June 10, 2015

expert reaction to study investigating screen time and bone health in adolescents

Low physical activity is a risk factor for a number of diseases, and researchers publishing in the BMJ Open have examined a possible association between activity and time spent in front of television or computer screens at weekends. The research team looked Norwegian adolescents and report that in boys, “screen time” was negatively associated with bone mineral density.

Dr Luis Gracia-Marco, Lecturer in the Children’s Health and Exercise Research Centre (CHERC), University of Exeter, said:

“Research on the effects of sedentary behaviours on bone mass during growth (i.e. adolescence) is of public health importance. Although a few cross-sectional studies have found negative associations between certain sedentary behaviours and bone-related variables, this is the first study showing significant associations between screen-based sedentary behaviour and bone with repeated measurements in adolescents.

“The present study has been performed in a large and representative sample of Norwegian adolescents and appropriate statistical methods have been used. The authors have controlled the effect of some potential variables.

“The present study adds new evidence in this research area. However, the results and conclusions about gender differences should be interpreted with caution for several reasons. Firstly, this was a self-reported screen time questionnaire. It did not distinguish between different screen modalities (different type of screen-based activities e.g. television, computer work, video games etc.). It is known that different screen modalities are associated to different energy expenditure or even different energy intake and this may induce some bias.

“Another limitation is the type of physical activity data used. As the authors discussed in the paper, physical activity is a strong predictor of bone mass. For this reason, the study would be more rigorous if physical activity had been objectively measured rather than self-reported by the adolescents. Usually those less active tend to overestimate their physical activity levels. However, this is understandable from a logistic/economic point of view given that the sample size was relatively large.

“Thirdly, the influence of soft tissues was not taken into account. The authors discussed the role of lean mass and fat mass on bone mass. From what we know, it is clear that lean mass is a very strong predictor of bone mass and its effect seems to be stronger than that of fat mass. However, neither lean mass nor fat mass were controlled in the analyses and the body mass index was used instead. A variable such as body mass index does not distinguish between lean, fat or bone mass and therefore its use may induce some bias in the results. The authors and press release also suggest that body fat distribution may help protect girls’ bones at this age, however this cannot be concluded from this study because body fat measurements have not been considered in the analysis.

“Overall, the conclusion from the present study is clear, however, it has to be a bit more specific when referring to the follow-up data. In this sense, only one variable (total hip) was found to be significant 2 years later. The title of the paper could be more accurate and clarify that screen-time was obtained (not just computer use) and follow-up data was analysed (not just cross-sectional data).”

Dr Benjamin Jacobs, of the Royal College of Paediatrics and Child Health, said:

“This is an important study showing that teenagers in Norway spend far more time in front of screens than the two hour recommended maximum, especially boys who had average of 4 hours on weekdays and 5 hours over weekends.

“The teenage boys with more screen time had significantly lower bone mineral density, lower vitamin D levels and higher BMI. This effect was found again when the study was repeated 2 years later. This relationship was not found in girls for reasons that are not yet clear.  However the low bone density in the boys who spend more than 6 hours a day in front of screens on the weekend is a major concern.

“This large important study adds further weight to the RCPCH’s drive to support paediatricians and others, to promote healthy lifestyles for young people in the UK. There are a number of ways they can do this – adequate exercise, screen time limited to less than 4 hours a day and appropriate vitamin D supplements to improve the bone health and general health of children in the UK.”

‘Leisure time computer use and adolescent bone health – findings from the Tromsø Study, Fit Futures: a cross-sectional study’ by Anne Winther et al. published in BMJ Open on Wednesday 10 June 2015. 

Declared interests

Dr Luis Gracia-Marco: “I declare I have no conflict of interest.”

Dr Benjamin Jacobs: Dr Ben Jacobs is also a consultant paediatrician.