Leptin's primary role may be signaling to the brain about insufficient energy reserves during fasting or pregnancy.

“the other view which was taken by Jeff flyer who was at Harvard at the time later became Dean he's still at Harvard Crimson Chua who was working with on this project or had worked on it and myself was that the protein was actually more important in its deficiency state as a signal to the brain that you didn't have enough energy to survive under circumstances of a fast or to have enough energy onboard to successfully complete a pregnancy”

“so that's why we were wondering right from the beginning why would nature invent something that would right it makes more sense that nature would say here's a hormone that if it's low is a kick in the pants to eat”