For Andrew Jenkinson, these stories, along with his experiences as a surgeon, became pieces in a jigsaw, slowly helping to explain why one person might put weight on more easily than another. And at the center of the picture is leptin, a hormone which helps to regulate our weight. The more fat we carry, the more leptin is in our bloodstream, which then sends signals to the brain, helping it understand how much energy we're storing. The problem comes

when leptin stops working, the fact that someone becomes obese or puts on too much weight, they have a high leptin signal, but for some reason the brain is not being able to read it. You imagine you're riding along in your car and you see that your petrol meter is flashing red, it's like empty. Okay, I better go to the petrol station as fast as possible. When you open the petrol cap and start filling up, actually you realize that the petrol tank's already full, the problem is the petrol tank meter is broken. And this is what leptin resistance is. The leptin level is high in someone who's obese, the gas tank is full, they have plenty of energy on board, but the brain is still giving them signals because it perceives the energy message from leptin is missing. The symptoms of that are quite a ravenous appetite and malaise, you know, feeling tired. People who don't understand obesity will say, well, actually you're obese because you eat too much and you're too lazy. You have a character flaw. I also maybe you don't have enough self control, but actually the heightened appetite and the low metabolism and feeling of malaise are symptoms of obesity. They're not the cause of it.