Leptin?

Leptin has been around for a while (discovered in 1994) and it's basically known as the "starvation hormone." leptin takes fat to the brain and let's the brain know that you have enough fats stored in your system to engage in normal, day-to-day activities.

The hope for leptin, when it was discovered, was that it could lead to weight loss by tricking the brain into thinking that it had enough fats stored in your body, even if your food intake was lower than what the body wants. A high level of leptin also decreases your appetite since your brain already thinks that it has enough nutrients to carry on its normal activities. It's also what drives your brain to crave fats/food when you are hungry, as your leptin levels are low

The problem with leptin is that high level of leptin overtime leads your brain to develop a leptin resistance, similar to type 2 diabetes where your body produces large amounts of insulin, but your body doesn't accept it. There is no "ceiling" for leptin and as your brain develops resistance to it, you continue to crave food even if your body is sending up sufficient level of leptin to your brain. Leptin doses are really only helpful if your body naturally is deficient in leptin or cannot produce leptin at all, but these are very rare cases.

I believe most leptin supplements don't actually even contain leptin, at least not in the form your body can process. However, some of them do contain other ingredients that may be beneficial to your body in other ways.

It's a hormone in your body. http://en.wikipedia.org/wiki/Leptin