How Many Carbs Per Hour for an Ultra?

For twenty years the advice was 60 grams of carbohydrate per hour, and that was that. It turned out to be an artefact of what people were being fed rather than a limit of the human body.

Where the 60 g/h number came from

Glucose crosses the intestinal wall via a transport protein called SGLT1. That transporter saturates. Feed a runner glucose-only drinks and oxidation rates plateau at roughly 60 g/h regardless of how much more you give them - the surplus sits in the gut, draws water in, and produces exactly the bloating and distress that gave sports nutrition its reputation.

Fructose uses a completely different transporter, GLUT5. Feed both together and you recruit two independent pathways, and the ceiling moves.

Studies using glucose-fructose mixtures at roughly a 2:1 ratio reliably show exogenous carbohydrate oxidation of 90 g/h, and work in trained athletes has pushed to 120 g/h with measurable benefits. In one trail marathon study, runners taking 120 g/h showed less muscle damage and better recovery of high-intensity running capacity than those at 60 or 90 g/h - and the great majority tolerated it without significant GI distress.

Nearly every modern sports nutrition product is now formulated around this. Check the label: if it lists maltodextrin and fructose, that is what it is doing.

So what should you actually take?

Not necessarily 120. The right number depends on duration, intensity, and how much work you have put into your gut.

The pattern to notice: the longest events do not use the highest rates. Three reasons. Intensity is lower, so a larger share of energy comes from fat. Absorption capacity degrades over very long durations. And palatability collapses - the practical limit at hour eighteen is what you can bring yourself to swallow, not what your intestine can transport.

The runners who need 100-120 g/h are those racing hard for four to eight hours, where carbohydrate availability is genuinely limiting and the gut is still fresh enough to cope.

Intensity is the hidden variable

Here is the thing most fueling articles skip. Digestion is gated by intensity.

As effort rises, blood is shunted from the gut to working muscle. Above roughly 75-80% of maximum effort, gastric emptying and intestinal absorption drop sharply. Whatever you swallow sits there.

This is why the most common cause of GI distress in an ultra is not the product - it is the first two hours. A runner who goes out too hard compromises their absorption for the rest of the day, then blames the gels.

The corollary is useful: if your stomach is failing, slow down before you change your nutrition. Ten minutes of hiking will often restore your ability to process food when swapping products will not.

Gut training: the protocol

Absorption capacity is trainable. Repeated exposure to high carbohydrate intake during exercise upregulates the transporters, increases gastric emptying rate, and reduces symptoms. It takes weeks, not days.

A twelve-week progression, embedded in your existing long runs:

Rules that make it work:

• Fixed timing beats intuition. Set a 20-minute repeat alarm. Eat on the beep whether you feel like it or not. Waiting until you are hungry means you are already behind.
• Start in the first 30 minutes. Do not wait until hour two. Your gut works best while it is fresh, and topping up a full tank is much easier than refilling an empty one.
• Same products you will race with. A different maltodextrin-to-fructose ratio, a different osmolality, a different flavour - all of these change your experience.
• Practise in race conditions. Heat impairs gut function significantly. A rate that works in October may not work in July.
• Log it. Rate, products, symptoms, effort. Patterns become obvious in writing that are invisible in memory.

Fluid and sodium, briefly

Carbohydrate does not work in isolation - too little fluid and concentrated carbohydrate sits in the stomach, and too much plain water dilutes your sodium.

Broad targets for moderate conditions: 400-600 ml per hour, with 500-800 mg of sodium per litre, adjusted up in heat and for heavy sweaters. Drink to thirst as your primary guide - it is a better regulator than a schedule for most people - and treat the numbers as a sanity check rather than a prescription.

There is more detail in the hydration and sodium guide, including why over-drinking is the more dangerous error.

Choosing products

Gels are the most concentrated and the fastest to take. Check the carbohydrate per gel - it ranges from 20 g to 40 g, which is the difference between two and four per hour. Take them with water unless they are specifically isotonic.

Drink mix is the easiest way to hit a high rate without chewing, and it delivers fluid and carbohydrate together. The trade-off is that your fluid intake and carbohydrate intake become coupled, which is awkward in heat when you need much more of one than the other. Many runners solve this with one flask of concentrated mix and one of plain water.

Chews and bars are more palatable over long durations and slower to absorb. Useful in the middle of long races when you cannot face another gel.

Real food becomes genuinely useful past about ten hours. Boiled potatoes with salt, rice balls, broth, sandwiches. Lower carbohydrate density, but you can actually eat them at hour fourteen, which is the constraint that matters by then.

Aid station food is a supplement, not a plan. Assume it will be different from what the website said.

A practical race plan

For a 6-hour mountain 50K at 75 g/h:

• Pre-race: 1-2 g/kg of carbohydrate three hours before. A small gel 15 minutes before the start.
• Every 20 minutes: 25 g of carbohydrate. That is one gel, or a large mouthful of concentrated drink mix, or a third of a bar.
• Every 15 minutes: drink to thirst, aiming for roughly 500 ml/h.
• Hours 4-6: switch at least half of the carbohydrate to something savoury or solid. Palatability, not physiology.
• Aid stations: refill, take broth if it is there, keep moving.

Write it on a card. At hour five you will not be doing arithmetic.

The mistakes

1. Starting too late. Eating from minute 20, not hour two.
2. Trying a new product on race day. Every gel you take in a race should have been taken in training at least three times.
3. Chasing 120 g/h without training for it. An untrained gut at 120 g/h is a vomiting incident with extra steps.
4. Blaming the fuel for a pacing error. If your stomach fails in an ultra, look at your first two hours before you look at your gels.
5. Ignoring palatability. The perfect fueling plan you cannot stomach at hour twelve is worth nothing. Build in variety and something salty.
6. Only practising fueling on easy long runs. Absorption at easy effort and race effort are different problems.

Fueling is one of the two things I change first when a runner comes to me after a race that fell apart - the other is the first two hours. If you want that worked out for your specific race and gut, that is what coaching covers.