Can resistance training reshape the gut microbiota?

The gut microbiota has gradually established itself as a key player in human health. Long confined to a digestive role, it is now recognized as a major regulator of metabolism, immunity, chronic inflammation, and even aging. The composition of intestinal bacterial communities influences the production of short-chain fatty acids, the permeability of the intestinal barrier, and the dialogue between the gut and other organs, including the muscles and brain.

Among the factors capable of modulating this microbiota, diet obviously plays a key role. But over the past decade, physical activity has also emerged as an important determinant. Studies conducted on endurance athletes have shown that regular endurance exercise is associated with greater bacterial diversity and an increase in certain species considered beneficial, particularly those involved in butyrate production. These observations have helped to reinforce the idea that movement, beyond its mechanical and cardiovascular effects, has a profound impact on the intestinal ecosystem.

On the other hand, the role of strength training remains much more uncertain. Resistance training induces physiological adaptations that are very different from those of endurance training: muscle hypertrophy, increased protein turnover, improved insulin sensitivity, and specific hormonal changes. However, the available data on its direct impact on the microbiota are scarce and often contradictory. Some studies report modest changes, while others observe no measurable effect. In this context, a central question remains: can resistance training alone reshape the gut microbiota in a way that is relevant to health?

The study

To answer this question, a recent study, published as a preprint (not yet peer-reviewed), set up a large-scale interventional study specifically targeting healthy adults who had been sedentary for at least one year. A total of 150 participants were included in the final analysis after exclusions related to consistency, data quality, or major confounding factors such as recent antibiotic use.

All participants followed a structured resistance training program for eight weeks, with two to three sessions per week, in fitness centers equipped with digitally controlled resistance training machines (EGYM) (cable row, lat pull, chest press, “Back trainer”, “Abdominal trainer”, leg curl, and leg press). These devices made it possible to precisely standardize the loads, execution speeds, and concentric and eccentric phases, while automatically recording all training parameters: weights lifted, repetitions, total volume, and individual progress. Two modalities were offered (one focused on “general well-being” and the other more focused on muscle development), but their overall loads and energy expenditure were comparable, which led the authors to group the participants together for the main analyses.

Biologically, stool samples were collected before the intervention, at four weeks, and at eight weeks. The composition of the microbiota was analyzed by sequencing the 16S rRNA gene, allowing for detailed characterization of the bacterial communities. At the same time, a targeted metabolomic analysis of the stool samples was performed to evaluate the metabolic products resulting from microbial activity. Participants were explicitly asked not to change their diet during the study period in order to isolate the effect of training as much as possible.

Finally, the researchers measured several indicators of physical performance, including maximum strength on the leg press, average strength gain across all exercises, and an EGYM index called “BioAge Strength” which compares muscle function based on performance to a database collected by this company, ultimately indicating whether your muscle strength corresponds to your biological age, or whether you are stronger or weaker… This approach made it possible to link microbiota adaptations not only to participation in training, but also to the actual extent of individual muscle adaptations.

Results & Analysis

In terms of physical fitness, the intervention produced the expected effects. A significant increase in leg press strength (+63kg on average) and a 24% increase in average strength (calculated from gains across all exercises in the program) were observed. These adaptations confirm that the training stimulus was sufficient and well tolerated, even in individuals who were initially sedentary.

However, among all participants, resistance training did not lead to an overall change in intestinal bacterial diversity (alpha diversity). Neither species richness nor the major profiles of microbiota composition appeared to be significantly different after eight weeks. Similarly, the metabolomic profile of feces remained broadly stable, suggesting the absence of massive transformation of detectable microbial functions in the short term. In addition, resistance training may influence host metabolism through pathways that are not necessarily reflected in fecal metabolites, including myokines, systemic inflammation, or changes in energy substrate utilization.

However, when the researchers looked at intra-individual changes (beta diversity), a modest but significant relationship emerged between microbiota adaptations and gains in muscle strength. In other words, it is not simply participation in training that appears to be decisive, but the body’s ability to respond to the stimulus. Participants with the greatest improvements in performance also showed the most marked changes in the composition of their microbiota.

In individuals who responded best to training in terms of strength gains, gradual, time-dependent changes were observed, becoming more pronounced by the eighth week. The enrichment of bacteria such as Faecalibacterium and Roseburia hominis, both of which produce short-chain fatty acids with anti-inflammatory properties, suggests that weight training may promote an intestinal microbial profile conducive to metabolic health and immune regulation. These fatty acids, particularly butyrate, produced by the bacterial fermentation of dietary fiber, have been implicated in maintaining the integrity of the intestinal barrier, regulating glucose homeostasis, and the biosynthesis and metabolism of amino acids (essential for protein biosynthesis), and are thought to exert systemic anti-inflammatory effects. Similar microbial adaptations have been documented in response to endurance training, highlighting that despite different exercise modalities, resistance training and endurance training may converge on common microbiome-mediated pathways that improve host metabolic and immune functions.

It is important to note that these transformations occurred, a priori, without dietary changes, which could reinforce the hypothesis of a specific effect of muscle strengthening on the intestinal ecosystem. On the other hand, the absence of clear variation in fecal metabolites suggests that these changes remain subtle, potentially localized, or that they require a longer intervention period to translate into measurable functional effects.

Practical applications

These results do not allow us to conclude that strength training systematically transforms the gut microbiota in all individuals. However, they suggest that resistance training could act as a modulator of the microbiota, particularly in people who respond strongly in terms of muscle growth. The microbiota may be sensitive to the biological intensity of the adaptation, rather than simply to the act of exercising.

This would also mean that the potential benefits of strength training on gut health are probably not automatic or immediate. They may depend on factors such as baseline level, actual progression, consistency, and perhaps even the individual’s ability to tolerate and integrate the mechanical stress of training. This interindividual variability reminds us that the microbiota is not a simple passive reflection of lifestyle, but a dynamic ecosystem influenced by multiple physiological signals.

In terms of overall health, these data reinforce the idea that resistance training is not limited to preventing sarcopenia or improving strength. It may also indirectly contribute to the regulation of chronic inflammation and metabolism through interactions with the microbiota. However, these effects appear to be modest in the short term and do not replace the well-documented impact of diet or endurance activity on microbial diversity.

In practice, incorporating resistance training into a health strategy remains entirely relevant, but it should be considered as one element of a broader approach, combining endurance, an appropriate diet, and long-term consistency. Above all, this study suggests that the muscle and gut interact more than previously thought, and that the profound physiological adaptations induced by training can, over time, leave a measurable imprint on our gut ecosystem.

Reference

Straub D, Englert T, Beller A, Stadelmaier J, Stahl M, Kilian J, Borzym J, Rotermund C, Akbuga-Schön T, Krakau S, Czemmel S, Weiler S, Pettenkofer M, Pettenkofer J, Maser U, Dammeier S, NieB AM, Enderle MD & Nahnsen S. Resistance training reshapes the gut microbiome for better health. bioRxiv [Preprint], 2025, doi:10.1101/2025.08.13.670057.