Leptin

Leptin is a hormone produced by fat cells that signals fullness to the brain and helps regulate appetite and energy expenditure. It is measured to assess metabolic function and the body's energy balance, particularly in weight management and metabolic disorders.

When elevated: Elevated leptin often occurs with obesity and excess body fat, as leptin levels correlate with total fat mass. High leptin can indicate leptin resistance, where the body does not respond properly to the hormone's appetite-suppressing signals, leading to continued overeating despite adequate energy stores. When low: Low leptin is uncommon in otherwise healthy adults but can occur with severe caloric restriction, malnutrition, or very low body fat. It may also be seen in certain genetic conditions and is associated with increased hunger and metabolic slowdown.

Leptin reflects energy availability and metabolic stress—relevant when training intensity or dietary restriction limits caloric intake relative to expenditure. Low leptin can signal inadequate energy for recovery, immune function, and hormonal balance; tracking it helps athletes avoid overtraining syndrome and optimize fueling strategies for performance and long-term health.