Great question, and that is what we call the leptin paradox. Most of the leptin in your body is produced by fat cells, so the more fat you have the higher your leptin levels. Logically you'd think that you then wouldn't be as hungry. But one of the biological mechanisms gets in the way, so to speak. High leptin levels are hindered from crossing a structure that's called the blood brain barrier. High triglycerides make it even worse, which is often the case when we're overweight. So, what you then have is all this, let's call it 'torso leptin' that can't cross the BBB to function as 'brain leptin'. The result is reduced and dysfunctional leptin signaling in the brain. So, that is the paradox — excess torso leptin, but it doesn't signal properly in the brain. The circulatory system is involved in how desired molecules cross the blood brain barrier, so getting a leptin injection doesn't do any good, and could in fact make matters worse by increasing your 'torso leptin' levels.