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.
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.
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.
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?
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.