Why Your Gut Bacteria Actually Matter When You're Racing

Most runners think about their gut in terms of what goes wrong on race day. Cramping, urgency, the mid-race sprint to a portaloo nobody wants to talk about. What's actually driving a lot of that isn't just what you ate that morning. It's the trillions of microbes living in your gut, and what your diet, training and wider gut environment is doing to them.

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So what does your microbiome have to do with running?

Your gut bacteria aren't just sitting there doing nothing while you train. They're involved in immune function, which matters when heavy training blocks are exactly when runners tend to get sick. They're involved in how nutrients are metabolised and used. And they're tied to the integrity of your gut lining, which becomes pretty relevant when you're bouncing around for six-plus hours straight and asking that lining to hold up under heat, fatigue and a stomach full of gels.

One of the more interesting findings in this space is a bacterium called Veillonella atypica. It increases in marathon runners' guts after racing, that part's observed directly in humans. Researchers then went further and isolated the bacteria, gave it to mice, and found it improved treadmill run time, with the effect traced to Veillonella converting lactate into propionate, which was associated with improved running performance in the mice. That's a demonstrated mechanism in mice, not yet a demonstrated performance effect in humans. We're nowhere near "take this probiotic and run faster" territory as a practical human intervention. But it's an interesting example of how closely gut function and endurance metabolism might be tied together, not just a vague wellness talking point.

Why race fuelling and everyday gut health aren't the same thing

Here's where I think a lot of runners get a bit confused. Carbohydrate during long efforts is critical for performance, that's not really up for debate. It helps maintain blood glucose, spare liver glycogen and delay fatigue. Diets consistently high in simple carbohydrates and low in fibre have been associated with changes in gut microbiome composition, although this is still an emerging area and most of the evidence isn't specific to endurance athletes.

Yes, higher-fibre foods can slow gastric emptying and increase the risk of gastrointestinal symptoms for some runners when eaten close to exercise.

No, that doesn't mean fibre should disappear from your diet altogether.

Certain fermentable fibres are used by gut bacteria to produce short-chain fatty acids, compounds tied to gut health and lower inflammation, and there's a positive association between overall dietary fibre intake and levels of beneficial bacteria like Bifidobacterium. The clearest evidence for what happens when fibre gets too low actually comes from research on bodybuilders rather than runners specifically, low-fibre, high-protein diets there were linked to fewer beneficial, SCFA-producing bacteria compared to both sedentary people. Low habitual fibre intake is a plausible way to reduce the substrates available to beneficial gut bacteria over time, which is exactly why timing fibre around training, rather than cutting it altogether, makes more sense.

This is where I see runners get stuck

This is something I see a lot with clients who run. Most of them already have some baseline gut symptoms, nothing dramatic, the kind of thing they've half normalised. What I see happen is those symptoms get noticeably worse when gels, chews and race fuel start making up a much larger proportion of their carbohydrate intake, while whole foods, fibre and plants drop away. I'm not saying the carbohydrate itself is the problem. It's that the overall dietary pattern has shifted. And the advice they've usually already been given is some version of "just train your gut harder."

I'm not against gut training, there's good evidence that repeated exposure to carbohydrate during exercise can improve tolerance and carbohydrate delivery. But I don't think every gut problem in a runner is simply something to push through and adapt to.

On microbiome mapping with clients like this, a pattern shows up often enough that I take it seriously. Everyone's microbiome is different, close to a fingerprint, so it's rarely the same specific organisms twice. But the pattern itself repeats: overgrowths of opportunistic bacteria that have been able to establish themselves because there isn't enough beneficial flora around to compete with them, alongside low sIgA (an antibody that's a key part of your gut's immune defence), and elevated zonulin, a marker that's been proposed as an indicator of intestinal permeability. That suggests there may be something going on underneath the race-fuelling problem, rather than the carbohydrate itself necessarily being the whole story.

The practical question is: if someone's gut is already unsettled, is the answer really more gut training, or is it stabilising what's going on first and then building fuelling tolerance on top of a calmer gut? In my experience, the second approach has been more useful than simply grinding through trial and error with gels alone.

So what should you actually do?

Train your gut with the actual fuel you're planning to race with, not just your training pace but your real gels, drinks and electrolyte products, the same way you'd rehearse pacing. Keep higher fibre foods away from the hour or two around training without cutting fibre from your week entirely. And if every long run ends in bloating, urgency or loose stools despite doing all of that, I don't think the answer is simply "train harder through it," I think that's worth actually investigating.

On the supplement side, there are some targeted interventions worth knowing about, not a stack everyone should take. Zinc carnosine, for example, has been shown in a small endurance-athlete trial to reduce markers of exercise-induced intestinal permeability. Saccharomyces boulardii has a broader clinical evidence base for some gastrointestinal conditions, but that doesn't mean either is something every runner needs. What gets used, and whether it's needed at all, depends entirely on the individual picture, not a generic list.

If your gut is the thing standing between you and a fuelling strategy that actually works on race day, that's exactly the kind of thing worth properly investigating rather than guessing at. Book a consult here.