Children who diet are more likely to gain weight in the long term than those who eat normally, researchers say.
The children may have binged on food
US doctors say it may be that children are affected by metabolic changes - but that it is more likely dieters resort to binge eating.
A study of over 16,000 boys and girls aged nine to 14 found those who said they were eating fewer calories and exercising more actually gained weight.
The research was published in the journal Pediatrics.
In the UK, the problem of child obesity is growing.
The Department of Health estimates that more than 8.5% of 6 year olds and 15% of 15 year olds in England are classified as obese.
In the US research, a team from the Brigham and Women's Hospital in Boston, Massachusetts followed the children from 1996 to 1998.
They monitored their eating habits by asking the children to fill out questionnaires.
They found that, around 30% of the girls and 16% of the boys were dieting to one degree or another when the study began.
Even though the dieters reported being more active and getting fewer calories than their peers, they gained more weight than non-dieters.
One girl who said she was a frequent dieter gained about 2 pounds (1 kg) per year more than other girls her age
who were not dieting.
Girls who dieted less often gained slightly less weight, but still more than non-dieters.
Similar differences were observed among the boys.
The researchers speculated that the children who dieted could have put on more weight because their
metabolism became more efficient, requiring fewer calories to maintain weight or become overweight.
But they said it was more likely that restrictive regimes were often not maintained for long periods and were often followed by binge eating.
The researchers said: "In that scenario it would be the repeated cycles of overeating between the restrictive diets that would be responsible for weight gain.
"Although medically supervised weight control may be
beneficial for overweight youths, our data suggest that for
many adolescents, dieting to control weight is not only
ineffective, it may actually promote weight gain."
They said young people and adults who were not severely overweight should be encouraged to adopt "a modest and therefore sustainable weight control strategy that includes physical activity and does not require severe restriction of total calories."