Long periods of intense exercise can change the composition of your gut bacteria, a new study has found.
The research looked at soldiers taking part in an intensive training programme and found that pro-longed exercise caused the protective barrier in their guts to become permeable.
In other words, the prolonged exertion triggered 'leaky guy syndrome' - a condition that could let harmful substances leak into the bloodstream.
With our gut health and overall health believed to be strongly linked, intense physiological stress could therefore raise the risks of many types of illnesses.
The new research is the first to investigate the response of gut microbiome - the term for the population of microbes in the intestine - during military training.
It provides a stark warning for endurance athletes and military personnel.
Most of us are aware that the bacteria in our gut play an important role in digestion. Furthermore, they are known to aid the production of certain vitamins - such as vitamins B and K - and play a key role in immune function.
But increasingly, research is emerging showing how poor gut health is linked to conditions ranging from irritable bowel syndrome, diabetes, high blood pressure, depression, obesity, childhood asthma, to colitis and colon cancer.
The study is the first to investigate the response of gut microbiome - the term for the population of microbes in the intestine - during military training.
It looked at a group of 73 Norwegian Army soldiers taking part in a military-style cross country skiing training programme.
The group skied 31 miles (51 km) while carrying 99-pound (45 kg) packs, across four days.
Before and after the training exercise, researchers collected blood and stool samples from the soldiers.
It was found that the microbiome and metabolites - the substance formed in or necessary for metabolism - in the soldiers' blood and stool altered 'significantly' by the end of the aggressive training period.
Furthermore, sucralose excretion in their urine samples rose considerably, indicating an increase in intestinal permeability (IP).
Scientists know that healthy intestines have a semi-permeable barrier, which acts as a defense to keep bacteria and other harmful substances out, while allowing healthy nutrients into the bloodstream.
It is thought that physical stress can increase IP, increasing the risk of inflammation, illness and symptoms such as diarrhoea.