Leptin is a hormone we've just recently started hearing about. Dr. Robert Lustig explains that leptin is a protein made in your body's fat cells. These fat cells send leptin through your bloodstream to your brain.
As long as the level of leptin is above your personal leptin threshold, the message to your brain is that your body's fat cells contain enough energy to do things. It tells your brain it's safe to go ahead.
Dieting means less fat in your body's fat cells, and it also means a lower level of leptin. When you go below your personal leptin threshold, your brain thinks you're starving. The body has ways of handling this apparent crisis, initiating a number of processes designed to keep you from starving, and raise the level of leptin back up above your personal leptin threshold. Unfortunately these processes can also sabotage your diet.
"Rather than taking supplements that haven’t been fully proven to help, overweight people have other options to aid leptin functioning, experts say. Lustig advises them to reduce resistance to insulin (a hormone that controls blood sugar) and to bring down high levels of triglycerides (a blood lipid)."
http://www.webmd.com/diet/features/the-facts-on-leptin-faq?ecd=wnl_day_060310