We are unlikely ever to see this warning displayed on our TV screens, but the evidence is convincing.

Researchers looking at a group of more than 50,000 women in the Nurses Health Study have found a strong association between sedentary behaviours particularly hours of TV watching, obesity and type 2 diabetes. (Publ. Journal of the American Medical Association, 9 April 2003)

A sedentary occupation reflected by long hours of sitting or standing at work was also significantly associated with obesity, whereas even light activities such as that done with housework and brisk walking were associated with a significantly lower risk of obesity and diabetes.

For every two hour increase in TV watching a day, women were found to have a 23% increased risk of obesity and 14% increased risk of type 2 diabetes. The study suggested that 30% of obesity cases and 43% of type 2 diabetes cases can potentially be prevented by following a relatively active lifestyle (less than 10 hours a week of TV watching and at least 30 mins brisk walking daily).

Similar results have previously been found in studies on men and it is well researched that there is an association between hours of TV viewing and obesity in children. So why is watching telly so bad for our health?

Firstly TV watching displaces more physical activities thereby reducing energy expenditure. Secondly people tend to eat whilst watching TV and thirdly often the things they eat tend to relate to the advertising which is for high fat, high sugar, low nutritional value food (ie. junk food). Watching TV has a significantly lower energy expenditure and lower metabolic rate than other sedentary activities such as sewing, reading, writing, playing board games and driving a car.

Public health messages that encourage increased levels of physical activity are being heard, but maybe more emphasis now needs to be placed on reducing sedentary behaviour particularly the number of TV viewing hours in our lives.