It's hardly surprising that spending hours a day sitting inert rather than running about does not make children fit. But research increasingly homes in on screen-watching as an independent and significant factor in child obesity: even more significant than diet and amount of physical activity. In fact, watching TV appears to generate more flab than other sedentary activities, such as reading.
To take one example, a study conducted at the Dunedin School of Medicine, University of Otago, New Zealand, tracking the television viewing habits and health of 1,000 children over 26 years. It found that children who watched more than two hours of television a day between the ages of five and 15 developed significant health risks many years later. The study concluded that 15 per cent of cases of raised blood cholesterol, 17 per cent of obesity, and 15 per cent of reduced cardiovascular fitness were linked to the television viewing that took place years before when the adults were children, irrespective of other factors.
But how does TV actually fatten us up? In the Journal of the American Medical Association, Harvard researchers reported that beyond merely displacing physical activity, TV slows our metabolism and burns fewer calories compared with other sedentary activities such as sewing, reading, writing or driving a car. Another study found that children's resting metabolic rate decreased as average weekly hours of TV viewing increased.
Watching television also makes us eat significantly more, even if we are not physically hungry. A recent US study found that even children who watched a below average amount of television (less than three hours a day for an average of 2.7 days a week) ate roughly the equivalent of an extra meal a day more than those who watched none.
This is not just because of all those tempting food advertisements so cunningly placed in the breaks. One of the reasons is that our brain is monitoring external, non-food cues - the television screen - rather than internal food cues telling us that we have eaten enough.
Experiments have found that when distracted in this way we continue to salivate in response to more and more food when normally we would not. A recent study concluded that watching television can disrupt the natural link between appetite and eating.
These findings occur at a time when 75 per cent of evening meals in the UK are eaten in front of the television. Even watching an hour of television a day is strongly linked with sleeping problems years later. And endocrinologists at the University of Bristol are finding a clear relationship between shorter sleep times and obesity in children because sleep loss is associated with abnormalities in metabolic hormones and an increased desire for high calorie food.
Fortunately, a new study by academics at the medical schools of Stanford University and the State University of New York offers good news. They studied the effects of screen-watching on the weight of 70 four to seven- year-olds in the fattest 25 per cent of the population.
The children were divided into two groups: one had its TV and computer viewing reduced by half; the other did not. After two years, there had been a significant reduction in the body mass index (BMI) of those who had halved their screen-viewing and relatively little in those who hadn't.
The academics conclude: "Reducing television viewing and computer use may have an important role in preventing obesity and in lowering BMI in young children," adding that putting a television in a child's bedroom might increase the risk of obesity more than televisions in family spaces.
Given the hysteria about our children being among the fattest in the world, it's time for schools and teachers to place child welfare above their concerns about offending anyone. So, make parents aware of the dangers and add the health issue of screen time to the PSHE syllabus
Dr Aric Sigman is a fellow of the Royal Society of Medicine and associate fellow of the British Psychological Society
Epstein, L.H., et al (2008) A Randomized Trial of the Effects of Reducing Television Viewing and Computer Use on Body Mass Index in Young Children, Archives of Pediatrics amp; Adolescent Medicine, 162:3, 239-245
Mark, A.E., Janssen, I. (2008) Relationship between Screen Time and Metabolic Syndrome in Adolescents, Journal of Public Health, 30:2, 153- 160
Temple, J.L. et al (2007) Television Watching Increases Motivated Responding for Food and Energy Intake in Children, American Journal of Clinical Nutrition, 85:2, 355-361
Other references can be found in Aric Sigman's Remotely Controlled: How television is damaging our lives (Vermilion, 2007).