One would be hard-pressed to find a competent health professional that did not recommend regular exercise for its health benefits. Along with smoking, being physically unfit is the most consistently significant behavior that reduces the quantity and quality of life. However, a recent review in the Mayo Clinic Proceedings suggests that marathoners, ultra-marathoners, and triathletes might be over-doing it. The review cites multiple research papers that associate extreme endurance training with structural damage to the heart and other abnormalities. So can extravagant hours of exercising be harmful? It is true that a few hardcore runners die each year in races or training, often from cardiac events. Such incidents reliably prompt ominous headlines. Like most topics in biomedical research, the real story is more complicated than a 140-character capsule allows. This article takes a closer look at the phenomenon of ultra-exercise and its health implications.
Endurance sports: the rising tide
Ever since Pheidippedes crushed a 26.2-mile run to deliver a message of victory to Athens, the marathon has been a symbol of elite fitness. In modern America, exercise disparity is alive and well. The last 30 years were witness to a dramatic rise in unfit and sedentary people paralleled by a growing cohort of the exercise-obsessed. From 2000 to 2009 alone, the number of marathon finishers jumped from 300,000 to about 500,000. A similar rise in triathlon and ultra-marathon (>50 km) participation has been observed. In general, these people live much longer and better lives. However, the rise in participation has brought two things with it. First is the ability to study endurance runners in larger numbers and greater detail. Second is an increased number of people who participate in marathons with poor preparation.
Finishing a marathon has become a major source of inspiration for many weekend warriors. Friends issue challenges to one another, or people may decide to run for charity. Although the challenge of a marathon is a great source of motivation, many people arrive unprepared for the demands of high-volume exercise. Increasing running mileage to train for a marathon is associated with an increased risk of musculoskeletal injury. However, here the focus shifts to the headlines which warn people that too much running damages the heart and may even cause premature deaths.
What does the science say?
The recent Mayo Clinic review examined research on both humans and laboratory animals who engaged in voluminous aerobic exercise. The results suggest high-volume training is associated with significant heart abnormalities and many mass media outlets quickly weighed in on the subject. Even alternative health guru Dr. Mercola got into the fray, proclaiming, “…even if you’re one of the lucky ones who does not end up suffering from a sudden cardiac event in the middle of a race, in the long run your heart health can still suffer.” It turns out most marathon runners are very lucky as only 28 people died out of 3.8 million marathons run from 2000 to 2009. By way of comparison, about 40 Americans die each year from being struck by lightning.
Aside from sensational headlines, the review (which I recommend reading in full) highlights several issues worthy of examination. Primarily, the extensive training that goes into high-endurance event preparation can change the anatomy of the heart. Much like weight training, big loads on the heart make the muscle itself thicken, especially the left ventricle. This condition is termed left-ventricular hypertrophy (LVH) and is illustrated below. The left ventricle is the largest chamber of the heart and pumps blood to the aorta. Therefore, its function is critical to proper circulation. While LVH in the presence of obesity is a risk factor for cardiovascular disease (CVD), it has not been shown to cause CVD. To date, it’s unclear if LVH from extreme exercise has the same association with disease and mortality as does LVH from obesity or diabetes.
Like other muscles, excess wear and tear on the heart causes it to lose some suppleness and become fibrotic, which hampers efficiency. Moreover, the longer the training or race and the more of it, the bigger the chance for muscle scarring. These underlying changes to heart muscle often confer an increased risk of abnormal heart rhythm (arrhythmias). However, it’s very difficult to tell if the arrhythmias show up only in those genetically pre-disposed or even if they end up affecting one’s ultimate cause of death.
What’s the big picture?
Right now, the best science can do is suggest an association between extreme training and heart complications in a very small fraction of the population. One would find a similarly troubling rate of changes to the spines of professional golfers or the shoulders of professional pitchers. Overuse can cause problems no matter what the activity. The real question to consider is if the risk is substantial enough to warrant a change in behavior or recommendations. The absolute death rate of running a marathon is very low, and there is not sufficient evidence to suggest training-induced heart abnormalities cause the few deaths we’ve seen. To wit, triathlons are substantially more hazardous than marathons. However, it’s things like drownings from swimming, not the longer work load of triathlons, that are responsible for the difference.
The review highlights a wide range of studies which suggest extreme endurance training is associated with impaired function of the heart. Since hundreds of hours go into training for a 2-5 hour marathon, it’s important to remember that the training and not the race is likely to be the most problematic. Therefore, if one is obsessed with ultra-endurance racing but concerned about his or her heart, the training should be examined. Runners can perform very well with reduced-volume training methods like intervals, weightlifting, and running hills. Reducing volume also lowers the risk of musculoskeletal injury. Dr. Mercola rightly notes that our hunter-gatherer ancestors were more likely to run hard in short bursts and walk for long periods in between. This paradigm can be replicated in interval training, which I outlined here.
The bottom line is that it’s easy to sensationalize stories of fitness freaks dying in a race. The problem comes when people use that rare occurrence to warn against one’s chosen form of regular exercise. Running remains a terrific choice for improving many aspects of public health, and the rare horror stories should not deter people. Extreme endurance athletes are advised to add variety to their training and lower their training volume for many reasons, including injury prevention and improved performance. Reducing the risk of death from extreme fitness is as much about safe training and event conditions as it is about concerns over heart damage. For the rest of us, the benefits of exercise are so overwhelming that there is no sound reason to think about stopping.
Max Prokopy is a Biochemical and Molecular Nutrition PhD student who has certifications and experience with training prep, collegiate and professional athletes with a particular focus on ice hockey.