Is yo-yo dieting -- repeatedly gaining and losing weight -- bad for me?

Yo-yo dieting has a concrete biological downside: fat cells appear to build up a kind of memory of earlier periods of excess weight. This memory lies in the way genes in fatty tissue are switched on or off, without the DNA itself changing. In mice, this meant that animals that had previously been overweight regained weight considerably faster once they started eating poorly again, compared with mice that had no such history. Whether this works in exactly the same way in humans has not yet been demonstrated, but the mechanism is biologically plausible.

On top of that, dieting itself lowers the resting metabolism: the number of calories your body burns at rest decreases. That makes it harder to maintain your weight after the diet ends. That combination -- a lower metabolism and more sensitive fatty tissue -- lays the foundation for the classic yo-yo pattern.

Large population studies find an association between large fluctuations in weight and a higher risk of cardiovascular disease and mortality. But there is a serious catch here: studies rarely distinguish between deliberate dieting and unintentional weight loss due to illness. Unwanted weight loss caused by an underlying condition explains part of that higher mortality. These are therefore associations, not proven cause and effect.

Two commonly heard fears turn out to be less well supported. The claim that yo-yo dieting permanently depletes muscle mass is not confirmed by the available studies: in obese British women who completed multiple rounds of a very low-calorie diet, no demonstrable permanent muscle loss was found. As for the liver, mouse research showed that yo-yo diets caused less liver inflammation than continuous poor eating, though that animal research does not translate directly to humans.

Psychologically, the picture is less rosy: yo-yo dieters more often report dissatisfaction with their bodies, lower self-esteem and a higher likelihood of binge eating. These are observational associations, but they are consistent. Finally, an official American expert panel concluded that the proven disadvantages of yo-yo dieting are not serious enough to advise people with severe obesity to stop making weight-loss attempts altogether. For that group, the benefits of even modest weight loss outweigh the risks.