Definition of Leptin

Leptin: A hormone that has a central role in fat metabolism. Leptin was originally thought to be a signal to lose weight but it may, instead, be a signal to the brain that there is fat on the body.

Hugely obese children have been found who have an inability to lose weight due to a genetic inability to produce leptin. Injections of leptin have had dramatic weight loss effects. It appears that, if one cannot make leptin, the brain thinks there is no fat and spurs the person on to eat more and more in a futile effort to get the leptin signal that fat is present. However, the effect of leptin on people who are obese but can make leptin normally appears to be different.

A study of leptin published in the Journal of the American Medical Association (JAMA) in 1999 returned somewhat disappointing results. It involved 54 people of normal weight and 73 who were obese. All injected themselves daily with leptin or, as a control, with saline (saltwater) for a month. Then the study continued with the obese subjects to assess leptin's ability to induce weight loss. The obese patients were given a weight-loss diet. There was little effect of leptin except at the highest dose tested, when 2 of 8 patients lost about 35 pounds in 24 weeks. The others lost much less and one gained weight. The 8 people on the highest dose of leptin were the last of a group of 18 patients who had been assigned to take it; the rest dropped out after a month of taking leptin because they could not stand to inject themselves with several tablespoons a day of a substance that irritated their skin and was painful to administer. The future of leptin in weight regulation remains to be determined.