The short answer
- For efforts beyond about two hours, getting carbohydrate from your stomach to your muscle is the limiting step, and the bottleneck is intestinal absorption.
- Using more than one type of sugar raises how much you can absorb and oxidise, because they use different transport routes.
- The gut adapts. Absorptive capacity increases when you practise taking carbohydrate in training instead of only on race day.
- Practise your race fuelling in training, at race pace, with the exact products you will use. Most stomach problems in races are unpractised stomachs.
- For anything beyond the basics, a sports dietitian is the right person. The position statement on this says so explicitly.
This one sits at the edge of what a physiotherapist should be telling you, so we will keep it to the part that is settled and say clearly where the line is.
Where the limit actually sits
You store enough carbohydrate for somewhere around ninety minutes to two hours of hard running. Past that, performance depends on how much you can take in while moving, and the constraint is not your stomach or your willingness to eat.
Two consequences follow, and they are the practical core of this article.
Mixing sugar types raises your ceiling. Glucose and fructose cross the gut wall by different transporters, so a product using both delivers more per hour than the same amount of glucose alone. This is why sports nutrition products list more than one carbohydrate source, and it is not marketing.
The gut trains. Absorptive capacity is not fixed. Practising carbohydrate intake during training increases what you can handle, which means race-day tolerance is something you build rather than something you have.
Roughly how much
The commonly used working bands, which any sports nutrition resource will give you: for efforts of about one to two hours, something in the region of 30 to 60 grams of carbohydrate per hour. For longer than that, more, with the higher end only achievable using mixed carbohydrate sources.
Treat those as starting points to test rather than targets to hit. Individual tolerance varies enormously, and the right amount for you is the most you can take without stomach trouble at race effort, which you find out by trying it.
Body size matters, effort matters, heat matters. A plan that works for a 90 kg runner in October is not the same plan for a 55 kg runner in July.
Why races go wrong
Almost every stomach disaster we hear about has the same cause: the first time the runner tried it was in the race.
New gel brand from the expo. A drink handed out on course that they had never tasted. A quantity they had read about but never practised. Combined with race-day nerves, a faster pace than training, and possibly heat, and the gut does what an untrained gut does.
The other half of the pattern is starting too late. Fuelling that begins when you already feel empty is trying to solve an absorption-limited problem after the deficit has opened. Starting early and regularly works better than a large amount taken once you feel it.
The failure mode that is not about race day
Running low on carbohydrate during a long run is a tactical problem. Chronically eating less than your training costs is a different and more serious one.
Low carbohydrate availability is one of the areas the IOC's consensus on Relative Energy Deficiency in Sport singles out as having grown in importance since its previous version2. If you find yourself consistently under-eating around a heavy training block, the fuelling of a single long run is the less important half of the problem. Our article on under-fuelling covers the rest.
Where our advice stops
Here, deliberately.
The joint position statement from the American College of Sports Medicine and the dietetic associations of the United States and Canada is explicit that athletes should be referred to a registered dietitian for a personalised nutrition plan3. We agree, and it applies to anything more specific than the principles above.
If you have a diagnosed gut condition, diabetes, a history of eating disorder, or you are chasing a performance target where the fuelling plan is part of the strategy, that is a dietitian's job and not ours.
What we will do is make sure the fuelling practice is built into the training plan, and that the long runs are structured so you get to rehearse it properly.
When to get it looked at
The last one matters. Fuelling that has started to feel fraught is worth addressing with someone qualified before it becomes a bigger problem.