Background: Aerobic fitness and long-term health
For the general population, the answer to this
question is a definitive yes, based on previous research. The real innovation
of this study was that it specifically examined people with extremely high
aerobic fitness. People in the top few percentiles of population-level aerobic
fitness don’t get there by genetics alone. As any distance runner is well
aware, becoming very fit requires a lot of intense and high-volume training.
Some cardiologists have hypothesized that this kind of intense training is
unhealthy. They point to research showing that biomarkers of heart damage increase after
running a marathon, and other work showing a potential “U-shaped curve” for physical activity levels
and cardiovascular disease risk.
The study details
The actual participants in this study were
122,000 men and women who underwent a standardized treadmill test of aerobic
fitness. The treadmill test progressed as most do, starting at an easy walk and
gradually increasing both the speed and the incline until the participant could
not continue any longer. This final stage of the treadmill test was used to
determine the person’s “peak METs,” or peak metabolic equivalent energy output.
The researchers tracked each study participant in
the Social Security Death Index, which (for fraud prevention purposes) is a
registry of all deaths of Americans who have social security numbers. By
monitoring which patients showed up in the death index, and when, the
researchers could determine who did and did not die during the course of the
study.
The findings
As with previous research, the authors of this
study found that people with better aerobic fitness, as measured by peak METs,
were less likely to die, even after controlling for factors like age, sex, body
mass, history of disease, smoking, and other potential confounding variables.
Most interestingly, the researchers found that
there was no upper limit to the benefits of physical fitness. The healthiest
group of people—in other words, those least likely to die from any cause—were
those that the authors classified as having “extreme cardiorespiratory
fitness.”
In the context of this study, the authors defined
this as scoring in the top 2.3% of all performers for their age and sex. These
extremely fit individuals were less likely to die compared to those who scored
in the 75th-97.6th percentiles for aerobic fitness, to the tune of a 23% lower
risk of death. This pales in comparison to the difference between the most and
least fit people, though: Compared to the top 2.3%, those in the lowest 25% of
aerobic fitness had five times the
risk of death!
Can your 5k time predict your lifespan?
One thing I love doing is trying to translate
research findings into something that’s more tangible and practical for people
who aren’t clinical researchers. What does it really mean to have a “peak MET two standard deviations above the
age and sex mean?”
You may have seen the term “MET” before, at the
gym on exercise equipment. It’s a standardized unit of energy expenditure,
where 1.0 METs is the energy expenditure of sitting still in a chair. METs are closely related to another unit of
energy expenditure that you might be more familiar with, which is VO2. One MET is equivalent to 3.5 ml of oxygen per
kg of body mass. What this means is that if you know someone’s peak metabolic equivalent, you can
easily figure out their VO2 max—all you have to do is multiply by
3.5. So, it’s pretty easy to turn the
cut-points for low, average, high, and elite aerobic fitness in this paper from
MET thresholds into VO2 max thresholds.
Once we have VO2max, we’re in more
familiar territory for runners. If you’ve read my book, Modern Training and
Physiology, you know that your VO2 max is a very important predictor
for your race performances. Now, coaches like me usually rail against VO2
max as the end-all-be-all of running performance, because VO2 max
does not differentiate very well between someone whose 5k PR is 16:00 and
someone whose 5k PR is 15:30. That being said, in this case, VO2 max is a pretty useful predictor of
running performance, if we are talking about magnitudes like a 30:00 versus a
20:00 5k (or a 30:00 versus a DNF).
You might see where this is going. What I want to
do is convert these fairly arcane public health measurements into something
that’s understandable for the everyday person. What better way than a 5k time?
If we convert the “elite” cutoffs in METs into VO2 max cutoffs, it’s
easy to run these through a race time predictor and come up with a goal 5k time
for “optimal longevity” (with some very serious caveats!). Let’s take a look at
what those elite aerobic fitness cutoffs look like before we talk about the
caveats.
After looking at these times, we can see the
disconnect between what’s “elite” at the population level and what’s “elite”
for a runner. A 21-year-old male who runs 18:25 for the 5k is certainly in good
shape, but he’s still nearly four minutes
shy of what he’d need to run to run at Division 3 Nationals in track—much less
D2 or D1.
In the context of health and longevity, this is a
good thing: even “extreme cardiorespiratory fitness” is well within the reach
of many (though certainly not all) people in the general population. One thing
I should reiterate is that the longevity benefits of being in the “elite”
fitness group are significant even after
adjusting for things like smoking, body weight, and other things you might
think could account for differences in fitness levels.
If these times seem out of reach for you, don't fret—the difference in survival between people of "high" and "elite" fitness was statistically significant, but very small compared to the differences between people who with "high" or "elite" fitness compared to those who were not fit.
As the figure above shows, the biggest differences in longevity are clearly seen when comparing those whose fitness is poor to those who are at least above average, or better.
If these times seem out of reach for you, don't fret—the difference in survival between people of "high" and "elite" fitness was statistically significant, but very small compared to the differences between people who with "high" or "elite" fitness compared to those who were not fit.
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Not so fast: The caveats
Now, for some caveats. First, my VO2
max to 5k time conversion is a tiny bit hand-wavy. Once you get to high levels
of running fitness, most of your improvements don’t come from better VO2
max; they come from improvements in running economy. That being said, I’m
fairly confident that my converted times are in the right ballpark.
The more important caveats have to do with the
nature of the causal relationship—or lack thereof—between high aerobic fitness
and a long and healthy life. This study measured the fitness levels of healthy
people, then followed them to see who died. Being extremely fit appeared to be
a good predictor of avoiding death in the future. The key question is what biological mechanism is responsible for
this association?
The easiest answer would be doing aerobic
exercise. After all, there is a strong case to be made that very few, if any,
29-year-old women are going to be in sub-20 minute 5k shape without doing a
good amount of aerobic exercise. But there are plenty of alternative mechanisms
that could contribute too. What about genetic variation? Some people are born
with a lot of what we call “talent,” the state of being highly aerobically
fit even when they do not exercise. These same genetic traits which make
someone a talented runner may also be responsible for different biological mechanisms that lead to a longer life: more flexible
arteries, resistance to metabolic disease, etc.
Strong aerobic fitness might merely be an indicator of these traits, not
the actual cause.
Now, the strongest circumstantial evidence still (in my opinion) supports the causal link between intense, high-volume training and a longer lifespan. At low, moderate, and moderately high doses, aerobic exercise is strongly protective against death. Moreover, alternative explanations, like the hypothetical genetic mechanisms I just laid out, would have to account for the fact that it is extremely rare, on the population level, for someone to reach these levels of “extreme cardiorespiratory fitness” with doing a solid amount of training.How many 45-year-old men are such fine genetic specimens that they can run a sub-20:00 5k fresh off the couch, compared to the number of 45-year-old men in sub-20 5k shape who train hard on a regular basis?
Unanswered questions about fitness and lifespan
When we talk about the potential health effects
of training too hard, we aren’t usually talking about a 40-year-old male who
runs 19:25 in the 5k. Usually we are talking about ultramarathoners, sub-16 5k
runners, Boston Marathon qualifiers, and other people who have a tendency to
hammer out hundred-mile weeks and long multi-hour running sessions. If anyone
is at risk of health problems from excessive exercise, it would be these
extreme outliers. Because of the nature of distributions, you can be sure there
are were a lot more 40-year-old men in 19:20 shape in this study than
40-year-old men in 16:20 shape, for example.
While these findings are promising, and suggest
that more really is better when it comes to fitness and exercise, there’s still
a need for research that focuses on those in the most extreme groups when it
comes to exercise volume and intensity.
Fortunately, there is a longitudinal
study happening right now on ultramarathoners that’s being run by
Stanford University and UC Davis. It might take ten or 20 years before we get
solid results from that study, though.
Finally, the question of causality still remains:
If you take someone who does not have
high aerobic fitness, and train them so they become aerobically fit, will they
live longer? This question can only be definitively answered in a
randomized controlled trial. Given the difficulty of following people for 20 or
30 years to observe mortality, testing this question directly might be
impractical. Instead, it might be possible to correlate changes in aerobic
fitness with changes in biological markers that we know are related to
longevity—for example, if we hypothesize that lower arterial stiffness is one
reason why extremely fit people live longer than those who are not fit, we
could conduct a two or five-year clinical trial to see if an intense,
high-volume aerobic training program would reduce arterial stiffness, compared
to a lower-volume, lower-intensity aerobic training program.
As for me? I’m putting my chips on “better
fitness = longer lifespan.” I’m turning 30 this year, so maybe I’ll hit the
roads on my birthday, just to make sure I can still crank out a 19:05 5k.
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