Domhnall MacAuley is a CMAJ Associate Editor and a professor of primary care in Northern Ireland, UK
Some years ago, out walking along the river, I met an ex-athlete friend . “I gave up running at 50,” he told me. "Too many of my friends were having heart problems”. This didn’t fit with what I believed about the benefits of exercise, so I didn’t give it a lot of thought.
More recently, however, when preparing a talk on sudden cardiac death, I came across a research paper from Sweden that followed up 52, 755 athletes who had completed the Vasaloppet a long distance cross country ski race (90k) and, curiously, showed that those who raced more often and faster, were more likely to have hospital admissions for cardiac arrhythmia. This seemed counter-intuitive. Surely those who were fittest should be healthiest. I worked hard to think of a confounding factor to explain it. But, the Physicians Health study, also found increased incidence atrial fibrillation in those who habitually exercised more vigorously.
I knew there was a J-shaped curve demonstrating the relationship between physical activity and mortality but I hadn’t considered it might apply to lifelong athletes (or, to me). I started to give it a little more thought.
A recent BBC investigative television programme asked if it is possible to do too much exercise, interviewing Professors Sanjay Sharma and Alejandro Lucia, two researchers I respect and whose work I know well. Sanjay, a cardiologist in London who has spent a major part of his career investigating the athlete’s heart and, in particular, sudden cardiac death, raised a number of questions about intensive endurance exercise. Alejandro, based in Madrid, who is an expert in high intensity endurance exercise, particularly among cyclists, took a contrary view, and cited his own research that found no ill effects in elite cyclists who had completed the Tour de France. Alejandro’s epidemiological perspective fitted with my own understanding of the literature but, didn’t directly address the individual risks of intensive endurance exercise as one gets older. I chased up some references and thought about it further.
Sanjay, in an article currently in press, gives some interesting and persuasive background. While recognising the benefits of exercise, and acknowledging that exercise related cardiac deaths in older people are more often due to coronary artery disease, he raised some concerns that, to me, seem reasonable. Intensive exercise raises troponins, one of the enzymes that we now recognise as being associated with ischemic heart damage. He suggested that recurrent high intensive exercise might increase the likelihood of cardiac fibrosis and the development of conduction disorders, including atrial fibrillation. This might fit with the findings of the Vasaloppet research, and might possibly explain some sudden unexplained cardiac deaths in fit older athletes. Coronary atheroma is, of course, always in the background as we get older, whether we are athletes or not.
So, how much is too much? James O'Keefe and his colleagues suggested dose-dependent benefits up to about 1 hour daily, but more than this might cause adverse cardiovascular effects in some individuals. Which begs the question....Should older athletes undertake high intensity exercise and compete in endurance events such as marathons or cross country ski races? And, are these risks due to the nature of individual events of long duration at submaximal intensity, or to high intensity training including interval training at close to maximal intensity? We don’t yet know.
Might this uncertainty make me reconsider my own level of training and competitive endurance sport? It has made me think about it seriously.