In mice, defects in the ob gene leads to mice with body weights as much as three times normal. The ob gene codes for a protein called leptin, which is normally synthesized in adipocytes and acts as a hormone, binding to a specific site in the brain. Obese mice lacking leptin eat as if perpetually starved. Conversely, injecting leptin into these mice causes them to lose weight dramatically. Obese humans, on the other hand, appear to contain high levels of leptin.

Serotonin controls the feeling of fullness after eating. The anti-obesity drug fenfluramine was originally prescribed because it increases serotonin levels, thus curing appetite. It turned out, however, that fenfluramine also did but serious damage to the heart and has since been withdrawn from the market. Despite this setback, biochemical research on obesity is one of the most active current research frontiers.