Sedentary Physiology Part 2 – Can Sitting Too Much Kill You?

Photo by independentman.

Welcome to our 5-part series delving into the fascinating research being performed in the emerging field of sedentary physiology.  Today in Part 2, we look at some of the health effects associated with excess sedentary behaviour.  For an introduction to the basics of sedentary physiology, check out Part 1.

Over the past few years research has suggested that being sedentary (e.g. sitting or lying down) for extended periods of time has a negative impact on your health. We know that people who sit for long periods of time are less likely to be physically active, and more likely to be obese, so it’s not that surprising that people who sit for extended periods of time would have increased health risk relative to people who sit less. But new research is suggesting that people who spend more time seated are at increased health risk even after control for these other factors. In other words, no matter your body weight or how much you exercise, sitting too much still results in increased health risk.

For example, a recent study by Canadian researchers published in Medicine and Science in Sport and Exercise looks at this very topic. The study, available here, was performed by Peter Katzmarzyk (a former instructor of both Peter J and myself during our time at Queen’s) and colleagues at the Pennington Biomedical Research Center, and examined the links between time spent sitting (at school, work, and at home) and mortality in a representative sample of more than 17,000 Canadians. They report that time spent sitting was associated with increased risk of all-cause and cardiovascular disease mortality (there was no association between sitting and cancer deaths). In fact, individuals who sat the most were roughly 50% more likely to die during the follow-up period than individuals who sat the least, even after controlling for age, smoking, and physical activity levels.

But wait – could it be that the people who sit more are only at risk because they also happen to be obese? Unfortunately body weight was only available in a subset of individuals in the present study, making this a difficult question to address. However, the authors did examine the association between sitting and mortality after control for body weight in this subset of individuals, and report that sitting remained a significant predictor of mortality. This suggests that all things being equal (body weight, physical activity levels, smoking, alcohol intake, age, and sex) the person who sits more is at a higher risk of death than the person who sits less. The authors go on to suggest that sitting for extended periods of time may alter certain physiological processes, such as lipoprotein lipase activity, which could explain the link between time spent sitting and mortality risk (we’ll get to this in Part 4 of the series!).

These findings linking sedentary behaviour with poor health are far from isolated.  For example, a similar longitudinal study from Australia reports that each hour of daily television viewing is associated with an 11% increase in the risk of all-cause mortality regardless of age, sex, waist circumference, and physical activity level. And as we summarize in a review paper published earlier this month, numerous epidemiological studies have linked sedentary behaviour with obesity, cardiometabolic risk, and even some cancers.

As you can see, the evidence is piling up that total sedentary time is a consistent predictor of negative health outcomes.  But does it matter how you accumulate that sedentary time?  For example, does sitting for 2 hours straight have the same health impact as sitting for 2 hours total, but with one hour of physical activity in the middle?  Be sure to check-in tomorrow for Part 3 (now online!) in our series to find out!

Travis

Katzmarzyk, P.T., Church, T.S., Craig, C.L., & Bouchard, C. (2009). Sitting Time and Mortality from All Causes, Cardiovascular Disease, and Cancer Medicine and Science in Sports and Exercise, 41 (5), 998-1005

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14 Responses to Sedentary Physiology Part 2 – Can Sitting Too Much Kill You?

  1. This is important information for upcoming study designs. For a researcher in exercise physiology working outside the exercise metabolism field, I know that I am doing a poor job when comes the time to recruit sedentary subjects :-) . It seems that they are not really sedentary ! Most likely, the differences I am usually reporting between clinical populations and controls are underestimated… From now on, I will at least pay more attention on how I am describing my control group (sedentary vs. lack of regular physical activity) !

    The next question would be: According to these findings, should we still use sedentary individuals in our control groups ??

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    • Travis says:

      Excellent! That is exactly the type of change in wording that we are hoping to see in the field! I’m hoping that soon the terms will become a bit more widely accepted and understood (hence doing a whole week of posts devoted to this one issue!), so that when a researcher talks about “sedentary” individuals we can be absolutely certain that we know what they are talking about. On a related note, do you use accelerometers to identify how active and/or sedentary subjects are? I might not have a chance to get into this later in the week, but obviously the self-reported measures tend to be weaker than the objective ones.

      I think the issue of who should form the control group is a different one – I know that some have argued that we should use physically active, non-sedentary controls in our studies, since these are the true “norm” physiologically. I’ve debated this a bit with colleagues when planning future studies as well. I think it depends on what question you are trying to get at – sometimes it makes more sense to use a sedentary control group, others a non-sedentary but insufficiently active control group, etc. What do you think makes the most sense?

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      • “On a related note, do you use accelerometers to identify how active and/or sedentary subjects are?”

        In the past, it was usually self-reported, but now we tend to use pedometers…accelerometers are, however, next on my list !

        “I think it depends on what question you are trying to get at – sometimes it makes more sense to use a sedentary control group, others a non-sedentary but insufficiently active control group, etc. What do you think makes the most sense?”

        Now that we know that “real” sedentarity” is that bad, and the fact that subjects recruited in our studies are rarely sedentary (according to the definition you use), I would argue that physically active subjects should be used as controls…in fact, it should be great to have both sedentary and physically active controls (if money is not a problem !!)

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  3. FrauTech says:

    How does the study control for other factors that are contributing to the sitting? As in age, smoking, weight and physical activity are not the only things that might cause a person to sit more. Some people have physical impairments that aren’t visible to the eye or aren’t obvious but might be causing them to sit more. And it’s possible these impairments are what contribute to an overall degradation in health. I mean, we all know the recommendations are to be more active, but it seems like this overlooks that there might be other health issues causing a portion of the population to be inactive and therefore increase their health risks.

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    • Travis Saunders, MSc, CEP says:

      So you’re suggesting that these people might sit more because they’re unhealthy, rather than being unhealthy because they sit more? That could be the case for some people, but when we get to the physiological mechanisms later this week, we’ll see that that is unlikely to account for the whole relationship.

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      • FrauTech says:

        I agree it wouldn’t account for the whole thing. I just think it’s oversimplifying to say “well if we all sat less we’d have longer lives.” That doesn’t deal with the cause, so it doesn’t necessarily solve the problem.

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        • Travis Saunders, MSc, CEP says:

          It may or may not deal with the cause – no one really knows just yet. And I would argue that standing from time to time is likely to have very few negative side effects, so it’s worth trying while we wait for more conclusive evidence.

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