Leptin is a hormone, known as the ‘satiety hormone’, because it plays an important role in appetite and weight control. It is mostly produced in white fat deposits in the body and secreted into the bloodstream, where it travels to the hypothalamus in the brain and decreases appetite. Leptin is thought to have two major functions; by binding to receptors in the hypothalamus, it counteracts the effects of neuropeptide Y (a neurotransmitter secreted by the hypothalamus that is believed to be important in stimulating appetite); and it also increases sympathetic nervous system activity, stimulating fatty tissue to burn energy.