Yo-yo effect

Yo-yo dieting or yo-yo effect, also known as weight cycling, is a term "yo-yo dieting" coined by Kelly D. Brownell at Yale University, in reference to the cyclical loss and gain of weight, resembling the up-down motion of a yo-yo. In this process, the dieter is initially successful in the pursuit of weight loss but is unsuccessful in maintaining the loss long-term and begins to gain the weight back. The dieter then seeks to lose the regained weight, and the cycle begins again.

Causes
The reasons for yo-yo dieting are varied but often include embarking upon a hypocaloric diet that was initially too extreme. At first the dieter may experience elation at the thought of weight loss and pride in their rejection of food. Over time, however, the limits imposed by such extreme diets cause effects such as depression or fatigue that make the diet impossible to sustain. Ultimately, the dieter reverts to their old eating habits, now with the added emotional effects of failing to lose weight by restrictive diet. Such an emotional state leads many people to eating more than they would have before dieting, causing them to rapidly regain weight. The process of regaining weight and especially body fat is further promoted by the high metabolic plasticity of skeletal muscle. The "Summermatter Cycle" explains how skeletal muscle persistently reduces energy expenditure during dieting. In addition, food restriction increases physical activity which further supports body weight loss initially. When food becomes available again, the thrifty program promotes the refilling of energy stores which preferentially occurs as catch-up fat.

Effects on health
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 body fat during the initial weight-loss phase (weight-bearing exercise is required to maintain muscle). After completing the diet, the dieter is likely to experience the body's starvation response, leading to rapid weight gain of only fat. This is a cycle that changes the body's fat-to-muscle ratio, one of the more important factors in health. A report by the American Psychological Association reviewed thirty-one diet studies and found that after 2 years of dieting up to a third of dieters weighed more than they did before they began the diet. One study in rats showed those made to yo-yo diet were more efficient at gaining weight. However the research compiled by Atkinson et al. (1994) showed that there are “no adverse effects of weight cycling on body composition, resting metabolic rate, body fat distribution, or future successful weight loss”, and that there is not enough evidence to show risk factors for cardiovascular disease being directly dependent on cyclical dieting patterns. Yo-yo dieting can have extreme emotional and physical ramifications due to the stress that someone puts on themselves to lose weight quickly. The instant gratification of losing the weight eventually gives way to old eating habits that cause weight gain and emotional distress.

Since there is “no single definition of weight cycling [that] can be endorsed”, it is almost impossible for research to draw specific conclusions about the actual effects of cyclical dieting, until it becomes more definitely defined.