The health benefits of exercise are well documented, but two new studies by researchers from Monash University in Melbourne, Australia, find exercising at extreme levels may do more harm than good – it could lead to blood poisoning.

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On analyzing blood samples from marathon runners taken immediately after the extreme endurance event, researchers identified markers of sepsis.

The studies – both led by Dr. Ricardo Costa of the Department of Nutrition and Dietetics at Monash – are published in the International Journal of Sports Medicine and Exercise Immunology Reviews.

Blood poisoning, also known as septicemia or sepsis, occurs when immune chemicals leak into the bloodstream, triggering an overactive inflammatory response. This can lead to blood clots and leaking vessels, impairing blood flow and preventing the body’s organs from receiving the required oxygen and nutrients.

According to the Centers for Disease Control and Prevention (CDC), more than 1 million people in the US develop sepsis each year, with around 28-50% of these individuals dying from the condition.

Infants and children, elderly individuals, people with chronic illnesses – such as cancer or AIDS – people who have suffered severe burns or physical trauma and those with a weakened immune system are at highest risk of sepsis.

However, the latest research from Dr. Costa and colleagues suggests extreme exercise may also be a risk factor for the condition.

The Physical Activity Guidelines for Americans recommend adults engage in at least 150 minutes of moderate-intensity aerobic activity or 75 minutes of vigorous-intensity aerobic activity each week.

But while it is estimated that almost half of us fail to meet these guidelines, some of us are on the other side of the fence, exercising to extremes.

According to Dr. Costa, exercising for more than 4 hours in one session is deemed extreme, as is engaging in repetitive days of endurance exercising. Such practices are common among people taking part in extreme endurance events, such as marathons, who often engage in these events during periods of high heat, putting extra strain on the body.

“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,” notes Dr. Costa.

For the studies, the team analyzed blood samples from 17 individuals who took part in a 24-hour ultra marathon and 19 people who took part in a multi-stage ultra marathon. Blood samples were taken from each participant immediately before and after the events, and all samples were compared with samples from control participants.

In almost all of the blood samples taken from study participants after marathon completion, the researchers identified markers identical to those found in blood samples of patients who are admitted to the hospital for sepsis.

“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,” Dr. Costa explains. “This then triggers a systemic inflammatory response from the body’s immune cells, similar to a serious infection episode.”

Note: Since endotoxins are not bacteria, for “…the naturally present bacteria, known as endotoxins…” a better phrase would perhaps be “…the naturally present bacteria, which SECRETE endotoxins…”

Interestingly, the team found that participants who were fitter and spent a longer period of time training for the marathons had higher levels of an anti-inflammatory cytokine called Interleukin 10 in their blood, which offset any negative health effects caused by the endotoxin-induced immune response.

Their findings, the team says, emphasize the importance of following guidelines when it comes to taking part in extreme endurance events. Dr. Costa adds:

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.

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.”

In future research, the team plans to further investigate how exercise affects gut integrity function in both high- and low-heat environments, as well as uncover ways in which exercise-induced gut damage may be prevented.

In October 2013, Medical News Today reported on a study suggesting marathon running may have a negative impact on the heart, reducing left and right ventricular function.