Eleven trained athletes rode twice for 150 minutes. On a personalised dose averaging 65 g/h, they burned as much ingested glucose at their peak as they did on a fixed 90 g/h. That’s 28% less fuel for the same result, and it suggests your carb absorption ceiling may sit well below the number on the gel box.
Here’s what the study showed, what it didn’t, and how to find your own number.
Where the 90 g/h number comes from
The 90 g/h rule is real guidance. Sports scientists note that a single carb source, like glucose alone, plateaus near 60 g/h. Mixing glucose and fructose uses two gut doorways instead of one. That’s why the guideline for sessions over about two hours is up to 90 g/h (Jeukendrup, 2017).
In short: one doorway caps your traffic, two doorways roughly double it. But a guideline is an average. Your gut isn’t an average.
Think of your gut as a highway on-ramp with a toll booth. Carbs queue up to enter the bloodstream. Past a certain rate, the cars don’t get through faster. They pile up at the ramp. That pile is the fullness, sloshing and cramping you feel at hour three.
What the 2025 study found about your carb absorption ceiling
Researchers led by Tim Podlogar gave 11 endurance-trained cyclists and triathletes (6 women, 5 men) a drink with a labeled glucose tracer. That tracer lets scientists see how much of the drink the body actually burns (Podlogar et al., 2025).
In the first ride, athletes took 90 g/h. Each person’s peak burn rate set their personal dose for the second ride. The team assumed 80% of intake gets oxidised, meaning burned for fuel.
The result: the personalised dose averaged 65 ± 10 g/h. Peak glucose burn was 0.91 ± 0.19 g/min on the personal dose and 0.90 ± 0.15 g/min on 90 g/h. Athletes also reported lower stomach fullness and lower effort ratings on the personal dose.
Translation: same fuel burned, less fuel swallowed, and a calmer stomach.
The Birmingham news report on the study says individual doses ranged from 49 to 80 g/h. We couldn’t confirm that range in the paper’s text, so treat it as a press-report figure. It still points the same way: the right dose varied a lot between people.
Why 90 g/h isn’t too high for everyone
Don’t over-read this. The 2025 study had only 11 people. It used glucose alone and measured no race or time-trial performance. It was a proof of concept.
Another study from the same lab found a different picture. Eleven highly trained men rode three hours on 120 g/h of fructose plus maltodextrin. They burned more ingested carbs than on 90 g/h: 1.51 vs 1.29 g/min on average (Podlogar et al., 2022). Peak burn hit about 1.6 g/min, and two riders reached 2.0 g/min.
So some guts take 120 g/h and use it. Others stall below 70. The authors flagged large differences between people.
One number can’t fit every gut. Yours has its own ceiling.
Intake isn’t oxidation
Here’s a trap. Eating 120 g/h doesn’t mean burning 120 g/h. A gram you swallow can sit in the gut or get stored.
From the 2022 data, 1.51 g/min works out to roughly 91 g/h burned, or about 75% of intake. At 90 g/h, 1.29 g/min works out to roughly 77 g/h, or about 86%. Efficiency fell as the dose went up.
| Intake (g/h) | Burned from drink (g/min) | Burned (g/h) | Share of intake burned |
|---|---|---|---|
| 90 | 1.29 | ~77 | ~86% |
| 120 | 1.51 | ~91 | ~75% |
Source: Podlogar et al. 2022, 11 trained men. Grams per hour and percentages are AthleteOS calculations (g/min x 60).
In short: each extra gel pays back less than the one before. If you want to turn a measured burn rate into a dose, the math is simple.
| Peak burn (g/min) | Burned (g/h) | Efficiency assumed | Dose to take (g/h) |
|---|---|---|---|
| 0.90 | 54 | 85% | ~64 |
| 0.90 | 54 | 80% | ~68 |
| 0.90 | 54 | 75% | ~72 |
| 0.90 | 54 | 70% | ~77 |
Burn rate (g/h) divided by efficiency gives your dose. Most athletes can’t measure burn rate without a lab, so use this as a way to understand the idea, not as a test.
Can gut training raise the ceiling?
Probably a bit, but the evidence is thin. In 16 cyclists, 28 days of high-carb intake improved how much ingested carbohydrate they burned (Jeukendrup, 2017). Stomach trouble is also common: roughly 30-50% of athletes report gut problems regularly, per the same review.
But training isn’t a guaranteed fix. In 19 elite race walkers, two weeks of a very high-carb diet and gut training at 90 g/h went with more upper-gut symptoms in the second half of exercise (King et al., 2022).
Gut training works like strength work. Go up slowly. Don’t jump from 60 to 120 in a week. For a deeper look at building the base that makes long sessions easier, see how Zone 2 builds your aerobic base.
Meet Dana: a case study
Take a runner I’ll call Dana. She’s 41, training for her first marathon, and runs 18-mile long runs. She followed a 90 g/h plan from a magazine. By mile 14, she felt full and queasy, and her pace slid. (Example only, not a study participant.)
Dana dropped to 60 g/h for her next long run and felt fine. Then she added 10 g/h each week. At 75 g/h, the bloating returned. She settled at 70 g/h. On race day she skipped the 90 g/h plan, held her pace through mile 22, and finished without the stomach crash.
Her ceiling wasn’t 90. It was 70, and she found it by testing.
How to find your number without a lab
You can’t measure oxidation at home. You can still run a smart test:
- Start at 60 g/h on a long session at race effort.
- Add 10 g/h every week or two.
- Log fullness, nausea and energy at each hour.
- When symptoms show up, drop back 10 g/h. That’s your working ceiling.
- Re-test after a few weeks of consistent practice.
Heat and hard effort shrink what your gut can handle. A pace that feels easy in cool weather may feel awful at 30 C, so test under race-like conditions. For more on heat, read how to train for a half marathon in hot weather.
Missed a long session while building up? Don’t cram carbs into the next one. Use a simple rule for making up a missed long run instead.
Putting it in your plan
AthleteOS builds a session fuel plan that sets your during-exercise carb target from session length and intensity. It then caps that target by your gut-training status: untrained, moderate or well trained. You can also set your own carb ceiling in g/h, and the plan respects it. If a session calls for more carbs than an untrained gut typically absorbs, the plan flags elevated GI-distress risk.
That’s a planning guardrail, not a lab measurement. It doesn’t measure oxidation. Use it with your own training log. You can try AthleteOS and set your ceiling.
The 90 g/h rule is a good place to start. Your own gut gets the last word.