Burnt Red Hen/Shutterstock.comLifting weights just two or three times a week can significantly change the trillions of bacteria living in your gut, and it might happen in as little as eight weeks.That’s according to a recent study – not yet peer-reviewed – finding that previously inactive people who began resistance training showed notable changes in their gut microbiome, the community of microbes living in the digestive system.Your gut is home to bacteria, fungi, viruses and other microscopic organisms, most of which live in the large intestine. These microbes help break down food that your body can’t digest on its own, allowing you to access more nutrients and vitamins.Some bacteria are considered beneficial because they’re often found in people who are healthy, both physically and mentally. They produce compounds that appear to support wellbeing.The makeup of your gut microbiome isn’t fixed. It changes based on factors such as what you eat, how old you are, how well you sleep – and, as this study shows, whether you exercise.Researchers at the University of Tübingen in Germany recruited 150 people who didn’t normally exercise and asked them to do resistance training two to three times a week for eight weeks. Participants used either lighter weights with more repetitions (15 to 20) or heavier weights with fewer repetitions (eight to ten).Both approaches produced similar improvements in strength and body composition. The exercises included chest presses, abdominal work, leg curls, leg presses and back exercises – two sets of each.The researchers collected stool samples at the start of the programme, after four weeks and after eight weeks to track changes in participants’ gut bacteria.Some people gained strength much faster than others. The researchers divided participants into “high responders” – the top 20%, who increased their strength by more than 33% on average – and “low responders” – the bottom 20%, who gained less than 12.2%.The biggest factor determining whether someone was a high or low responder appeared to be their initial strength level.But the researchers also found something interesting: the people who gained the most strength showed subtle but significant changes in their gut bacteria that the others didn’t.High responders showed increases in 16 types of bacteria and decreases in 11 others. Two bacteria in particular stood out: Faecalibacterium and Roseburia hominis.Both produce butyrate, a type of compound called a short-chain fatty acid. These compounds are created when gut bacteria break down fibre, and they serve multiple purposes: they provide energy for the body and help maintain a healthy gut lining, which prevents harmful bacteria from entering the bloodstream.Similar increases in these bacteria have been found in other studies looking at exercise and the gut. However, in this study, the researchers didn’t find an actual increase in short-chain fatty acids in the stool samples – only more of the bacteria that produce them.Not that simpleIt’s tempting to label certain bacteria as “good” or “bad”, but it’s not that simple. Throughout the study, some bacteria typically associated with good health decreased, while others previously linked to poor health increased.This highlights an important point: everyone’s microbiome is unique. The same bacteria might perform different roles in different people, depending on the individual and their overall health. Everyone’s gut microbiome is unique. Helena Nechaeva/Shutterstock.com We also can’t say for certain whether the changes in gut bacteria caused the strength gains, or whether getting stronger caused the bacterial changes. Studies like this can show associations, but they can’t prove cause and effect – the microbiome is influenced by too many factors to control them all.Diet, for instance, has a major effect on gut bacteria. Participants were told not to change their eating habits during the study, but it’s extremely difficult to accurately track what people eat. It’s possible that some high responders changed their diet as they became more focused on fitness, and this could have contributed both to their bacterial changes and their strength gains.What we can say with more confidence is that exercise appears to benefit overall physical and mental health and should be part of a healthy lifestyle regardless of what it does to your gut microbes.This was a small study that still has to go through the peer-review process of being officially looke at by other scientists. But it has the potential to add to growing evidence that our lifestyle choices, including how much we move, can influence the microscopic world living inside us.Rosie Young does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.