Every weekend the parks are packed with them — hordes of Lycra-clad athletes in training for a marathon or, increasingly, a triathlon. The popularity of these gruelling endurance events has soared recently, as a generation tires of the tedium of gym workouts and looks for other ways to test itself. A record 8,000 people took part in this year’s London triathlon — a sevenfold increase since 2001 — while 40,000 ran the last London Marathon, compared with a mere 7,000 in the inaugural race.

But such endurance events can be risky for both the participant who is ill-prepared and the more experienced athlete. During last week’s BUPA Great North Run, four participants, aged between 28 and 52, collapsed in separate incidents around the 13.1-mile (21km) course. Their deaths are being linked to the heat.

Experts have recently identified a condition called “post extreme endurance syndrome”, or PEES, as a cause of death among triathletes, marathon runners and practitioners of other sports, and the American College of Sports Medicine (ACSM) has just published an advisory document on how to avoid the condition.

Dr Michael Bergen, a physiologist at the Medical College of Georgia and a member of the ACSM advisory team, says that even slight rises in temperature can be risky. “Large sweat losses, insufficient fluid intake and consequent fluid deficits can impair performance and may increase the risk of hyperthermia and heat injury,” he says. Wearing costumes to raise money for charity increases the likelihood of heat stress by adding to the body ’s workload and impairing its ability to cool itself.

Dr Bergen explains that athletes, especially those who are less than optimally fit and/or who overexert themselves, could experience muscle fibre damage because of an excessive build-up of potassium in the blood. “It can lead to life-threatening kidney failure and can sometimes be fatal,” he says.

But there are factors other than heat that can make endurance events risky for some. At the University of Arkansas school of medical sciences, Dr Hilary Ann Peterson, an expert in emergency medicine, found that more than a third of the entrants in Iron Man triathlons who sought medical attention had experienced dizziness, vomiting, low blood pressure, raised heart rate and decreased body temperature. Some had collapsed, or required intravenous fluid replacement after the race was over.

It was not only novices or the ill-prepared who were prone to extreme endurance syndrome, Dr Peterson discovered; some elite athletes who pushed themselves to physical limits were one and a half times more likely to suffer. “People are experiencing some really significant injuries at these events,” she says. “We have seen only a few fatalities in marathons so far. But with more people participating in endurance events, and with those participants having less experience, you have to ask youself whether we will see more.”

There is no single cause of the syndrome; however, chronic dehydration and heat exhaustion or gorging on too much water (which causes hyponatraemia — diluted body salts) are potential triggers. Another possible cause is a phenomenon unique to endurance activity in which the body shunts blood first to the muscles and then to the skin for cooling, but fails to deliver enough blood to the kidneys and intestines. This cuts of f blood flow to the vital organs and can lead to serious complications such as the bowel ischemia that one competitor in a Hawaii triathlon suffered — he collapsed and later had to have inches of his bowel removed.

Dr Dan Tunstall Pedoe, a cardiologist at St Bartholomew’s Hospital, in East London, and the medical director of the London Marathon, says that most adverse effects of completing long-distance events are temporary, if sometimes a little unpleasant. “There are a lot of body adaptations after a hard endurance test,” he says. “Hypothermia is a risk, and many people are unable to consume even a drink without vomiting.”

A shift in circulating blood volume to the legs may cause oedema, or painful swelling, that can be eased by lying down and raising your legs above your head.

According to the ACSM panel, anyone attempting an endurance challenge should exercise caution. Do not even start, it advises, if you have suffered from a recent illness, vomiting, diarrhoea or fever; all of these increase the risk of heat stress and other problems. Dr Bergen urges competitors to begin “well hydrated, well rested and well nourished, and with a normal resting body temperature”.

In warm conditions, clothing should be light, and the less worn the better. If you suffer dizziness, a headache or feel faint, very hot or very cold when you are running, then you should call it a day. As Dr Peterson says: “Your body tells you when it has had enough. Listen to it.”

Visit www.acsm.org for more information.