Leptin Part II: How Does this Hormone Affect Weight?

In “Leptin Part I,” we saw that leptin works like a thermostat -an adipostat - that tells our brain that we have sufficient energy stored in our fat cells. However, the questions is, if leptin works like a thermostat, why do we keep gaining weight? The problem is that overweight people have large amounts of leptin, but their brains aren't getting the important signal to stop eating.

"How come the brain doesn't get it? That phenomenon is called ‘leptin resistance,'" says Dr. Lustig,M.D., professor of pediatrics at the University of California., who has done research on the subject. Leptin resistance is similar to insulin resistance in type 2 diabetes, in which the pancreas produces large amounts of insulin, but the body doesn't respond to it properly. Leptin levels can keep going higher as people get fatter. "We all have a leptin floor; the problem is, we don't have a leptin ceiling," Lustig says.

But in leptin-resistant people, the reward system doesn't cue a person to stop eating when leptin levels rise, Lustig says. "The leptin is being made by the fat cells, the fat cells are trying to tell the brain, ‘Hey, I don't need to eat so much,' but the brain can't get the signal. You feel hungrier and the reward doesn't get extinguished. It only gets fostered, and so you eat more and you keep going and it becomes a vicious cycle. If your brain can't see the leptin signal, you're going to get obese." Rather than taking supplements that haven’t been fully proven to help, overweight people have other options to aid leptin functioning, experts say. One option is to reduce resistance to insulin (a hormone that controls blood sugar) and to bring down high levels of triglycerides (a blood lipid).Reducing high triglyceride levels helps, too, Lustig says. Too much triglyceride interferes with leptin’s journey from the blood to the brain via a leptin transporter that allows the hormone into the brain. “When you’re insulin-resistant, you have high triglyceride [levels]. That’s one of the hallmarks,” Lustig says. “Triglyceride seems to block leptin transport into the brain. In order to make your leptin work, you have to let the signaling occur. The only way to let the signaling occur is to get your triglyceride down.”