Your Gut Microbiome Is Listening to Your Training Blog Banner
You’ve probably had the client who does everything right. They turn up on time, hit their sessions, eat a sensible diet, sleep seven or eight hours a night. And yet something is off. Progress stalls. Recovery feels heavier than it should. Bloating, sluggish mornings, the odd unexplained energy crash mid-session. Most of us reach for the usual levers first. Tweak the programme. Check their calories. Look at their sleep hygiene. But there is one variable sitting quietly underneath all of it that very few personal trainers ever discuss with their clients and the latest research is making it pretty clear that it is far more important than we used to think.
That variable is the gut microbiome. The trillions of bacteria, fungi and other microbes living in your digestive tract that help break down food, train your immune system and produce compounds that travel right around the body. For a long time this sat in the realm of gastroenterologists and nutrition researchers. It felt like something that belonged in a different lane to what we do on the gym floor. But over the last couple of years, a wave of new research has made it impossible to ignore. Exercise changes the gut microbiome in real and measurable ways. And the microbiome, in turn, changes how your clients respond to training.
Many of us find our clients chasing the next supplement, the next protocol, the next hack. This one is different. It is not a pill you can sell and it is not a quick fix you can coach in a single session. But the principles are simple enough to weave into how you already work and understanding the basics should give you the confidence to have better conversations about food, training and recovery with every client on your books.
Before we get into what exercise does to it, a quick plain-English recap here maybe worth it. Your gut microbiome is the community of microbes living mostly in your large intestine. Estimates put the number of bacterial cells in and on the human body at around 38 trillion, which is roughly the same order of magnitude as the number of human cells (Sender et al., 2016). The bulk of them live in the gut and collectively they carry hundreds of times more genes than your own genome does.
The headline metric researchers keep coming back to is diversity. A healthy gut tends to be one where lots of different bacterial species coexist in reasonable numbers, each doing slightly different jobs. Lower diversity is associated with a whole list of problems including inflammation, insulin resistance, obesity and even mood disorders. When researchers look at athletes compared to sedentary controls, one of the first things they see is that athletes tend to have more diverse microbiomes, with higher levels of certain beneficial species. Which raises an obvious question. Is that because the training created the microbiome or because people with good microbiomes end up drawn to training? The latest research is leaning pretty firmly toward a two-way relationship and both directions are worth coaching around.
Exercise is one of the more reliable ways we know of to change the composition of the gut microbiome in humans. A comprehensive systematic review published in the Journal of Physiology (Cullen et al., 2023) pulled together human studies on this and found a consistent pattern. Exercise tends to increase short-chain fatty acid producing bacteria, boost microbial diversity in sedentary individuals and enrich species associated with a healthier metabolic profile. Short-chain fatty acids are compounds like butyrate, propionate and acetate that gut bacteria produce when they ferment fibre. They feed the cells lining your colon, help regulate inflammation and even play a role in how your body handles glucose.
What is particularly noteworthy is that you do not need to be an elite endurance athlete to see these shifts. Moderate-intensity aerobic training, even just three sessions a week for six to eight weeks, has been shown to change the microbiome in previously sedentary adults. A 2025 systematic review and meta-analysis in Frontiers in Microbiology looked specifically at exercise in people with obesity and type 2 diabetes and found that exercise consistently increased beneficial taxa including Akkermansia muciniphila and Faecalibacterium prausnitzii, two species that come up again and again in the gut health literature (Lin et al., 2024).
Akkermansia muciniphila in particular is one to know about. It sits in the mucus layer lining the gut wall and appears to help strengthen that barrier, reducing what researchers call intestinal permeability. Lower levels of Akkermansia are linked to metabolic disease and a recent 2025 randomised controlled trial on home-based exercise training in postmenopausal women with type 2 diabetes showed that structured training significantly increased Akkermansia abundance and those increases were correlated with improvements in HDL cholesterol and cognitive function (Vahed et al., 2025). So we are not just talking about gut discomfort. We are talking about systemic outcomes your clients care about.
For a long time, the research on exercise and the microbiome was weighted heavily toward aerobic work. That has started to shift. A 2024 rapid review in BMC Sports Science, Medicine and Rehabilitation noted that while the number of studies specifically on resistance training was still small, early results suggested that lifting weights might reduce zonulin, a marker of intestinal permeability and increase mucin production, both of which point to a healthier gut barrier (Wagner et al., 2024).
A more recent 2025 study looking at resistance training found time-dependent changes in the gut microbiome, with the most notable shifts showing up in people who made the biggest gains in strength. Faecalibacterium and Roseburia hominis, both linked to anti-inflammatory benefits, were significantly enriched in the higher-responder group. This is an early signal and the sample sizes are still small, but it lines up with what we would expect. Training that drives adaptation also drives change in the bacterial communities that depend on you for fuel and housing.
The practical point here is that whatever kind of training your clients are doing, whether it is strength work, aerobic conditioning or a mixed-modality programme, they are shaping their microbiome with every session. They just do not know it.
Here is where things get really interesting. The microbiome does not just respond to training. It influences how well you respond to training in the first place.
The most famous study in this space came out in Nature Medicine in 2019, when researchers looking at the stool samples of Boston Marathon runners noticed that a genus of bacteria called Veillonella became more abundant after the race (Scheiman et al., 2019). When they looked closer, they realised Veillonella atypica specifically feeds on lactate, the same lactate your clients’ muscles produce during hard training. The bacteria metabolise lactate into propionate, a short-chain fatty acid that seems to have ergogenic effects. When researchers gave this specific strain to mice, the mice ran about 13% longer on a treadmill before exhaustion. Put another way, a gut bug that eats lactate made the animals better at tolerating it.
A 2025 randomised controlled trial in 151 healthy adults built on this, looking at eight weeks of Veillonella atypica supplementation and finding that the treated group reported less fatigue interference and higher voluntary physical activity levels (Aras-Hira et al., 2025). The mechanism appears to tie into dopamine signalling in the brain, which is consistent with other recent work suggesting the gut microbiome influences exercise motivation itself. Think about that for a moment. Some of the reason your client pushes through a tough set or skips a session in favour of the sofa, may come down to the bacteria in their gut shaping the reward signals in their brain.
We should not overstate what any of this means yet. The translation from mice to humans is never clean and a single probiotic strain is a long way from a full dietary strategy. But the principle is now well established. The microbiome is not just a passive passenger. It is part of the training-adaptation system and it responds to and talks back to everything you are asking your clients to do.
If exercise shapes the microbiome, food shapes it even more. And this is where a lot of personal trainers stop feeling comfortable, because nutrition and gut health sit at the edge of what we are qualified to talk about. Understanding the principles however, is what counts. Giving clinical advice to someone with IBS or IBD is far more important for a registered dietitian to handle. But coaching the fundamentals of a gut-friendly eating pattern sits squarely within what a qualified personal trainer or nutrition should be able to do.
The single biggest driver of microbiome diversity in most studies is dietary fibre diversity. Not grams of fibre in isolation, but the variety of plant sources. The American Gut Project, the largest microbiome dataset of its kind, found that people who ate more than 30 different plant foods in a week had significantly more diverse microbiomes than people eating fewer than 10 (McDonald et al., 2018). That includes fruits, vegetables, legumes, whole grains, nuts, seeds, herbs and spices. The so-called 30 plants a week target is a useful anchor to give clients.
Fermented foods are another big one. A Stanford study published in Cell showed that a 10-week fermented food diet including things like yogurt, kefir, kimchi, sauerkraut and kombucha increased microbiome diversity and reduced markers of systemic inflammation (Wastyk et al., 2021). That is a much more reliable effect than what most off-the-shelf probiotic supplements manage to produce. Polyphenols, the plant compounds in berries, dark chocolate, green tea, coffee and olive oil, also appear to feed beneficial bacteria and support diversity.
On the protein side, the question is more nuanced. High-protein diets are a normal part of most of our clients’ programmes and there is nothing inherently problematic about that. But there is some evidence that very high intakes of animal protein, particularly from processed meats, can shift the microbiome in less favourable directions if they come at the expense of plant foods. The fix is not to cut protein. It is to keep plant diversity high alongside it. Pair the chicken with a big varied salad, the steak with roasted vegetables and a grain, the whey shake with some berries and nuts on a handful of days.
And the big one. The probiotic supplement hype is pretty much running ahead of the evidence. Most over-the-counter probiotics contain a small handful of strains that may or may not survive the trip to the gut and may or may not colonise once they get there. For most healthy clients, the return on a daily probiotic pill is far lower than the return on building a varied, fibre-rich diet. If a client asks about supplements, steer them toward food first and the supplement aisle last.
Let’s bring this back to what you actually do on Monday morning with your next client. The gut microbiome is not a new system to train around. It responds to the same things we already coach. Consistency. Variety. Good food. Enough recovery. Low chronic stress. Sleep. Read that list again and you will notice it could have come straight out of any introductory personal training course module. Because the fundamentals of gut health and the fundamentals of everything else we do are pretty much the same fundamentals.
Here is a simple framework to weave into client conversations. Train consistently, because the microbiome responds to regular training over weeks not single sessions. Eat widely, aiming for 30 or more different plant foods in a typical week, not because the number is magic but because it nudges clients toward variety. Include fermented foods a few times a week where they are tolerated. Keep protein high but pair it with plants. Prioritise sleep, because the microbiome follows a daily rhythm just like the rest of the body and chronic sleep deprivation erodes diversity. Manage stress, because chronic psychological stress shifts microbial populations and thins the gut barrier. And be cautious with supplements, especially broad-spectrum probiotics that promise the earth.
There is one more thing that is worth being crystal clear on. If a client is dealing with anything clinical, persistent diarrhoea, blood in their stool, severe bloating, unexplained weight loss or a diagnosed condition like IBS, IBD or coeliac disease, they need a GP and a registered dietitian, not a personal trainer who has read an interesting article. Know where your lane ends. The role of the PT is to coach the habits that support a healthy gut alongside everything else, not to treat gut disease.
Here is the main point. The gut microbiome is not a fringe topic any more. It is showing up in mainstream sports science journals, in mainstream metabolic health research and in the conversations your clients are starting to have with you. The PTs who can have an informed, grounded chat about what the research actually says will stand out from the ones who either dismiss it as a fad or pretend to know more than they do.
And the deeper you go into applied nutrition and exercise science, the more these systems start to feel less like separate topics and more like the same topic viewed from different angles. Training, nutrition, sleep, stress, gut health. They are all one system. Coach one of them well and the others move in the right direction on their own.
If this week’s article has made you want to dig further into the applied nutrition side of coaching, the TRAINFITNESS Nutrition & Exercise Specialist and Master Diplomas™ and our Level 4 Nutrition Coach qualification are the routes that will take you there. These are not generic nutrition courses. They are built for fitness professionals who want to specialise in applied nutrition science and give their clients credible, evidence-informed guidance on everything from macro and micronutrients to gut health, hydration, supplementation and more.
You will cover the science in depth, look at the latest research and finish with the credibility to charge premium fees for a service your clients already want and are currently getting from influencers online. There has never been a better moment to add this to your CV.
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