Wednesday, December 16, 2009
Image by Dailyinvention.
One of my personal interests is the relationship between sedentary time (e.g. the amount of time that we spend sitting) and chronic disease risk. Several interesting papers have come out in the past few years suggesting that spending too much time sitting down is a risk factor for obesity, chronic disease, and even death, independent of physical activity levels. In other words, no matter how physically active you are, the more time you spend sitting, the greater your risk of death and disease.
This is a very new area of study, so a lot of questions remain unanswered. For example, consider two situations:
1) An individual who does a 60 minute run every morning, but spends the rest of the day being completely sedentary.
2) An individual who spends all day on their feet, walking and moving at a slow pace, but never raising their heart rate above 100 beats per minute.
All else being equal, which of these individuals is at a lower health risk? Up until a few years ago, I think almost all physiologists would have said that the first condition was much better than the second - in other words, as long as you're meeting the physical activity guidelines, it doesn't matter what you do with the rest of your day. But several recent studies have started to bring that into question, and a new study in the International Journal of Obesity makes it increasingly difficult to believe that performing 60 minutes per day of structured exercise somehow inoculates us against the damage done by excessive amounts of sedentary behavior.
In this new study, Dr GF Dunton and colleagues at the National Cancer Institute used a phone survey to collect information on a sample of 10,000 non-underweight Americans above the age of 21. Participants self-reported their height and weight, as well as the amount of time that they had spent engaging in both physically active and sedentary behaviors in the past 24 hours. Not surprisingly, time spent engaging in sedentary behaviors like watching TV and driving in a car were associated with increased body weight, while high levels of physical activity were associated with lower body weights. But what is really interesting is that sedentary behaviors altered the relationship between physical activity and obesity. To help illustrate, I have recreated a graph from the paper below.
Adapted from Dunton et al., 2009.
As you can see, in individuals who reported watching TV for less than one hour per day, those who performed at least one hour of moderate to vigorous physical activity (MVPA) had significantly lower BMIs. However, in individuals who reported spending more than an hour per day watching TV, BMI was not lower in those who performed more than one hour of MVPA per day. In other words, for individuals who spent too much time watching TV, physical activity was no longer associated with body weight. Interestingly, while time spent watching TV seems to influence the relationship between physical activity and BMI, physical activity did not seem to affect the relationship between TV watching and BMI.
There were also interesting interactions found for active vs sedentary modes of transportation. I find these things tricky to describe in text, so I have re-created one other graph to illustrate the findings.
Adapted from Dunton et al., 2009.
As you can see, among those who performed no active transportation, individuals had similar BMIs regardless of how much time they spent commuting by sedentary means. However, in individuals who performed at least some active transportation (defined as one minute or more), there was a positive relationship between sedentary transportation and BMI.
So what does it all mean? Well, the results of this study suggest that if you spend too much time watching TV, you are likely to have a higher BMI regardless of how much physical activity you perform. Similarly, if you spend >80 minutes in the car commuting everyday, you are likely to have a high BMI even if you also do some active transportation. These findings (along with other studies we have discussed in the past) suggest that no matter how much exercise you perform, it is important to minimize the amount of time you spend sitting.
Now this study has some important limitations that are worth mentioning. First off, the information was self-reported, which tends to make things a bit messier. In these situations body weight tends to be under-reported, while people may forget about some bouts of sedentary behavior (it's not that easy to remember all the time you spent sitting down or watching TV in the past 24 hours). Perhaps even more importantly, all of these results are cross-sectional. So people may be heavier because they watch too much TV, or they may watch more TV because they are heavier. Same thing with physical activity - a lack of physical activity may be a cause of weight gain, but weight gain may also cause a reduction in physical activity (my guess is that it's probably a bit of both). Now that's not to say that cross-sectional studies aren't interesting or important (Peter and I have published several cross sectional studies), but it's definitely worth keeping in mind. Just to say that one more time, this study does not prove that too much sedentary time causes obesity. But it's a very cool paper nonetheless.
So let's return to my question at the top of this post - is it better to get an hour of physical activity but spend the rest of the day being sedentary, or to spend the entire day engaging in very low intensity physical activity? Given the limitations I just mentioned, this study obviously doesn't settle the issue one way or the other. But it is one more piece of evidence suggesting that no matter how much physical activity you perform on a daily basis, too much sedentary time (and especially too much TV) is a very bad thing.
Which do you think is worse - too much sedentary time, or too little physical activity?
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Dunton, G., Berrigan, D., Ballard-Barbash, R., Graubard, B., & Atienza, A. (2009). Joint associations of physical activity and sedentary behaviors with body mass index: results from a time use survey of US adults International Journal of Obesity, 33 (12), 1427-1436 DOI: 10.1038/ijo.2009.174
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