Welcome to our 5-part series delving into the fascinating research being performed in the emerging field of sedentary physiology. In Part 1 we discussed the basics of sedentary physiology, in Part 2 we discussed the relationship between total sedentary time and negative health outcomes, and in Part 3 we examined how interruptions in sedentary time may be protective health benefits. Today we look at the mechanisms underlying these relationships.
There are a number of mechanisms – both physiological and behavioural – which are thought to link excess sedentary behaviour with increased risk of obesity, metabolic dysfunction, and even death. Many of the physiological mechanisms are consistent across behaviours (e.g. as far as your skeletal muscle is concerned, sitting in front of the TV is likely to be quite similar to sitting in the car or at a desk), while the behavioural mechanisms may to vary a bit depending on the context. Let’s begin by looking at the behavioural factors.
Quite obviously, when you are engaging in sedentary behaviours, 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. And there is some evidence that sedentary behaviour can displace physical activity. For example, a recent study suggests that active transportation to and from school in the Toronto area dropped by roughly 20% in the past 2 decades, while the amount of sedentary commuting (e.g. commuting by car) doubled during the same period. However, 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 behaviour 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.
In addition to reducing our energy expenditure, sedentary behaviours 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. The data was self-reported, but nonetheless these are pretty interesting findings, and suggest that television commercials are likely an important mechanism linking screen time with obesity risk. 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, 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 behaviour – 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, sitting (and TV watching in particular) seems to put us in situations where we choose to eat more than we would otherwise.
The evidence discussed above suggests that sedentary behaviour may result in both increased caloric intake and decreased energy expenditure through behavioural changes. In other words, these mechanisms are at least nominally under our personal control (e.g. some would argue that while we may be exposed to food ads on TV, we can still choose not to buy those products, nor to eat while being sedentary). However, other evidence suggests that sedentary behaviour also has negative health impacts through physiological mechanisms which are completely outside of our conscious control. For example, our Canadian colleague J-P Chaput has reported that both seated mental work (which is not dissimilar to a typical day at school or at work) and video games significantly increase caloric intake, while having little impact on energy expenditure. Other research has shown that scientists increase their food intake (especially their intake of fatty foods) while preparing grant applications, in comparison to other periods of less intense mental work. How is this possible? Mental work requires glucose, which may influence blood sugar levels and therefore appetite. And stressful situations such as grant applications may also increase stress hormone levels, which are also known to influence hunger and food intake. This research suggests that engaging in sedentary behaviours may not just put us in situations making over-consumption more likely – they may actually result in physiological changes that increase hunger and make us seek out energy dense foods.
Can these effects be negated by low-intensity physical activity? In other words, would video games have less impact on hunger if they were played while being physically active? No one knows for sure, but it is likely that the discrepancy between energy intake and energy expenditure would be smaller than when they are played by people who are being completely sedentary.
Other studies suggest that sitting 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 behaviour – not unlike a work day for many individuals. Sedentary behaviour 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. 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.
What’s the take-home message?
Although it’s impact on increased food intake and energy expenditure (through both behavioural and physiological mechanisms) is very important, sedentary behaviour is also likely to result in rapid changes in skeletal muscle function, causing dramatic increases in metabolic risk, even for lean or otherwise physically active individuals. The good news? Animal research suggests that simply walking at a leisurely pace may be enough to rapidly return these metabolic risk factors to normal levels. While there are a number of ongoing studies in humans on this very subject, as of right now we have a very poor understanding of the physiological impact of a single day of sedentary behaviour on metabolic health in humans. Similarly, it is unclear what intensity or volume of physical activity (if any) can prevent these negative changes. This issue is of particular interest to me personally, and one that I am hoping to explore during the remainder of my PhD.
Throughout the course of this week we have now looked at both the epidemiological and physiological evidence linking sedentary behaviour with increased health risk. Where is future research heading? That is the focus Part 5 of our series on Sedentary Physiology, which will be published tomorrow (now online here).
Zimmerman, F., & Bell, J. (2009). Associations of Television Content Type and Obesity in Children American Journal of Public Health, 100 (2), 334-340 DOI: 10.2105/AJPH.2008.155119