BioEd Online

Athletes who carry one type of a well-known 'fitness' gene might actually push themselves so hard that they tire out their hearts. That's the finding from a study of individuals who competed in one of the most gruelling races in the world.

Exercise experts have long assumed that heart muscles, unlike those of our legs and arms, don't tire. But a few studies of athletes have hinted that the heart also gets worn down by many hours of extreme exercise.

To find out how bad it can get, cardiologist Euan Ashley of Stanford University School of Medicine in California, and his colleagues studied the participants of an 'adventure race' that took place in Scotland, UK. Teams of four battled to run, cycle, kayak, orienteer and swim across some 480 kilometres, snatching only a few hours sleep over several days.

Ashley and his team measured how well competitors' hearts worked before the race using echocardiography (an ultrasound of the heart), and then re-did the measurements as some of them staggered across the finish line more than 90 hours later. The researchers offered the shattered finishers doughnuts as an incentive to take part.

They also took blood samples to see which variant of a known 'fitness' gene the athletes carried. This gene makes angiotensin-converting enzyme (ACE), which promotes the production of a hormone that narrows blood vessels. The variant that results in lower ACE activity is more common among elite, endurance athletes and seems to boost the body's stamina.

Tired at heart

The team report two main results. First, they found that the hard-core exercise did indeed tire out the athletes' hearts, to the point where their hearts lost around 10% of their ability to pump blood. This is a far greater drop in performance than that previously observed, after shorter races. "Ten percent could easily be the difference between winning and losing," Ashley says.

To Ashley's surprise, the team also found that the hearts of athletes carrying two 'endurance' copies of the ACE gene tired out more. These people's hearts lost around 13% of their power, whereas carriers of one copy or none lost closer to 8%.

Ashley proposes that people with two copies of the endurance variant may be able to push themselves longer and harder than those with the other genetic variant, with the result that they tire out their heart.

The drop in heart function does not appear to be damaging, however, because the group found that the athletes' hearts had returned to normal within a day or two. And there was no apparent relationship between heart function and race times; with all the athletes pushing their entire bodies to a point of exhaustion, it seems that muscles or lungs could have proven their limiting factor.

Stretching the limit

Ashley says that trainers and athletes, particularly in the growing field of adventure racing, should be aware that their bodies can outstrip their hearts. In theory, it might even be possible to specifically train heart muscle, although most exercise does this anyway by getting the heart pumping.

The study may also bear insights for those studying heart disease. During a condition called atrial fibrillation, for example, the heart can speed up and eventually tire and weaken. The new results imply that this happens whenever the heart is over-exerted for too long, and that it may not have long-lasting repercussions once corrected.

Ashley says that he next wants to find out what causes the heart to tire after being pushed to the edge: a high heart rate, too much adrenaline, oxygen deprivation or some other explanation? The results are published in the Journal of the American College of Cardiology¹.

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