Last week a fascinating study was published by SBRN member David Dunstan and colleagues in Australia, which examined the acute (e.g. short-term) impact of uninterrupted sitting on metabolic health. In this new study, individuals with overweight or obesity were asked to perform 3 separate conditions in random order.
- Uninterrupted sitting – participants sat for 5 consecutive hours
- Sitting plus light intensity breaks – similar to the uninterrupted sitting condition, except that participants had a 2 minute walk break at a light intensity every 20 minutes throughout the day
- Sitting plus moderate intensity breaks – similar to the light intensity breaks condition, except that the breaks were at a moderate intensity
The figure below nicely demonstrates the basic protocol for the three conditions.
In all three conditions participants were given a standardized 760 calorie test drink at baseline (for reference, that’s about the same as a medium McDonald’s triple-thick milkshake), and had blood taken every hour to determine the glucose and insulin response. This is pretty similar to an oral glucose tolerance test, except that the test drink included both sugar and fat, while an oral glucose tolerance test involves only sugar. This sort of drink will produce a spike in insulin and glucose levels in the blood, but a healthier person will have a lower spike than an unhealthy person. A big spike in glucose or insulin levels suggests that your body has to work harder to get sugar into your muscles, which is a sign of insulin resistance and a risk factor for diabetes.
So what happened?
Plasma insulin and glucose levels were higher on the day of uninterrupted sitting, in comparison to the days with light or moderate intensity breaks. And not just a bit higher – more than 20% higher! I’ve graphed the average insulin and glucose levels during each condition below.
Even more fascinating is that the groups seem to diverge almost immediately – in the figure below, you can see that the glycemic response to the test meal during the uninterrupted sitting seems to be distinctly higher than the other two conditions even just 1 or 2 hours into the session.
While that may seem surprising, another recent study published in Applied Physiology, Nutrition and Metabolism found that sitting for 2 straight hours (e.g. 120 consecutive minutes) following a standardized meal increased the glycemic response by >45%, in comparison to a combination of 40 minutes of very light intensity walking and 80 minutes of sitting. In other words, these people had a clinically significant increase in metabolic risk simply because of an extra 40 minutes of sitting following the test meal.
What does this mean?
Taken together, these studies strongly suggest that sitting for several consecutive hours has a measurable and negative impact on metabolic health (at least in individuals with overweight or obesity). This could go a long way to explaining the relationship between sedentary behaviour and mortality – if you are engaging in uninterrupted sitting for periods of a couple hours on a regular basis, that could be exposing your body to elevated glucose and insulin levels following every meal, which over the long term could have serious consequences.
On the bright side, these studies also suggest that simply taking an occasional walk break at a very light intensity could substantially reduce the impact of prolonged sitting. In the Dunstan study participants walked at just 3.2 km/h (2 mph), which is a leisurely stroll for most able-bodied individuals. In addition, participants were asked to identify how hard they were walking in the light intensity condition using the Borg scale. The scale goes from 6 to 20, with 6 being “no exertion at all” and 20 being “maximal exertion”. The average rating was 8, which falls between “extremely light” and “very light”. In other words, these participants were not “exercising” in any way – they were just standing up and walking around at a very easy pace, just as you might when walking from your desk to the washroom.
These studies are of particular interest to me since my thesis work is examining similar issues in children. With any luck I will have some data to report from that study later this year. Along with other members of our research group I have also recently completed a systematic review on the acute impact of sedentary behaviour, which found a surprisingly large body of evidence linking short bouts of sedentary behaviour with increased metabolic risk (e.g. reduced insulin sensitivity and increased fat levels in the blood). That review is currently in press, although it is currently available as a provisional pdf for those who don’t mind the formatting.
For those interested in learning more about the new study by Dr Dunstan and colleagues in Australia, I have embedded below a presentation by Dr Neville Owen who co-authored the above paper. Thanks to Dr Owen and SBRN member Ernesto Ramirez for recording the session and making it available online. And if you are interested in learning more about the impact of acute and chronic bouts of sedentary behaviour, be sure to check out the Sedentary Behaviour Research Network.
Dunstan, D., Kingwell, B., Larsen, R., Healy, G., Cerin, E., Hamilton, M., Shaw, J., Bertovic, D., Zimmet, P., Salmon, J., & Owen, N. (2012). Breaking Up Prolonged Sitting Reduces Postprandial Glucose and Insulin Responses Diabetes Care DOI: 10.2337/dc11-1931
Nygaard, H., Tomten, S., & Høstmark, A. (2009). Slow postmeal walking reduces postprandial glycemia in middle-aged women Applied Physiology, Nutrition, and Metabolism, 34 (6), 1087-1092 DOI: 10.1139/H09-110