Welcome to our 5-part series delving into the fascinating research being performed in the emerging field of sedentary physiology. Today, we’ll start with an introduction. For Part 2 in our series, click here.
As regular readers will know, sedentary physiology is one of our favourite topics here at Obesity Panacea. Later this week we will be examining the relationship between sedentary behaviour (aka behavior) and health, as well as the mechanisms that are thought to mediate this association. But before we delve into those deeper topics, I’d like to give a bit of background.
What is sedentary behaviour?
Sedentary behaviours are those characterized by very low energy expenditure – typically those requiring 1.5 METs or less. MET stands for metabolic equivalent, and is equal to 3.5 ml/kg/minute of oxygen consumption. All you really need to know is that by convention, 1 MET is considered to be the resting metabolic rate while sitting quietly. In contrast, moderate physical activity is defined as an intensity between 3-6 METs, while anything above 6 METs is considered vigorous exercise. Given that sedentary behaviours are those with an energy expenditure of 1.5 METs or less, these are activities that are burning roughly the same number of calories as sitting quietly – e.g. not much at all. With few exceptions, this means that any time you are sitting or lying down (watching TV, playing traditional videogames, using the computer, reading, driving in a car, etc), you are likely engaging in sedentary behaviour. This does not mean that riding an exercise bike (which involves sitting) is sedentary, since it also involves an energy expenditure above 1.5 METs. Ditto for certain energy-intensive activities in the prone position. The figure below is from a recent paper published by several colleagues and myself, which illustrates the entire “activity continuum”, ranging from completely sedentary to highly active. The figure also differentiates the focus of traditional exercise physiology research (e.g. moderate and vigorous physical activity), from the focus of the emerging field of sedentary physiology.
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 later this week, accumulating evidence suggests that 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. Consider the following figure from the same paper as above:
As you can see, one individual (the white circles) performs a single bout of structured physical activity, but then remains completely sedentary throughout the rest of their waking hours. In contrast, the individual represented by the black boxes accumulates a similar volume of structured physical activity, but dramatically less sedentary time. And the evidence that I will present later this week suggests that the individual represented by the black boxes is likely to have significantly lower metabolic risk than the one represented by the white circles, even though they are both meeting physical activity guidelines.
All this to say that it is important that we recognize that sitting too much is not the same as exercising too little (a line which I am happy to borrow from Marc Hamilton, one of the leading researchers in this area). As the above figure illustrates, an individual can be both highly active and highly sedentary at the same time, and it is thus important that we distinguish between the two whenever possible.
How much sedentary time do we accumulate on a daily basis?
The short answer? A lot!
Several studies have aimed to quantify the amount of time that both adults and children spend sitting on a regular basis, and all have found it to be an incredibly large portion of the day. For example, a recent nationally representative survey of more than 5700 Americans found that the average sedentary time was just over 8 hours per day. That equates to roughly half of our waking hours.
The situation in children is, unfortunately, no different. There is evidence that children in both Canada and the USA 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 behaviour are in addition to the hours and hours (and hours) that kids spend sitting at school. In fact, a recent study by our colleagues Lindsay Nettlefold and Ashlee McGuire 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).
Ok, so hopefully we are now on the same page with respect to what sedentary behaviour is, what it is not, and how it dominates most of our waking hours. What is the health impact of this ubiquitous sedentary behaviour? The short answer is that it’s certainly not good. For the details, be sure to check back tomorrow for Part 2 (now online) in our series on sedentary physiology.