I know it can be confusing since there is only a one-letter difference between these two words. It’s worth paying attention to what they mean or represent, however. Leptin is an extremely vital and valuable hormone. When you hear the word Leptin, think “thin.” Really. Because it comes from a Greek word meaning thin. Made by fat cells, this substance—known as the “satiety hormone” is designed to put on the hunger brakes. Leptin is opposite—no surprise—from grehlin, its opposing hormone known as the “hunger hormone.” Both hormones can bind to receptors in the hypothalamus to regulate appetite: Grehlin pushes you to eat; leptin says that you have enough energy and need no more food. More tomorrow.