One-third of US adults are obese. A big part of the environment that encourages weight gain is the availability and consumption of calorie dense, high-fat foods. In particular, what we eat can alter the brain regions that regulate body weight. Leptin is a key hormone secreted from fat tissue and its main function is to inhibit appetite. Eating a high-fat diet reduces the actions of leptin in the brain. This miscommunication can lead to increased food intake and weight gain. Evolutionary speaking, we are designed to want to eat foods high in fat and gain weight because it made it easier to survive times when food was scarce. That is no longer a real concern since food is always available, but we still have a biological desire to eat calorie dense foods.
Cell Metabolism, Feb 2012