Time spent sitting may raise chronic disease risk

"Stand up if you want to stay healthy, warn researchers," the Daily Mail reports, as a large study has found an association between time spent sitting down and chronic disease.

The study surveyed middle-aged Australian men at one point in time. It found that after adjusting for other factors associated with disease (such as body mass index and physical activity levels) men who said that they sat for more than four hours a day were at increased risk of being diagnosed with a chronic disease, including cancer, heart disease, diabetes and high blood pressure.

When these chronic diseases were examined separately, sitting for more than six hours a day was associated with increased odds of diabetes, and sitting for at least eight hours a day was associated with increased odds of high blood pressure.

However, this study cannot prove that increased time spent sitting down leads to the development of chronic diseases – other factors may have been involved. And it cannot tell whether increased sitting time occurred before or after the development of chronic diseases. People with a chronic disease such as diabetes may have a more sedentary lifestyle as a result of their condition.

Despite its limitations, this study seems to add to the growing body of evidence that physical inactivity is bad for you.

Where did the story come from?

The study was carried out by researchers from the University of Western Sydney, Australia, and Kansas State University in the US. No sources of financial support for this study are explicitly stated.

The study was published in the peer-reviewed International Journal of Behavioural Nutrition and Physical Activity.

The study was reasonably well-reported by The Daily Telegraph and the Daily Mail, but there were several mistakes and omissions in the reporting.

First, both headlines talk about sitting down increasing the risk of cancer. However, although the study found a link between increased time spent sitting and the odds of developing any chronic disease, when cancer was examined by itself no significant association was found. This means the results seen could have been the result of chance.

Second, most of the reporting focused on the potential risk faced by office workers, but the study looked at time spent sitting – not occupation.

The research suggests that all groups of men who spend long parts of the day sitting down – whether they are unemployed, or work as bus drivers or air traffic controllers – have the same potential increase in risk.

What kind of research was this?

This was a cross-sectional study. It aimed to explore the association between sitting time and a range of chronic diseases in middle-aged Australian males.

Cross-sectional studies highlight possible associations between behaviour and health outcomes, but they cannot prove a cause and effect relationship.

Because information in cross-sectional studies is only taken at one point in time, this study cannot determine whether increased sitting time is associated with the development of chronic diseases or whether chronic diseases are associated with increased sitting time.

Cohort studies, where people are followed over time, are needed to determine which came first.

Even in a cohort study it would be difficult to pinpoint time sitting down as the influencing factor on the risk of chronic disease, as various health, lifestyle and behavioural factors may have an effect.

What did the research involve?

This study used information from 63,058 men aged between 45 and 64 years old who were living in New South Wales, Australia.

The men were asked to complete a questionnaire reporting:

• whether they had ever been told by a doctor that they had a chronic disease (cancer – excluding skin cancer, heart disease, diabetes or high blood pressure)
• how much time they spent sitting each day
• how much walking, moderate and vigorous activity they performed each week
• highest educational qualification
• household income
• smoking status
• height and weight (to calculate body mass index)
• functional limitation, which is the extent to which an individual’s health limits their ability to perform daily functional activities (measured using the Medical Outcomes Study Physical Functioning scale)

The researchers then examined the odds of having each, or any, of the following chronic diseases:

• cancer (excluding skin cancer – risk factors for skin cancer are different from those for most other types of cancer)
• heart disease
• diabetes
• high blood pressure

For each chronic disease they calculated the risk associated with the following categories of sitting time:

• less than four hours
• four to less than six hours
• six hours to less than eight hours
• at least eight hours sitting time a day

The researchers adjusted their analyses to account for the following confounders: physical activity, age group, educational qualification, household income, smoking status, BMI and functional limitation.

What were the basic results?

Any chronic disease

The researchers found that increasing sitting time was associated with increased odds of any chronic disease. After adjusting for potential confounders, the researchers found that, compared with men who reported sitting for less than four hours a day, the odds of having any chronic disease were:

• 6% higher in men who reported sitting for between four and six hours a day
• 10% higher in men who reported sitting for between six and eight hours a day
• 9% higher in men who reported sitting for at least eight hours a day

Diabetes and high blood pressure

When chronic diseases were analysed individually, it was found that men who reported sitting for between six and eight hours a day had significantly increased odds (15%) of diabetes compared with men who reported sitting for less than four hours a day. Men who reported sitting for at least eight hours a day also had significantly increased odds of diabetes (21%) and significantly increased odds of high blood pressure (6%) compared with men who reported sitting for less than four hours a day.

Cancer or heart disease

There were no significant associations between sitting time and cancer or heart disease.  

How did the researchers interpret the results?

They state that: “Independent of physical activity, BMI, and additional covariates, sitting time was significantly associated with diabetes and overall chronic disease in this sample of Australian males.”

Conclusion

The results of this study suggest that there is an association between longer periods of time spent sitting down and diabetes and overall chronic disease in middle-aged Australian males.

Previous research has also found that the amount of time spent sitting down (‘sedentary behaviour’) is a risk factor for health, and that this risk factor is independent of the amount of time spent performing physical activity. This large study, which took into account several potential confounding variables, adds to this body of evidence. However, this study has limitations:

• Due to the cross-sectional study design, we cannot tell whether increased sitting time occurs before or after the development of chronic diseases.
• All of the data was self-reported by the men in the study. This means that it could have been affected by recall bias, or by under- or over-reporting.
• Not all people who were invited to complete the questionnaire did so. This means that there could have been a ‘selection bias’ in the recruitment of participants. This could have skewed the results either way – healthy people may have been more likely to respond or, alternatively, men more worried about their health may have been more likely to respond.
• It should also be remembered that this study was performed in middle-aged Australian men, and it is unclear whether the results can be generalised to other populations.

Despite these limitations, this study offers further support for the current physical activity recommendations for adults and the fact that physical inactivity can be bad for you.