Wednesday April 20 2011
The number of people who work at a desk all day is growing
“Having a desk job for 10 years can almost double bowel cancer risks,” reported the Daily Mirror. It said that a study has also found that it doesn’t matter how much recreational activity you get outside work, the risk is still the same.
The story is based on an Australian study of 918 people with large bowel (colon) cancer and 1,021 controls without cancer, which looked at their occupational and lifestyle habits. Participants, between the ages of 40 and 79, who had spent 10 years or more in a sedentary job had almost twice the risk of cancer of the last section of the colon (distal colon) than those who never had a sedentary job. The risk was independent of any recreational physical activity such as going to the gym.
However, these findings do not conclusively show that prolonged sitting causes bowel cancer. The study had several limitations. Only 45 people with cancer of the distal colon and 96 controls had worked in a sedentary occupation for more than 10 years, and statistical comparisons between such small numbers can be inaccurate. The study also relied on people recalling their lifetime recreational physical activity levels and only used job titles to assess how much time people spent sitting.
Growing numbers of people work in jobs that involve prolonged periods of sitting. The health effects of this type of work, and strategies to modify any adverse effects, are important areas for further research.
Where did the story come from?
The study was carried out by researchers from the University of Western Australia, Perth. It was funded by the Australian National Health and Medical Research Council. The study was published in the peer-reviewed American Journal of Epidemiology.
The study was covered by several newspapers and was generally reported accurately. The Daily Express headline that 10 years in a desk job “could be lethal” was misleading, as the study did not look at mortality.
What kind of research was this?
This case control study set out to investigate whether sedentary work is associated with specific types of colorectal (large bowel) cancer. This type of study compares people with a disease or condition with those who do not have it (the control group). Researchers then collect information about both groups to ascertain whether any differences could affect the risk of disease. This type of study is usually retrospective, meaning that it takes people who already have the disease in question and examines their histories. As evidence, case control studies are considered weaker than cohort studies, which follow disease-free people over a number of years, to see which ones develop disease. A cohort design would be possible for an outcome such as bowel cancer as it is a common disease.
The authors say that research suggests that prolonged sitting may increase the risk of some chronic diseases, including colorectal cancer. However, these previous studies did not take into account how much recreational physical activity people also did, and whether it affected their cancer risk.
What did the research involve?
Researchers recruited 918 patients with confirmed colorectal cancer between 2005 and 2007, through the Western Australian Cancer Registry. Participants were aged between 40 and 79 and had been diagnosed with cancers in different sites of the bowel. As a control group, 1,021 controls who were matched for sex and age to the people with cancer were recruited from the Western Australian electoral roll.
Information was collected from both groups about their lifetime job history – from first job to retirement – and on their lifestyle, diet, level of recreational physical activity and medication use.
The degree of occupational physical activity was calculated by classifying each job into one of five categories, using an accepted classification rating. The categories were sedentary (e.g. bookkeepers), light activity (e.g. teachers and housewives), medium (e.g. nurses), heavy (e.g. plumbers) and very heavy activity (e.g. miners). The researchers also calculated the number of years spent in each level of job and recorded whether the work was part time or casual. The number of years the participants spent in sedentary work was categorised as none, more than zero but less than 10 years, and 10 or more years.
The researchers excluded 48 cancer cases and 25 controls because of missing data, leaving 870 cases and 996 controls for the analysis. They used statistical methods to analyse possible associations between the amount of sedentary work and the risk of cancers of the proximal colon, distal colon and rectum. The proximal colon is the first part of the colon after the small bowel (including the ascending colon and transverse colon). The distal colon is the lower part of the colon (including the descending colon and sigmoid colon, which leads into the rectum).
What were the basic results?
The researchers found that, compared with those who did not spend any time in sedentary work, people who spent 10 or more years in sedentary work had almost twice the risk of distal colon cancer (adjusted odds ratio 1.94, 95% confidence interval 1.28 to 2.93). This association was independent of how much recreational physical activity people did and was seen even among the most recreationally active participants.
A similar association was observed for cancer of the rectum, but this did not reach statistical significance (adjusted odds ratio 1.44, 95% confidence interval 0.96 to 2.18).
Proximal colon cancer had no association with sedentary work.
How did the researchers interpret the results?
The researchers say these results suggest that long-term sedentary work may increase the risk of distal colon cancer and rectal cancer, independent of physical activity. They suggest several plausible biological mechanisms through which this might happen. One of these is that prolonged sitting has been shown to increase blood glucose levels and decrease insulin sensitivity. These are both thought to promote colorectal cancer. Sedentary behaviour has also been linked to a greater risk of diabetes and obesity, both risk factors for this type of cancer.
The researchers say their findings have occupational health implications, especially since advances in technology have led to increasing amounts of sitting at work.
This is a well-conducted study, using confirmed cases of bowel cancer and comparing their occupational and lifestyle habits with people without cancer. The researchers took into account several other factors that might influence the risk of bowel cancer, such as lifetime recreational physical activity and diet, age, smoking and body mass index (BMI). The findings have several limitations that need to be considered when interpreting the results:
- Cancer cases were taken from the Western Australia Cancer Registry. As it is mandatory to register all cases of cancer in Australia we can be fairly certain that the study captured all people in this area diagnosed with bowel cancer between 2005 and 2007. Controls were randomly selected from the electoral role, which means they should be representative of the general population. However, as only 46.5% of controls invited to participate responded, it is possible that they have certain behavioural or lifestyle factors influencing their decision to participate that differs from those who chose not to. Therefore, the controls may not be representative of the general population without cancer. It should also be noted that as this study was conducted in Australia, results may differ from what would be observed in other populations worldwide.
- It is not necessarily the case that people with the same job title would have the same levels of activity in their jobs, and basing sedentary levels on job title is not an ideal way of assessing them. For example, nurses were categorised as having medium occupational activity, but the amount of activity a nurse has will differ depending on the type of nursing that they do.
- The study relied on people remembering and self-reporting their recreational activity over many years.
- The researchers did not take into account the amount of sedentary behaviour outside work, such as in the home or using transport.
- Only 45 people with cancer of the distal colon had worked in a sedentary occupation for more than 10 years while196 did no sedentary work. Similarly, 96 controls did sedentary work for more than 10 years compared with 805 controls who did no sedentary work. These figures calculate to give an odds ratio that a person who does sedentary work for more than 10 years has almost twice the odds of having cancer than a person who does no sedentary work. However, though the overall study sample is large, the number of people working in sedentary jobs for more than 10 years was fairly small, and statistical tests using such small numbers can be inaccurate. Assessment of a larger number of people who had worked in a sedentary job for over 10 years would give a more accurate indication of the true size of the risk association.
- Most importantly, it is possible that prolonged sitting itself is not having the effect upon bowel cancer risk, but that it is a marker for another possible risk factor that is actually behind the association, for example, obesity, poor diet or low levels of vitamin D.
Technological advances mean that increasingly large numbers of people are engaged in desk jobs that involve sitting down for much of the day. The health effects of this type of work and strategies needed to modify any adverse effects is an important area worthy of further research.