Too much exercise can trigger blood poisoning, scientists have warned.

Taking part in extreme endurance events causes intestinal bacteria to leak into the bloodstream, leading to serious infection, a new study revealed.

Researchers said their findings came after analysing people who participated in fitness events, including ultra-marathons, and multi-stage marathons which span consecutive days.

Dr Ricardo Costa, of the University of Monash, said: 'Blood samples taken before and after the events, compared with a control group, proved that exercise over a prolonged period of time causes the gut wall to change.

Extreme endurance events like ultramarathons and multi-stage marathons can causes intestinal bacteria to leak into the bloodstream, triggering blood poisoning, a study has found

'This allows the naturally present bacteria, known as endotoxins, in the gut to leak into the bloodstream.'

This then triggers an inflammatory response throughout the body from it's immune cells, similar to a serious infection, he added.

As part of the study, Dr Costa and his team examined people taking part in a 24-hour ultra-marathon and multi-stage ultra-marathons.

These are events in which people run for longer than the standard 26 miles (42km), and often run for long distances on consecutive days.

'Nearly all of the participants in our study had blood markers identical to patients admitted to hospital with blood poisoning,' Dr Costa said.

'That's because the bacterial endotoxins that leach into the blood as a result of extreme exercise, triggers the body's immune cells into action.'

And they found that it is people who jump into endurance events with little preparation that are most at risk.

People who haven't done the training and you're unfit - these are the people who can get into trouble Dr Richard Costa, of the University of Monash

They discovered fit and healthy individuals who follow a steady training program to build up to extreme endurance events develop immune mechanisms to counteract the body's inflammatory reaction, without any side effects.

This meant they were protected against developing blood poisoning.

But those who were unfit and who carried out little training put their bodies under enormous strain, above the immune system's protective capacity.

With elevated levels of endotoxins in the blood, the immune response can be far greater than the body's protective counter action, Dr Costa said.

In extreme cases, it leads to sepsis induced systemic inflammatory response syndrome, a condition which can be fatal if it is not diagnosed and treated promptly.

The study is the first to identify a link between extreme endurance exercise and the stress it may place on the gut.

Dr Costa said anything over four hours of exercise and repetitive days of endurance exercise is considered extreme.

'Exercising in this way is no longer unusual - waiting lists for marathons, Ironman triathlon events and ultra-marathons are the norm and they're growing in popularity,' he said.

'It's crucial that anyone who signs up to an event, gets a health check first and builds a slow and steady training program, rather than jumping straight into a marathon, for example, with only a month's training,' he added.

People following a steady programme are less likely to develop blood poisoning as their bodies are able to counteract the negative reaction. But those who have done little training are at risk, experts warned

The research team found that people who were fitter and trained over a longer period of time leading into the ultra-marathon event had higher levels of Interleukin 10 - an anti-inflammatory agent - which allowed them to dampen down the immune response which caused the negative health impacts.

'The body has the ability to adapt and put a brake on negative immune responses triggered by extreme endurance events.

'But if you haven't done the training and you're unfit - these are the people who can get into trouble,' Dr Costa said.

Next, Dr Costa and his team want to investigate to what degree exercise, with and without heat, impacts the gut's function.

They will also investigate and develop strategies for individuals to prevent and manage damage to the gut caused by stress from heat and exercise.