How to Avoid GI Issues During Endurance Racing
Ask any group of marathon runners or long-course triathletes what ruined their last race and a surprising number will not say cramp, pacing or the weather. They will say their stomach. The research backs that up: depending on the event, somewhere between 30% and 90% of endurance athletes report gastrointestinal symptoms during competition, and in long-distance triathlon it sits at the higher end of that range. Bloating, nausea, side stitch, reflux, the desperate scan for a portaloo at 30km — these are not rare misfortunes, they are the single most common reason athletes fail to finish events they were physically trained for.
The good news is that GI issues in endurance racing are largely trainable and largely preventable. They are not a fixed personality trait of your gut. They are the predictable result of asking a digestive system with reduced blood flow to process a fuel load it has never practised, at a concentration it cannot handle, while you are mildly dehydrated. Fix those four variables and most people's symptoms disappear. Here is how to do it properly.
What Causes GI Issues During Endurance Racing
There are three mechanisms at work, and they compound each other.
Blood flow redistribution. When you exercise hard, your body sends blood to working muscle and skin. Splanchnic (gut) blood flow can fall by up to 80% at intensities above roughly 70% of VO2 max. A gut running on a fifth of its normal blood supply absorbs slowly and tolerates poorly. This is why symptoms cluster in the back half of a race and why they spike when you surge.
Osmolality and unabsorbed carbohydrate. If you take in more carbohydrate than your intestinal transporters can shift, the excess sits in the gut lumen and pulls water in with it. That is the mechanism behind the sloshing, bloated feeling and the sudden urgency. A drink mixed at 12% carbohydrate concentration is not "extra fuel" — it is a laxative you paid for.
Dehydration and mechanical stress. Losing more than 2% of body mass in fluid measurably worsens gastric emptying and increases symptom severity. Add the mechanical jostling of running, which is why runners report more lower-GI symptoms (urgency, diarrhoea) while cyclists report more upper-GI symptoms (reflux, bloating) from the aerodynamic hip angle compressing the abdomen.
Train Your Gut the Way You Train Your Legs
Your intestinal carbohydrate transporters are adaptable. Studies on gut training consistently show that repeated exposure to high-carbohydrate feeding increases SGLT1 transporter expression and reduces symptom scores within two to four weeks. You would not attempt a hilly 100-mile sportive without ever riding a hill. Do not attempt 90g of carbohydrate per hour without ever having practised it.
A protocol that works for most people:
- Weeks 1–2: take 30g of carbohydrate per hour on every session over 90 minutes. Get comfortable eating while moving.
- Weeks 3–4: increase to 45–60g per hour on your two key long sessions each week.
- Weeks 5–8: push your longest session to your target race rate — 60g/hr for a marathon, 80–90g/hr for a 70.3 or full-distance triathlon or a big day on the bike.
- Race week: nothing new. Not one item.
Two details matter enormously here. First, train the gut at race intensity, not just race duration — a gut that copes at endurance pace may fail at threshold. Second, use the exact products you will race with. Different carbohydrate sources, sweeteners and textures behave differently, and "an energy chew is an energy chew" is how people end up ill at kilometre 32.
Get the Carbohydrate Blend and Concentration Right
This is where most avoidable GI issues in endurance racing originate. Glucose is absorbed via the SGLT1 transporter, which saturates at around 60g per hour. Take in more glucose than that and the surplus simply sits there. Fructose, however, uses an entirely separate transporter, GLUT5. Combine the two and total absorption rises to 90g per hour and, in gut-trained athletes using ratios closer to 1:0.8 glucose to fructose, up to 120g per hour.
The practical upshot is that a mixed-source fuel is not a marketing gimmick — it is the single most effective thing you can do to raise your carbohydrate intake without raising your symptom risk. Multiple trials comparing glucose-only against glucose-fructose blends at matched intake find lower nausea and bloating scores in the blended group, alongside higher oxidation rates.
Concentration matters just as much as composition. Keep any drink you are relying on for fuel between 6% and 8% carbohydrate — that is 60–80g per litre. Above roughly 10%, gastric emptying slows sharply. If you want more carbohydrate than your bottle can carry at that concentration, take the extra as a solid or semi-solid alongside plain water, rather than mixing your bottles double-strength.
Fluids, Fibre and the Details Around Race Day
Aim for 500–800ml of fluid per hour in temperate conditions, adjusted upwards in heat and for heavy sweaters, with 300–800mg of sodium per litre. Sodium supports fluid retention and gastric emptying; plain water at high volumes does neither well.
In the 24–36 hours before a race, shift to a low-residue diet. Cut high-fibre foods, pulses, large salads and anything with sugar alcohols. You are not depriving yourself of anything useful — you are simply not carrying two days of undigested fibre around the course. Keep the carbohydrate loading in place, just choose lower-fibre sources: white rice, white bread, potatoes without skins, ripe bananas.
Two more things worth knowing. Avoid NSAIDs entirely around racing — ibuprofen has been shown to increase intestinal permeability and worsen GI damage during exercise, and the "preventative pre-race painkiller" is genuinely one of the worst decisions in amateur endurance sport. And be honest about caffeine: it is performance-enhancing, but in doses above roughly 3mg/kg it increases gut motility in a lot of people. If you are prone to urgency, keep race-day caffeine modest and, again, practise it.
What We Use at FTP Endurance Fuel
We built our pastilles around this problem rather than around it. They use a mixed glucose–fructose carbohydrate base so you are recruiting both transporters rather than hammering one, and they are deliberately chewable and low-volume rather than a thick syrup that arrives in the stomach as a concentrated bolus. Each pastille is a small, countable dose, which makes it far easier to feed steadily — two every fifteen minutes rather than a whole gel every forty — and steady feeding is consistently better tolerated than large infrequent hits.
The Mixed Fruit Pastilles are our default racing fuel, and the 1kg format exists precisely so gut training is not financially painful — you need to eat a lot of carbohydrate over an eight-week block, and rationing your fuel because it is expensive is the most common reason athletes never actually complete the adaptation. If you would rather test tolerance before committing, the Taster Pack is the sensible starting point.
Stomach problems on race day feel like bad luck. They almost never are. Pick a fuel with a mixed carbohydrate source, keep your drinks between 6% and 8%, drink 500–800ml an hour with sodium in it, go low-residue for the day and a half before the gun, and spend eight weeks teaching your gut to handle the load you intend to give it. Do that and GI issues stop being the thing that decides your race. Start your gut training block with a Taster Pack, or go straight to a 1kg pack if you already know eight weeks of consistent fuelling is what you need.
