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Phys Ed Gretchen Reynolds on the science of fitness.

Being physically active may encourage beneficial germs to thrive in your gut, while inactivity could do the reverse, according to an innovative new study. The findings suggest that, in addition to its other health benefits, frequent exercise may influence our weight and overall health by altering the kinds of organisms that live inside of us.

In recent years, there has been an explosion of interest in the role that gut microbes play in whole-body health. A multitude of studies have shown that people with large and diverse germ populations in their digestive tracts tend to be less prone to obesity, immune problems and other health disorders than people with low microbial diversity, and that certain germs, in particular, may contribute to improved metabolic and immune health.

But little science had examined the interplay between physical activity and gut bugs in people. So, for a study published this month in Gut, researchers at University College Cork, part of the National University of Ireland, and other institutions, set out to learn more by turning to a group of people who exercise a lot: the national rugby team of Ireland.

“We chose professional athletes as a study group, because we wanted to be sure not to miss any effect of exercise and needed a group who were safely performing at the extremes of human endeavor,” said Dr. Fergus Shanahan, an author of the study who is a professor of gastroenterology and director of the Alimentary Pharmabiotic Center at University College Cork.

Forty of the players agreed to participate. At the time of the study, the men’s national team was in preseason training and the players were exercising strenuously for several hours every day.

For the sake of comparison, the researchers also recruited two groups of healthy adult men, none of them athletes. One group consisted of men with a normal body mass index. Most of the men in this group exercised occasionally but lightly.

The men in the final group were generally sedentary and had a body mass index that would qualify them as overweight or obese. This group was included, Dr. Shanahan said, because the rugby players, although supremely fit, were physically huge, with body masses well above normal. The researchers wanted to compare their gut microbes to those of men whose weight was similar, if not their musculature.

The scientists drew blood and collected stool samples from all of the men, rugby players and non-athletes alike. The volunteers also completed lengthy questionnaires about their exercise routines and diet, and spoke with a nutritionist about their typical daily food intake.

Then the scientists analyzed the men’s blood for markers of muscle damage and inflammation, which would indicate how much each volunteer had — or had not — been moving and exercising recently. The scientists also used sophisticated genetic sequencing techniques to identify and enumerate the particular microbes living in each man’s gut.

As it turned out, the internal world of the athletes was quite different from that of the men in either of the control groups. The rugby players had considerably more diversity in the make-up of their gut microbiomes, meaning that their intestinal tracts hosted a greater variety of germs than did those of the other men, especially the men in the group with the highest B.M.I.

The rugby players’ guts also harbored larger numbers of a particular bacterium, uneuphoniously named Akkermansiaceae, that has been linked in past studies with a decreased risk for obesity and systemic inflammation.

Interestingly, the rugby players’ blood showed low levels of markers for inflammation, even though the men were exercising intensely. Their muscles were being pummeled but, in physiological terms, recovering well.

The men in both of the control groups, on the other hand, especially those with the highest B.M.I.s and who rarely exercised, had relatively low numbers of Akkermansiaceae in their guts and elevated markers for inflammation in their bloodstreams.

These findings “draw attention to the possibility that exercise may have a beneficial effect on the microbiota,” Dr. Shanahan said, in ways that improve bodily health.

However, the results are still preliminary, he said. This study was small and, because of its methodology, the researchers can’t determine how exercise alters gut germs or tease out the effects of intense exercise from those of diet. The rugby players consumed far more calories than did the other men, with a much larger percentage of their diet consisting of protein. Such nutritional differences can affect which microbes thrive in the gut. The athletes also were training at a level that few of us would be able or willing to emulate.

Dr. Shanahan and his colleagues have begun a follow-up study examining whether and how moderate exercise changes the gut environment in both men and women. The results should be available later this year.

But even in advance of those findings, he said, it seems likely that any amount of exercise should make your gut more welcoming to the bacteria that you want residing there.