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Founded in 1970 by the "Father of Aerobics"
Kenneth H. Cooper MD, MPH

 
 
 

Can physical activity counteract mortality risk if you sit during most of your waking hours?

Posted in

Wednesday, Sep 01, 2021

In countries like ours that have a high standard of living, much of our waking time is spent sitting.

Most of us sit while getting to and from work. Many occupations involve sitting for a large portion of the workday. Upon arriving home from work, many individuals spend a significant amount of time either watching TV or doing social media before going to bed. An important and unanswered question is: can enough physical activity decrease the negative effects of a high amount of daily sitting time?

To answer this question, researchers combined 16 of the best studies in this area. All the studies contained detailed data regarding sitting time, physical activity, and mortality. Over one million individuals provided the necessary data at baseline. The group was divided into 4 categories of sitting time and 4 categories of physical activity, so that mortality risk could be examined in all 16 possible combinations of these two variables. The categories were as follows:

Sitting Time
<4 hours/day, 4-5.9 hours/day, 6-8 hours/day, >8 hours/day

Physical Activity
>35 MET-hours/week, 16.1-35 MET-hours/week,  2.5-16 MET-hours/week, <2.5 MET-hours/week

What is a MET-hour?

Resting energy expenditure is defined as 1 MET. So, if you were exercising at 5 METs, it means that you are expending 5 times the amount of energy than you would at rest. If you exercised at 5 METs for 1 hour, then you would accumulate 5 x 1 = 5 MET-hours during that exercise session. The easiest way to determine your MET level during physical activity is to use the Compendium of Physical Activities.

The participants were followed for about 10 years, during which 84,609 died. In the Figure below, the risk of all-cause mortality for all 16 possible combinations of daily sitting time and weekly physical activity are shown.


Each data point represents one of the 16 possible combinations of daily sitting time and weekly physical activity. We will use the data point on the far left as our reference, meaning that group had the lowest mortality risk. That data point represents men and women who sat for less than 4 hours per day and accumulated more than 35 MET-hours per week of physical activity. You can see by looking at the next 3 data points that mortality risk did not change much with increased daily sitting time within that category.  At the other end of the spectrum, notice that the data point at the far right showed the highest mortality risk; that group was about 60% more likely to die during follow-up when compared to our reference group. This data point represents men and women who sat for more than 8 hours per day and accumulated less than 2.5 MET-hours per week of physical activity.


Some good news for that lowest physical activity category was that mortality risk decreased substantially with decreased sitting time. Persons in the lowest physical activity group who sat less than 4 hours per day had only a ~20% increase in mortality risk when compared to the reference group. Remember that persons in the same physical activity group who sat for more than 8 hours per day had a 60% increased risk. We can probably assume that the low sitting/low physical activity group did a lot of standing during the day rather than sitting. An example of such an individual would be a cashier at a grocery store or someone working behind the counter at a fast food restaurant.

The most impressive finding in this meta-analysis was that high levels of weekly physical activity (>35 MET-hours per week) completely eliminated the mortality risk associated with increased levels of daily sitting.

35 MET-hours per week would be equivalent to performing moderate intensity exercise such as brisk walking 60-75 minutes every day. It is also important to note that for the 3 remaining categories of weekly physical activity, there was an increased risk of mortality with increased sitting time within each of the categories. This was especially true in the 2 lowest physical activity categories, as is clearly seen in the Figure. Unfortunately, the number of Americans in the lowest physical activity category is very high, while the number in the highest physical activity category is very low.

In Conclusion

So, with regard to how to decrease your mortality risk, it seems you don’t need to worry very much about sitting time if you are highly physically active. If you are very inactive and don’t plan to change much, you should try to decrease your daily sitting time. For example, you might try taking several 2 minute walking breaks during the day and stand rather than sit while on the phone. However, you would get more bang for your buck by getting out of that lowest category of physical activity. The American College of Sports Medicine and the American Heart Association recommend getting between 500 and 1000 MET-minutes (8 to 17 MET-hours) per week of moderate intensity physical activity for health benefits. Sit less, stand more, and get moving!


Reference

Ekelund, U., Steene-Johannessen, J., Brown, W., Fagerland, M., Owen, N., Powell, K., Bauman, A., Lee, I. (2016). Does physical activity attenuate, or even eliminate, the detrimental association of sitting time with mortality? A harmonized meta-analysis of data from more than 1 million men and women. Lancet, 388:1302-1310.