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Too Much TV Reduces the Benefits of Physical Activity

Wednesday, December 16, 2009 Posted by Travis Saunders

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?

Travis Saunders

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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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11 Response to "Too Much TV Reduces the Benefits of Physical Activity"

  1. OleanderTea Said,

    "...too much time sitting down is a risk factor for obesity, chronic disease, and even death..."

    I'm fairly certain that everyone's risk (and, indeed, experience) of death is 100%, even if they've never seen a TV.

    Posted on December 16, 2009 at 1:28 PM

     
  2. Travis Saunders Said,

    For clarification, it is increased risk of death in a finite period (12 years in this case).

    :)

    Travis

    Posted on December 16, 2009 at 1:37 PM

     
  3. Caroline Said,

    I found this quite an interesting read. The one thing that came to mind while reading this was the fact that it may not be the actual sitting that increases mortality risk, but rather what might be done while sitting (i.e. mindless eating). I'm guessing they didn't measure that in the study, did they?

    Posted on December 16, 2009 at 3:30 PM

     
  4. Travis Saunders Said,

    @ Caroline,

    That's a great point, and there are a few interventions that suggest you may be right (I'm hoping to discuss them in the near future). In the meantime, you might want to check out this recent paper by Gary Goldfield and colleagues.

    There are also some cool studies suggesting that sitting actually has very specific and rapid effects on muscle metabolism, which are likely to also play a role in the links between sedentary behaviours and obesity/disease. There is a great paper on the topic by Lionel Bey, and it can be found here.

    Travis

    Posted on December 16, 2009 at 3:41 PM

     
  5. Claude Said,

    Great article Travis!

    Good reminder to take advantage of the free time during the holidays to add more activity to my day!

    Posted on December 16, 2009 at 5:26 PM

     
  6. Anonymous Said,

    You mention that too much sedentary time, in particular TV watching time, is a risk factor for disease and mortality. What about other sedentary activities... e.g. people using computers, people with office jobs, etc. Also, are there studies on occupation-related obesity risk?

    Posted on December 16, 2009 at 7:21 PM

     
  7. Travis Saunders Said,

    @ Anonymous,

    Great questions. In general, sedentary behaviours are bad. For example, studies that measure total sedentary time by having people wear pedometers find that it is strongly associated with things like obesity and chronic disease risk. TV appears to be even worse than some other sedentary behaviours though, because it is often accompanied by snacking on high fat foods as Caroline suggested, and also tends to expose people to advertisements for "bad" foods. There is even some research suggesting that we burn less calories when watching TV than during other sedentary behaviours.

    One of the most famous studies on physical activity actually looked at the heart disease risk of bus drivers (who spend most of the day sitting) and ticket takers (who spend most of the day walking up and down the aisle) in the middle part of the 20th century. Just as you'd expect, those who walked more had dramatically lower health risk than those who spent the day sitting down. I don't have a link off hand, but it's one of the first papers to really show that physical activity has health benefits. It may turn out that we have simply underestimated the negative impact of being sedentary all day long. If anyone has a link to the study I am referring to, please feel free to post it below.

    Travis

    Posted on December 16, 2009 at 8:06 PM

     
  8. Gina Said,

    Thanks for this informative post, Travis. So what you're saying is that even if I'm watching The Biggest Loser, I'm probably becoming The Bigger Watcher?? Darn! Here I was thinking that it was totally OK to surf the couch guilt-free after my daily workout. Go on, keep raining on my parade. (great post though, seriously)

    Posted on December 16, 2009 at 9:45 PM

     
  9. Travis Saunders Said,

    @ Gina,

    Research like this actually scares me a bit, as I tend to fall into the category of the person who does a solid workout, followed by several hours of sitting at a computer. I bike to work most of the year, and take frequent breaks to stretch my legs, so it's not like I'm completely sedentary during the workday, but I do start to wonder how my life as a desk jockey impacts my longterm health :)

    Posted on December 16, 2009 at 10:09 PM

     
  10. Anonymous Said,

    It has been shown that physical activity and actual fitness are too different things. People who are physically fit have a stronger inverse relation with heart disease risk http://healthresearch.lbl.gov/vs.html
    Perhaps this research would be better using a treadmill test to determine fitness rather than reported hours of activity. I run on average about 50 minutes per day, about 35 miles per week, my BMI is about 22.8. I currently sit at a desk for most of my workday (except during my lunch when I run). I calculated last week that I burned over 5,000 calories running (43.5 miles). It is hard to believe that an hour in front of the TV is negatively equal to my 5,0000 calories, unless every time I sit in front of the TV for an hour, I consume over 1,000 calories in snack foods.

    Posted on January 13, 2010 at 1:05 PM

     
  11. Travis Saunders Said,

    @ Anonymous,

    You are absolutely correct that fitness and physical activity are two different things, although they obviously interact. Which one you use really depends on the question that you want to ask. Physical activity is a behaviour which is largely (but not exclusively) under the control of the individual, but fitness is an outcome which depends not only on physical activity, but also on genetic factors. For that reason I am personally more interested in physical activity, since it has a more relevant public health message (better to focus on something that people can more easily change).

    It's not necessarily that TV time offsets the calories that you would expend during your run. When you sit for an extended period of time, it affects the lipoprotein lipase activity in your muscles, which results in increased blood fat, and reduced HDL (good) cholesterol. These are both bad things, and surprisingly, this is NOT just the opposite of what happens when you exercise. This is probably why we are starting to see these strange interactions between TV time and physical activity. There is a terrific paper by Marc Hamilton that does a great job of examining how both exercise and sedentary time affect metabolic health.

    Travis

    Posted on January 13, 2010 at 11:00 PM

     

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We are PhD students in the School of Kinesiology and Health Studies at Queen's University in Kingston, Ontario. Our research focuses on the relationships between obesity, physical activity, and health risk. This blog is our attempt to consider the many "cures" for obesity that we read about on a daily basis. Enjoy.

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The opinions expressed here belong only to Peter and Travis and do not reflect the views of any organization. Any medical discussion on this page is intended to be of a general nature only. This page is not designed to give specific medical advice. If you have a medical problem you should consult your own physician for advice specific to your own situation.

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