AI 速览
- Intestinal absorption, not muscle burning, sets the ceiling for carb intake: the SGLT1 transporter that moves glucose saturates at roughly 60 g per hour.
- Fructose uses a separate transporter (GLUT5). Combining glucose and fructose at a 2:1 ratio lets riders absorb up to 90 g/hr and burn more of it as fuel.
- Maltodextrin is a chain of glucose molecules: same transporter as glucose, same ceiling, but less sweet and easier on the stomach at high concentrations.
- For rides under 2.5 hours, 30–60 g/hr of any carb is enough. Beyond 2.5 hours, the 60–90 g/hr multi-transportable ratio measurably extends time to exhaustion.
/ 01
Why there is a ceiling at all
Eating carbohydrate during cycling preserves blood glucose, spares liver and muscle glycogen, and extends time to exhaustion. The question is not whether to eat carbs but how much the gut will actually absorb. The bottleneck is the intestinal brush border, where specific transporter proteins move single sugar molecules out of the gut and into the blood.
The sodium-glucose linked transporter (SGLT1) moves glucose and galactose. It saturates at around 60 g per hour — once the transporter is fully loaded, more glucose in the gut does not become more glucose in the blood. Instead it sits in the intestine, draws water in by osmosis, and produces the bloating, cramping, and diarrhoea that every cyclist has met after a heavy gel day.
/ 02
Three carbohydrates compared
Carbohydrate sources for cycling and how they reach the blood
| Carb | Transporter | Absorbed (g/hr) | Sweetness | Best use |
|---|---|---|---|---|
| Glucose (dextrose) | SGLT1 | ~60 (shared) | High | Base of any mix |
| Maltodextrin | SGLT1 (after digestion) | ~60 (shared) | Low | High-concentration drinks, less cloying |
| Fructose | GLUT5 | ~30 (separate) | Very high | Stack on top of glucose |
| Sucrose (table sugar) | SGLT1 + GLUT5 (after split) | Up to ~90 combined | High | Cheap 1:1 ratio source |
| Galactose | SGLT1 | ~60 (shared) | Low | Niche; no advantage over glucose |
/ 03
The 2:1 multi-transportable ratio
In the mid-2000s Asker Jeukendrup's group showed that feeding glucose and fructose together at a 2:1 (or 1:0.8) ratio raises total carb oxidation to roughly 1.7 g/min — about 100 g/hr — versus 1.0 g/min for glucose alone. The mechanism is exactly the two-transporter story: SGLT1 moves the glucose, GLUT5 moves the fructose, and the two pools merge in the liver before reaching muscle as fuel. Riders in blinded trials rode longer and reported lower perceived effort at the same power.
Practically, a 2:1 mix means 60 g glucose plus 30 g fructose per hour, or 70 g glucose plus 35 g fructose for a bigger rider pushing toward the 90 g/hr ceiling. Many commercial mixes already use this ratio — read the label and look for 'multiple transportable carbohydrates' or a fructose component listed in the ingredients.
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Dosing by ride duration
Carb intake should scale with ride length and intensity. Below 60 minutes there is no measurable performance benefit from eating on the bike — endogenous glycogen is enough. From 1–2.5 hours, 30 g/hr sustains blood glucose. Past 2.5 hours, the science is unambiguous: 60–90 g/hr of a multi-transportable mix extends time to exhaustion and protects power output in the final hour. Stage racers and ultracyclists routinely train the gut to tolerate 90–120 g/hr over consecutive days.
Carbohydrate intake targets by ride duration (multi-transportable mix)
| Duration | Carb (g/hr) | Calories/hr | Ratio (glu:fru) | Format |
|---|---|---|---|---|
| <60 min | 0 | 0 | — | Water only |
| 1–2 hr | 30 | 120 | Any | One bottle sports drink |
| 2–2.5 hr | 30–60 | 120–240 | 2:1 preferred | Drink + 1 gel/hr |
| 2.5–4 hr | 60–90 | 240–360 | 2:1 mandatory | Drink + gels + solids |
| Stage race / ultra | 90–120 | 360–480 | 2:1, trained gut | Liquid-dominant, low fibre |
/ 05
Real-food and DIY options
Table sugar (sucrose) is half glucose and half fructose, dissolved in water for free — a 60 g serving in a 500 ml bottle gives roughly the same 1:1 ratio as a premium sports drink at one-hundredth the cost. Maltodextrin sold in 1 kg bags from any supplement retailer forms the base of most homebrew mixes: 50 g maltodextrin, 25 g fructose, a pinch of salt, flavour to taste. Rice cakes with jam, boiled potatoes with salt, and fig rolls are the pro-peloton staples for long stages because they deliver carbohydrate without the sweetness fatigue of pure sugar.
Whatever the source, the rule is the same: train the gut. Practise your race-day fuelling plan in training blocks at race intensity for at least four sessions before the event. A trained gut tolerates 90 g/hr where an untrained one vomits at 60.
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