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The facts on leptin: FAQ

Hormone leptin and obesity

Leptin has sometimes been referred to as the "obesity hormone" or "fat hormone", but also the "starvation hormone". Scientists discovered leptin in 1994 and it was quickly touted as a weight loss treatment. To this day leptin supplements are marketed on the internet and elsewhere, but what exactly is leptin?

What is leptin?

Research on leptin describes it as a hormone that plays a "pivotal role" in regulating food intake and energy output.

Some researchers suggest leptin is our "starvation hormone" - rather than our "obesity hormone".

Leptin is a protein that's made in the fat cells, circulates in the bloodstream and goes to the brain. Research suggests it may be the way that fat cells tell the brain that the body’s energy thermostat is set correctly and that enough energy is stored in fat cells to engage in normal metabolic processes.

Researchers think that when leptin levels are at a certain threshold - probably genetically set - the brain senses that the body is energy sufficient. That 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 also engage in expensive energy processes, such as puberty and pregnancy.

However, when people diet they eat less and their fat cells lose some fat, which then decreases the amount of leptin produced. This may lower levels below your personal threshold, leading your brain to sense that you are in a state of starvation.

Starvation triggers several processes within the body that drive leptin levels back up. One includes stimulation of the vagus nerve - regarded as your energy storage nerve - which runs between the brain and the abdomen.

Research suggests when the vagus nerve is turned on you get hungrier. The vagus nerve appears to be designed to make you take up extra energy and store it in your fat. This generates more leptin in order to re-establish your personal leptin threshold. In other words, it causes you to eat and get your leptin back to your personal level.

How does leptin affect weight?

If leptin works like a thermostat - an adipostat - 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.

Researchers suggest this phenomenon is called "leptin resistance". This may be similar to insulin resistance in type 2 diabetes, when the pancreas produces insulin but the body doesn't respond to it properly.

So as people get fatter, leptin levels keep going higher. In other words, we all have a leptin floor but are missing a leptin ceiling.

Leptin resistance occurs when your leptin is high and you are overweight, but your brain cannot see it. In other words, your brain is starved while your body is obese. Some researchers have referred to this as brain starvation.

Not only does leptin appear to be part of the hunger system, it may also be part of the reward system. When your leptin levels are low, food appears even more rewarding. When your leptin levels are high, it may extinguish the reward system, so you don't eat as much and food looks less appealing.

Researchers argue that in leptin-resistant people the reward system fails to cue a person to stop eating when leptin levels rise. When the brain fails to get the signal to stop eating, people feel hungrier and eat more. Studies suggest this creates an ongoing cycle, leading to obesity.

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