Children who consistently watch more than two hours of TV a day are more likely to become overweight, smoke and suffer from high cholesterol as adults, according to a new study.

Unlike in previous short-term studies, researchers in New Zealand studied 1,000 children; 73 of them were tracked from birth into adulthood.

The researchers found watching an average of one to two hours of TV on weeknights between the ages of five and 15 was linked to:

• Higher body-mass index.
• Lower cardio-respiratory fitness.
• Increased smoking.
• High cholesterol.

TV advertising tends to promote an unhealthy diet.

Robert Hancox of University of Otago and his team factored out influences such as mom and dad's smoking habits and weight, family income and the children's size at age five.

They estimate among 26-year-olds, seven per cent of overweight, 15 per cent of raised blood cholesterol, 17 per cent of smoking, and 15 per cent of poor fitness can be attributed to watching television for more than two hours a day during childhood and adolescence.

The study appears in this week's issue of the medical journal, The Lancet.

"Our results suggest that excessive television viewing in young people is likely to have far-reaching consequences for adult health," Hancox said in a release.

"We concur with the American Academy of Pediatrics that parents should limit children's viewing to 1-2 hours per day; in fact, data suggest that less than one hour a day would be even better."

He acknowledged it may be difficult to restrict viewing, adding parents need support from society.

"[T]he data presented by Hancox and colleagues strengthen the case for a ban on food advertisements aimed at children," wrote David Ludwig, a pediatrician at Harvard Medical School, in a commentary accompanying the study.

Ludwig noted genetic, environmental and psychosocial factors contribute to obesity, adding its complexity is not an excuse for inaction.