Athletes have much higher quantities of a specific bacteria in their guts than lazy people, research has found.
Scientists compared the stool samples of 15 athletes who ran the Boston Marathon and 'sedentary' people.
Veillonella bacteria was abundant in the guts of the athletes. However, the bacteria was significantly less common in the other humans.
Experts now believe the bacteria enhance exercise capacity and hope the findings could lead to a probiotic supplement in future.
Researchers at the Joslin Diabetes Center, Boston, say a supplement would help those who struggle to exercise, such as type 2 diabetics.
Veillonella break down lactic acid, which causes 'a stitch' during strenuous activity, into the fatty-acid propionate, the scientists found.
The body is thought to use propionate to maximise its athletic performance - tests on mice showed it improved their exercise capacity.
Specific bacteria reside only in the guts of athletes and enhance their ability to exercise (stock)
Dr Aleksandar Kostic, assistant investigator, said: 'The microbiome is such a powerful metabolic engine.
'What we envision is a probiotic supplement that people can take that will increase their ability to do meaningful exercise.'
He added that this would 'therefore protect them against chronic diseases including diabetes', heart disease and an early grave.
The microbiome is linked to many disorders, including inflammatory bowel diseases, autoimmune conditions, and even obesity.
However, how microscopic bugs in the gut influence wellbeing is poorly understood, the researchers wrote in the journal Nature Medicine.
Exercise has long been recognised as being crucial to overall health, particularly that of our hearts.
However, many people with cardiovascular diseases or type 2 diabetes struggle to stay active.
The researchers therefore set out to uncover whether our microbiomes influence our ability to stay fit.
This began in 2015, when the academics collected faecal samples from 15 runners of the Boston Marathon.
Samples were gathered every day of the weeks before and after the race. They were compared against samples collected from 10 sedentary individuals.
'One of the things that immediately caught our attention was this single organism, Veillonella, was clearly enriched in abundance immediately after the marathon in the runners,' Dr Kostic said.
'Veillonella is also at higher abundance in the marathon runners [in general] than it is in sedentary individuals.'
To confirm their findings, the researchers supplemented mice with Veillonella that was isolated from the runners' stool samples.
Five hours later, the rodents were made to 'run to exhaustion' on a treadmill.
The animals given Veillonella had a
