Tuesday March 3 2009
Time spent watching TV was used as an indicator of a sedentary lifestyle
“Children who spend more than two hours a day in front of the television in early childhood are twice as likely to develop asthma as those out playing,” The Daily Telegraph reported. The newspaper said that scientists who carried out the study believe it to be the first suggested link between the condition and a “couch potato lifestyle at a young age”.
This study did have strengths, including its size (about 3,000 children were included in the analysis) and its prospective nature. However, it has some limitations, such as the fact that TV viewing was only assessed once, and it was the only form of sedentary behaviour assessed. Further research will be needed to investigate the suggested link between sedentary behaviour and asthma risk before any firm conclusions can be drawn. However, it is already clear that encouraging physical activity has health benefits for children and adults.
Where did the story come from?
Dr A Sheriff and colleagues from the University of Glasgow and other universities in the UK carried out this research. The work was funded by the Medical Research Council. The study was published in the peer-reviewed medical journal Thorax.
What kind of scientific study was this?
This was an analysis of data from a prospective cohort study which followed 14,541 pregnancies in the Avon region. The Avon Longitudinal Study of Parents and Children (ALSPAC) aimed to determine whether there is a relationship between sedentary lifestyles and the risk of developing asthma. The researchers used the amount of time spent watching TV as an indicator of a sedentary lifestyle.
As part of ALSPAC, TV watching was assessed by a questionnaire at about 3.5 years of age (39 months). At the time (mid-1990s), watching TV was likely to be the main sedentary behaviour because video games and personal computers were not widely used. The questionnaire asked how long TV was watched during the week and at the weekend (possible answers were none, less than an hour a day, 1-2 hours a day, or more than two hours). As weekday and weekend viewing were found to have strong links, weekday viewing was used as the sole measure of TV viewing and sedentary behaviour.
At age 11.5 years, sedentary behaviour was measured using an accelerometer, a device that is attached to the body and objectively measures movement. This method was not used for the measurements at 3.5 years because accelerometers were not widely in use at the time.
Parents filled in questionnaires about wheezing symptoms in their child at age six months, and then annually thereafter. Asthma was defined as having been diagnosed with asthma by a doctor by 7.5 years of age with symptoms, and/or treatment in the previous year at 11.5 years of age.
The analysis only included the 3,065 children with no wheezing symptoms reported before the age of 3.5 years (the point at which TV viewing was measured), and with information about asthma presence or absence at 11.5 years. The researchers looked at whether TV viewing at age 3.5 years was related to the risk of developing asthma. Results were adjusted to take into account body mass index at age 11.5 years, maternal smoking during pregnancy, maternal history of asthma and allergies, as well as various social and lifestyle factors.
What were the results of the study?
Most children watched one to two hours of TV a day at age 3.5. Of the children who did not have wheezing at this age, 6% went on to develop asthma by age 11.5. About 9% of children who watched over two hours of TV a day at age 3.5 developed asthma by age 11.5. This was compared with 5.6% in those who watched one to two hours of TV a day, 4.2% in those who watched less than an hour a day, and 5% among those who watched no TV. This represented an increase of about 80% in the risk of developing asthma in those who watched over two hours of television a day compared to those who watched one to two hours.
TV viewing at age 3.5 years was not indicative of sedentary behaviour at age 11.5 years.
What interpretations did the researchers draw from these results?
The researchers concluded that “longer duration of TV viewing in children with no symptoms of wheeze at 3.5 years of age was associated with the development of asthma in later childhood”.
What does the NHS Knowledge Service make of this study?
The prospective nature of the study, its relatively large size, and the exclusion of children who had wheeze before TV viewing was measured are strengths of the study. However, there are a number of limitations to consider:
- As with all studies of this type, the differences in rates of asthma between the groups compared may be related to factors other than TV viewing (called confounding factors). Although the authors adjusted their analyses for these, such as maternal history of asthma, there may still be effects from these or other unknown confounders.
- TV viewing was measured at one age only, and may not be indicative of TV viewing habits at other ages, which may have varied.
- TV viewing was used as an indicator of sedentary behaviour, as it was thought this would be the main form of sedentary behaviour in the absence of widespread access to games consoles or computers. However, there are other sedentary behaviours, and including questions about them may have been a better indicator of overall behaviour.
- Just under 60% of the ALSPAC group provided sufficient data for inclusion in this analysis. Results for this subgroup of participants may not be representative of the full group.
- Parents' reports of their children's TV watching may not have been accurate.
Further research will be needed to investigate the suggested link between sedentary behaviour and asthma risk before any firm conclusions can be drawn. However, it is already clear that encouraging physical activity benefits to the health of children and adults.