According to The Daily Telegraph, we should “limit TV watching to two hours to live longer”, while the Daily Mail has said we should “stand up if [we] want to live longer”.
Both stories are based on a US study that aimed to estimate the effects that behaviour such as sitting down and watching television (“sedentary behaviour”) has on life expectancy. The researchers analysed the results of a range of studies as well as a national survey.
The results of the research are modelled on the assumption that sedentary behaviour would lead to a reduction in life expectancy. This is a reasonable assumption to make, as it is well known that lack of exercise can have a negative impact on your general state of health.
The statistical model used by the researchers estimated that the average life expectancy would be two years longer if adults reduced their time sitting by three hours a day, and 1.4 years longer if they reduced television viewing to less than two hours a day.
This study doesn’t prove that sedentary behaviour is solely responsible for shortened life expectancy. Other factors such as smoking, diet and illness may contribute as well. Furthermore, the fact that the study was looking at people in the US does not mean that its findings necessarily apply to people in the UK.
Finally, these findings do not alter the current UK guidance for adults to take 150 minutes of moderately intense activity each week.
Where did the story come from?
The study was carried out by researchers from the US Pennington Biomedical Research Centre, Louisiana, and Harvard Medical School. The researchers reported that they received no funding. The study was published in the peer-reviewed online journal, BMJ Open.
This story was picked up by several newspapers and online media, most with attention-grabbing headlines. Reassuringly, most articles went on to mention the limitations of the research. The Mail noted that these findings may cause concern among office workers who spend a lot of the day at their desks.
What kind of research was this?
What did the research involve?
For the meta-analysis, the researchers identified five prospective cohort studies among 460 abstracts that looked at this question. They selected those that examined associations between sitting or watching television and all-cause mortality. They only included studies that reported relative risk along with 95% confidence intervals. They then pooled the results and adjusted them for differences in age and gender across groups.
The researchers also reviewed evidence of how common (prevalent) it was for American adults aged 18 and older to spend time sitting and watching television each day. These estimates were obtained from a nationally representative survey (US National Health and Nutrition Examination Survey) using data from 2009-2010. The participants were asked: “How much time do you usually spend sitting on a typical day?” and given the options of:
- less than three hours
- three to six hours
- more than six hours
To assess their amount of television viewing, the participants were asked: “Over the past 30 days, on average how many hours per day did you sit and watch TV or videos?” They were given the options of:
- less than two hours
- two to four hours
- more than four hours
The researchers then combined the results of the meta-analysis and the estimated prevalences to come up with a population-attributable fraction. This is a theoretical estimate of the effects of a risk factor on an outcome at the population level; in this case, all-cause mortality. It represents what could be expected if the people who were inactive became active.
What were the basic results?
The researchers identified two studies that investigated the association between sitting and all-cause mortality and three studies looking at associations between television viewing and all-cause mortality. Following statistical analysis, the main results of this study were:
- The estimated gains in life expectancy were two years for reducing excessive sitting to less than three hours per day (95% confidence interval 1.39 to 2.69).
- The estimated gains in life expectancy were 1.38 years from reducing excessive television viewing to less than two hours per day (95% confidence interval 0.48 to 2.51).
- 27% of all deaths in the US are partly caused by adults spending too much time sitting down.
- Nineteen per cent of all deaths in the US are partly caused by adults watching too much TV.
How did the researchers interpret the results?
The researchers concluded: “Reducing sedentary behaviours such as sitting and television viewing may have the potential to increase life expectancy in the USA”. The researchers did note that their estimates are only theoretical but suggested that the findings could provide an important public health warning.
In response to the study findings, Professor David Spiegelhalter of Cambridge University pointed out that: “Few of us currently spend less than three hours sitting each day, and so this seems a very optimistic target.”
In conclusion, the research does not alter the current advice in the UK for adults to have 150 minutes of moderately intensive activity such as cycling or fast walking each week. This research provides some evidence to put numbers on the link between sedentary behaviours such as sitting or watching television with life expectancy. It’s important to note that this type of research cannot determine that one causes the other; only what the size of an effect might be when given the link. Although the researchers corrected for confounders such as age and gender, nothing else was adjusted for.
Furthermore, there may have been other factors at play, such as smoking or illness, which influenced the results. There are other limitations to this study, some of which the authors noted:
- This study was based on the results of US participants, and therefore its findings may not apply to the UK population.
- Sedentary behaviours were identified by self-report, which makes the results less reliable. It is possible that participants did not accurately report their activity patterns and people may have felt more inclined to describe themselves as more active than they actually were.
- Life expectancy as noted by the authors is a population statistic and does not apply to individuals. The authors noted that these estimates are theoretical estimates only.
Consequently, the Mail’s headline “Stand up if you want to live longer,” is misleading to the reader and should be interpreted with caution, although most of us could benefit from being more active. Information about the recommended amount of physical activity can be found at NHS Choices.
- Katzmarzyk PT, I-Min L. Sedentary behaviour and life expectancy in the USA: a cause-deleted life table analysis. BMJ Open. Published online June 9 2012.