Melbourne: Over four hours of exercise daily can cause intestinal bacteria to leak into the bloodstream, leading to blood poisoning, new research says.

For the research, the experts monitored people participating in a range of extreme endurance events, including 24-hour ultra-marathons and multi-stage ultra-marathons, run on consecutive days.

‘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, allowing the naturally present bacteria, known as endotoxins, in the gut to leak into the bloodstream,’ said the research led by Ricardo Costa from Monash University in Australia.

‘This then triggers a systemic inflammatory response from the body's immune cells, similar to a serious infection episode,’ the study authors said.

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

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

Significantly the study found that individuals who are fit, healthy and follow a steady training programme to build up to extreme endurance events, develop immune mechanisms to counteract this, without any side effects.

However individuals who take part in extreme endurance events, especially in the heat and with little training, put their bodies under enormous strain above the body's protective capacity, the research said.

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 are growing in popularity,’ Costa said.

‘It is 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.

Thus, the 24-hour ultra-marathon study, published in the International Journal of Sports Medicine and the multi-stage ultra-marathon study, published in Exercise Immunology Reviews, both by Costa's team, reinforces current guidelines for people wanting to take part in extreme endurance events.