While exercise has its health benefits, evidence is mounting that long-term strenuous exercise can actually be bad for the heart. By Michael Lim

MR A was in his fifties and came for a review as he was about to take part in a seven-day marathon. In preparation for this ultra-marathon, he was running 40 kilometres a day. He wanted to know if it was safe for him to go ahead with this race.

While exercise has long been actively encouraged and considered beneficial to health, the impact of strenuous and prolonged physical activity on the body was less clear. Now, as medical science advances and imaging technology improves, the impact of strenuous exercise is gradually becoming clearer.

There is substantial data to show that exercise can help to reduce weight, decrease “bad” cholesterol, elevate “good” cholesterol, improve blood vessel elasticity and strengthen the heart. Analysis of data from the Framingham Heart Study, a research study that has followed more than 5,000 residents of one Massachusetts town for more than 40 years, suggested that residents who exercised moderately lived longer than those who were sedentary. However, there is as yet no data to suggest that intense physical exercise will increase life expectancy.

Just as exercise can strengthen the muscles, the heart becomes stronger with exercise, and is therefore able to pump more blood through the body with every beat. As a result, the resting heart rate of an athlete is also slower as less effort is needed to pump blood.

Although there is evidence to show that exercise can be beneficial for healthy individuals and in patients with heart disease, there is increasing evidence that long-term strenuous exercise can increase the risk of developing abnormal heart rhythms. Both abnormal heart rhythms arising from the upper and lower chambers of the heart have been associated with sustained intense exercise, including the potentially life-threatening abnormal heart rhythm, ventricular tachycardia. What was less certain in the past was whether these abnormal rhythms were due to underlying heart diseases, with intense exercise triggering the onset of abnormal heart rhythms, or whether sustained intense exercise caused damage to the heart muscle.

A Harvard study on the impact of 90 days of vigorous training on young athletes showed that exercise produces significant changes in cardiac structure and function, and the type of change varies with the type of exercise performed.

At the end of 90 days, both endurance athletes and strength athletes had significant increases in heart size. For endurance athletes, the left and right lower heart chambers (ventricles) expanded. In contrast, in the strength athletes, the heart muscle of the left ventricle thickened and became stiffer.

While changes to the heart are well documented, what also needs to be documented is that these changes result in damage and scarring of the heart, thereby making it more susceptible to abnormal heart rhythms.

Heart muscle fibrosis

A recent animal study evaluated whether sustained intensive exercise training induces potentially adverse changes to the heart muscle, thereby creating a potential substrate for abnormal heart rhythms. The study showed that scarring (fibrosis) of the heart muscle occurred after sustained intensive exercise, with changes to the heart function and increased inducibility to abnormal heart rhythms. For the first time, intensive exercise was shown to induce heart muscle scarring and increases the likelihood of abnormal heart rhythms.

This evidence was further strengthened by a recent study comparing life-long, competitive endurance veteran athletes greater than 50 years with age-matched controls and young athletes. Newer techniques in magnetic resonance imaging (MRI) of the heart were used to identify early signs of fibrosis within the heart muscle. While none of the younger athletes or the older non-athletes had fibrosis in their hearts, 50 per cent of the older lifelong athletes showed heart muscle fibrosis.

Severe fibrosis can lead to stiffening or thickening of portions of the heart, which can contribute to irregular heart function and, eventually, heart failure. This study was the first to prove that sustained intensive exercise can cause permanent scarring of the heart muscle with potentially serious detrimental effects.

In addition to the risk of dying from abnormal heart rhythms, patients with underlying narrowing of the heart arteries are also at risk of developing a sudden heart attack during intense exercise such as running a marathon. Intensive exercise can result in a tear in the inner lining of the narrowed segment of the heart artery, triggering a series of reactions culminating in the formation of a blood clot to seal the tear in the lining. If the clot is large enough, it can occlude the residual lumen of the artery, causing a heart attack.

Those who “collapse” from a heart attack during intense physical exercise such as a marathon will generally not survive as the milieu in the body during intensive exercise increases the susceptibility of recurrent abnormal heart rhythms and makes it very difficult to resuscitate the individual. As a result of sustained intensive exercise, the body is dehydrated, accumulates acidic substances and is starved of oxygen. When this is exacerbated by the failure of the heart to pump blood to the body to supply oxygen, the situation deteriorates rapidly and is very difficult to reverse even with prompt resuscitation measures.

While moderate exercise can have potential benefits, sustained intense exercise can potentially cause permanent scarring of the heart with adverse consequences. Hence, those who exercise frequently and intensely should get themselves properly assessed by the doctors before they develop irreversible damage to their hearts.

In addition to the risk of dying from abnormal heart rhythms, patients with underlying narrowing of the heart arteries are also at risk of developing a sudden heart attack during intense exercise such as running a marathon.