You've felt it. Three, maybe four hours into a long ride, and it isn't your legs that go first. It's your stomach. The thought of swallowing one more gel turns your mouth sour. You're bloated, faintly nauseous, and the bottle you know you should be drinking feels like a punishment. So you stop fuelling. And forty minutes later, the legs go too, because the tank you stopped topping up is empty.
This is GI distress, and it ends more long rides and sportives than poor fitness ever does. You can carry the best fuel in the world, but if your gut won't take it on, none of it reaches your legs. The good news, and there's plenty of it here, is that GI distress is almost always caused by a handful of fixable mistakes. Let me break down what's actually going wrong, and how to keep your stomach open when it counts.
Your gut is a fuel pump, and it can be overwhelmed
Start with the basic picture, because it explains everything that follows. The carbohydrate you eat on the bike has to be emptied from your stomach, absorbed across the gut wall, and shuttled into the blood before it ever helps your legs. That whole chain has a maximum rate. Push fuel in faster than the gut can process it, or do anything that slows the gut down, and the fuel backs up. A backed-up stomach is a sick stomach. Nausea, bloating and cramps are simply the signals that the pump is overwhelmed.
So the entire game of avoiding GI distress is about not overwhelming that pump. There are five ways riders overwhelm it, and once you see them you'll recognise your own bad days immediately.
Mistake one: your drink is too concentrated
This is the single most common cause, and almost nobody suspects it. You buy a high-carb drink mix, you want the carbs, so you tip a big scoop into a small bottle and reason that stronger is better. It isn't. A drink mixed too concentrated — too much sugar in too little water — sits in your stomach like syrup. The body won't empty it until it's diluted, so it pulls water into the gut to do the diluting, which leaves you bloated, sloshing and sick, and can even make you more dehydrated.
The fix is to respect the concentration. Most drink mixes have a sensible carbohydrate-to-water ratio printed on the tub for a reason. If you want more carbohydrate per hour, the answer isn't a thicker bottle — it's taking some of your carbohydrate as a more concentrated source like a gel with a separate mouthful of plain water to dilute it on the way down. Strong syrup in a small bottle is how you spend the back half of a ride feeling sick.
Mistake two: you're relying on glucose alone
Here's where it gets genuinely useful. Your gut absorbs glucose through one transporter and fructose through a completely different one. Lean entirely on glucose and you max out that single pathway fairly quickly, and the excess sits in the gut causing trouble. But blend glucose and fructose together — at a ratio of roughly 1 to 0.8 — and you open a second lane on the motorway. Asker Jeukendrup's research on these multiple transportable carbohydrates is the reason every serious modern fuel uses a glucose-fructose blend, and it's why riders can now absorb 90 grams an hour and beyond without the gut rebelling.
The practical point: check your fuels. A product built on a glucose-fructose mix will sit far better at high intakes than one built on glucose or maltodextrin alone. If you've been getting sick at higher carbohydrate amounts, the blend may be your problem, not the quantity.
Mistake three: fibre, fat and protein in the wrong place
What you eat before and during the ride matters as much as how much. Fibre, fat and a big slug of protein all slow the stomach down — they're digested slowly by design, which is brilliant for a normal meal and a disaster right before or during hard riding. A high-fibre breakfast, a fatty pastry at the café stop, a protein-heavy bar mid-ride: each one parks in your stomach and blocks the gate for the carbohydrate you actually need to clear.
The fix is timing. In the two to three hours before a long ride, eat carbohydrate that's low in fibre and fat — white bread, a plain bagel, white rice, a ripe banana, the boring stuff. Save the wholegrain, the nuts and the big protein hit for after. And on the bike, during the ride itself, keep it almost entirely to carbohydrate. The gut on a hard ride is not the place for digestion-heavy food. This is exactly the discipline the pro chefs build into race fuelling — simple, fast-clearing carbohydrate when it's go time.
Mistake four: you let yourself get dehydrated
Dehydration and GI distress are linked far more tightly than most riders realise. When you fall behind on fluid, your body protects the working muscles and the skin — for cooling — by pulling blood away from the gut. Less blood to the gut means slower digestion and worse absorption, which means the fuel you're taking on just sits there. This is why stomach trouble so often appears late in hot rides: you quietly fell behind on drinking, the gut blood flow dropped, and the whole system seized up.
The fix is to drink to your sweat rate from the start, not to catch up once you're already in trouble. If you don't know your sweat rate, the electrolytes and sweat-rate guide shows you how to find it with a set of scales and one ride. Get the fluid and the sodium right and you keep the gut supplied with the blood it needs to do its job.
Mistake five: you went out too hard
The last one is the simplest and the most overlooked. Intensity itself shuts the gut down. The harder you ride, the more blood gets diverted to your legs and away from digestion, and above a certain effort the gut more or less stops absorbing altogether. Hammer the first two hours of a five-hour ride and you may find that by the time you want to fuel, your stomach has already closed for the day.
The fix is pacing, and it's free. Ride the early hours of a long event genuinely controlled — the discipline I bang on about for long climbs applies to your gut as much as your legs. Keep the effort steady and aerobic in the first half and your gut stays open and absorbing. Blow the doors off early and you're not just risking your legs, you're risking your ability to eat for the rest of the day.
Train the gut so race day isn't a test
There's one more piece, and it underpins all of the above. The gut's capacity to absorb carbohydrate is trainable. The transporters that move fuel across the gut wall actually increase with repeated practice, which is the whole reason a Tour rider can take on 120 grams an hour while an untrained amateur is heaving at 60. If race day is the first time your gut has ever been asked to process big fuelling, it will fail.
So practise. Use your long training rides to rehearse exactly the fuelling you'll use in the event — the same products, the same amounts, the same timing — and build the quantity up gradually over weeks. I've written the full ramp in how to train your gut to absorb 90 to 120 grams an hour, and it's the difference between a gut that copes on the day and one that quits. Nothing new on race day, including how much you ask your stomach to do.
If it hits mid-ride: the rescue
Prevention is the whole game, but you'll still have days when the gut turns on you anyway, so here's what to do when it happens rather than just suffering to the finish.
The instinct is to keep forcing fuel down because you know you need it. Resist it. If your stomach has shut down, more fuel just adds to the backlog and makes it worse. Instead, back off the pace for a few minutes — drop the effort right down — to let blood return to the gut. Stop the solids and the thick mixes, and switch to small sips of plain water or, the old domestique's trick, a few mouthfuls of flat cola, which is light, easy to absorb and gives you a little sugar and caffeine without overwhelming anything. Give it ten or fifteen easy minutes. Nine times out of ten the gut settles, the nausea lifts, and you can start fuelling again gently, in smaller doses than before. A short truce with your stomach mid-ride saves the day far more often than gritting your teeth and forcing another gel into a system that's already full.
The short version
GI distress isn't bad luck and it isn't a weak constitution. It's a gut being overwhelmed, and it's almost always one of five things: a drink mixed too strong, glucose with no fructose, fibre and fat clogging the gate, dehydration starving the gut of blood, or an intensity it can't cope with. Fix those, train the gut in advance, start fuelling early and take it in small regular doses, and you keep the pump running all day.
Get this right and the gels stop being a thing you dread and go back to being what they're meant to be: fuel that actually reaches your legs.
For the absorption-building protocol, read gut training for cyclists, and for how much to take on once your gut can handle it, carbohydrate per hour for cyclists. Got a fuelling problem you can't crack? Bring it to the community on Skool.