No, Endurance Exercise Won’t Kill You
Athens, Greece
September 12, 490 b.c
This is the site of the first known fatality related to endurance running. Upon arrival to Athens, running the revered distance of 26 miles to deliver a message of a military victory against the Persians, Pheidippides, our Greek hero, collapsed, most likely due to sheer exhaustion on behalf of his previous effort.
“Joy to you, we’ve won” he said, and there and then he died, breathing his last breath with the words “Joy to you”.
This is the first case report of an endurance sport-related death. While an awe-inspiring anecdote, our knowledge of human limits, physiology, and the sheer number of participation in endurance sports prevents us from concluding that, directly or indirectly, running a marathon will “kill you”. Pheidippides story is provocative. Perhaps it tells a tale of what the human spirit can accomplish if truly pushed to the limits (although I don’t suggest exerting yourself to the point of death). Far from a warning for everyone to avoid running, this tale has created a “lust” around the modern 26.2 mile distance — Pheidippides lives on as hordes of runners, in one way or another, try to recreate his heroic effort each weekend.
Indeed, U.S marathon running, and endurance sport participation in general, has hit a revival. This fact is supported by the nearly 507,000 marathon finishers in 2016. These numbers indicate rising participation, an increase in people involved in chronic endurance exercise training for the sake of competition and health.
Is there a downside to this participation? Opponents (critics?) of endurance running like to cite the infrequent cases of modern-day Phedippides as reasons as to why endurance running is dangerous. Indeed, a “comprehensive” list of marathon-related deaths on Wikipedia contains 47 cases of “death by marathon” in the U.S and elsewhere, up until 2016.
A list of the causes of death in these marathons reveals an underlying pattern. Many, but not all, relate to cardiac-related issues (i.e. heart attack, arrhythmia, underlying congenital heart abnormality). We know exercise, and especially a marathon, stresses the heart, among other systems. A list like this leads one to propose that endurance running may in fact be detrimental to the cardiovascular system— it must be, to have resulted in these deaths?
We choose our statistics, however, and the “black swans” are always what is noticed. Indeed, these 47 deaths were over the span of some 35 years, over which some 4 million + people finished a marathon. What this amounts to is around .9–1.6 deaths/100,000 marathon finishers. Marathon running is far from a potent risk factor for an acute cardiovascular event, as evidence by the statistics. While it is undeniable that lack of preparation, underlying cardiac issue, or freak accident may make one susceptible to irreversible injury or death, we cannot ignore the facts— very few people on a population wide level suffer from life-threatening incidents after endurance exercise or related activities.
However, provocative findings such as the ones cited above compel us to at least investigate potential mechanisms as to why endurance activity participation may be detrimental in the short term as well as when participated in chronically. Indeed, another argument against high intensity long-term endurance training is that it may actually lead to unfavorable adaptations such as the athlete’s heart (discussed later). Even adaptations such as this have been shown to be primarily physiological rather than pathophysiological in nature. The central question becomes not “is exercise good for us?” — for we know that this answer is a resounding yes. However, “is more better?” has become a topic of discussion in terms of cardiovascular disease prevention and enhancement of lifespan and healthspan. Do the adverse effects of endurance exercise outweigh the benefits if one fails to abide by the law of “everything in moderation?”
In an elegant review from which the proceeding discussion references, Eijsvogels et al compile convincing evidence for the case that acute and chronic endurance exercise post no great harm, and in fact may confer a long-term benefit, to cardiovascular health.
Does exercise pose an acute cardiovascular risk?
As mentioned previously, a large amount of endurance sport-related deaths occurred due to a cardiac incident during or following a bout of activity. Indeed, vigorous exercise is known to increase the risk (in the near-term) of sudden cardiac death. However, it is noted that this increased risk manifests only in individuals with an underlying cardiac disease. For instance, younger (< 30 or 40 years of age) individuals who suffer from sudden cardiac death during exercise are usually found to have an inherited or congenital cardiac condition, also known collectively as cardiomyopathies. 44% of deaths among athletes in the U.S were shown to be due to hypertrophic cardiomyopathy — an inhertied heart condition in which the heart is enlarged.In older adults, reasons for death often include further cardiovascular issues such as coronary artery disease. These conditions make one greatly susceptible to risk of injury or death during high exertion activity, and prohibits drawing the same conclusions about health effects of exercise in asymptomatic individuals. One study examining the risk of sudden cardiac death in male physicians (a “healthy” cohort) concluded that, while the risk of an SCD was 16.9% higher during vigorous vs. lower intensity exercise, the absolute risk was still extremely low — 1 death per 1.42 million hours of vigorous exercise.
We do have evidence that one’s risk of suffering from a cardiac incident during exercise goes DOWN as you get fitter, and this makes intuitive sense. Running a marathon or vigorous endurance race “unprepared” would seem to increase one’s susceptibility to injury or death. Observations support this. When comparing the risk of an SCD during exercise to the amount of baseline physical activity individuals participated in, it was shown that risk dropped with increasing levels of activity. Additionally, among a study of 84,888 women, those who exercise 2 or more hours per week were shown to have a decreased risk of suffering an exercise-related cardiac event. In the long term, exercise thus seems to be protective in terms of health as well as lowering one’s risk for SCD during exercise.
Why might exercise increase the risk of a cardiac event? While I won’t touch on mechanisms, it deserves a word here to note that vigorous exercise does place quite the demand on the heart. While our hearts are robust, an underlying substrate of cardiac disease or arrhythmia may be the straw that breaks the camel’s back. Lower cardiac function (i.e. due to cardiac fatigue processes, which only occur after ultra-long term exercise) may be one way in which one’s risk for injury rises during or after exercise. As we activate our sympathetic nervous system and release epinephrine and norepinephrine into our systems (that “fight or flight” response), heart rate and the force of the heart contraction both increase (feel your own heart after sprinting 100m, and this becomes clear). It is proposed that, over a long-term exercise bout, our body, in particular the heart, may be unable to handle the increased physical demands and may reduce its function over time. Not to worry — unless your exercise routinely lasts >3 hours, many of these detrimental processes are unlikely to take place.
Evidence tells us that the risk of an exercise-related event, among healthy individuals who regularly participate in exercise, is extremely low. Even among individuals with underlying heart conditions or CVD, vigorous exercise doesn’t seem to pose any substantial threat.
Long term exercise training and the heart: are the adaptations detrimental or beneficial?
Most of the argument over whether long-term endurance training is harmful centers around a concept referred to as the “athletes heart”. In short, the athletes heart refers to adaptations of the heart including mild to moderate enlargement of the ventricles and the heart walls, enhanced left ventricle filling, enhanced heart mechanics (twisting and untwisting during contraction), and enlargement of the atria (see figure below). In contrast to the negative left ventricular hypertrophy that is characteristic of many cardiovascular disease conditions (characterized by a thicker left ventricle wall, but a ventricle that “shrinks” causing the heart to under-perform) — the “eccentric” hypertrophy that occurs with training has been referred to as a positive adaptation, allowing more blood to be filling and then ejected from the ventricle. This allows for more blood delivery during exercise and makes the body more suited to the task.
Trained athletes show increased left ventricle size compared to untrained, normal individuals. A study cited in the review notes that out of 1,3000 Italian athletes who underwent an evaluation, 45% had a left ventricle diameter greater than the upper limit of normal. An interesting note — the authors indicate that the many of the heart chamber size differences among studies may be due to “genetic selection” or otherwise innate characteristics that may have naturally selected individuals to participate or excel in endurance sports — and may not be due to the training itself.
A few other key findings in athletes — improved diastolic (filling) function and more compliant arteries — provide evidence that heart function may actually be improved with long term training, or otherwise present no clinically significant detrimental effects.Many of heart’s adaptations discussed above, the authors note, are “required” for the superior exercise performance of trained endurance athletes.
Can exercise be maladaptive? That is the question this review attempts to answer, with evidence, which is often conflicting.
In general, exercise training is able to reduce all of the major risk factors for coronary artery disease and lower morbidity and mortality, among other benefits. In those who take part in vigorous endurance exercise, however, the risk factor picture may get complicated. For instance, the presence of atherosclerotic plaques in 90% of marathon runners has been noted, as well as increased coronary artery calcium scores, which are shown to be related to the increased risk of a cardiac event. These observations provide some evidence that long term endurance exercise participation may provide no enhancement in CVD risk reduction. However, where there is a line of evidence for one conclusions, there are several for the opposite. Indeed, increased physical activity has also been associated with a reduction in calcium scores.
Other potential deleterious adaptations of the heart to vigorous long-term exercise involve structural adaptations that may increase risk for CVD or CV-related events. For instance, fibrosis of the heart, in which collagen and cell-matrix accumulate and lead to cardiac stiffening, has been reported to be present in veteran endurance athletes.
Arrhythmias (abnormality in heart contraction/relaxation) are also common findings in life-long endurance athletes. While low-levels of exercise are associated with a decreased risk of arrhythmias (hence the benefits of moderate-exercise for everyone), a U-shaped relationship is proposed to exist, such that increasing or abnormally high levels of physical activity may increase the prevalence of arrhythmias among athletes. As was stated before, the enlargement of the left ventricle in athlete’s may be one mechanism behind this. Athletes have intrinsic differences in their heart’s electrophysiological properties, and this is well known (resting HR, for one, is dramatically reduced with training). Thus, much of the functional changes observed in studies are hard to interpret clinically and may represent adaptations that allow for increased endurance performance without an increase in cardiovascular risk — the authors routinely note that more experimental studies need to be conducted before evidence is concluisive as to the long term impact of exercise on the athlete’s heart.
Endurance exercise and a longer life
Not to sound like a broken record, but again, it must be noted that moderate-intensity exercise (i.e. adhering to the recommended guidelines) LOWERS the risk of cardiovascular disease, morbidity, and mortality. Despite few studies demonstrating increased acute risk of death among athletes, the overwhelming evidence points to a longevity benefit among lifelong endurance athletes. In one study assessing 13,016 runners and a control group (sedentary or recreationallu fit)of 42,121 individuals, running was shown to reduce death from all causes by 30% and death from cardiovascular-related complications by 45%.
Those who had participated in running for a longer period of time showed even greater longevity benefits. Furthermore, a study involving Finnish elite skiers demonstrated an increase in life-span of 2.8–4.3 years when compared to a group of normal men. This increase in lifespan is consistent with others cited in the review, and provides evidence that participation in endurance exercise throughout life indeed has benefits related to lifespan enhancement.
“…doses of habitual exercise above the recommended physical activity guidelines may improve health and stimulate longevity in the long run”
A thorough examination of the literature would still note provide us with enough evidence for or against the participation in long term endurance exercise training. Where on study says “do,” another says “don’t.” It is nearly impossible to conduct experimental studies of exercise in humans — as these would require controlled exercise programs over the lifespan and the correct measures of adaptive/maladaptive changes along with it. While animal models are useful in this regard (mouse lifespans are considerably shorter), they still don’t provide us with the depth of insight required to elucidate mechanisms of endurance exercise related adaptations in humans. We can let epidemiological and long-term cohort studies provide our theories, but they will remain just that.
Exercise, in sum, appears to have profound beneficial effects in nearly everyone who takes part. Some is better than none, and more is better than some. Whether or not “more” above the typically recommended dose confers additional benefits has yet to be studied. The way I see it, there is no replacement for the enjoyment (for those who do enjoy it) of endurance exercise and the depth it provides to life. That, I think, is more important than any physiological adaptation can explain.
Sources:
Eijsvogels et al. Are There Deleterious Cardiac Effects of Acute and Chronic Endurance Exercise?Physiol Rev 96: 99 –125, 2016 Published November 25, 2015; doi:10.1152/physrev.00029.2014
Race Image- http://framework.latimes.com/2015/03/12/los-angeles-marathons-first-years/#/0
Cell image- https://www.baker.edu.au/research/laboratories/experimental-cardiology/project-myocardial-fibrosis
