In-race carbohydrate
Evidence: strong
Scale carbohydrate to duration: 30-60 g/h up to about 2.5 hours, up to 90 g/h beyond that using glucose plus fructose. Train the gut to tolerate it first.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
Taking carbohydrate during prolonged running improves performance, and the amount that helps scales with how long the effort lasts. The consensus dosing, from the IOC and the underlying fuelling literature, is graded by duration (Burke et al. 2011; Jeukendrup 2014):
- Under 45 minutes: none needed.
- 45 to 75 minutes: a small amount, or even a carbohydrate mouth rinse, is enough. The benefit here is central, sensed by receptors in the mouth, not metabolic, because muscle glycogen is not limiting over about an hour.
- 1 to 2.5 hours: 30 to 60 g per hour.
- Beyond about 2.5 to 3 hours: up to 90 g per hour, but only from multiple transportable carbohydrates.
The guidelines are absolute amounts in grams per hour, not scaled to body mass, because exogenous carbohydrate oxidation does not track body size (Burke et al. 2011).
The 60 to 90 g ceiling
A single carbohydrate such as glucose or maltodextrin saturates the intestinal SGLT1 transporter at about 60 g per hour, so taking more does not raise oxidation (Burke et al. 2011). The limit is absorption in the gut, not the muscle. Adding fructose, which is absorbed by a different transporter, GLUT5, lifts oxidation to over 90 g per hour, roughly a third higher, at an optimal glucose-to-fructose ratio near 2:1 (Jeukendrup 2014). The performance benefit of this combination over glucose alone shows up specifically in efforts of about 2.5 to 3 hours and longer.
Gut training
The gut adapts to carbohydrate intake. About two weeks of repeated high-intake feeding roughly halves malabsorption and gastrointestinal symptoms and improved running capacity in one trial, by upregulating the intestinal transporters and improving gastric emptying (Miall et al. 2018). This matters more for runners than cyclists, who carry less gut jostling, so rehearsing race fuelling in training is part of the work, not an afterthought.
The frontier
Very high intakes of 100 to 120 g per hour are tolerable for some trained athletes and may reduce muscle damage (Viribay et al. 2020), and elite cyclists now report similar figures in racing. But no trial shows that intakes above 90 g per hour improve performance over 60 to 90 g, and one study found 112.5 g per hour added no oxidation over 90 and slightly lowered power (King et al. 2018). The absorption mechanism is solid; the performance payoff of the highest intakes is contested.
A fuelling plan
Marathon fuelling, worked through
A three-to-four-hour marathon sits in the range where 60 to 90 g/h helps. That is roughly one gel (about 22 to 25 g) every 20 to 30 minutes, or the equivalent from drink mix and chews, started early rather than once you feel empty. To go near the top of the range without gut trouble, use products blending glucose and fructose, and rehearse the exact plan on long runs so the gut is trained and nothing is a surprise on race day.
Practise it first
The gut is trainable but unforgiving if ignored. High intake attempted cold, for the first time on race day, is the classic cause of mid-race stomach distress. Treat fuelling as a skill to train, not a race-morning decision.