Do you feel like you are constantly hungry? Is it possible you could be overeating? According to WebMD, the culprit could be leptin, a hormone produced by fat cells secreted into your bloodstream. Once it's in your bloodstream, it goes to your brain and acts like a thermostat, signaling to your brain that your fat cells have enough energy stored, and you therefore can stop eating.
Overweight people have more leptin in their bodies because they have more fat cells stored, but they are less sensitive to leptin's thermostat effect, which is known as leptin resistance. "In leptin resistance, your leptin is high, which means you're fat, but your brain can't see it," said Robert H. Lustig, MD, professor of pediatrics at the University of California, San Francisco. "In other words, your brain is starved, while your body is obese. And that's what obesity is: it's brain starvation."
In cases of leptin resistance, leptin is being made by fat cells and the fat cells are trying to signal to the brain that it doesn't need to eat so much, but the brain can't get the signal. You feel hungrier, so you eat more and it becomes a vicious cycle. If your brain can't see the leptin signal, you will feel hungry and eat unnecessary foods, resulting in weight gain or even obesity.
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When leptin was first discovered in 1994, it was used to treat obesity because scientists believed that if people were leptin-deficient, giving them leptin would raise leptin levels, which would signal to the brain to stop overeating. But leptin as a cure for obesity only works in extremely rare cases in which people produce no leptin at all and overeat, therefore becoming obese. Otherwise, no amount of leptin will allow you to overcome the leptin resistance.