Carbohydrate Intake in Endurance Sport
The runners who tell you they "don't really fuel" in training are the same ones who hit kilometre 32 of a marathon and discover exactly how much that decision cost them. Not because they bonked dramatically. Because they spent 18 weeks of long runs at 40g of carbohydrate per hour and then asked their body to perform at 80g on race day — and it refused.
Carbohydrate intake in endurance sport is one of the most straightforward performance variables available. The science is clear. The mechanisms are understood. The application is trainable. What most athletes get wrong is not the knowledge. It's the practice.
The ceiling for carbohydrate absorption during exercise isn't fixed at 60g/hr. With the right carbohydrate ratio and a gut that's been progressively trained, 90–120g/hr is physiologically achievable. The problem is that most athletes never test above what's comfortable in training, then wonder why race-day performance hits a wall that fitness alone couldn't break.
"The research on carbohydrate absorption during endurance exercise changed how I thought about fuelling entirely. The idea that you can reach 90 to 120g of carbohydrate per hour — using both intestinal transporters simultaneously — means the ceiling most athletes accept isn't physiological. It's a training gap. You train the legs progressively. The gut needs exactly the same approach. Build the intake, test it at race intensity, and don't leave that work until the week before the race."
— Alistair Brownlee, Two-time Olympic Triathlon Champion and founder of truefuels
Why the 1:1 glucose-fructose ratio matters for carbohydrate intake
The gut has two primary carbohydrate transporters. SGLT1 handles glucose and is sodium-dependent. GLUT5 handles fructose and operates on a separate, independent pathway. When carbohydrate intake is glucose-only, SGLT1 saturates at roughly 60g/hr, creating an absorption ceiling that limits performance regardless of how much carbohydrate is available.
A 1:1 glucose-to-fructose ratio activates both pathways simultaneously. That's the mechanism behind the 90–120g/hr ceiling that 13C oxidation research on trained endurance athletes has established. It's not a marketing claim. It's what happens when two absorption pathways run in parallel rather than one being overwhelmed.
The truefuels Performance Gel is built on a 1:1 ratio — 40g of carbohydrate per gel, unflavoured, with no artificial sweeteners or preservatives that raise osmotic load. Two gels per hour gives 80g of carbohydrate via both transporters. Three gels per hour pushes to 120g for well-trained athletes whose guts can handle it. Most competitors use 1:0.8 ratios with 20–30g per gel, meaning athletes need 4–5 gels per hour to approach similar intake — more to carry, more to manage, and more flavour fatigue risk over long events.
Carbohydrate targets by event type
Carbohydrate demand in endurance sport varies significantly with duration, intensity and individual training status. These are working targets, not prescriptions — they should be tested in training before being applied in racing.
| Event / session type | Duration | Carb target (g/hr) | Source approach | Gut training priority |
|---|---|---|---|---|
| Easy aerobic run or ride | <75 min | 0–20g | Pre-session meal sufficient | Low |
| Steady long run | 90 min–2hr | 40–60g | Gel every 30 min from start | Medium |
| Marathon race | 3–5hr | 60–90g | Gel every 20–25 min, consistent | High |
| Long ride / sportive | 2.5–5hr | 60–90g | Gels + drink; start within 20 min | High |
| Half Ironman (70.3) | 4–6hr | 60–90g | Bike-focused; gels + system | High |
| Full Ironman | 8–14hr | 60–80g | Consistency over ambition | High |
| Threshold / interval session | <90 min | 30–50g | One gel at start if key session | Medium |
Before exercise: the foundation that determines everything
Pre-session carbohydrate loading doesn't mean a heroic meal the night before. It means consistent carbohydrate intake across the 24–48 hours before a key session, combined with a race-morning meal timed correctly.
Muscle glycogen storage is the target. Starting a long run or race at near-maximal glycogen means a longer period before stores become limiting and a larger buffer against the consequences of imprecise in-race fuelling. For most endurance athletes, a carbohydrate-led meal 2–4 hours before the start — low fibre, low fat, familiar — plus a small top-up in the 15 minutes before the gun (a single truefuels gel works cleanly here: 40g fluid carbohydrate, no additives, quick to absorb) sets the correct platform.
truefuels Training Bundle
20 gels and 20 electrolytes — the fuel to rehearse your full race protocol across a training block, so race day becomes execution, not experiment.
See the bundle →During exercise: start before you need it
Carbohydrate taken during exercise becomes available in blood within 15–20 minutes. That lag is why athletes who wait until they feel flat are always fuelling for a problem that's already 20 minutes old.
For sessions and races over 75 minutes, start carbohydrate intake within the first 20–30 minutes and continue at consistent intervals. Even spacing — every 20–25 minutes — delivers steadier blood glucose than reactive boluses when energy drops. For athletes pushing towards the 90g/hr ceiling, the 1:1 ratio in truefuels gels is the mechanism that makes high hourly intake manageable. Both SGLT1 and GLUT5 working simultaneously means neither pathway reaches saturation.
Hydration changes what carbohydrate can do
Carbohydrate absorption is not independent of hydration status. When the gut receives concentrated carbohydrate without adequate fluid, gastric emptying slows, the osmotic gradient works against absorption and GI distress becomes more likely. The practical consequence: high carbohydrate intake works best when fluid delivery matches it.
This is one reason the sodium in the truefuels gel earns its place twice. It enhances glucose co-transport via SGLT1 — improving carbohydrate absorption directly — and it supports the fluid retention that makes consistent drinking possible. In hot conditions or for heavy sweaters, adding the truefuels Active Hydration sachet alongside the gel system covers the additional sodium demand without overloading carbohydrate concentration in the drink.
Gut training: the variable that decides the ceiling
The 90–120g/hr ceiling is real, but it is not automatic. It requires deliberate training over multiple long sessions. The gut adapts to higher carbohydrate intake the same way muscles adapt to higher training loads — progressively, with response to the stress applied.
The process:
1. Identify your current sustainable hourly carbohydrate intake in long sessions (the level at which you finish without GI issues).
2. Build progressively — increase by 10–15g/hr every 2–3 weeks across long sessions.
3. Test at target race intensity, not just easy efforts. Tolerance at easy pace often overstates what's achievable when effort rises.
4. Track pace stability, perceived exertion, stomach response and finish state. A session where intake was right leaves you tired. One where intake was wrong leaves you bonked, nauseous or unable to eat for hours afterwards.
Athletes who arrive at race day with gut training matched to their carbohydrate targets rarely hit the wall late. Athletes who train at 40g/hr and race at 80g/hr usually do.
After exercise: the recovery window that protects the next session
Post-exercise carbohydrate intake restores muscle glycogen and supports the adaptation that training is designed to create. The practical priority: take carbohydrate within 30–60 minutes of finishing key sessions, especially when the next training session is within 24 hours.
For athletes in high training load periods with back-to-back long days, this window is not optional. Poor recovery nutrition compounds across a training week faster than most athletes realise. The sessions that feel hardest on day four or five often reflect poor nutrition decisions made on day two.
Carbohydrate intake in endurance sport isn't a race-week concern. It's a year-round training habit. Build the protocol in training and race day becomes execution rather than experiment.
The fuel to rehearse your race protocol in training.
20 gels and 20 electrolyte sachets — everything needed to build and test your carbohydrate and sodium plan across a training block, so race day becomes execution, not experiment.
See the Training Bundle →Find the carb intake your event actually demands.
The Fuelling Protocol tool calculates your hourly target — then helps you train your gut up to it across your long sessions.
