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“TV habits 'can predict kids' waist size and fitness,'” reported BBC News, while the Metro carried this balder headline: “Two-year-olds who watch too much television ‘will get fat’.”

So does the amount of time your toddler spends glued to CBeebies determine whether they’ll be a chunky child? While this study suggests there might be an association between TV watching and waist size in later life, it would be premature to state that there is a direct link.

The headlines are based on a Canadian study of just over 1,300 children. It assessed their TV viewing at around two and four years old (as recalled by their parents) and then:

their ability to jump from a standing start at age eight
their waist circumference at age 10

The researchers found that each additional hour the children watched TV between the ages of two and four was associated with a 3mm reduction in jump length at the age of eight and a 0.5mm increase in waist circumference at the age of 10. This means that the effects of each additional hour of TV viewing on waist size certainly don't amount to the “inches” suggested in some reports.

The study has some limitations that mean its findings should be viewed with a degree of caution, such as the fact that children who watch more TV may differ from children who watch less TV in other ways too. Although it seems logical that children who spend more time in sedentary activities could have less muscle strength and larger waist circumferences, this study cannot conclusively attribute this to TV watching.

Where did the story come from?

The study was carried out by researchers from the University of Montreal in Canada. No sources of funding were reported. The study was published in the peer-reviewed International Journal of Behavioural Nutrition and Physical Activity.

Although most news sources reported the actual results of the study appropriately, several focused on the idea of TV adding “inches” in their headlines. This made the association between TV and waist circumference sound larger than it is: an additional inch of waist circumference at the age of 10 would be associated with about a (somewhat unrealistic) additional 50 hours of weekly viewing at the age of two, based on the results seen in the study.

The headlines contrast with those of June 21, when the papers reported research that supposedly showed that children who watched a lot of sport on television were often encouraged to be more physically active (TV viewing boosts kids' activity claim examined).

What kind of research was this?

This was an analysis of data collected as part of a cohort study that looked at child development. The current analysis looked at whether TV viewing in young children was related to muscle fitness and waist circumference later in childhood.

This study design is the best way to look at whether one factor might potentially influence another, as it establishes which factor comes first. The limitation to this sort of study is that TV viewing might not be the only factor to influence the outcome. Researchers can use statistical methods to try to remove the effects of other factors (such as overeating) that they know could affect the results. However, the effects may not be fully removed, or there may be other unknown factors having an effect.

What did the research involve?

The researchers took a random sample of 2,837 infants born between 1997 and 1998 from Quebec, Canada. Of this sample, 1,314 children (46%) were followed up to the age of 10 and could be included in the final analysis. When the children were aged 29 months (two years, five months) and 53 months old (four years, five months) their parents were asked how much time their child spent watching TV each day. When the child was 29 months old, the parents were also asked whether their child overate, and how often they took part in physical activity. Overeating and activity were rated between 1 (never) to 5 (several times a day).

The researchers tested the children’s leg muscle strength by measuring how far they jumped forward from a standing position at the age of eight. They were shown how to do the jump, and jumped twice, with the longer measurement taken. The researchers then measured the children’s waist circumference at the age of 10. The measurements were taken three times and the average measurement was used in the analyses.

The researchers took several factors into account in their analyses, including:

birth weight
gender
family income at the start of the study
maternal education
immigration status
maternal body mass index when the child was 17 months old
overeating and physical activity

What were the basic results?

At the age of 29 months, the children watched an average of 8.8 hours of TV a week, ranging from a minimum of about 33 minutes a week to a maximum of 56 hours a week. At the age of 53 months, the children watched an average of 14.9 hours of TV a week, ranging from a minimum of about one hour a week to a maximum of 49 hours a week.

At the age of eight, the children could jump on average about 117.3cm from standing (range 23cm to 184cm). At the age of 10, the average waist circumference was 64.8cm (range 48cm to 114cm).

More TV viewing in early life was associated with less leg muscle strength on the jumping test at the age of eight, and higher waist circumference at the age of 10. Every additional hour of weekly TV viewing at the age of 29 months was associated with jumping 0.4cm less far at the age of eight. Every hour’s increase in weekly TV viewing between 29 months and 53 months was additionally associated with jumping 0.3cm less far.

Watching TV at the age of 29 months was not significantly associated with waist circumference at the age of eight. However, every hour’s increase in weekly TV viewing between 29 months and 53 months was associated with an additional 0.05cm waist circumference. This difference was only just statistically significant.

How did the researchers interpret the results?

The researchers concluded that watching too much TV in early childhood may compromise muscle fitness and waist circumference in children as they approach puberty.

Conclusion

This study has found an association between TV viewing in early childhood and leg muscle strength and waist circumference in later childhood.

The study has the advantage that it measured TV viewing first and then measured waist circumference and leg strength later, rather than measuring them all at the same time. This makes it more likely that TV viewing could be influencing the outcomes measured. However, there are limitations to the study, including:

Children who watch more TV may also differ from children who watch less TV in other ways; for example, their diet may be different. Such differences could be influencing the results. The researchers did try to take some other factors into account, such as overeating and physical activity, but these were measured only once in childhood and in a very basic way. This means that the researchers’ adjustments may not have completely removed the effects of these or other unknown factors (confounders)
TV viewing was only assessed by asking parents on two occasions. The parents’ responses may not have been very accurate, particularly if they felt their child’s TV viewing, if excessive, could reflect badly on them. The children’s viewing may also have changed over time.
Only a single measure of muscle strength was used, and this may not reflect the children’s overall muscle strength or fitness.
The average difference in standing long-jump length and waist circumference associated with each additional hour of TV viewing on was small – only 4mm jump length and only 0.5mm in waist circumference.
The study did not assess whether the children were over the recommended waist circumference for their age, so we do not know if these children would be considered to be an unhealthy size.
Over half of the original random sample of infants was not followed up. If all of the children were included this could have affected the results.

Overall, this study by itself cannot prove that TV viewing is directly causing the differences reported. However, the authors said that their findings suggest the need to explore this further. In particular, they suggested that future studies could look at longer-term fitness and health effects. At all ages, it is important to have enough physical activity to remain strong and healthy.

Analysis by NHS Choices. Follow Behind the Headlines on twitter.