Leptin is often described as the hormone that tells the brain how much energy is stored in body fat. This review examines the factors that regulate leptin production and finds that leptin levels are influenced not only by the amount of body fat a person carries, but also by hormonal and nutritional signals. The authors conclude that insulin is one of the primary drivers of leptin production, while fasting, catecholamines, and sympathetic nervous system activity suppress it. Cortisol appears to amplify insulin's effects, helping explain why leptin levels are often elevated in obesity despite persistent hunger and weight gain.

The review also highlights that leptin responds to meals on a timescale of hours, rising several hours after eating and falling rapidly during fasting, even before meaningful changes in body fat occur. High-carbohydrate diets produced larger leptin excursions than high-fat diets, likely due to their greater effects on insulin secretion. The authors argue that chronic hyperinsulinemia may contribute to the elevated leptin levels commonly observed in obesity.

Rather than acting as a primary controller of metabolism, leptin appears to function as a downstream signal reflecting the body's metabolic state. Insulin and other hormones influence how much leptin fat cells produce, while leptin in turn communicates information about energy availability to the brain. In this view, elevated leptin is often a consequence of chronic overnutrition, enlarged fat stores, and persistent insulin signaling rather than the root cause of metabolic dysfunction itself.