The tragic death of Claire Squires in the last mile of the London marathon has not only touched the hearts of thousands of people – as her JustGiving donations of over £660,000 from 56,000 people (at the time of writing) shows – but has made people wonder just how safe running a marathon is. (Her death was the 10th in the event since it began in 1981, and the first since a 22-year old fitness instructor died in 2007.) Well, the short answer is it’s surprisingly safe.

I’ll start with an interesting observation which is that anecdotal evidence often states that more people start to walk in the first mile or two of a half marathon than a full one, possibly because it is easier to be blasé about a shorter distance – typically as a dare, bet or spur-of-the-moment decision. Such decisions are made when deciding to enter a full marathon of course but there is possibly more chance of common sense prevailing with time when faced with the full distance.

However, moving away from an anecdotal view, there are a number of key points to be made here. The first is that if you take 30,000 people and see what happens to them over 3 hours – exercising or not – some of them will experience heart problems simply by the laws of average. The next is that elevated levels of fitness do not by themselves exclude you from heart disease. It is certainly the case that regular running reduces the overall risk of sudden death in people with latent heart disease, but it can increase the risk of sudden death during exercise for those with undiagnosed heart disease that predisposes to sudden death. Because sudden death in public races makes for high-profile media coverage the public can get a distorted impression of the relationship between exercise and sudden death.

One reputable study followed the London marathon for 23 years and over that time there were seven cardiac deaths and five other runners who suffered heart attacks but survived – all five were subsequently found to have previously unknown heart disease. Overall, the study found that the risk of death in the London marathon was one in 67,414 – a risk comparable to many daily activities. This finding is also borne out by an analysis of the annual Marine Corps and Twin Cities marathons held over a 30-year cumulative period that showed the risk for sudden cardiac death associated with intense physical effort was exceedingly small at 1 in 50,000. The overall prevalence of sudden cardiac death during the marathons was only 0.002% – very much lower than for several other variables of risk for premature death calculated for the American population as a whole.

So, as a group, runners have a lower risk of cardiac death than the general population and so I continue to promote running as a form of exercise to my patients. However, if they are over 45, or have risk factors such as smoking, high blood pressure, high cholesterol levels, obesity, diabetes or a family history of heart disease – and suddenly think that running either a half or full marathon with little training is a good idea – they need to get a medical MOT first.