Monday October 15 2012
It is estimated that 4 in 5 people in the UK have a desk job
‘Sitting for long periods increases the risk of diabetes, heart disease and death, researchers suggest’ BBC News reports.
The news is based on the findings of a review which summarised the results of all the observational studies that had looked at the association between the time spent sitting or lying down whilst awake (sedentary behaviour) and the risk of diabetes, cardiovascular disease, and death due to cardiovascular disease (such as heart attack) or any cause.
The link between worsening health outcomes and time spent sitting first became apparent in the 1950s when researchers found that London bus drivers were twice as likely to have heart attacks as their bus conductor colleagues.
The researchers were concerned that, due to changes in lifestyle and employment, the health problems associated with sedentary behaviour are likely to have worsened. They cite the findings of a 2011 study showing that the average adult now spends 50-60% of their day in sedentary pursuits.
The main findings of the study were that, compared to the shortest time spent sedentary, the longest time spent sedentary was associated with a:
- 112% increase in risk of diabetes
- 147% increase in cardiovascular events
- 90% increase in death due to cardiovascular events
- 49% increase in death due to any cause
This study cannot show that sedentary behaviour is the direct cause of the increases in risk. Nevertheless, it certainly seems to reinforce the recommendations that adults should perform at least 150 minutes of moderate-intensity aerobic activity every week.
Our ancestors did not have had to worry about going to the gym when they were spending 12 hours a day working in a coal mine or picking turnips. These days, however, for most of us, the working day provides very little opportunity for exercise and so we need to compensate for that fact.
Where did the story come from?
The study was carried out by researchers from Loughborough University and the University of Leicester. The primary author is being funded for a PhD in the Department of Cardiovascular Sciences, University of Leicester.
The study was published in the peer-reviewed medical journal Diabetologia.
The research was well-reported by the BBC, Daily Mail and Daily Express.
Both the Mail and the BBC included a number of useful quotes from the researchers involved in the study. For example, Professor Stuart Biddle (one of the research team, and Professor of Physical Activity and Health at Loughborough University), is quoted as saying ‘'There are many ways we can reduce our sitting time, such as breaking up long periods at the computer at work by placing our laptop on a filing cabinet (and then use them while standing up). We can have standing meetings, we can walk during the lunch break, and we can look to reduce TV viewing in the evenings by seeking out less sedentary behaviours’.
What kind of research was this?
This was a systematic review and meta-analysis looking at the association between the time spent sitting or lying down whilst awake, and the risk of diabetes, cardiovascular disease, and death due to cardiovascular disease (such as heart attack or heart failure) or any cause.
This study design is the best way to summarise what all of the existing research says on a research question. However, even a well-conducted systematic review (which this was) that has identified all relevant observational studies is likely to contain inherent limitations due to the differences in the study design, included populations, method of assessing exposures and outcomes, and duration of follow-up.
Such studies also cannot demonstrate cause and effect as there may be other confounding factors associated with both sedentary behaviour and disease risk (for example smoking, alcohol, diet, or socioeconomic factors) which the individual studies may not all have taken into account.
What did the research involve?
The researchers searched literature databases for cross-sectional and prospective cohort studies looking at the association between the time spent sitting or lying down whilst awake and health outcomes in adults.
The researchers assessed the quality of the studies. They then extracted data on the outcomes associated with the highest sedentary time compared with the lowest, and pooled the results of the studies. Where available, the researchers extracted the results that had adjusted for the greatest number of potential confounding factors that may be influencing the results (for example age, sex, education, smoking status, and diet).
What were the basic results?
The researchers included 18 studies (with a total of 794,577 participants) that had examined the association between the time spent sitting down and the risk of health outcomes (16 prospective cohort studies and two cross-sectional studies). The studies examined the association between sedentary time and diabetes (ten studies), cardiovascular disease (three studies), cardiovascular mortality (eight studies), and all-cause mortality (eight studies).
The studies were performed in a range of countries, including Australia, England, Canada, Germany, Japan, Scotland and the US. The researchers judged 15 of the studies to be of high quality.
All of the studies used a self-reported measure of sedentary time.
After pooling the results of the studies, the researchers found that the greatest time spent sitting compared with the lowest time spent sitting was associated with a:
- 147% increase in the risk of cardiovascular events (relative risk [RR] 2.47; 95% confidence interval [CI] 1.44 to 4.24)
- 112% increase in the risk of diabetes (RR 2.12; 95% credible interval [CrI] 1.61 to 2.78) - a credible interval differs from a confidence interval, in that rather than being based purely on the data provided by the study, it also takes into account prior data
- 90% increase in the risk of cardiovascular mortality (hazard ratio [HR] 1.90; 95% CrI 1.36 to 2.66)
- 49% increase in the risk of all-cause mortality (HR 1.49; 95% CrI 1.14 to 2.03)
Despite the fact that the studies were from a range of countries, and that each study was performed differently, the time spent sedentary was consistently associated with poorer health outcomes.
The researchers then limited their analyses to only include results that had controlled for the amount of physical activity people performed.
Although this changed the relative risks and hazard ratios, it did not change the conclusions: that sedentary time is associated with an increased risk of poor health outcomes. This suggests that the increase in risk seen is not due to the fact that people who spend longer sitting also perform lower amounts of moderate to vigorous physical activity.
How did the researchers interpret the results?
The researchers conclude that “sedentary time is associated with an increased risk of diabetes, cardiovascular disease, and cardiovascular and all-cause mortality; the strength of the association is most consistent for diabetes”. The researchers go on to propose that the results of their study suggest “that substituting sedentary behaviour with standing or light-intensity physical activity may reduce the risk of chronic disease and mortality, independently to the amount of [moderate-to-vigorous intensity physical activity] taken”.
In this systematic review, the results of all observational studies which have looked at the association between the time spent sitting or lying down whilst awake and the risk of diabetes, cardiovascular disease, and death due to cardiovascular disease or any cause, were pooled. This was done in order to determine the risks associated with sedentary behaviour.
Increased sedentary behaviour was associated with increased risk of all health outcomes.
The researchers also found that this effect was not mediated by the amount of moderate-to-vigorous physical activity people performed, and suggest that to reduce risk of diabetes, cardiovascular disease and death, people should try and substitute sedentary behaviour with standing or light-intensity physical activity.
As one researcher put it in a interview, “You can go for a 30 minute run every day, but if you're sitting around for the rest of the day, you're not doing yourself any favours.”
A systematic review and meta-analysis, like this study, is the best way to summarise what all of the existing research says on a research question. Still, both techniques, and this study, are subject to a number of inherent limitations:
- cross-sectional and cohort studies – the analysis which the study was based on – can never prove a direct casual effect, just detect association
- there may be other confounding factors not taken into account, such as age, smoking, alcohol, diet, presence of other (comorbid) diseases, and socioeconomic factors (though the researchers did try to take such factors into account whenever possible)
- all of these studies relied on self-reporting on sedentary behaviour – self-reporting is notoriously prone to inaccuracies
- each of the studies analysed and reported sedentary behaviour in different ways
Despite these limitations, this was a well-conducted and useful study. It would strongly suggest that many of us need to find ways to compensate for our desk-job lifestyles.
Performing moderate-to-intense physical activity (at least 150 minutes per week) and minimising the amount of time spent sitting is already recommended by the Department of Health. In addition, decreasing the amount of time we spend sitting, for example by holding ‘standing meetings’ could also be beneficial.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.