By Chris Carmichael,
Founder and Head Coach of CTS
It was with great pleasure that I read this recent article from Alex Hutchinson, this one by Amby Burfoot, as well as the research they’re based on. Every few years during the past decade or so, we’ve seen headlines claiming high volumes of exercise cause damage to the heart and increase the risk of early death for lifelong athletes. But in the field, those of us working with the exact population that would be dropping dead haven’t seen “the bodies in the streets”, as Burfoot put it. At the risk of falling victim to confirmation bias, this new research provides evidence that extremely high volumes of exercise don’t increase all-cause or cardiovascular disease mortality risk.
The original study by Laura Defina et al. (2018), examined data from 21,758 healthy men who underwent testing at the Cooper Clinic in Texas between 1998 – 2013. Subjects were divided into groups based on self-reported levels of physical activity: < 1500 MET-min/week, 1500 – 2999 MET-min/week, and 3000+ MET-min/week, and classified again based on whether they had more or less than 100 AU of Coronary Artery Calcification (CAC).
As a matter of reference, a MET is a measure of work based on a multiple of your basal metabolic rate. According to the Compendium of Physical Activities, cycling at 16-19mph is a 12 MET activity, meaning that 60 minutes of cycling at 16-19mph would yield 720 MET-minutes. That means 3000 MET-minutes cycling at 16-19mph would equate to a little more than 4 hours on the bike per week.
The study found that the high exercise group (3000+ MET-min/week) had more coronary artery calcification than groups with lower amounts of physical activity. (As a side note, the mean for the high exercise group was actually a bit more than 4600 MET-min/week, which is the equivalent of 6.4 hours at 16-19mph on the bike.) More CAC is often viewed as a marker for increased heart disease risk because higher CAC leads to greater stiffening of coronary arteries and is often associated with the presence of plague, which leads to blockages and clots. But despite higher CAC levels, subjects in the high exercise group were about half as likely to die from CVD over the next 10 years. And there was no difference in 10-year mortality within the high exercise group based on CAC level, meaning even the high-volume exercisers who developed more CAC didn’t increase their risk of heart disease or death.
All of that is well and good, but 6 hours a week isn’t that much for a moderately trained endurance cyclist. The research got a lot more interesting when they re-analyzed the data looking at very-high volume athletes. Out of the roughly 2154 people in the original 3000+ MET-min/week group, 66 were found to exercise at 10,000+ MET-min/week (equivalent to nearly 14 hours/week at 16-19mph) on the bike if they rode steady, and even more hours when you factor in variable intensity), and had been doing so for more than two decades. In the subsequent analysis, the extraordinary volume group did not have any increased risk of CVD or death compared to the original 3000+ MET-min/week group.
The investigators concluded that “very high volume (≥10,000 MET∙min/week) exercisers are not at increased risk of cardiovascular or all-cause mortality.”
Key Takeaways for Longtime Cyclists
Your heart health is determined by a lifetime of choices
As I wrote about in 2015 and continue to confirm with cardiologists, cardiac surgeons, and electrophysiologists, pinning down the exact cause of heart disease or a cardiac event is very difficult – particularly in populations older than 50 years old – because there are so many variables in play. There’s genetics, your level of lifestyle stress in earlier years, alcohol or drug use in your 20s (or now or anytime in between), the way you ate at various times of your life, how much you exercised earlier in life, and many more.
In other words, the fact you’ve been a cyclist for 20 years or even longer is only one factor among many, which is why no one has been able to definitely say long-term high-volume exercise damages the heart and increases an older athlete’s risk of CVD or sudden death.
Of course, that doesn’t mean exercise is an ironclad protection against heart disease. As Benjamin Levine, a senior author on the paper from UT Southwestern Medical Center and Texas Health Presbyterian Hospital in Dallas, told Amby Burfoot for this Washington Post article, “Exercise is not magic, particularly not for those trying to overcome a lifetime of bad habits. Even high-volume exercisers who are strong and fit are vulnerable. Anyone who develops symptoms while training should consult their doctor.”
The Benefits of Improved Cardiorespiratory Fitness Outweigh Any Risks
I have yet to find any physician or researcher who disagrees that the health and longevity benefits of increased physical activity and improved cardiorespiratory fitness outweigh the risks of damaging the heart through exercise. The original study from Cooper Clinic just adds to the weight of literature supporting the benefits of exercise for heart healthy and longevity. Both the medium (1500-2999 MET-min/week) and high (3000+ MET-min/week) exercise groups had lower mortality risks compared to the <1500 MET-min/week group.
This study didn’t look at Atrial Fibrillation
One key difference between this study and my previous article on long-term exercise and the risks of heart damage is that this study looked at coronary artery calcification and the previous article was a response to headlines connecting long-term exercise to increased risk of atrial fibrillation.
Four years later, I still stand by my previous article: 1) Cardiac problems are still extremely rare for CTS Athletes. If high volumes of high intensity exercise increased the occurrence of Afib, I’d expect it would be noticeable and alarming for large-scale coaching companies. 2) Atrial fibrillation diagnoses are increasing in the overall population, particularly because the massive Baby Boomer generation is now in the age group where Afib is more common. 3) Diagnostic tools have improved and athletes are more likely to notice even slight Afib, compared to sedentary people with less body awareness.
This study didn’t look at the intensity of the exercise completed
One potential limitation of this study is that it looked at the volume of exercise performed, but not necessarily the intensity of that exercise. Just like you can do 1500 kilojoules of work on the bike in 90 strenuous minutes or 3 moderate hours on the bike, you can achieve 10,000 MET-min/week with either a smaller volume of very strenuous exercise or a larger volume of moderate exercise.
However, a 2014 meta-analysis of 10 studies encompassing more than 42,000 elite and professional athletes found that elite athletes lived longer than the general population and had reduced CVD and cancer mortality. Though purely anecdotal, my own experience lines up with this conclusion. I was an elite athlete and have spent my life in the company of elite and professional athletes. I’m 59 years old. If high amounts of high intensity exercise reduced life expectancy, I would be going to a lot more funerals.
We ride for more than heart health
The hard part about digging too deeply into studies about disease risk factors associated with exercise is that it often misses the point. I haven’t been riding my bike for 50 years because I wanted to reduce cardiac disease risk factors, or because I wanted to burn calories or fat. I doubt those are the core reasons why you’re riding, either. We ride because we love it, and we ride hard because it’s more fun to be fast and that’s what it takes to be able to ride fast. Thankfully, this new research indicates our passion for cycling isn’t likely to kill us, but I don’t know if I would change anything even if it was.