Children should watch no more than two hours of TV a day to protect their future health, scientists have warned.
Children should play more instead of watching TV
A Lancet study found adults who had been avid TV watchers as children were more likely be overweight, to smoke and to have high cholesterol.
Researchers from the University of Otago, New Zealand, said parents should 'lead by example' and turn the TV off.
Other experts said the study bolstered the argument for banning junk food ads during children's programmes.
The researchers followed 1,000 children born in 1972 and 1973 from the ages of three up to the age of 26.
Every two to three years, they interviewed their parents, then in adolescence the children themselves, to see how much TV they watched each day.
When the study participants were 26, the researchers assessed their adult health.
They checked blood pressure and cholesterol levels, participants' height, how fit they were, whether they smoked and their body mass index (BMI) - which indicates whether a person is a healthy or unhealthy weight.
It was found watching more than two hours of TV a day as children and teenagers was linked with an increased risk of being overweight, smoking and having poor cardiovascular fitness - a measure of how well the heart is working.
It was found 17% of weight problems, 15% of raised
blood cholesterol, 17% of smoking and 15% of poor cardiovascular fitness could be linked to excessive TV viewing in childhood and adolescence.
However no link was found between TV and blood pressure.
The associations remained after adjustment for factors such as social
background, BMI at age five, parents' BMI, parental smoking, and physical
activity at 15 years old.
Benefits for adults
Dr Robert Hancox, who led the study, said: "Although the adult health indicators that we have found to be associated with child and adolescent television viewing are unlikely to result in clinical health problems by the age of 26 years, they are well
established risk factors for cardiovascular illness and death later in life.
"Our results suggest that excessive television viewing in young people is
likely to have far-reaching consequences for adult health."
He said he backed American Academy of Pediatrics calls for parents to limit
children's viewing to one to two hours per day.
Dr Hancox added: "In fact, data suggest that less than one hour a day would be even better."
He said that it may be difficult for some parents to impose such a regime and
would need a lot of support and encouragement.
But he added: "Adults are likely to obtain health benefits themselves if they lead by example and turn off the television.
"We believe that reducing television viewing should become a population health priority."
In an accompanying article in The Lancet, Dr David Ludwig, from Boston's Children's' Hospital, said: "Measures to limit television viewing in childhood and ban food advertisements aimed at children are warranted, before another generation is programmed to become obese."
A spokeswoman for CBBC, which broadcasts children's TV programmes for six to
12-year-olds, said average children's television viewing in the UK had fallen
from 166 minutes a day in 2001 to 154 minutes in the first three months of this
She said: "It's perhaps too simplistic to point the finger at TV viewing. There may be far more complex reasons why children sit around at home more and take too little exercise, for instance the fact that we live in a risk-averse society."
She added children cycled much less than they did 20 years ago and spent more time using computers.
"Sedentary behaviour should be a cause for concern, but we should sound a
note of caution about looking for a single factor behind it."
June Davison, Cardiac Nurse at the British Heart Foundation said: "Children should participate in at least one hour of moderate intensity physical activity a day.
"However, only around two thirds of children aged two to15 in England reach even this minimum level. If this trend continues, problems like coronary heart disease will occur in even younger patients."
She added: "This study highlights that it is never too early for parents to encourage their children to adopt a healthy lifestyle."