It seems an almost regular occurrence in cycling that a rider is forced to retire as a result of heart issues. The past year has provided several such examples: Australian veteran Michael Rogers called timed on his career in April 2016 following the discovery of a previously unknown arrhythmia; Johan Vansummeren retired two months later due to a heart anomaly; and then, just two weeks ago, Gianni Meersman was forced off the bike due to a cardiac arrhythmia.

And then there are the tragic stories of riders losing their life due to heart troubles: 22-year-old Belgian Continental-level rider Daan Myngheer died in March 2016 following a heart attack at the Criterium Internationale and 21-year-old Dutch amateur Gijs Verdick died in May last year. Former Australian champion Will Walker nearly lost his life at the 2014 Australian Road National Championships, an incident that forced him to quit racing for good.

So is it the case that elite athletes are more susceptible to life-threatening heart conditions? What differences are there between the hearts of elite athletes and the general population? And is there such a thing as too much exercise?

Following on from articles written for CyclingTips several years ago, cardiologist Associate Professor Andre LaGerche from Baker IDI Heart and Diabetes Institute looks at the latest research and considers the research that’s yet to be done.

Let’s get the most important things out of the way first. Exercise has massive positive health benefits. It has been well-proven that mild and moderate exercise can prolong life and reduce serious health problems. More specifically, exercise is associated with:

– Lower risk of heart attacks and stroke

– Reduced diabetes

– Fewer cancers (particularly breast and bowel cancer)

– Lower rates of depression

– Improved blood pressure and healthier cholesterol profile.

As a result, international guidelines recommend 150 minutes of moderate to vigorous exercise per week. However, in Australia just 44% of adults and 19% of 5-17 year-olds meet national physical activity recommendations, according to a 2012 study.

The public health issue is clear: we need more people engaged in regular exercise.

Too much exercise?

There is current controversy about whether you can do too much exercise and start to cancel out some of the health benefits of exercise. This debate is fuelled by the fact that there are some ‘injuries’ that are more common when very large amounts of exercise are performed.

Like a tennis player is at risk of elbow injuries (“tennis elbow”) or a mountain runner can develop Achilles’ tendinopathy, endurance exercise places a considerable demand on the heart and it has been argued that it may be at increased risk of an “overuse” injury as a result.

Although we have mountains of evidence on the benefits of exercise, most of this is accumulated from people performing exercise in amounts far less than that of competitive or highly active amateur athletes.

A famous epidemiologist named Steven Blair once coined the term “asymptote of benefit” referring to the fact that the benefits of exercise seemed to plateau at a certain ‘dose’. This dose was the amount of training that enabled someone to jog slowly (10 metabolic equivalents or VO2 = 35 ml/min/kg).

One thing is very clear: the health benefits in going from unfit or sedentary to moderately fit are much greater than from going from average fitness to super fit.

As demonstrated in the figure below, there are diminishing health benefits with improving fitness, but this does not mean that there is no return.

The health of the highly trained

In the really highly trained endurance athlete, there are a number of changes that take place in the heart and blood vessels. Most of these changes are very healthy but it is becoming clear that there may be some ‘side-effects’ associated with endurance sport training and racing.

But, even in the very most trained, the benefits still outweigh the risks.

Landmark studies from Finland reported fewer cardiovascular and cancer deaths in former elite athletes who had competed at an international level between the years 1920 and 1965, as compared with the general population. Similar findings were recently documented in Tour De France cyclists. However, the extent to which these improved outcomes can be attributed to exercise is difficult to ascertain given that there were also important differences in smoking, alcohol consumption, obesity and socio-economic class.

In other words, we don’t know whether endurance athletes enjoy great health because of all of the training or because, in general, they live very healthy lives.

So what about the ‘side-effects’ of endurance sports training? This is where things become a little controversial and I would encourage readers to keep an open mind. There are many commentators on this topic with very strong opinions and it is important to remember that the science is not watertight.

One clear fact is that endurance exercise has a profound effect on the heart. The heart of a well-trained athlete can be twice as large as that of a non-athletic individual.

Heart size

Training causes heart enlargement (often referred to as “athlete’s heart”) and it is the major adaptation that enables an athlete to pump larger volumes of blood to the muscles during exercise.

This is a critical determinant of athletic performance as greater intensity exercise requires more fuel and this requires oxygen to burn. The oxygen is supplied in the blood and hence, the more blood pumped to the muscles the better the capacity for performance.

There is a strong relationship between the size of an athlete’s heart and their VO2 max, a measure of the maximum amount of oxygen the body can use during exercise. This is an important determinant of exercise performance but not the be all and end all. There are many other physical and psychological factors that define an athlete’s performance. Nonetheless, elite endurance athletes have almost universally high VO2 max values.

So is it good or bad to have a big heart? From a sports performance perspective the simple answer is that it is good to have a big heart. Bigger engine = more blood flow. However, in many heart diseases a big heart is associated with problems and so many heart specialists have wondered whether athletes with very big hearts may be at slightly higher risk of some problems.

One of the major problems that we see with big hearts (in non-athletic settings) is more heart rhythm problems. Thus, there has been concern that this may also be true for athletes. The strongest evidence for this is for the heart rhythm problem called “atrial fibrillation”.

Atrial fibrillation (AF) is the most common heart rhythm disorder in people of middle-age or older. Instead of a normal regular conduction of electrical signals through the heart, there is a random firing of electrical signals from the upper chambers of the heart (the atria) and this causes an irregular heartbeat that is often more rapid than would normally be appropriate for the level of activity.

Some patients are completely unaware that they have AF whereas other patients are greatly troubled by an uncomfortable sense of an irregular heart rhythm, fatigue and breathlessness. AF is not life-threatening but can be very annoying and is also associated with an increase in the risk of stroke.

It is not a good thing to have but, on the other hand, AF can be quite effectively treated in most cases.

Almost all studies have observed a higher rate of AF amongst endurance athletes as compared with non-athletic individuals. Importantly, this excess in AF has not been observed in female athletes and the difference between genders is not simply explained by the lesser proportional representation of females in endurance sport cohorts.

In a systematic review combining data from six contemporary studies, the risk of AF in athletes was estimated to be five times that in matched non-athletic controls. Similar findings have been observed in very large population studies but the relative risk is more around a two-fold increase.

My personal view in interpreting the findings from multiple studies is that endurance exercise is clearly associated with an increase in AF risk but that the risk is closer to two-fold than five-fold.

Heart attacks in cyclists

So what about other heart rhythm problems? What about the risk of athletes dying suddenly? There have been many sad examples of athletes dying very suddenly and way too young, not least the recent cases of Daan Myngheer at 22 years old and Gijs Verdick at 21.

The reports often talk about the athlete having died of a “heart attack” or “cardiac arrest”. The most common cause of a heart attack in the general population, especially among those of middle-age or older, is a sudden clot in the arteries in the heart, limiting blood flow to the heart. In the young athlete, the most common cause of a cardiac arrest is a heart arrhythmia.

In young athletes such as Myngheer or Verdick, the most common causes of cardiac arrest would be a heart muscle disorder that they were born with – conditions such as hypertrophic cardiomyopathy (a thicker-than-normal heart wall) or arrhythmogenic right ventricular cardiomyopathy (a weakening of one of the heart’s chambers). These are very uncommon conditions that are passed down through the family. Usually these conditions can be identified on simple cardiac tests such as an electrocardiogram (ECG).

This simple test measures the electrical patterns coming from the heart and takes only a few minutes to perform. It is now a compulsory test for professional cyclists competing at Pro Continental level.

It is very important to remember that these events are very uncommon – around 1 in 100,000 athletes. It is highly debatable as to whether they occur any more frequently in athletes than in non-athletes.

It is going to be a long time before we get any clarity on whether athletes are at greater risk of cardiac arrest from arrhythmias because, thankfully, these events are so uncommon that it would require a massive study to detect an excess.

Some argue that the link between cardiac enlargement and atrial fibrillation can be extended to imply that the heart changes seen in athletes may cause other arrhythmias but, at present, we just do not know whether this is the case.

So what can we do to find out?

As mentioned above, many sporting bodies such as the UCI are addressing this issue by regularly assessing athletes to see whether they have any heart abnormalities that may increase their risk of problems. They also mandate that professional athletes are checked regularly, presumably with the rationale that they may be able to detect any problems that develop with years of training that may present an issue.

The problem is that the tests we have are not very good at predicting who will run into problems.

As discussed earlier, the changes in an athlete’s heart are profound and we do not know which features should prompt us to be concerned. It is certainly not as simple as to restrict those with the biggest hearts. If we did that we would be suspending a lot of athletes and 99.9% of them would be completely healthy.

Further research needed

It is very convenient for a cardiovascular researcher to say that more research is needed! However, quite simply, exercise is an essential part of life and many of us are very passionate about the joys of really pushing the body to its limits. In that context, it worries me that we do not know the answers to the following questions:

– Why do athletes get more atrial fibrillation?

– Are serious arrhythmias more common in athletes?

– What is the long-term consequence of having an enlarged heart?

It is for this reason that we are leading a large international study called the Prospective Athlete’s Heart Study (or Pro@Heart for short). This is a collaboration between researchers in Australia, Belgium, France and possibly expanding to Norway and the United States.

The concept is relatively simple. We will comprehensively assess young elite athletes and follow their health and performance for many years. This is a more scientific means of determining whether changes due to training (such as increases in heart size) are associated with an increase in the risk of heart rhythm problems.

We will also be assessing factors such as whether genetics is associated with heart size, performance and the risk of arrhythmias.

In addition to the questions that we have set out to answer, it is likely that the Pro@Heart study will provide a wealth of new information on the effect of sport on the heart. The prospective study design (referring to the fact that we will study the natural history across time) enables us to assess things such as how rapidly the heart gets larger with training and whether it returns to normal size when training stops.

It will also enable us to look at the relationship between exercise dose, heart size and heart rhythm problems in a far less biased manner than studies that have looked backwards after people have developed problems. We should develop the most comprehensive data on the link between sport and both health benefits and side-effects.

We are collaborating with Australian and international experts to reserach the association between endurance sport and arrhythmias. We have significant funding support from the National Health and Medical Research Council of Australia and should have some important results within the next 3 -4 years and beyond. More information is available at www.ProAtHeart.org.

In summary though, here’s what we know:

– Mild and moderate exercise is undoubtedly good for us and more people need to be doing it

– It’s not yet clear whether there is such a thing as “too much exercise” for the heart.

– Training causes enlargement of the heart, which is good for performance. There is some speculation that this enlargement may also slightly increase the risk of some heart rhythm problems such as atrial fibrillation.

– We know that AF is more common among endurance athletes compared with non-athletic folks. The reasons for this excess is not clear.

– Heart attacks in young athletes are most commonly caused by heart arrhythmias, but it’s highly debatable whether such heart attacks happen more frequently in athletes than non-athletes.