothalamus and central nervous system. (source) It regulates appetite, metabolic rate, reproductive function, immune activity, and the body's willingness to expend energy — making it a master regulator of physiological homeostasis.
For performance, leptin levels reflect the adequacy of energy availability relative to training demands. Chronically low leptin — seen in athletes with low body fat or those in caloric deficit — signals energy deficiency to the brain, triggering suppression of non-essential functions including thyroid output, sex hormone production, and bone turnover. (source) This is a hallmark of Relative Energy Deficiency in Sport (RED-S), a condition that significantly impairs both performance and long-term health.
