Friday June 27 2014
Is watching TV really dangerous?
“Watching TV for three hours a day can be deadly – doubling your risk of dying early,” the Mail Online reports.
The website reports on a study involving a relatively large group of Spanish university graduates. The participants were asked to self-report time spent on three types of sedentary behaviour: TV viewing, computer use and time spent driving.
They were then followed for between 2 and 10 years to see if any of the participants died prematurely, and if so, if there was a significant association between premature death and types of sedentary behaviour.
In their analysis, the researchers took into consideration potential confounding factors, such as the age, smoking status and total energy intake of participants.
The main finding from this study was that the risk of death was doubled for participants reporting three or more hours of TV viewing a day, compared to those reporting less than one hour a day. Time spent using a computer or driving was not significantly associated with risk of early death.
This unexpected association with TV watching, but not other forms of sedentary behaviour, could be due to the very small sub-group of people who died during the follow-up period – just 0.7% of the cohort. In such a small sample size, there is a significant possibility that any association is simply down to chance.
Where did the story come from?
The study was carried out by researchers from the University of Navarra, in Pamplona, Spain. It was funded by various Spanish government grants, the Navarra Regional Government and the University of Navarra. The study was published in the peer-reviewed, open-access Journal of the American Heart Association, so it is freely available to read online.
The study was picked up by The Mail Online, which appropriately reported the methods and findings, but failed to adequately discuss the study's limitations. It also failed to put the risks of increased premature death into a useful context for readers. During the length of the study, only 0.7% of participants died prematurely – equivalent to around 1 in 142 people.
What kind of research was this?
This was a dynamic prospective cohort study looking at the associations between three types of sedentary behaviour (TV viewing, computer use and time spent driving) and all-cause death in a group of Spanish university graduates. It is called a dynamic study because recruitment to the study is permanently open.
A cohort study examines how particular exposures affect outcomes in groups of people over time. A prospective study looks at these exposures and measures outcomes of interest in these people over the following months or years. Results from prospective studies are usually considered as more robust then retrospective studies, which either use data that was collected in the past for another purpose, or ask participants to remember what has happened to them in the past.
What did the research involve?
This study used data from the wider “Sun Cohort” research. The Sun Cohort is a multi-purpose prospective cohort study, using Spanish university graduates as participants, that was designed to assess the association of diet or lifestyle with the rate of several diseases and death. Recruitment of participants began in 1999.
The researchers collected information on participants through self-administered questionnaires sent by mail at baseline and every two years. The baseline questionnaire included items to assess TV viewing, computer use and time spent driving. Each of these items had 12 possible categories for responding, ranging from “never” to “more than nine hours a day”.
Information on weekday and weekend use was measured separately, calculated to provide data over a week (five weekdays, two weekend days) and divided by seven, to give each participant’s total time spent per day.
Additional information was captured on the participants':
- medical history
- sociodemographic factors
- body measurements
- physical inactivity
- smoking status
- dietary habits
- adherence to the Mediterranean dietary pattern
In December 2012, there were 20,572 participants who had completed the baseline questionnaire and had been followed up for at least 2 years, up to 10 years. Participants who reported diabetes, cardiovascular disease or cancer at the baseline assessment were excluded from the analyses. Also excluded were those with missing data on TV viewing and those people who were not followed up (who dropped out). Accounting for these exclusions, the researchers carried out their analyses on a total of 13,284 participants.
The main outcome of interest was death from any cause. Most deaths were reported to be identified from next of kin, work associates and postal authorities. The Spanish National Death Index was also checked every six months.
The researchers then used statistical techniques to analyse the data. They considered participants with the lowest exposure time at baseline (the lowest time spent viewing TV, or the lowest amount of time spent driving) as a comparison group to higher levels of exposure. In their analysis, the researchers provided results with varying types of adjustments.
The most adjusted results took into account the following confounders:
- smoking status
- total energy intake (kcal/day)
- Mediterranean diet adherence
- baseline body mass index (BMI in kg/m2)
- leisure time physical activity (metabolic equivalent tasks (METs) per week)
What were the basic results?
There were 13,284 participants (61.6% female) included in the analysis, who had an average age of 37 years and were followed for a median of 8.2 years. There were a total of 97 deaths registered from all causes among these participants (0.7%). The researchers say the expected number of deaths from this population was estimated at 128 for this sample size.
At baseline, participants spent on average:
- 1.6 hours watching TV a day (standard deviation [SD] 1.3)
- 2.1 hours using computers a day (SD 2.1)
- 0.9 hours driving a day (SD 1.2)
The main findings from this study were that in the most adjusted analyses:
- TV viewing was positively associated with all-cause death. The risk of death was doubled for participants reporting three or more hours of TV viewing per day compared to those reporting less than one hour a day (Incidence rate ratio (IRR) 2.04, 95% confidence interval (CI) 1.16 to 3.57). When analysed in a different way, each additional two hours of TV viewing had an incidence rate ratio of 1.40 (95% CI 1.06 to 1.84)
- time spent using a computer or driving was not significantly associated with death
How did the researchers interpret the results?
The researchers conclude that, in this study, TV viewing was directly associated with all-cause mortality. They said, however, that computer use and time spent driving were not signiﬁcantly associated with higher mortality.
When discussing the findings of this study, lead researcher Professor Miguel Martinez-Gonzalez from The University of Navarra is reported as saying the findings "are consistent with a range of previous studies where time spent watching TV was linked to mortality”.
This prospective cohort study provides some limited evidence of an association between TV viewing and death from all causes among a group of relatively young Spanish university graduates. It found that the risk of death was higher for people who watched three or more hours of TV a day compared to people who watched less than an hour a day. Computer use and time spent driving was not found to increase the risk of death.
This study included a relatively large number of people followed prospectively, and it attempted to adjust results for several potential confounders, such as energy intake, age and smoking status.
However, the main limitation of this study is that only the baseline information on participants' total daily time spent on TV viewing, computer use and driving was analysed in its association with risk of early death. Therefore, this study is based on data collected at one point in time and does not reflect changes to the participants' time spent in these activities over the years they were included in the study. A more appropriate analysis would have also considered time spent in these activities at each of the two-year follow-ups.
An additional limitation worth noting, is that time spent in these three activities was collected via self-reporting, so there is a possibility that participants inaccurately reported time spent in these activities.
There is always the possibility that other factors may be influencing the results. As the researchers note, there is a possibility that eating and drinking is more likely to occur with TV viewing than with computer use and driving. However, the researchers say that the associations hardly changed after adjustment for energy intake and of these two factors.
Another important potential confounder could be the health and disability of the people taking part. For example, people with poor health and a disability may be more likely to spend more time viewing the TV, and also more likely to die early. However, as the researchers say, the cohort was fairly young, and they also excluded people with diabetes, cardiovascular disease, and cancer and baseline. This may reduce the possibility of ill health and disability confounding the results.
Another possibility is that they may only be chance observations.
Despite the large sample of over 13,000 people, because of the relatively young age of the population, there were only 97 deaths during follow-up – just 0.7% of the cohort. Examining lifestyle factors associated with such a small number of deaths increases the possibility of chance observations.
However, despite its limitations, the study supports established health recommendations to limit sedentary time and take regular physical activity. Current recommendations are that healthy adults should take at least 150 minutes of exercise a week.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.