Friedman and colleagues discovered leptin (from the Greek word leptos, meaning thin) in 1994 and ushered in an explosion of research and a great increase in knowledge about the regulation of the human feeding and satiation cycle. Leptin is a 16-kD protein produced predominantly in white subcutaneous adipose tissue and, to a lesser extent, in the placenta, skeletal muscle, and stomach fundus in rats. Leptin has myriad functions in carbohydrate, bone, and reproductive metabolism that are still being unraveled, but its role in body-weight regulation is the main reason it came to prominence.

Since this discovery, neuromodulation of satiety and hunger with feeding has been found to be far more complex than the old, simplistic model of the ventromedial hypothalamic nucleus and limbic centers of satiety and the feeding centers of the lateral hypothalamus. Potentially, leptin sensitizers may assist in changing feeding habits.

The major role of leptin in body-weight regulation is to signal satiety to the hypothalamus and thus reduce dietary intake and fat storage while modulating energy expenditure and carbohydrate metabolism, preventing further weight gain. Unlike the Ob/Ob, mouse model in which this peptide was first characterized, most humans who are obese are not leptin deficient but are instead leptin resistant. Therefore, they have elevated levels of circulating leptin. Leptin levels are higher in women than in men and are strongly correlated with BMI.