How much TV children watch accurately predicts whether they will go on to become overweight, a study suggests.
The effect of watching TV on weight appeared stronger in girls
It has previously been shown television is linked to weight gain as children are less active and eat while watching.
Researchers at New Zealand's University of Otago looked at how much TV children aged five to 15 watched.
The International Journal of Obesity study found the 41% who were overweight or obese by the age of 26 were those who had watched most TV.
A study by the same team published last year suggested children should watch no more than two hours of TV a day to protect their future health.
They warned then that adults who had watched a lot of TV as children were more likely to go on to be overweight, to smoke and to have high cholesterol.
Situation 'worse now'
In this latest paper, they monitored TV watching and body mass index - calculated by dividing weight in kilograms by the square of height in metres.
A BMI of 25 or over is considered overweight, and one of over 30 obese.
All the 1,000 children studied were born between April 1972 and March 1973.
At age five, seven, nine and 11, parents were asked how much TV they watched. At ages 13 and 15, the teenagers themselves were questioned.
Between the ages of five and 15, children were found to watch an average of 2.33 hours of TV per weeknight.
Aged 13 to 15, they watched an average of 24.6 per week.
At each age, the amount of TV watched was consistent with the child's BMI.
The links were stronger in girls, which the researchers say may be linked to the differences in lifestyle and physical make-up of between teenage boys and girls.
Writing in the International Journal of Obesity, the researchers, led by Dr Robert Hancock, said: "Although the effect size appears small, the correlation between television viewing and BMI is stronger than reported correlations between BMI and diet or physical activity."
They add that the situation is likely to have worsened for children since this group were studied - when New Zealand had just two television channels, both of which had limited viewing hours.
Dr Hancock and his team added: "Since then, the number of free-to-air TV channels increased from two to four, the time given to advertising increased, pay satellite television become available, viewing hours have increased to provide 24-hour coverage, more homes have video or DVD players and many homes have acquired a computer and internet access.
"Thus, despite the apparently heavy television use in this cohort, the opportunities for 'screen-time' are far higher for children today.
"We believe that watching TV is an important contributing factor to the current epidemic of childhood obesity."
Dr Ian Campbell, chairman of the National Obesity Forum, said: "What this study does is highlight the importance of what we do in everyday life, and the way we bring up our children, on future health.
"There is so much that adults can do to help children live healthier lifestyles.
"Sensibly limiting hours of TV watching would be a good start."
He added: "TV watching is not the cause of obesity, but it's certainly a part of the cause of the problem."
A spokeswoman for the National Heart Forum said: "The findings of this study are consistent with those of other similar studies, in that it shows a weak but positive association between TV watching and obesity."