But the latest research indicates that the link between appetite and weight is more complicated in human beings. Studies showed that the mice were obese because of a broken gene which prevented their bodies from producing a substance that warned their brains that they were gaining weight. But when scientists injected a substance called leptin, the mice seemed to hear the alarm bells, and began to lose weight. When geneticists at Thomas Jefferson University tried to apply that research in humans, they couldn't even find the so-called OB genes in overweight people. What's more, leptin production in most obese people studied seemed to work at the same levels as in lean people.
Researchers believe that eating more food produces more body fat, which boosts production of leptin.
The leptin tells the brain to lose its appetite, so people eat less food and body fat drops. In obese people, the system works well initially. Body fat levels rise, and so do leptin levels. But that means simply giving overweight people extra leptin may not make a difference. What the leptin research has highlighted is that obesity is more complicated than was previously believed.