Let's get America Moving Again, Part IV: Can’t we just exercise?
So far in this series we have only discussed the importance of increasing habitual physical activity in our daily lives. For example, in part one, we discussed the advantages of an active commute in comparison to driving. In part two, we discussed how most Americans are not meeting physical activity recommendations and how even a little physical activity can go a long way. In part three, we discussed the concept of a habitually sedentary exerciser. However, there is always some gray area in science. Here, in part four, we will discuss a large study that concluded it may be ok to sit at your desk for 8 hrs if you exercise.
In 2012, the prestigious medical journal The Lancet published its own series on physical (in)activity and concluded that physical inactivity is an important modifiable risk factor for chronic diseases (like type 2 diabetes and heart disease) on par with obesity and tobacco. The goal of the Lancet’s Sedentary Behavior Working Group is to encourage policy makers around the world to take physical activity more seriously and provide sufficient resources and funding to implement national policies. So clearly this group is interested in increasing physical activity and reducing sedentary behavior.
In a 2016 report they published a meta-analysis including 16 prospective cohort studies seeking to address the following question: “If one is active enough, will this attenuate or even eliminate the detrimental association of daily sitting time with mortality?” So, essentially this analysis compiled data from a bunch of studies that took measures at a baseline and followed participants’ lifestyle patterns for several years. With data from over a million people, the researchers re-analyzed data from each study using predefined categories of sitting time, TV-viewing time, and physical activity. All the measures were assessed by different questionnaires (self-reported). The categories are below.
The physical activity requirement to be included in the analysis was at least “moderate”. The researchers listed two examples: walking briskly at about 3 or 3.5 mph or bicycling at ~10 mph.
The key finding of the meta-analysis was that in the most physically active individuals, a high sitting time did not increase their risk of mortality. So, what did the authors conclude? Well here are some highlights from their interpretation section:
High levels of moderate intensity physical activity (ie, about 60–75 min per day) seem to eliminate the increased risk of death associated with high sitting time.
*I think it is important to note that these results could potentially be repeated with lower levels of higher intensity exercise (e.g., 30-45 minutes of high intensity effort like sprinting and lifting weights).
These results provide further evidence on the benefits of physical activity, particularly in societies where increasing numbers of people have to sit for long hours for work and may also inform future public health recommendations.
Now while I don’t disagree with these remarks, I do think they need some more context. Let’s take a look at data from the study:
I have recapitulated table 2 from the meta-analysis like any reasonable nerd would do to illustrate a point.
Let’s quickly orient ourselves. MVPA on the far-left means moderate to vigorous physical activity. The four yellow boxes in the 2nd column from the left are the reference groups. In each case the reference group was the group who sat the least and as we move to the right this means the group of individuals sat more. What you will notice is that individuals with the highest activity levels are essentially protected from increased risk of all-cause mortality no matter how much they sit. For all other groups, more sitting = higher risk, except in the group that got 50-65 minutes of moderate to vigorous physical activity where 6-8 hours of sitting did not increase risk but 4-6 hours did? Scientific findings aren’t always perfect. Anyways, the most important thing here is that only the group getting an hour a day or more were protected from sitting. As we discussed earlier in this series, the common physical activity recommendations in the U.S say, “at least 30 minutes most days of the week”. Well according to the table in this huge study, that won’t cut it!
The findings of this study suggest 25-35 minutes per day do not protect against increased mortality with increased sitting. So essentially unless you are a hard-core exerciser, your exercise session will not protect you from excess sitting. This would support the argument(s) discussed in part three that we need to increase physical activity throughout the day as a single 30-minute bout will not cut it. Other caveats from this analysis:
- These protective effects of MVPA were not seen for increased television time. So, a specific type of person who sat a lot was protected. What that may suggest is that those who sat a lot as a group were largely represented by hard working people with sedentary jobs who made exercise a priority. If we speculate this, then these folks were also more likely to be highly educated, affluent, and likely to eat a better diet, all of which could influence the results (1,2,3).
- These data were all based on questionnaires. People are not always the best at filling out questionnaires (myself included) because we remember ourselves in a more positive light than reality. The sheer volume of participants included in this meta-analysis was very impressive. However, the ideal would be a study where participants actually tracked their physical activity, recorded their foods as they ate, recorded their activities (e.g. on computer, on phone, etc), and wore sleep monitors to objectively track everything. This will likely not happen soon.
- If we take a step back and look at the big picture this analysis was only looking at death! Sure, it’s great not to die but what would the findings have been if we looked at incidence of high blood pressure high blood sugar, overweight and obesity? The point is we have no metric for quality of life based on this study alone. Certainly, some exercise is better than none, I wouldn’t want to discourage anyone who is currently exercising even 10-15 minutes per day. However, these findings emphasize that perhaps meeting the bare minimum or below in regard to exercise may not counteract the negative effects of excessive sitting on our health.
In the next post, I will quickly wrap up the series and provide some tips on how we might be able to build more activity into our daily lives.