Obesity has become a true pandemic. In the United States, over two thirds of adults are obese or overweight. The prevalence of obesity has doubled since 1980. The increase in the prevalence of obese and overweight individuals has happened too rapidly for it to be due to an alteration in the genome. The gastrointestinal, sensory (taste and olfaction), and brain feeding mechanisms that developed during the past 2 million years were highly adaptive for ancestral hunter-gatherers living in an environment with limited high-density foods and periods of food deprivation. Today, however, humans in industrialized countries live in what has been called an "obesogenic environment." The nonhomeostatic brain reward circuitry that was acquired during evolution to seek out and eat as many nutritionally high-dense foods as possible is able to overrule the physiological inhibitory mechanisms that were designed to limit meal size and weight gain.