A few months back I wrote an article about the amount of exercise necessary to maximise life expectancy. That article, based on observational studies, concluded that you could get 90% of the longevity enhancing benefits of exercise with just half an hour of moderate intensity walking per day, which is nice for those, like me, who aren’t even remotely interested in intense sporting activities. When the other boys were outside kicking a ball around, I was lying in my bed reading books, so any scientific evidence supporting that lifestyle makes me happy.
However, all the studies discussed in that previous article were focused on the total number of MET’s (metabolic equivalent task) performed per week. In case you don’t know what a MET is, here’s what I wrote about it in my earlier article:
One MET is defined as the amount of calories a person expends in one hour when at rest. Walking at a slow pace expends two METs. Walking at normal walking speed (3 miles or 5 km per hour) expends three METs. Bicycling at average speed (11 miles or 17 km per hour) expends six METs. Running at a speed of 6 miles per hour (10 km per hour) expends 10 METs. Generally, activity that expends three to six METs is considered moderate intensity, while activity that expends six METs or higher is considered vigorous intensity.
In order to be able to compare apples with apples, the studies discussed in the previous article were just looking at total MET’s expended per week. This led them to conclude that optimal life expectancy is achieved if you do more than 35 METs per week (equivalent to twelve hours of walking at a normal pace) but less than 75 METs per week (yes, too much exercise is bad for you).
Those studies didn’t compare different intensities of exercise, so they didn’t say whether someone who expends 50 METs per week running will live longer (or shorter) than someone who expends 50 METs per week walking. That’s where a new study that was recently published in JAMA Internal Medicine comes in.
This was a big observational study, involving 400,000 people in the United States. The authors reported no conflicts of interest. In order to be included in the study, participants had to be over the age of 18, and they had to have a decent level of physical function (those requiring help with activities of daily life were excluded).
People with heart disease, stroke, or cancer were also excluded from the study, which is a shame, because it would be interesting to see whether people with serious health problems can benefit from vigorous exercise. My guess is that they were excluded out of fear that their participation would confound the results – these people are often not able to perform vigorous physical activity due to their illness, and they also have a short life expectancy due to their illness. This could make lack of exercise look worse than it is if the effect isn’t corrected for.
Participants were selected at random from the US population and asked to fill out a questionnaire. The first participants were recruited in 1997 and the last participants were recruited in 2013. They were then followed over time to see whether they died or not. The median amount of follow-up per participant was ten years, and the average age at the time of inclusion in the study was 43 years.
The two central parts of the questionnaire were a question asking people how much time they spend per day engaging in light to moderate exercise during leisure time (defined as exercise that doesn’t cause significant sweating or an increased pulse or breathing rate), and how much time they spend per day engaging in vigorous exercise during leisure time (defined as exercise that does cause sweating, or an increase in pulse or breathing rate).
I’m sure you can already see some problems with this study. The first and most obvious is that the data is entirely observational – there is no randomization and no control group. That means this study can at best show a correlation. It cannot say anything about cause and effect. As mentioned already, people who are less healthy will probably engage in less vigorous physical activities, but if they then die earlier, is it because of their underlying poor health or because of their lack of vigorous exercise. What is the hen and what is the egg? Observational studies can never say.
The second is that the main input in the study is a questionnaire. And here there are multiple problems. One is that the questionnaire was only filled in once. Over the course of the last ten years I’ve varied a lot in how much intense exercise I do per day on average, from an hour or two per day to none at all. How accurate is a single snapshot from a person’s life? And how predictive can a single questionnaire really be ten years down the line?
Another problem is recall bias, which is basically that people have crappy memories, and can easily over- and underestimate things. It is easy to imagine people overestimating the amount of intense exercise they do due to wishful thinking, or underestimating the amount due to poor memory.
This problem is made worse by the the way the questions in this specific study were designed. Note that they were asking specifically about exercise during leisure time. What about exercise outside of leisure time? Are our bodies somehow only affected by the exercise we do in our free time, but not exercise we engage in at work? And what about if I take the bike to work? Is that leisure time or not? It is easy to imagine different people interpreting the question differently, and offering up different answers, which could muddy the results.
To me, only asking about exercise during leisure time is a big problem, which could hugely impact the results. Lots of people do a lot of walking at work, for example, but little walking in their leisure time. In the data used in this study, they would appear to be engaging in a lot less moderate exercise than they really do.
Anyway, the data gathered from the questionnaire were then used to figure out what proportion of exercise during leisure time was vigorous, and what proportion was moderate, and this was then correlated with the probability of dying.
For some reason the researchers decided to divide up participants in to categories based on proportion of vigorous to moderate exercise, with six different categories in total. So there was a group that did no vigorous exercise, a group that spent between 0 and 25% of their exercise time doing vigorous exercise, a group that spent between 25% and 50% of their time doing vigorous exercise, and so on, up to a group that reported only doing vigorous exercise.
As anyone who’s taken a basic statistics class knows, you shouldn’t take a continuous variable and turn it in to a series of categories, because in doing so you lose a lot of information. Instead of seeing any real difference that happens to exist between someone who spends 26% of their time engaging in vigorous exercise and someone who spends 49% of their time engaging in vigorous exercise, these two people end up in the same category, i.e. they get treated as if they are doing the same proportion of vigorous exercise, and the information about differences ends up being destroyed. Why did the researchers do this? Who knows.
Around 10% of respondents reported only doing vigorous exercise. At first I thought this seemed very strange. It must mean either that 10% of the population are so unfit that they break out in a sweat at the slightest movement, or that they are literally running everywhere.
Then I realized that it was due to the way the questionnaire was formulated, only asking about exercise during leisure time, and only asking patients to count exercise that lasted at least ten minutes. So for people who literally take the car everywhere, even to the gym, and never go for walks, or for people who don’t interpret walking to the gym as “leisure”, it’s perfectly possible get up to 100% vigorous exercise. But, as mentioned, our bodies don’t know the difference between “leisure” exercise and “non-leisure” exercise, so this is a problem that seriously reduces the ability of the study to produce useful results.
Almost 10% of participants in the study died over the course of follow-up. This proportion was highest in the group that engaged in no vigorous exercise. After adjusting for known confounders (age, gender, ethnicity, education level, marital status, BMI, smoking, alcohol… yes that is a long list, but then consider all the confounders that weren’t included and you see why observational studies are problematic!) they reached the following results:
Those spending between 0% and 25% of their leisure exercise time on vigorous exercise were 11% less likely to die over the course of follow-up. Those spending between 25% and 50% of their time on vigorous exercise were 10% less likely to die. Those spending 50 to 75% of their time on vigorous exercise were 17% less likely to die. Those spending 75 to 99% of their time on vigorous exercise were 15% less likely to die. And those spending literally all their leisure exercise time engaging in vigorous exercise were 10% less likely to die than those not doing any vigorous exercise at all.
I am of course talking about relative risk reductions, not absolute risk reductions, so for example, a 10% reduced risk of dying in a group where 10% die means that you’ve actually reduced your risk from 10% to 9%. Personally, that’s not enough to get me to put on spandex and head to the gym.
Ok, let’s analyze those results. The optimal proportion of vigorous to moderate exercise seems to be somewhere between 50% and 75%, with a “whopping” 17% reduction in risk (i.e. taking you from a 10% risk of dying over ten years all the way down to an 8.3% risk of dying), compared with not doing any vigorous exercise at all. Doing both less, and more, vigorous exercise seems to reduce longevity. Which makes quite a bit of intuitive sense. The “Born to Run” guy (Micah True) died of a heart attack at the age of 58 – apparently all that running did him more harm than good.
So spending somewhere between 50% and 75% of your leisure exercise time doing vigorous exercise appears to be optimal for longevity. However, although the difference between no vigorous exercise and some vigorous exercise could possibly be considered statistically significant (with a p-value of 0,02, which would be considered significant if you were only looking at one relationship, but really shouldn’t be considered significant when you are looking at lots of relationships, as the researchers have done in this study), and the difference between no vigorous exercise and a lot of vigorous exercise is statistically significant (p-value 0,001), the difference between doing some vigorous exercise and a lot of vigorous exercise does not appear to be statistically significant (although no p-values are provided for this, there is significant overlap between confidence intervals). So, it seems that some vigorous exercise is better than none. Whether a lot is better than some is not possible to say with any certainty from the data that have been presented.
And to be honest, even the conclusion that some is better than none rests on shaky ground, due to the observational nature of the study. Although the researchers have tried to adjust for confounders, it’s impossible to get rid of every single confounder there is, and it is possible that the apparent 11% risk reduction seen in those doing some vigorous exercise is entirely due to an unmeasured confounder. As I mentioned in the earlier article about exercise and longevity, healthy behaviors travel together, and so do unhealthy behaviors, so in observational studies it is often the case that you think you’re seeing an effect of one supposedly healthy behavior, when you are in fact seeing the effect of some entirely different healthy behavior. The researchers in this study didn’t attempt to adjust for dietary factors, and as I’m sure pretty much all readers of this blog know, few things matter as much as diet when it comes to longevity, so that’s a pretty big confounder to not adjust for.
Before we get in to the final little part of the article, I need to mention that the official US exercise recommendations are 150-299 minutes of moderate exercise per week, or 75-149 minutes of vigorous exercise per week. In other words, if you do vigorous exercise, you can spend half as much time doing it. Other than that, the two options are considered equivalent from a health perspective in the official recommendations.
When the researchers looked at the absolute amount of time doing either moderate or vigorous exercise, and compared those doing the recommended 150-299 minutes of moderate exercise against those instead doing the recommended 75-149 minutes of vigorous exercise, they didn’t find any meaningful difference.
Among those doing the recommended amount of moderate exercise, the relative risk of dying decreased by 17%, while among those instead doing the recommended amount of vigorous exercise, the risk decreased by 20%. The difference was not statistically significant.
So, what can we conclude from all this? This study suggests that doing vigorous exercise on a regular basis may slightly prolong life, as compared to just doing moderate exercise. Thankfully the evidence is weak enough that I can stick to walking, carrying babies around, and pram pushing – no need to head to a spin class quite yet.
For reasons that are beyond my understanding, some of you actually enjoy vigorous exercise. If that is the case, then remember the conclusion from my earlier article, that total MET’s spent engaging in exercise per week should not go above 75, if you want to maximize longevity. What that means is that you should not be engaging in vigorous exercise for more than one hour per day, assuming you do absolutely no moderate intensity exercise.
But the above described study suggests that just doing vigorous exercise is worse for longevity than mixing vigorous exercise with moderate exercise. So, if you walk at a moderate pace for half an hour a day, then you’ve expended around 11 METs per week on walking, which leaves around 65 MET’s per week that can be expended on vigorous activities, equivalent to six and a half hours of running. Your body won’t love you if you do more than that.