Yo-yo dieting is the repeated cycle of losing weight through dieting and then regaining it — down and up again like the toy. Each cycle typically ends with the dieter heavier than before, because restrictive diets are hard to sustain and the body’s metabolism adapts to scarcity by becoming more efficient. Beyond frustration, weight cycling is linked to loss of muscle mass, gallstones, hormonal disruption, and a strained relationship with food; some studies associate it with worse cardiovascular outcomes than stable overweight. The pattern reveals the flaw in diet-as-temporary-punishment thinking. Lasting weight control comes from permanent, moderate changes to eating and activity — not from repeated campaigns.