“Watching the World Cup on TV could kill you,” according to the Daily Express.
The news story is based on a study that followed over 13,000 adults (average age 61) for almost 10 years to see how their level of TV viewing was associated with their risk of death. After adjusting for various lifestyle factors that could affect the relationship, the reseachers calculated that each extra hour of TV viewing a day raised the risk of dying from any cause by 4% and of death from cardiovascular disease by 6%. However, a further analysis suggests that greater body fat among those watching more TV may explain these increases.
There are some problems with the study, such as only surveying TV viewing on a single occasion, which may not reflect a person’s lifetime viewing habits. Greater sedentary TV viewing and increased mortality were also associated with issues such as less physical activity and greater energy consumption through food. This study did not look at any risk associated with one-off rises in viewing time or the tension from watching football matches, and its results do not suggest that a ‘binge’ of increased viewing at one point in time is going to increase your risk of dying.
Where did the story come from?
This study was conducted by researchers from Medical Research Council’s Epidemiology Unit, Cambridge, and the Department of Public Health and Primary Care at the University of Cambridge. The study was funded by several sources including the Medical Research Council and British Heart Foundation and published in the peer-reviewed International Journal of Epidemiology.
While most newspapers have correctly reported the findings of this research, some present them in a confusing context, even suggesting that watching more TV on a single occasion will raise an individual’s risk of death. This research was concerned with TV viewing as a marker of a sedentary or unhealthy lifestyle in the long term, rather than any danger from a short-term increase in viewing such as watching the world cup.
Some newspapers also suggest that each hour of TV viewed increases the risk of death rather than clarifying that it is each additional hour a day that was associated with a rise in risk.
What kind of research was this?
This was a cohort study that aimed to examine the relationship between TV viewing and mortality due to any cause, but specifically from cancer and cardiovascular causes. A cohort study is the best way to assess cause and effect and, in this case, the research has rightly excluded those with cardiovascular disease and cancer at baseline. It also has a lengthy follow-up of about 10 years. However, many of the measures, including TV viewing, were reported by participants on only one occasion, and may not be reflective of participants’ behaviours over their lifetime or even the decade-long study.
What did the research involve?
This research used the Norfolk section of The European Prospective Investigation into Cancer and Nutrition (EPIC) study, a large ongoing study conducted across 10 countries. Between 1993 and 1997, EPIC Norfolk recruited 25,633 adults (aged 45 to 79 years old), who lived in Norfolk and who were followed up three to five years later in 1998 to 2000.
At this follow-up session the researchers conducted a comprehensive physical activity questionnaire featuring questions on work and leisure activities. This was used to calculate physical activity energy expenditure (metabolic equivalents x hours/week). The questionnaire also asked about time spent watching TV each week, with the sum total calculated using four questions on viewing patterns before 6pm, after 6pm, on weekdays and on weekends.
After excluding those who had a self-reported history of stroke, heart attack or cancer at this follow-up, and excluding those who did not complete full assessments, they were left with 13,197 men and women (average age 61.5 years). They then followed these adults for a further 9.5 years (on average) until 2009, identifying all deaths and causes of death by this time using data from the Office of National Statistics.
The researchers examined how the risk of mortality changed with each additional hour of TV viewing a day. They carried out numerous statistical models adjusting for various confounding factors that could be affecting the relationship. These include total physical activity energy expenditure (PAEE), education level, smoking status, alcohol consumption, blood pressure and cholesterol medications, body measurements, personal and family medical history, and total energy intake (estimated from a food-frequency questionnaire).
What were the basic results?
A total of 1,270 adults died during follow-up (725 men and 545 women): 373 of these were due to a cardiovascular cause and 570 due to cancer. Those who died from any cause were generally less physically active, had a poorer health profile for several other measures (including waist circumference and BMI) and watched, on average, 0.4 hours more TV a day than survivors. People who died from cardiovascular causes watched 0.6 more hours a day, and people who died from cancer watched 0.3 more hours a day than survivors.
Following adjustment for confounders, each hour increase in TV viewing a day was associated with a borderline significant 4% increase in risk of death from any cause (hazard ratio [HR] 1.04, 95% confidence interval [CI] 1.01 to 1.09) and 7% increase in risk of cardiovascular death (HR 1.07, 95% CI 1.01 to 1.15). However, there was no relationship with deaths from cancer (HR 1.04, 95% CI 0.98 to 1.10). When the researchers added waist circumference to the factors adjusted for in the analyses, the relationship between TV viewing and risk of death from any cause or cardiovascular cause was no longer significant.
The researchers compared the risk of death due to any cause in those watching the most and fewest hours of TV. They estimated that you could expect a 5.4% reduction in overall mortality if people who watched greater than 3.6 hours of TV a day watched less than 2.5 hours a day instead.
Other factors associated with watching the highest amount of TV each day were current smoking, low educational level, greater BMI, greater use of blood pressure and cholesterol tablets, less physical activity and lower alcohol consumption.
How did the researchers interpret the results?
The researchers suggest that public health recommendations should consider advising a reduction in TV viewing, whilst advocating physical activity.
This large cohort study of over 13,000 people has found that greater TV viewing is associated with a slight increase in risk of death from any cause over an almost 10-year follow-up. However, the findings need to be interpreted correctly:
- The relative increase in risk of death with each hour increase in TV viewing was only small at 4% and was also of only borderline significance, as was that for cardiovascular mortality. The fact that taking into account waist circumference when carrying out the analyses made them non-significant suggests that the effect seen for TV viewing may be because people who watch more TV are more likely to be carrying more fat around their waists.
- Other factors were independently associated with both greater TV viewing and greater mortality risk, such as: smoking, low educational level, greater BMI and waist circumference, greater use of blood pressure tablets, less physical activity and higher energy consumption. Although these factors were adjusted for in the analyses, they and other factors may still have been affecting the results.
- All of the measures assessed in this study (with the exception of height and weight) were self-reported and liable to some recall bias and inaccuracy.
- TV viewing itself was only self-reported on one occasion. This may not reflect a lifetime pattern for the individual. These associations are more likely to be found with longer-term patterns, e.g. greater sedentary TV viewing over a lifetime can reasonably be expected to be associated with other factors detrimental to health, such as less physical activity, higher energy intake compared to expenditure, and possibly other medical health problems.
- Lastly, it is worth noting that these people were, on average, followed up between the ages of 60 and 70. Different patterns of TV viewing and physical activity levels may be expected in younger cohorts, for whom you may also expect different relationships with mortality.
The identified relationship is very unlikely to mean that a binge of increased viewing at one point in time (e.g. the World Cup, as the Express mentions) is going to increase your risk of dying. The key message is that more time spent in sedentary activities reduces time spent in physical activity, and it is greater physical activity and balanced energy intake to energy expenditure that holds the key to improved health.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Daily Telegraph, 24 June 2010
Daily Mail, 23 June 2010
Financial Times, 23 June 2010
Daily Express, 23 June 2010
Links to the science
International Journal of Epidemiology, June 23 2010