Children who watch more than five hours of television a day are twice as likely to be obese as those who watch an hour or less, a new study shows.

Furthermore, turning off the television could prevent some children from being overweight.

"Watching television requires no energy in excess of resting metabolic rates, and it may reduce the time spent in more energy-expensive activities" such as active playing, Dr. William H. Dietz and Steven L. Gortmaker write in the journal Pediatrics.

The researchers also cite television's heavy advertising of junk food as contributing to the obesity of young viewers.

Most TV stars are slim and trim, the study notes, which "may indirectly suggest to children that eating and drinking high caloric foods is of little consequence with regard to weight."

The researchers studied data of more than 10,000 children gathered during the 1960s.

The most profound effects of watching television appeared in 12- to 17-year-olds. In that group, each extra hour of television increased the number of obese children by 2 percent. About 10 percent of those who watched an hour or less were obese, compared with about 20 percent of those who watched more than five hours.

For younger viewers, the effects seemed to occur later, in adolescence.

"The prevalence of obesity could be reduced," the authors conclude, "by a reduction in television viewing and an increase in other activities."