Yo-yo dieting is the successive gain and loss of weight due to excessive dieting. The reasons are varied, but often include embarking upon a diet that was initially too extreme. At first the dieter may experience elation at the thought of loss, and proud of their rejection of food. Over time, however, such extreme diets cause effects such as depression or fatigue, that make the diet impossible to sustain. The dieter reverts to their old eating habits, having learnt nothing about nutrition, and with the added emotional effects begins to gain weight rapidly again.

This kind of diet is associated with extreme food deprivation as a substitute for good diet and exercise techniques. As a result, the dieter may experience loss of both muscle and fat during the initial weight-loss phase (exercise is required to maintain muscle). After completing the diet, the dieter is likely to experience the body's famine response, leading to rapid weight gain of only fat. This is a dangerous fat-cycle that changes the body's fat to muscle ratio, one of the more important factors in health.