Stopping children from watching TV really does make them healthier and less fat, according to one of the few studies to observe the effects of intervening directly in their watching habits.

"Television viewing is related to consumption of fast food and foods and beverages that are advertised on television. Viewing cartoons with embedded food commercials can increase choice of the advertised item in pre-schoolers, and television commercials may prompt eating," Leonard Epstein of the State University of New York in Buffalo and his colleagues wrote in the journal Archives of Paediatric and Adolescent Medicine.

He and his team investigated whether encouraging or coercing children into cutting down their viewing time made them healthier.

They started with 70 children aged between four and seven who were in the most obese 25% of the population, according to measurements of their body mass index. In half of the children's homes the team fitted limiting devices to their TVs and computers which prevented them from watching for more than a pre-set amount of time. The children could decide when and what they watched, but if they exceeded their weekly allowance the device would shut down until the end of the week.

Each month, the team reduced the children's TV time by 10% until it had dropped by a half compared with the original score of 25 hours. On average the restricted children watched 17.5 hours less TV a week, compared with 5.2 hours less for the group that did not have any restrictions.

The team found that in the group who had their TV watching curtailed, the children's daily calorie intake dropped by more than 300 from 1,550, or around 150 more than the drop in the control group.

The TV-restricted group also lost weight and ended up with an age-adjusted BMI more than 0.2 lower, from around 19.2, after two years than at the start. Their reduction was more than 0.1 higher than the control group.

The first group were also less sedentary, but they were not more active. The authors state that fitting a device to limit TV viewing time takes the pressure off busy parents who are trying to police their children's activity.

"Using technology to modify television viewing eliminates parental vigilance needed to enforce family rules and reduces the disciplinary action needed if a child exceeds his or her sedentary behaviour limits," they wrote.