An analysis of long-term dieting studies carried out by US psychologists has revealed that most weight-loss diets don’t work in the long term. Professor Traci Mann, from the University of California in Los Angeles, conducted a rigorous analysis of 31 long-term studies to find out whether successful dieters were able to maintain their new weight in the long-term.

It was found that obese subjects were able to lose 5 to 10 % of their starting weight in the first six months of the studies. However, 2 to 5 years after weight loss, the majority of subjects had regained all of this weight. An unfortunate 30-60% of subjects even ended up heavier than they had been at the start. Only a minority of subjects were able to maintain their new weight in the long-term. The best results were seen for dieters involved in formal weight-loss programmes, as opposed to those trying to lose weight on their own. Dieters who were physically active on a regular basis were also more successful.

The researchers concluded that most dieters in these studies would have been better off not going on a diet in the first place. The health benefits of weight loss are only achieved when the weight is kept off. Weight loss followed by weight regain may actually represent a health risk.

While this analysis of dieting efficacy offers a bleak picture, the reality could, in fact, be worse. Poor methodology in some of the studies meant that subjects were self-reporting weight, while follow-up rates in 8 of the 31 studies were less than 50%. Both of these factors can serve to exaggerate weight loss while dieting and play down weight regain.

For more information see:
Mann T et al (2007). Medicare’s search for effective obesity treatments: diets are not the answer. American Psychologist Vol 62,  issue 3; pg  220–233.

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