From The Race Across America, a 3,000-mile 12-day cycle demanding more than 20 hours per day in the saddle, to the Iditarod Trail Invitational, where participants run, bike, sled, or ski their way across 1,000 miles of Alaskan snow, ultra endurance sporting events are becoming increasingly popular.

Since I ran my first ultra marathon in 2010, the sport has taken me on many weird and wonderful adventures around the world. This includes taking part in the renowned Marathon Des Sables – a 150-mile footrace through the Sahara Desert carrying all food and supplies on my back.

Both as an athletic competition and a personal accomplishment, ultra endurance events truly test the limits of physical and mental endurance. However, research suggests that long-term participation might cause adaptations in the body that are more commonly linked to cardiovascular disease. These include structural and functional changes in the heart and blood vessels, as well as electrical changes in the cardiac nerves and possible damage to the heart tissue.

About 90% of people who undertake ultra marathons also suffer symptoms of gastrointestinal distress, including cramping and diarrhoea. And for some participants, despite their training and supreme fitness, these nasty side effects can result in them not completing the event.

But although there’s a great deal of emerging research in these areas, there is still a lot we don’t know about how the body responds to extreme endurance exercise – and specifically to challenges that last several days or weeks.

The 100 peaks challenge

As a physiologist, I’ve always been fascinated by the body’s responses to physical stress. But actually getting hold of robust data on ultra endurance exercise is difficult because of the low numbers of people willing to endure events in remote locations and extreme environments.

So when I encountered a team of athletes preparing for a unique ultra endurance event in the UK – The 100 Peak Challenge – I jumped at the chance to be involved. The event was modelled on the Three Peaks Challenge, a footrace to ascend and descend the three highest mountains in England, Scotland and Wales – the three peaks was then taken to the extreme to become 100 peaks. This was to be contested over 25 consecutive days, with the added difficulty of cycling on pushbikes between the five base-camps in England, Scotland and Wales.

With a total distance of 840 miles and a cumulative elevation of 34,000m – which is equivalent to four ascents of Mount Everest – there are no competitive statistics on the challenge because it’s never been attempted. As the English explorer, Sir Ranulph Fiennes, said:

It’s quite possibly the most demanding endurance challenge ever staged in the United Kingdom.

The man behind the idea for The 100 Peaks Challenge was Karl Rushen, whose brother Lloyd, a serviceman with the British Army Special Forces, was tragically killed in action – 25 days into his third tour of Afghanistan. In memory of his brother, Karl wanted to embark upon a series of endurance challenges around the country in aid of military charities.

Human endurance

I collaborated with several experts across the UK to explore physical responses to the challenge. To do this, we carried out tests before and after the challenge – including cardiac ultrasound, blood vessel structure and function, and respiratory airway function. And we also travelled with the team collecting data in the field.

We looked at the impact of this extreme event on the respiratory system (lungs and airways), the cardiovascular system (the heart and blood vessels) and nutrition (dietary profiles and gut biology). My specific interest was to investigate what happened to lung function when strenuous exercise was repeated over multiple, consecutive days.

What we found was that the 100 Peaks Challenge resulted in progressive reductions in respiratory and cardiovascular function. This had not recovered in the athletes after two days of rest. There was also evidence that airway obstruction had occurred – and three of the team developed symptoms of upper respiratory tract infection. The progressive decline in function that we saw with time suggests that, if the team had have continued for longer, their function may have diminished sufficiently to force them to stop.

We also found that the event resulted in decreased function of the small blood vessels, which in extreme cases can result in elevated blood pressure and increased cardiovascular risk.

Throughout the challenge, we profiled the athletes’ diets using food diaries to calculate their calorie intake. And our preliminary estimates showed that each athlete was burning between 4,000 to 6,000 calories a day.

These findings show the true gruelling effects these types of sporting events can have on a human body. And it was only by starting the 100 Peaks with supreme fitness that the athletes were able to continue performing and complete the challenge. The plan is to now use this data to help inform future training for extreme endurance challenges. Because inevitably – despite the risks – people will continue to want to challenge themselves in some of the most unique ways possible.

Nick Tiller, Lecturer in Applied Physiology, Sheffield Hallam University.

This article first appeared on The Conversation.