Screen time and child obesity

“Limiting children’s television and computer use helps them to lose weight,” The Daily Telegraph reports. The Guardian also publishes an article on a US study on 70 children all of whom were among the most obese for their age, as measured by body mass index (BMI). The newspaper says that by limiting the amount of time that TV and computers were used at home, the children’s daily calorie intake was reduced by “more than 300 from 1,550”.

Although the intervention in this study led to changes in BMI that might be considered minor, it was also linked to a decrease in calorie intake. However, although the children were less sedentary they were not more active. Screen time appears to be an important modifiable cause of childhood obesity and the researchers concluded that cutting down on screen time rather than cutting calories seems to reduce weight.

As this study has shown that less TV does not necessarily increase physical activity, parents who want to get their children doing more may have more success presenting physical activity as a fun first choice, rather than just as an alternative to watching TV.

Where did the story come from?

Dr Leonard Epstein and colleagues from the University of New York at Buffalo and Stanford Prevention Research Center in California carried out the research. The study was supported by a grant from the US National Institute of Diabetes and Digestive Diseases. The study was published in the peer-reviewed medical journal: Archives of Pediatrics and Adolescent Medicine.

What kind of scientific study was this?

This was a randomised controlled trial of 70 children aged between four and seven in the US.

Newspaper adverts, flyers and direct mailing that targeted families with children in that age range were used to recruit likely candidates. To be included, the children had to have a BMI (weight in kilograms, divided by height in metres squared  - kg/m2) that placed them within the top 25% of children of their age and sex. Children with a medical condition that prevented physical activity were not allowed to take part in the study. They also had to have unlimited access to TV and related sedentary activities and to watch TV or play computer games for at least 14 hours a week.

The 70 families that took part agreed to have a monitoring device added to every computer and TV in their home. In order to be turned on, any appliance that the device was attached to needed to have a four digit PIN code input. Each member of the family was given a PIN code that was kept secret from other members of the family.

The families were randomly allocated to two groups, an intervention group and a control group. The intervention group were set a weekly budget of time they could watch TV and use computers and when this budget was reached, the appliance would not work. As an incentive, the children were given $0.25 (up to $2 per week) for every half hour that they were under budget. Each month, the budget was reduced by 10% of the child’s original screen time, until it reached half of what they had been watching at the beginning of the study. Other incentives and praise were also used to reinforce healthy behaviour.

The children in the control group had no budget and had free access to TV and computer games. They also received $2 per week for participating in the trial and had similar routine measurements taken.

In addition to monitoring the children’s TV and computer use, the researchers recorded physical activity with an “Actigraph monitor” attached to the child. Average activity levels were taken from three randomly selected weekdays, from school until bedtime, and one day at the weekend. Food intake was assessed using a validated food frequency questionnaire, which asked parents about the food eaten by the child over the previous month. The researchers also collected other information about the child such as weight and height and characteristics of the neighbourhood, such as distance to school, perceived safety and socio-economic status of the family.

The study ran for two years. All the families that took part completed the six-month period, but three dropped out before the end of a one-year review.

What were the results of the study?

At the end of the study, the children in the control group had managed to reduce their screen time by an average of 5.2 hours a week. In comparison, the children in the intervention group had reduced their “screen time” by an average of 17.5 hours a week.

The children in the intervention group also had greater reductions in BMI (about 0.1 units of the age adjusted BMI). They also consumed fewer calories, (energy intake was reduced by about 200 kilocalories) compared with the control group. These results were statistically significant. The intervention worked better among families of low socioeconomic status.

The researchers note that although the change in television viewing was related to change in energy intake, it was not related to any change in physical activity.

What interpretations did the researchers draw from these results?

The researchers conclude that “Reducing television viewing and computer use may have an important role in preventing obesity and in lowering BMI in young children, and these changes may be related more to changes in energy intake than to changes in physical activity.”

What does the NHS Knowledge Service make of this study?

This randomised controlled trial demonstrated the effects of a complex behavioural intervention and has used reliable objective measurements where possible. The study does have some limitations, which the authors acknowledge:

  • The study only included children who were already at or above the 75th percentile of BMI for their age; the results may therefore not apply to children who are less overweight.
  • The TV monitoring device accumulates the total hours of screen time, but cannot provide data about when the appliance was turned on, or who was watching it. Although each family member had their own PIN number, the researchers do not describe how they accounted for TV viewing as a family, or if computer games were played outside of the home, at friends’ houses, for example.
  • The researchers suggest that a food diary, where the participant or parent records everything eaten, may have been preferable to relying on the parents recalling what was eaten in the previous month. However, this would have been more labour intensive for the families involved.

This study's findings challenge the commonly held belief that less TV results in an improvement in health by encouraging more physical activity in children. This study instead suggests that children, and particularly children from lower socio-economic groups, reduce their energy intake (i.e. how much they consume) as a response to restrictions in TV viewing.

Screen time appears to be an important modifiable cause of childhood obesity, but exactly how it exerts an effect remains to be established. As this study has shown that less TV does not necessarily increase physical activity, parents who want to get their children doing more healthy physical activities may have more success presenting it as a fun first choice, rather than just as an alternative to watching TV.

NHS Attribution