Some information provided by News Medical.
Leptin is a protein hormone with important effects in regulating body weight, metabolism and reproductive function. Leptin is expressed predominantly by fat (adipose) cells. Smaller amounts of Leptin are also secreted by cells in the epithelium of the stomach and in the placenta. Leptin receptors are concentrated in areas of the hypothalamus known to be important in regulating body weight, as well as in T lymphocytes (an immune response white blood cell) and vascular endothelial cells (where plaque deposits occur). Elevated levels of Leptin in obese patients also correlate with the following:
- Development of insulin resistance
- Elevated triglycerides
- Diabetes
- Cold intolerance
A decreased amount of Leptin correlates with depressed immune function, poor nutrition, and infertility. Thyroid dysfunction can also lead to abnormal serum Leptin levels.
Mechanism and Actions of Leptin
Leptin 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 thus vital in the regulation of reproduction.
Leptin and Body Weight
Recent studies of obese and non-obese humans demonstrated the relationship of increased serum Leptin concentrations with an increased percentage of body fat. It appears that as fat cells increase in size due to the accumulation of triglyceride, they synthesize more and more Leptin.
Leptin’s effect on body weight is mediated through its effect on the hypothalamic centers that control feeding behavior, hunger, body temperature, and energy expenditure. The hypothalamus uses the Leptin concentration to determine the need to eat and sends the appropriate hormonal signals. Patients with abnormal serum Leptin levels can also manifest diabetes, and show cold intolerance, depressed immune function and infertility. Blood concentrations of Leptin are usually increased in obese humans, suggesting that they are in some way insensitive to Leptin, rather than suffering from Leptin deficiency.
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