It's been called the "obesity hormone" or "fat hormone" -- but also the "starvation hormone." When scientists discovered leptin in 1994, excitement arose about its potential as a blockbuster weight loss treatment. Even today, the Internet is loaded with sites that sell leptin supplements. Any truth to those pitches? And what exactly is leptin?
WebMD asked two experts on leptin to discuss how this hormone affects weight and appetite, as well as other aspects of health.
Q. What is leptin?
"Leptin is not our obesity hormone. Leptin is our starvation hormone," says Robert H. Lustig, MD, professor of pediatrics at the University of California, San Francisco and a member of the Endocrine Society's Obesity Task Force.
Leptin is a protein that's made in the fat cells, circulates in the bloodstream, and goes to the brain. "Leptin is the way your fat cells tell your brain that your energy thermostat is set right," Lustig says.
"Leptin tells your brain that you have enough energy stored in your fat cells to engage in normal, relatively expensive metabolic processes," he says. "In other words, when leptin levels are at a certain threshold -- for each person, it's probably genetically set -- when your leptin level is above that threshold, your brain senses that you have energy sufficiency, which means you can burn energy at a normal rate, eat food at a normal amount, engage in exercise at a normal rate, and you can engage in expensive processes, like puberty and pregnancy".
But when people diet, they eat less and their fat cells lose some fat, which then decreases the amount of leptin produced.
"Let's say you starve, let's say you have decreased energy intake, let's say you lose weight," Lustig says. "Now your leptin level goes below your personal leptin threshold. When it does that, your brain senses starvation. That can occur at any leptin level, depending on what your leptin threshold is."
"Your brain senses that and says, ‘Hey, I don't have the energy onboard that I used to. I am now in a starvation state,'" Lustig says.
Then several processes begin within the body to drive leptin levels back up. One includes stimulation of the vagus nerve, which runs between the brain and the abdomen.
"The vagus nerve is your energy storage nerve," Lustig says. "Now the vagus nerve is turned on, so you get hungrier. Every single thing the vagus nerve does…[is] designed to make you take up extra energy and store it in your fat. Why? To generate more leptin so that your leptin can re-establish its personal leptin threshold... It causes you to eat and it causes you to get your leptin back to where it belongs."