Why triathletes and other endurance athletes get GI issues - and how to actually fix them
Gut distress derails more endurance athletes races than bad pacing, bad weather, and punctures combined. Here's what I've learned — from both sides of the finish line.
Pip Taylor · APD · Accredited Sports Dietitian · Former Professional Triathlete
“I raced professionally for over a decade and spent years with my own gut issues before I understood what was actually causing them. Now, as an Accredited Sports Dietitian with a Masters in Nutrition & Dietetics and an IOC Diploma in Sports Nutrition, I work with athletes from age group to elite level — and gut problems are still one of the most common and most fixable things I see. This is what I wish someone had told me at the start of my career.”
70% of endurance athletes experience GI issues during racing at some point. GI issues are the #1 cause of slow down or DNF in long distance triathlon events
12 weeks of consistent gut training and smart nutrition decisions, can meaningfully improve absorption capability and help recuce instances of GI problems.
What's Actually Happening in Your Body
When you race hard, your body makes an executive decision: blood goes to the muscles, the heart, the lungs. Everything else — including your digestive system — gets a fraction of its normal supply. At race intensity, gut blood flow can drop by 60–80%. That's not a small reduction. That's your gut essentially running in emergency mode.
Without adequate blood supply, gastric emptying slows, the small intestine loses absorptive capacity, and the gut lining becomes temporarily more permeable. The fancy term is splanchnic hypoperfusion. What it feels like is nausea, cramping, bloating, and an urgent need to find a portaloo at kilometre 28 of the run.
And here's the thing that makes it worse: most athletes are training one way and racing another. Long sessions on water and a half a banana. Races on gels, sports drink, cola, and whatever's being handed out at aid stations. Your gut has never seen that combination. It has never been asked to process that volume at that intensity. Why would it cope?
"The gut is not failing you. It's doing exactly what an untrained organ does when you ask too much of it, too suddenly, under extreme stress."
The absorption ceiling most athletes don't know about
There's a second mechanism that catches a lot of athletes out. Your small intestine absorbs carbohydrate via specific transport proteins. The main one — responsible for glucose — maxes out at around 60 grams per hour. Full stop. Any glucose beyond that rate sits unabsorbed, draws water in, and starts to ferment. That's your bloating and cramping right there, in a single sentence.
Why glucose + fructose changes everything
Glucose and fructose use completely different transport pathways. Glucose uses SGLT1 (max ~60g/hr). Fructose uses GLUT5. Combined in roughly a 2:1 ratio, trained athletes can absorb up to 90g of carbohydrate per hour — because both channels are running
simultaneously. This is why modern performance gels use maltodextrin + fructose blends, and why the gut needs to be trained to handle this volume. Most athletes haven't done that training.
The Things That Make It Worse
Blood flow reduction is the baseline problem. Pile on top of it any of the following and you're in real trouble:
Heat and humidity — increases dehydration and compounds every gut symptom significantly
Dehydration — even mild fluid loss measurably worsens GI function during exercise
Running — the mechanical impact of footstrike adds physical stress to an already compromised gut. This is why run-leg gut issues are so much worse than bike-leg gut issues
Pre-race nerves — the gut-brain connection is real. Stress hormones directly disrupt gut motility before the start gun even fires. I've had athletes with perfect nutrition arrive at the start line and spend 20 minutes in the toilet queue
NSAIDs — this is a big one that doesn't get enough airtime. Ibuprofen and aspirin increase gut permeability and thin the protective mucous lining of the stomach. Taking ibuprofen for race-day aches is, from a gut perspective, genuinely dangerous. Don't do it
The Good News: Your Gut Will Adapt If You Train It
Everything I've described above is trainable. That's the part I want you to really sit with, because it changes everything.
The gut is an organ. Like your heart, your lungs, and your legs, it responds to progressive, systematic training stimulus. With consistent work over 12–16 weeks:
Your stomach increases its volume tolerance — it learns to hold more without discomfort
Intestinal transport proteins upregulate — literally more absorption capacity develops
Gastric emptying improves — food moves through faster and more efficiently under stress
Your whole gut becomes more resilient to the demands of hard racing
I've watched athletes who'd had gut problems for years — who were convinced they were "just someone who couldn't fuel on the run" — completely transform their racing with 12 weeks of targeted gut training. Not because they found a magic gel. Because they trained their gut the same way they trained everything else.
Five Things You Can Do Starting This Week
Fuel every key long session with your race nutrition, starting now. Not sometimes. Every key session over 60 minutes. Start conservatively - half a banana or one gel - and build from there. Your gut needs repetition, not occasional exposure.
Switch to 2:1 glucose-to-fructose products. Check your gel labels. You want maltodextrin or glucose paired with fructose. Pure maltodextrin products at high doses will hit that absorption ceiling fast.
Practise drinking on a schedule. Gut training also means training your stomach to accept and process fluid under load. Every 15– 20 minutes on the bike, drink something — whether you feel like it or not. You won’t always need to drink to schedule - but for now - it’s all about getting volume in to the gut and getting used to the feeling.
Drop the NSAIDs. If you're taking ibuprofen routinely around training or racing, please stop. Speak to a sports medicine doctor about what's causing the pain that needs managing.
Lock in your race-day product list now. Every gel, bar, sports drink and chew you plan to use on race day needs to be tested in training first. Race day is an execution day, not a testing day.
WHEN TO LOOK BEYOND NUTRITION
✓ GI symptoms persist despite 12+ weeks of consistent gut training
✓ You experience significant symptoms at rest, not just during
exercise
✓ There's a pattern of specific food triggers outside of training
✓ Unexplained weight loss, or blood in stool — see a doctor
✓ You suspect an underlying condition like IBS, coeliac disease, or food intolerance
If any of those apply to you, reach out. Sometimes gut issues are a training problem. Sometimes they're a medical problem in disguise. An Accredited Sports Dietitian can help you figure out which one you're dealing with.
Gut problems end good races every single weekend. They end careers. And the hardest part is that most of them are entirely avoidable — not with the right gel brand or the perfect race-day strategy, but with the same patient, progressive training approach you've applied to everything else.
Your gut will adapt. You just have to give it the chance.