Conventional wisdom holds that eating too many calories and exercising too little cause obesity, a belief that gave rise to the “Eat Less, Move More” approach to weight loss. Drawing on decades of research, Fung debunks these pervasive myths. Eating too much may be the proximal cause of weight gain, but it is not the ultimate cause. Further, exercising does not prevent obesity. “Eat Less, Move More” implies that obesity is a choice or a personal failing because it frames an individual’s daily choices as the cause of weight loss or gain, ignoring underlying factors. Fung contests these views, arguing that obesity results from a hormonal imbalance of hyperinsulinemia.

Homeostasis, or the body’s tendency toward equilibrium, explains why caloric reduction doesn’t yield weight loss. Studies reveal that the body adapts to reduced calories metabolically: The heart rate slows, blood pressure and body temperature fall, and the heart volume decreases to conserve energy. The body also responds hormonally to caloric reduction by increasing hunger hormones and decreasing satiety hormones. Dieters initially lose weight when they reduce their food intake. However, the body soon adapts, causing them to plateau and regain the lost weight, and then some.