> For all you deluded ones who believe that sitting instead of daily life is > Zen. :-)
Edgar > > Can sitting too much kill you? > > By Travis Saunders | Jan 6, 2011 10:39 AM > > We all know that physical activity is important for good health—regardless of > your age, gender or body weight, living an active lifestyle can improve your > quality of life and dramatically reduce your risk of death and disease. But > even if you are meeting current physical activity guidelines by exercising > for one hour per day (something few Americans manage on a consistent basis), > that leaves 15 to 16 hours per day when you are not being active. Does it > matter how you spend those hours, which account for more than 90% of your > day? For example, does it matter whether you spend those 16 hours sitting on > your butt, versus standing or walking at a leisurely pace? Fortunately or > unfortunately, new evidence suggests that it does matter, and in a big way. > > What is sedentary behavior? > > Before we go any further, it’s important that we define the term "sedentary > behavior". Sedentary behavior is typically defined as any behavior with an > exceedingly low energy expenditure (defined as <1.5 metabolic equivalents). > In general, this means that almost any time you are sitting (e.g. working on > a computer, watching TV, driving) or lying down, you are engaging in > sedentary behavior. There are a few notable exceptions when you can be > sitting or lying down but still expend high energy expenditure (e.g. riding a > stationary bike), but in general if you are sitting down, you are being > sedentary. > > The above definition may seem rather intuitive, but this is not the way that > the term sedentary has been used by exercise science researchers for the past > 50 years. Up until very recently, referring to someone as sedentary meant > simply that they were not meeting current guidelines for physical activity. > In simple terms, if you were exercising for 60+ minutes/day, you were > considered physically active. If you were exercising 10 minutes/day, you were > sedentary. Case closed. But as we will discuss below, sedentary time is > closely associated with health risk regardless of how much physical activity > you perform on a daily basis. Further, it is entirely possible to meet > current physical activity guidelines while still being incredibly sedentary. > Thus, to quote researcher Marc Hamilton, sitting too much is not the same as > exercising too little. (if you take only one thing from this post, let it be > that quote from Dr Hamilton). Which is why it is so important that when we > use the term "sedentary", we are all on the same page about what that means. > > Now that we know what sedentary behavior is, let’s look at its relationship > with health risk. > > Epidemiological Evidence > > In 2009 Dr Peter Katzmarzyk and colleagues at the Pennington Biomedical > Research Center published an influential longitudinal paper examining the > links between time spent sitting and mortality in a sample of more than > 17,000 Canadians (available here). Not surprisingly, 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 deaths due to cancer). But what is fascinating is that the relationship > between sitting time and mortality was independent of physical activity > levels. 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. Further > analyses suggested that the relationship between sitting time and mortality > was also independent of body weight. 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 above findings linking excessive sitting with poor health are far from > isolated. For example, a similar longitudinal study from Australia reports > that each hour of daily television viewing (a proxy of sedentary time) 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 my > colleagues and I summarize in a recent review paper (PDF), numerous > epidemiological studies have linked sedentary behavior with obesity, > cardiometabolic risk, and even some cancers. > > New evidence also suggests that in addition to the quantity of sedentary > time, the quality of sedentary time may also have an important health impact. > For example, Genevieve Healy and colleagues examined this issue in > participants of the Australian Diabetes, Obesity and Lifestyle (AusDiab) > Study. A total of 168 men and women aged 30-87 years wore an accelerometer > (an objective measure of bodily movement) during all waking hours for 7 > consecutive days, which allowed the researchers to quantify the amount of > time that participants spent being sedentary, as well as how frequently they > interrupted these sedentary activities (e.g. standing, walking to the > washroom, etc). > > What did they find? > > The greater the number of breaks taken from sedentary behavior, the lower the > waist circumference, body mass index, as well as blood lipids and glucose > tolerance. This was true even if the total amount of sedentary time and > physical activity time were equal between individuals—the one who took breaks > more frequently during their time at the office or while watching television > was less obese and had better metabolic health. Importantly, the breaks taken > by the individuals in this study were of a brief duration (<5 min) and a low > intensity (such as walking to the washroom, or simply standing). > > Taken together, the epidemiological evidence strongly suggests that prolonged > sitting is an important health risk factor. But what explains these > relationships? Let’s now look at the multiple mechanisms linking sedentary > time with increased health risk. > > Mechanisms > > Reduced Energy Expenditure > > Quite obviously (and by definition), when you are sedentary, you are not > being physically active. And so one common assumption is that people who sit > more are at increased health risk simply because they are getting less > physical activity. However, somewhat surprisingly, sitting time and physical > activity do not appear to be related for most people. For example a paper > from the European Youth Heart Study published in PLoS Medicine reports no > association between physical activity and TV watching in a sample of nearly > 2000 children and teenagers, and other reports suggest that there is little > evidence that sedentary behavior displaces moderate or vigorous physical > activity. So while it makes intuitive sense that being sedentary reduces > energy expenditure, it is likely through the reduction of very light > intensity physical activity (e.g. standing, walking at a slow pace), rather > than by reducing the volume of what we typically think of as exercise. This > may also help explain why the relationship between sedentary behavior and > health risk are often independent of moderate or vigorous physical activity. > > Increased Food Intake > > In addition to reducing our energy expenditure, sedentary behaviors may also > promote excess food intake. For example, a recently published study in the > American Journal of Public Health suggests that the amount of commercial > television (e.g. television with advertisements) that children watch before > the age of 6 is associated with increased body weight 5 years down the road, > even after adjustment for other important variables including physical > activity, socio-economic status and mother’s BMI. In contrast, watching > non-commercial television (DVD’s or TV programs without commercials) showed > no association with body weight. Similarly, it has also been reported that > each hour of daily television watching in children is associated with an > increased consumption of 167 calories per day (PDF), mainly through increased > consumption of high calorie, low nutrient foods (e.g. the foods most commonly > advertised on television). Much of this is likely just a learned > behavior—watching TV exposes us to food ads promoting unhealthy fare, which > is likely to have a disproportionate influence on younger viewers. Just as > importantly, people may just really enjoy munching on food while relaxing on > the couch. Either way, excess sitting (and TV watching in particular) seems > to put us in situations where we choose to eat more than we would otherwise. > > Physiological Adaptations > > I don’t think the mechanisms described above—that sitting too much may lead > to reduced energy expenditure and increased food intake—will come as much of > a surprise. But what I find truly fascinating is that sedentary behavior also > results in rapid and dramatic changes in skeletal muscle. For example, in rat > models, it has been shown that just 1 day of complete rest results in > dramatic reductions in muscle triglyceride uptake, as well as reductions in > HDL cholesterol (the good cholesterol). And in healthy human subjects, just 5 > days of bed rest has been shown to result in increased plasma triglycerides > and LDL cholesterol, as well as increased insulin resistance—all very bad > things. And these weren’t small changes—triglyceride levels increased by 35%, > and insulin resistance by 50%! > > These negative changes are likely related to reductions in the activity of > lipoprotein lipase, an enzyme which allows muscle to uptake fat, thereby > reducing the amount of fat circulating in the blood (it also strongly > influences cholesterol levels—the details can be found here). Animal research > has shown that lipoprotein lipase activity is reduced dramatically after just > six hours of sedentary behavior—not unlike a typical day at work or school > for many individuals. Sedentary behavior may also reduce glucose transporter > protein content in the muscle, making it more difficult for glucose to be > taken into the muscle and resulting in higher blood sugar levels. What is > most interesting to me personally is that these physiological changes in > skeletal muscle have little or nothing to do with the accumulation of body > fat, and occur under extremely rapid time-frames. This means that both lean > and obese individuals, and even those with otherwise active lifestyles, are > at increased health risk when they spend excessive amounts of time sitting > down. > > Should we be concerned about the health impact of sedentary behavior? > > Yes. > > Western society is built around sitting. We sit at work, we sit at school, we > sit at home, and we sit in our cars as we commute back and forth. In fact, a > recent survey reports that the average American accumulates more than 8 hours > of sedentary behavior every day—roughly half of their waking hours. The > situation in children is, unfortunately, no different. There is evidence that > children in both Canada and the USA (PDF) accumulate more than 6 hours of > screen-time (time spent in front of the TV, computer, or other screen-based > device) on a daily basis. Keep in mind that screen-time is almost exclusively > sedentary (active video games excluded), and that all these hours of > sedentary behavior are in addition to the hours and hours (and hours) that > kids spend sitting at school. In fact, a recent study reports that roughly > 70% of class time, including physical education class, is completely > sedentary (while slightly better than class time, children were also > sedentary for the majority of lunch and recess). > > In short, given the consistent links between sedentary behavior and both > death and disease, and the ubiquity of sedentary behavior in our society, we > should be very concerned about the health impact of sedentary behavior. > > What is the take-home message? > > There is a rapidly accumulating body of evidence which suggests that > prolonged sitting is very bad for our health, even for lean and otherwise > physically active individuals. The good news? Animal research suggests that > simply walking at a leisurely pace may be enough to rapidly undo the > metabolic damage associated with prolonged sitting, a finding which is > supported by epidemiological work in humans. So, while there are a lot of > questions that remain unanswered (e.g. Is there a “safe” amount of daily > sedentary time?), the evidence seems clear that we should strive to limit the > amount of time we spend sitting. And when we do have to sit for extended > periods of time (which, let’s face it, is pretty much every single day for > many of us) we should take short breaks whenever possible. > > Finally, if you take only one thing from this post, let it be this—sitting > too much is not the same as exercising too little. >
