US scientists conducting a comprehensive review of dieting research have concluded that dieting does not work.

The study is published in the April edition of American Psychologist, the journal of the American Psychological Association.

Researchers at the University of California, Los Angeles (UCLA), reviewed 31 long-term studies lasting between 2 to 5 years.

UCLA associate professor of psychology and lead author of the study, Traci Mann said:

“You can initially lose 5 to 10 percent of your weight on any number of diets, but then the weight comes back.”

“We found that the majority of people regained all the weight, plus more,” she added.

The researchers found a very small minority of study participants managed to sustain weight loss, while the majority put all the weight back on, and more in the longer term. Dr Mann stated:

Diets do not lead to sustained weight loss or health benefits for the majority of people.”

Dr Mann and colleagues sought to determine the long term effects of dieting and address the question “Would they have been better off to not go on a diet at all?”.

So they analyzed every study they could find that followed people on diets for 2 to 5 years. Studies that take less than 2 years are “too short to show whether dieters have regained the weight they lost,” they said.

They discovered that it would have been better for most of them if they had not gone on a diet at all.

“Their weight would be pretty much the same, and their bodies would not suffer the wear and tear from losing weight and gaining it all back,” explained Dr Mann.

Their findings show that:

  • People on diets typically lose 5 to 10 per cent of their weight in the first 6 months.
  • But 33 to 66 per cent regain more than what they lose within 4 to 5 years.

Dr Mann and colleagues suspect the real situation is actually even worse; the figures do not really reflect reality, making diet studies look better than they are. They say there are a number of reasons for this:

  • Many participants phone or mail their results in themselves, without an impartial assessor.
  • A lot of studies have a below 50 per cent follow up rate; and the people who put on a lot of weight are less likely to stay in touch.

UCLA graduate student of psychology and co-author of the study, Janet Tomiyama said that “Several studies indicate that dieting is actually a consistent predictor of future weight gain.”

One study in particular that they looked at found that men and women who took part in a weight reduction programme gained significantly more weight than those who did not over the same period of time.

Tomiyama mentioned another study, this time looking at links between lifestyle and weight in 19,000 healthy older men over four years. This study found that, “One of the best predictors of weight gain over the four years was having lost weight on a diet at some point during the years before the study started,” she said.

Also, in many studies with control groups, the people in the control group very often were better off than the participants who dieted.

Dr Mann suggests that eating in moderation and exercise do make a difference. Although they were not looking at exercise in particular, Dr Mann said that:

Exercise may well be the key factor leading to sustained weight loss. Studies consistently find that people who reported the most exercise also had the most weight loss.”

One study following obese patients discovered that:

  • Among those followed for under 2 years, 23 per cent of patients had regained their weight loss.
  • Among those followed for more than 2 years, 83 per cent had regained their weight loss.

Another study found that 50 per cent of dieters weighed 11 pounds (5 kilos) more than their starting weight 5 years after their diet.

Among the health hazards of repeated weight loss and regain are cardiovascular disease, stroke, diabetes and altered immune function, said Dr Mann and colleagues.

They said more research is needed on the effects of weight loss and regain on health, and say scientists do not fully understand the underlying factors involved in this complex relationship.

Dr Mann quoted her mother, who herself has tried to diet many times, without success. Dr Mann’s mother said her daughter’s findings were “obvious”.

Although this study reviewed 31 long term dieting projects, they did not look into specific diets.

The researchers are of the opinion that weight loss programmes are not good value for money in the treatment of obesity.

“The benefits of dieting are too small and the potential harm is too large for dieting to be recommended as a safe, effective treatment for obesity,” said Dr Mann.

Between 1980 and 2000, the proportion of obese Americans has doubled, from 15 to 31 per cent of the population.