Leptin is a cell-signalling hormone vital in the regulation of appetite, food intake and body weight. Studies have shown that an absence of leptin in the body or leptin resistance can lead to uncontrolled feeding and weight gain.

Mechanisms and actions of leptin

Leptin acts as a hormone that modulates the size of the adipose tissues in the body. It regulates food intake and body weight. Leptin also acts on specific receptors in the hypothalamus to inhibit appetite through both counteractive and stimulatory mechanisms:

• Leptin counteracts the effects of a feeding stimulant released in the gut called neuropeptide Y as well as the effects of a cannabinoid neurotransmitter called adandamide which stimulates appetite.
• Leptin also promotes the synthesis of an appetite suppressant called α-melanocyte-stimulating hormone

When fat mass decreases, the level of plasma leptin falls so that appetite is stimulated until the fat mass is recovered. There is also a decrease in body temperature and energy expenditure is suppressed. By contrast, when fat mass increases, so do leptin levels and appetite is suppressed until weight loss occurs. In this way leptin regulates energy intake and fat stores so that weight is maintained within a relatively narrow range.

Leptin also plays an important role in regulating and modulating the onset of puberty. For example undernourished and thin women take longer to reach puberty than heavier girls. Thin girls often fail to ovulate or release an egg from an ovary during menstruation cycles. Reproductive growth and fat stores are therefore vital in the regulation of reproduction.

Researchers know that leptin plays a role at least to some degree in the onset of puberty as individuals with an absence of leptin or a lack of response to leptin do not experience puberty, with their bodies remaining pre-pubescent for life.