Exercise as it relates to Disease/The relationship between sedentary behavior and mortality

This is an analysis of the journal article ‘Sitting time and mortality from all causes, cardiovascular disease, and cancer’ by Katzmarzyk et al, (2009).



What is the background to this research?
It is well known that regular physical activity is associated with reduced mortality and morbidity. It also reduces the risk of developing cardiovascular disease, coronary heart disease, obesity, type 2 diabetes, cancer and osteoporosis. For this reason the physical activity recommendations focus on increasing exercise participation. Recent studies have found that increased sedentary behaviour is also associated with metabolic syndrome, cardiovascular disease, obesity and type 2 diabetes.

In the modern society there is a greater opportunity for sedentary behaviour such as watching television, using the computer and sitting in the car for transportation. It is reported that children and adults in the United States spend an average of 7.7 hours per day sedentary.

Therefore the purpose of this study was to determine the relationship between sedentary behaviour and mortality rates from all causes. It is also important to identify if excessive sedentary behaviour in individuals who meet the physical activity recommendations can cause adverse health effects. If this is the case then future physical activity guidelines need to include recommendations addressing sedentary time.

Where is the research from?
This study used data from the 1981 Canada Fitness Survey which included people from urban and rural areas of every province of Canada. There are no potential conflicts of interest from the authors in this study that may bias the results. The authors have previously conducted studies looking at adverse health effects of physical inactivity and obesity.

What kind of research was this?
This is an observational study where the authors prospectively examined sitting time and mortality in a representative sample of Canadians.

What did the research involve?
The participants in the study were a sample of 17,013 Canadian men and women aged 18–90 years, who took part in the 1981 Canada Fitness Survey. Aboriginal people living on reserves, institutionalized persons, armed forces personnel living on bases and residents of the Territories and remote areas were excluded from participating in the study.

Baseline measurements were conducted as shown in the table below:

Participants mortality status was then observed for a maximum of 12.9 years. Mortality was recorded and grouped into categories:
 * Cardiovascular disease.
 * Cancer.
 * Other (respiratory disease, injuries and violence, mental and nervous system disorders, digestive system disorders, others).

Strengths of the study

 * It was a prospective study compared to a retrospective study. Prospective studies are classified higher because the investigators have control over what outcomes are measured and this reduces any potential bias.
 * Included a large sample size of participants.
 * An exclusion criteria was used.
 * Adequate baseline measurements where taken.

Limitations of the study

 * This type of study did not consider if lifestyle variables changed over the follow up time period. It is likely that over the 12.9 years these variables would have changed in many of the participants and this would influence the results.
 * The participants were not randomly selected which indicates potential for selection bias from the authors.
 * The measurements of daily sitting time and physical activity were obtained by self report. Studies have shown that self reported measurements of sedentary time are under reported and physical activity levels to be over reported when compared to objective measurements. This indicates that baseline measurements of sitting time and physical activity are likely to be inaccurate.

What were the basic results?


During the follow-up interval there were 1,832 deaths (759 from cardiovascular disease, 547 from cancer and 526 from ‘other’ causes). A total of 204,732 person-year of follow up was accumulated.

Daily sitting time was positively associated with increased mortality rates, except cancer. Additionally, the association between sitting time and mortality was independent of physical activity levels.

When grouped with BMI, the highest mortality rates were observed in obese individuals who reported sitting for almost all of the time.

I believe that the authors claims are appropriate as more recent studies have found similar results with an association between sedentary behavior and mortality.

What conclusions can we take from this research?
From these results we can assume that there is some increased risk of mortality in people who sit for longer periods of time throughout the day. Those with higher BMI’s are most at risk as the highest mortality rates observed were in obese individuals who spend most of the day sitting.

Results also found that people who met the physical activity recommendations but also had increased sitting time were still positively associated with mortality risk. This result may suggest that sitting for long periods throughout the day cannot be compensated for with physically activity. It also indicates that prolonged sitting throughout the day has its own independent risk factor on mortality irrespective of physical activity levels. This suggests that future physical activity guidelines need to consider recommendations on daily sitting time.

A recent Meta Analysis study also found that prolonged sedentary time is independently associated with reduced health outcomes regardless of physical activity level. Other research has found that breaking up long periods of sitting can help improve glucose and insulin levels and reduce the risk of cardiovascular disease. Since these results have been obtained changes in Physical Activity guideline have been made. The current Australian Physical Activity Recommendations now include recommendations on sedentary behaviour.

Practical advice
The advice I would take from this study is that sitting for long periods can cause adverse effects on people's health. However due to modern day society and certain occupations, sitting is often unavoidable. Therefore trying to break up long periods of sitting as well as maintaining regular physical activity is the best practical advice.

Further information/resources

 * Current Australian Physical Activity Guidelines: http://www.health.gov.au/internet/main/publishing.nsf/content/health-pubhlth-strateg-phys-act-guidelines#apaadult
 * Ideas to help increase physical activity and reduce sedentary behaviour: http://www.health.gov.au/internet/main/publishing.nsf/Content/ti-18-64years