Gut Training: How the Stomach Becomes the Bottleneck on Long Distances

The recent history of sports nutrition looks like a race of numbers: 60 grams of carbohydrate per hour, then 90, now 120. But between "eating 120 grams" and "using 120 grams" sits an organ people usually remember only when it rebels. The gut is not a passive pipe but a system with a limited throughput. And it has a very specific bottleneck.

Where exactly it runs out

Glucose and maltodextrin are absorbed in the small intestine via the SGLT1 transporter. Its capacity is finite: at roughly 60 grams of glucose per hour it reaches saturation. However much more you eat, it cannot be absorbed any faster — the remainder travels further down the gut, pulls water along with it and delivers that familiar scenario involving a search for bushes.

The workaround is well known and elegant: fructose uses a different transporter — GLUT5. It does not compete with glucose for the same door. That is why a blend of glucose (or maltodextrin) with fructose lets total absorption push past 90 grams per hour and approach 120.

This principle is exactly what all modern sports nutrition labelled "2:1" or "1:0.8" is built on — those are the ratios of sources, chosen so that both transporters are loaded.

What the measurements showed

The key study here is the work by Hearris et al. (Journal of Applied Physiology, 2022). Trained cyclists received 120 g of carbohydrate per hour at a glucose-polymer to glucose-fructose ratio of 1:0.8, in four different formats: drink, gel, jelly chew and a combination. The work lasted three hours at an intensity of about 95% of the lactate threshold.

The result: a high rate of exogenous carbohydrate oxidation, comparable across all formats, and minimal gastrointestinal discomfort. The practical conclusion is important and often gets lost: the delivery form barely matters. Gel, drink or jelly chew — the body handles them the same way. Choose whatever is easier to carry and nicer to eat.

The second half of the conclusion is less convenient: these were trained cyclists with adapted guts, not random recreational athletes.

Does the gut adapt

Yes, and that is not a metaphor. A regular high carbohydrate load increases the density and activity of SGLT1 — that is, it literally increases throughput. Changes in absorption were recorded after roughly two weeks of a repeated feeding protocol.

Here, however, begins the zone where I have to be careful. Popular figures such as "an untrained gut handles 30–45 g/h, a trained one 90–120" and schemes like "start at 30 g/h and add 10 g each week over 6–8 weeks" are practical protocols from the coaching literature, not the result of large randomized trials. They capture the direction correctly; the precision of the numbers is an open question. Treat them as a reasonable starting point, not as law.

How to apply this

Train your fuelling on long sessions, not on race day. That is the only way to find out what your particular stomach tolerates. The rule "nothing new on race day" exists for a reason.
Build up gradually. Jumping from 40 g/h straight to 100 is a reliable way to ruin your day.
Use a blend of sources. Glucose-only products will hit the SGLT1 ceiling regardless of your training status.
Don't chase 120 g/h if the distance is shorter. The top figures make sense for multi-hour work; in a one-hour race other things decide the outcome.
Choose the format by convenience. Since oxidation is comparable, pick whatever does not interfere with your breathing and does not stick to your hands.
Bear in mind that heat and high intensity worsen tolerance. Blood flow is redistributed away from the gut towards muscles and skin; what goes down easily on a calm long run may not go down at race pace.

The calculator below will help you work out how much carbohydrate you actually need for a given distance — but remember that calculated requirement and tolerance are two different things, and you will have to establish the second one in training.

Key points

Glucose absorption runs into the SGLT1 transporter, with saturation at around 60 g/h.
Fructose goes via GLUT5 and does not compete with glucose, so a blend of sources raises the ceiling to 90–120 g/h.
In the 2022 study, trained cyclists taking 120 g/h achieved high oxidation equally in the form of a drink, gel, jelly chew or a combination — the format is not the deciding factor.
The gut adapts: a regular load increases SGLT1 density, with shifts recorded after roughly two weeks.
The popular "+10 g per week" protocols are practice, not RCT data; use them as a guideline, not as a standard.
Train your fuelling on long sessions; heat and high pace worsen tolerance.

Sources: Hearris M.A. et al. “13C-glucose-fructose labelling reveals comparable exogenous CHO oxidation during exercise when consuming 120 g/h in fluid, gel, jelly chew or co-ingestion”, Journal of Applied Physiology, 2022. “Increased exogenous but unaltered endogenous carbohydrate oxidation with combined fructose-maltodextrin ingested at 120 g/h versus 90 g/h at different ratios”. https://pmc.ncbi.nlm.nih.gov/articles/PMC9560939/. Podlogar T. et al. “Personalised carbohydrate feeding during exercise based on exogenous glucose oxidation: a proof-of-concept study”, Performance Nutrition, 2025. https://doi.org/10.1186/s44410-025-00003-9