If you started riding twenty years ago you were told 60 grams of carbohydrate an hour, and that was the end of the conversation. If you started last year you’ve probably seen 120 on a gel wrapper and assumed the sport had lost its mind.
Both numbers are real. What changed in between is worth understanding, because it decides whether you should be chasing the big one.
Glucose doesn’t cross your intestinal wall on its own. It rides a transporter called SGLT1, and there are only so many of them. Feed a rider more and more glucose during exercise and the amount they can actually burn from that drink flattens out around 60 grams an hour, about a gram a minute. Past that you’re not fueling, you’re just carrying it. Often unpleasantly.
That number held for years because it’s a hard physical limit, not a matter of trying harder.
The second door
Fructose uses a different transporter. Asker Jeukendrup’s group worked out that if you combine glucose and fructose you open two channels at once, and the amount of ingested carbohydrate you can oxidize jumps by as much as 65% over glucose alone. The ceiling wasn’t the gut, it was the single-door design.
That’s where 90 grams an hour comes from, and why sports drinks started listing ratios on the label. It’s the standing recommendation for efforts past about two and a half hours, and for most people it’s the useful target.
Then Viribay and colleagues went further, in the field, with elite mountain marathon runners taking 120, 90 or 60 grams an hour through an actual race. The 120 group came out with markedly lower markers of muscle damage. A companion study on a trail marathon found better neuromuscular function and a better return of high intensity running capacity afterward at 120 as well.
Those are small studies on elite athletes who had clearly practiced eating like that, and one race is not a training block. Treat 120 as evidence that the ceiling is higher than we thought rather than as a target for your Saturday ride.
The part nobody wants to hear
Your gut is trainable, and if you don’t train it, none of this applies to you. Jeukendrup made the case directly in 2017: transporter expression, gastric emptying and comfort all adapt to regular practice. Which means the athlete who reads about 120 grams an hour and tries it for the first time on race morning is going to spend the back half of the day looking for a hedge.
Gut training isn’t mysterious. Pick your long ride of the week and treat it as the fueling rehearsal, not only the endurance one. Start a block at whatever rate feels easy now, maybe 50 or 60 grams an hour, and add ten grams every week or two, always on the same ride so you’re comparing like with like. Watch the last hour, which is where tolerance actually shows itself, not the first.
Two things go wrong. People jump the rate too fast and blame the carbohydrate when the real fault was the step size, and people only ever practice on easy days, so the stomach never learns to empty under real intensity. Push the rate up on a ride that has some hard work in it, because that’s the condition you’re training for.
The mix in the bottle matters as much as the rate. The early two-transporter drinks came in a 2:1 ratio, two parts glucose to one part fructose, because that matched the assumed capacity of each channel. Newer products pushed the fructose share up toward 1:0.8, closer to even, and that’s what most high-rate mixes now use. The reason is comfort as much as oxidation. Leftover glucose sitting in the gut is what pulls water in and turns a long day sour, and a more balanced blend leaves less of it stranded. So if you’ve hit 90 grams an hour and felt bloated rather than fueled, check the ratio before you blame the total.
Fuel is the number people chase, but it rides in on fluid and sodium, and those have limits of their own. Sweat rate and sweat sodium vary far more between people than carb tolerance does, so the same bottle that keeps one rider even leaves another cramping or waterlogged. No population number fixes this. You find your sweat rate by weighing yourself before and after a long ride in the heat, and you work outward from what you learn.
There’s a thread connecting this to the durability research. When Clark’s group looked at what happens to critical power during prolonged exercise, carbohydrate intake during the ride blunted the decline. Eating isn’t only about not bonking. It’s about which athlete shows up at hour four.
The rate should scale with the day. A 90 minute endurance ride doesn’t need 90 grams an hour. A six hour race at tempo needs everything you can absorb. OSO sizes carbohydrate, fluid and sodium per session against how long and how hard you’re actually going, so the number moves with the work instead of sitting there as a slogan you half remember.
Start at 60. Get comfortable. Work toward 90. Only chase 120 if you’re racing long enough that it changes the outcome, and only after your gut has agreed to it in training.