How much do you really know about how the human body works—how it reacts to food, exercise, nutrition, and the environment? While most of us have read about at least one fad diet, we're left wondering about the greater biochemistry, psychology, sociology, and physiology of the obesity crisis in the United States. Gut Reactions by chemist Simon Quellen Field shows us how our bodies react to food and the environment, how our brain affects what and how much we eat, and why some diets work for some people but not for others based on genetics, weight history, brain chemistry, environmental cues, and social pressures. It explores how our hormones affect hunger and satiety and interact with the brain and the gut, and it explains the addictive nature of foods that interact with the same dopamine and opioid receptors in the brain that cocaine, heroin, amphetamines, and nicotine do. Whether you're looking to lose weight, put on muscle mass, or simply understand how your metabolism or gut microbiome is affecting your food cravings, Field has a scientific answers for you.