Lifting weights just two or three times a week can significantly alter the trillions of bacteria that live in your gut — and it can happen in just eight weeks.
According to a recent study—not yet peer-reviewed—previously sedentary people who began resistance training showed notable changes in their gut microbiome, the community of microorganisms that live in the digestive system.
The intestine is home to bacteria, fungi, viruses and other microscopic organisms, most of which inhabit the large intestine. These microorganisms help break down foods that the body cannot digest on its own, allowing access to more nutrients and vitamins.
Some bacteria are considered beneficial because they are often found in healthy people, both physically and mentally. They produce compounds that appear to promote well-being.
The composition of the gut microbiome is not fixed. Changes depending factors such as diet, age, sleep quality — and, as this study shows, physical exercise.
Researchers at the University of Tübingen in Germany recruited 150 people who did not exercise regularly and asked them to perform resistance training two to three times a week for eight weeks. Participants used lighter weights with more reps (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 bench presses, abdominal exercises, leg curls, leg presses, and back exercises — two sets of each.
Researchers collected stool samples at the beginning of the program, after four weeks, and again after eight weeks to monitor changes in participants’ gut bacteria.
Some people gain strength much faster than others. The researchers divided participants into “high responders” — the top 20 percent, who gained strength by more than 33 percent, on average — and “low responders” — the bottom 20 percent, who gained less than 12.2 percent.
The main factor that determined whether someone was a high or low responder appeared to be their initial level of strength.
But researchers discovered something interesting: The people who gained the most strength showed subtle but significant changesin their intestinal bacteria, which were not observed in the others.
High responders showed increases in 16 types of bacteria and decreases in 11 others. Two bacteria stood out in particular: Faecalibacterium and Roseburia hominis.
Both produce butiratoa type of compound called short-chain fatty acid. These compounds are created when intestinal bacteria break down fiber and have multiple functions: they provide the body with energy and help maintain a healthy intestinal lining, preventing harmful bacteria from entering the bloodstream.
Similar increases in these bacteria have been observed in other studies on exercise and the gut microbiome. However, in this study, researchers found no actual increase in short-chain fatty acids in stool samples — just an increase in the bacteria that produce them.
It’s not that simple
It’s tempting to label certain bacteria as “good” or “bad,” but it’s not that simple. Over the course of the study, some bacteria normally associated with good health declined, while others previously linked to poorer health increased.
This highlights an important point: Each person’s microbiome is unique. The same bacteria can play different roles in different people, depending on the individual and their general health.
Furthermore, the Food, for example, has a huge impact on intestinal bacteria. Participants were instructed not to change their eating habits during the study, but it is extremely difficult to accurately control what people eat.
What can be said with more confidence is that exercise appears to benefit physical and mental health in general and it should be part of a healthy lifestyle — regardless of what it may do to your gut bacteria.
This was a small study, which still needs to go through the scientific review process by other researchers. But it has the potential to add to the growing evidence that our lifestyle choices — including how much we move — can influence the microscopic world that lives inside us.
