Children who watch more than two hours of television a night seem to be at higher risk of becoming smokers or being fat, out of shape or having high cholesterol (search) as adults, according to a new study.

Watching TV in childhood and adolescence has long been linked to adverse health indicators, including obesity, poor fitness and high cholesterol, but the study published Friday in The Lancet was the first to track a group from birth to adulthood.

Dr. David Ludwig (search), director of the obesity program at Children's Hospital in Boston, and Steven Gortmaker, a sociology lecturer at the Harvard School of Public Health, said the data indicate television viewing in childhood has "serious long-term consequences" and strengthen "the case for a ban on food advertisements aimed at children." Neither was connected with the study.

The researchers from the Dunedin Multidisciplinary Health and Development Research Unit (search) assessed some 1,000 people born in Dunedin, New Zealand, in 1972-73, at regular intervals until age 26. They investigated associations between childhood TV viewing and body-mass index, or BMI, cardio-respiratory fitness, cholesterol level, smoking status and blood pressure.

They found that even an average weeknight viewing of one to two hours between the ages of 5 and 15 was associated with higher body-mass indices, lower cardio-respiratory fitness, increased smoking and raised cholesterol.

This was the case even after they adjusted for such factors as family economics, the smoking habits and weight of the parents, and the children's size at age 5.

The study found that among 26-year-olds, 17 percent of overweight, 15 percent of raised cholesterol, 17 percent of smoking and 15 percent of poor fitness could be attributed to watching television for more than two hours a day during childhood and adolescence.

The researchers noted that, as in any observational study, they couldn't prove TV viewing caused health problems.

"Television viewing might be a marker for some unidentified determinant of adult health, and individuals who have a natural tendency to obesity and poor physical fitness might prefer to watch television than do other activities," they wrote.

The researchers said several childhood behaviors — including physical activity and diet — could explain the association between TV viewing and health.

For example, watching television could affect fitness and obesity by taking the place of more active pursuits, they said, adding that TV advertising in New Zealand also tends to promote an unhealthy diet.

The researchers said watching TV might also influence other behaviors, such as cigarette smoking, which "we found to be significantly associated with television viewing."

Although TV advertising of tobacco was banned in New Zealand before study members were born, programs have continued to show frequent images of smoking during children's viewing time, they said. Tobacco sponsorship for sports events continued until 1995.

The report said it could not define a safe level of TV viewing because it couldn't find enough people who watched no television to serve as a control group, but those who watched an hour or less a day were the healthiest.

The American Academy of Pediatrics (search) recommends that parents limit their child's viewing to two hours a day.

"Clearly, obesity is a complex condition, with numerous genetic, environmental and psychosocial contributing factors. However, (this) should not be an excuse for inaction," Ludwig and Gortmaker wrote in a separate commentary in the Lancet.

"Measures to limit television viewing in childhood and ban food advertisements aimed at children are warranted, before another generation is programmed to become obese."