Pontzer's groundbreaking studies with hunter-gatherer tribes show how exercise doesn't increase our metabolism. Instead, we burn calories within a very narrow range—nearly 3,000 calories per day, no matter our activity level. This was a brilliant evolutionary strategy to survive in times of famine. Now it seems to doom us to obesity. The good news is we can lose weight, but we need to cut calories. Refuting such weight-loss hype as paleo, keto, anti-gluten, and even vegan, Pontzer discusses how all diets succeed or fail—for shedding pounds, a calorie is a calorie. At the same time, we must exercise to keep our body systems and signals functioning optimally, even if it won't make us thinner. Hunter-gatherers like the Hadza move about five hours a day and remain remarkably healthy into old age. But elite athletes can push the body too far, burning calories faster than their bodies can take them in. It may be that the most spectacular athletic feats are the result not just of great training, but of an astonishingly efficient digestive system.