Positive Health Online

You are training hard to run marathons, and you are even dreaming of ultras. But some people (mostly non-runners) keep warning you that you could be doing too much... Of course you laugh and do not believe them, but could too much running be bad for your heart? Everybody agrees that moderate exercise is good for you, but as apparently healthy people have suddenly died during a prolonged vigorous effort, those nagging questions keep popping up.

Statistics show that there are only about 0.8 sudden cardiac deaths for 100.000 marathon runners, which corresponds to two deaths for a million hours of vigorous exercise. Nevertheless, this is seven times more than during less strenuous exercise, and it is disturbing that an activity which is meant to improve your health could kill you.

In people younger than 35, a sudden death is usually due to hypertrophic cardiomyopathy, while in older people it is more often caused by atherosclerosis.  Hypertrophic cardiomyopathy is a congenital abnormality, characterised by an inappropriate, asymmetrical thickening of the heart muscle. It is inherited, but there are milder and more severe forms. It is estimated to occur in 0.05 to 0.2% of the population. 

In this article we concentrate on sudden deaths caused by atherosclerosis.

Why are the Short Time Risks of a Strenuous Run So High?

In 2008, Stefan Moehlenkamp and Frank Breuckmann stirred the discussion with a cardiac magnetic resonance imaging (CMR) study of marathon runners. CMR is the best method we actually have to examine a heart’s volume, mass and function. It allows us to evaluate coronary artery calcification, which gives us a good idea of the amount of atherosclerosis. By administering an intravenous contrast product, we can detect regions of delayed blood perfusion, which may correspond to fibrosis (scarring) of the heart muscle. This can trigger sudden rhythm irregularities, especially during intense exercise, and lead to a sudden cardiac death.

Stefan Moehlenkamp and Frank Breuckmann compared risk factors such as cholesterol, blood pressure and body weight, and the amount of coronary artery calcification between 108 recreational marathon runners and a control group. The study group consisted of healthy men older than 50, who had completed at least five marathons in the last three years. As expected, the runners had a lower blood pressure and body weight, more HDL (‘good cholesterol’) and less LDL (‘bad cholesterol’), but to their surprise they had as much coronary artery calcification as their sedentary controls.  The researchers explained this mismatch by the life style of some of the athletes. They concluded that marathon runners cannot consider themselves immune from cardiovascular disease, and that vigorous exercise is not an excuse for an unhealthy life style. Atherosclerosis is indeed a very slow progress that starts early in life, even if it only manifests itself much later, and cannot be completely wiped away by a few years of running.

Heart Muscle Damage

In a second study the same researchers looked at the blood perfusion in the hearts of 102 seasoned marathon runners and the same number of matched controls. They observed more areas of fibrosis in the runners than in the controls (12% versus 4%).

They thought that at least some of these lesions were due to vascular disease, and they argued that during vigorous exercise, tiny pieces of a plaque could be ripped off and carried away by the blood stream, and then obstruct small vessels farther downstream and damage the heart.

However, cardiovascular disease could only explain part of the findings.

To avoid any influence of a previous unhealthy lifestyle, Dr Wilson and colleagues compared the hearts of lifelong veteran endurance athletes with those of young athletes and sedentary controls. Only the veteran athletes showed fibrosis. This does not prove yet that hard training and racing causes heart scarring, only that there is an association. Dr Benito therefore made healthy rats run vigorously for 16 weeks, which would correspond to about 10 years of intense endurance training for humans, and compared their hearts with those of sedentary rats. Most of the exercised rats had developed fibrosis, but the hearts of the sedentary ones did not show any changes.

So What Could Cause This?

Several investigators have examined runners just after finishing a marathon by cardiac magnetic resonance imaging, electrocardiography, ultrasound and blood samples. They showed a dysfunction of the right heart and an increase in biomarkers, suggesting heart muscle damage, such as troponin, but no signs of injury due to reduced blood supply.

The right atrium receives blood from veins draining the muscles and transfers it to the right ventricle, which pumps it to the lungs for oxygenation. During exercise the volume of blood coming back to the heart increases, leading to a dilatation of the right atrium. In addition, the pressure in the pulmonary artery also increases, making it harder for the right ventricle to pump blood into it. During prolonged intensive exercise, the right heart has to work very hard to deal with this and the muscle starts to make a kind of wringing motion during each beat. This normalizes quickly after the effort and could be the reason for the temporary increase in biomarkers. However, some researchers think that this repeated hard work, dilatation and recovery could lead to fibrosis after many years of long distance running. It is still unclear if this would put the athletes at a higher risk of heart rhythm disturbances. 

Data from ‘Real’ Life

In April 2011, a Norwegian group published a study comparing the mortality of 122 life-long cross country skiers with that of the general population over 30 years. The death rate was 31% in the skiers group and 40% in the general population. Recently, Spanish scientists published a study showing that Tour de France’ cyclists lived on average 8 years longer than the general population.

It is of course possible that the situation is different for marathon running, or that the longevity is due to other factors such as a healthier lifestyle or a more favourable genetic make-up, but it shows that vigorous exercise is not linked with an early death, and suggests that the fibrosis is innocent.

What does this Mean for Me?

It is all about risk management. Long distance running will dramatically reduce your chances of suffering a heart attack, but during and just after strenuous exercise you are at a higher risk, especially if you have already some heart disease. But even if you do, you are less likely to die from it than your sedentary counterparts. Thus, no matter how you look at it, it is much safer to continue training.

Most people who suffered an attack reported symptoms before the event. If you feel chest discomfort, palpitations, breathless or anything unusual, stop immediately and seek help.

Keep running, but look after your heart, and this includes diet, check-ups, tests and awareness.

If you have no cardiac problems at all and always had a healthy lifestyle, could you exercise so much that your heart gets weaker? Probably yes, but you would have to train much more than elite endurance athletes.

References

B Benito, G. Gary-Jordi, A. Serrano-Molar. Cardiac arrhythmogenic remodelling in a rat model of long-term intensive exercise training. Circulation 123: 13-22. 2011.

F.  Breuckmann, S.Moehlenkamp, K. Nassenstein. Myocardial late gadolinium enhancement: prevalence, pattern, and prognostic relevance in marathon runners. Radiology 251(1): 50-57. 2009.

S Moelenkamp, N Lehmann, F Breuckmann. Running: the risk of coronary events. Eur heart J 29 (15): 1903-1910. 2008.

T.G. Neilan, J.L. Januzzi. Myocardial injury and ventricular dysfunction related to training levels among non elite participants in the Boston Marathon. Circulation 114: 2325-2333. 2006.

D. A. Redelmeier, J.A. Greenwald. Competing risks of mortality with marathons: retrospective analysis. BMJ 335(7633): 1275-1277. December 2007. 

JE Trivax, BA Franklin, JA Goldstein. Acute cardiac effects of marathon running. J Appl Physiol 108(5): 1148-53. May 2010.