The amount of TV a child watches can predict whether he or she will become obese or overweight, a study suggests.

Furthermore, the study indicates that TV viewing has a greater impact on body mass index (BMI) than that usually found for either diet or inactivity.

Watching TV should be regarded as an important contributing factor in the current childhood obesity epidemic, according to researchers from New Zealand's Otago University, whose results have just been published in the latest issue of the International Journal of Obesity.

Findings included a stronger association in adolescent girls between television watching and obesity than boys, though the reason why this is unclear.

"We found that TV viewing is more strongly associated with an increased BMI than diet or activity levels have been reported to be," says study co-author Dr Bob Hancox.

The new findings by Dr Hancox and fellow Otago Associate Professor Richie Poulton, are the latest to emerge from the long-running Dunedin Multidisciplinary Health and Development Study of 1,000 children born in 1972 and 1973.

After recording the viewing habits and BMI of the children at regular intervals between three and 15 years of age, the researchers found a clinically significant relationship between BMI and television viewing in childhood.

Dr Bob Hancox: "After taking a range of other factors into account, our research showed that time spent watching television is a significant predictor of BMI and being overweight."

"After taking a range of other factors into account, our research showed that time spent watching television is a significant predictor of BMI and being overweight," says Dr Hancox.

The study adds further evidence of a cause-and-effect relationship rather than a just an association, as the researchers were able to compare earlier amounts of TV viewing with later BMI measures, and take into account factors such as socio-economic status and parental tendency to be overweight.

"While there are a number of factors involved in childhood obesity, the strength of the association we found with TV viewing was greater than that commonly found for the effects of nutritional intake and physical activity. Clearly further work is needed to unravel all the critical factors and how they interact," added Dr Hancox.

"In the meantime, significantly reducing the amount of television children watch would be an important first step in attempts to tackle the current epidemic of childhood obesity."

Previous studies by the research unit have linked too much TV viewing in childhood to a range of health problems in young adulthood, as well as lower educational achievement as adults.