Why Ultrarunners Eat So Much Sugar – and Why Your Long Run Should Too

Ultrarunners eat sugar because carbohydrate is your muscles' fastest fuel, and you store only enough for about 90–120 minutes of steady running. Eating carbs on the move keeps blood sugar up and holds your pace; research supports up to 90 grams per hour for long efforts. If your long run lasts more than about 90 minutes, eat 30–60 grams of carbohydrate every hour – it makes the run easier and trains your gut for race day.

Key takeaways

• Your body stores only around 500 grams of carbohydrate as glycogen, mostly in muscle. Long efforts drain it.
• Taking in carbs during exercise delays fatigue. This has been shown repeatedly since the 1980s.
• Guidelines: about 30–60 g/hour for 1–2.5 hours, up to 90 g/hour for 2.5+ hours using glucose plus fructose.
• Your gut is trainable. Two weeks of practice can noticeably cut stomach problems.
• "Sugar during exercise" and "sugar on the sofa" are different things. Context matters.

Why carbohydrate is the long-run fuel

Your body runs on a blend of fat and carbohydrate. Fat stores are huge – even a lean runner carries enough for days of walking. But fat is slow to break down. As your pace rises toward marathon effort, your muscles lean more and more on carbohydrate, because it releases energy faster per litre of oxygen.

The catch is storage. Most adults hold roughly 400–500 grams of glycogen in muscle and about 80–100 grams in the liver. That's somewhere around 2,000 calories of fast fuel. At a steady long-run pace, you can burn through a large share of it in two hours.

This was first shown directly in the 1960s. Swedish researchers including Jonas Bergström and Bengt Saltin used muscle biopsies in a 1967 study to show that endurance time tracked closely with how much glycogen was in the muscle at the start. More glycogen meant longer before exhaustion.

When glycogen and blood sugar run low, you know the feeling: heavy legs, foggy head, a pace that collapses. Marathoners call it "the wall." Ultrarunners call it "a bonk" – and they plan their entire race around avoiding it.

What the research says about eating while you run

Carbs during exercise delay fatigue

A classic 1986 study by Edward Coyle and colleagues had trained cyclists ride at about 70% of their VO2 max until exhaustion. Without carbohydrate they lasted around three hours; with carbohydrate they kept going for roughly an hour longer. Since then, dozens of trials in running and cycling have pointed the same way.

Even a mouth rinse helps on shorter efforts

In 2004, James Carter, Asker Jeukendrup and David Jones found that simply rinsing a carbohydrate drink around the mouth – and spitting it out – improved a one-hour cycling time trial. The likely reason: receptors in the mouth signal to the brain that energy is on the way. For runs under an hour, that's about all you need.

The modern numbers

In a 2014 review in Sports Medicine, Asker Jeukendrup set out duration-based guidelines that most sports scientists still use:

• Under about 45 minutes: no carbohydrate needed.
• 45–75 minutes: a small amount or a mouth rinse.
• 1–2 hours: up to 30 g/hour.
• 2–3 hours: up to 60 g/hour.
• More than 2.5 hours: up to 90 g/hour, using "multiple transportable carbohydrates."

That last phrase matters. Glucose is absorbed through a gut transporter called SGLT1, which tops out around 60 grams per hour. Fructose uses a different transporter. Mix them – roughly 2 parts glucose (or maltodextrin) to 1 part fructose – and you can absorb and burn more, with less sloshing in your stomach.

Why ultrarunners take it so far

Ultras last 6, 12, even 30 hours. Pace is lower, so fat does more of the work, but carbohydrate still powers every climb and keeps the brain sharp. Surveys of 100-mile finishers find that most of them report some gut problems during the race, so ultrarunners have become experts in eating on the move: gels, sports drink, cola, boiled potatoes, rice balls, bananas, dates and sweets at aid stations.

Some ultrarunners famously go plant-based. Scott Jurek, who wrote about his record Appalachian Trail run in North, built his diet around whole plant foods. Even so, carbohydrate-heavy fuel remained at the heart of how he kept moving.

"But isn't sugar bad for you?"

For everyday eating, health agencies advise keeping added sugars low – the World Health Organization recommends less than 10% of daily energy. That advice is about sedentary snacking.

During a two-hour run, your muscles are pulling glucose out of your blood at a high rate. Sugar taken then is burned almost immediately as fuel. It doesn't behave like a soda at your desk.

What about low-carb diets? In a 2017 study in The Journal of Physiology, Louise Burke and colleagues put elite race walkers on a very low-carb, high-fat diet. Their bodies got much better at burning fat, but they became less economical, and their race performance did not improve. Groups on higher-carb diets did improve. For hard, fast efforts, carbohydrate still wins.

If you have diabetes or another metabolic condition, talk to your doctor or a registered sports dietitian before changing how you fuel.

Train your gut like a muscle

Your digestive system adapts to what you ask of it. In a 2017 study, Ricardo Costa and colleagues had endurance runners practise eating carbohydrate during runs for two weeks. Gut symptoms dropped by around 60%, and their performance in a follow-up distance test improved by about 4–5%. The group that practised with a placebo got no such benefit.

That's why fuelling belongs in training, not just on race day. For more on what causes stomach trouble mid-run, read our guide to runner's gut and how to fix it.

A 6-week gut-training plan

• Weeks 1–2: 30 g/hour on your long run. Start at 30 minutes, then take something every 20–30 minutes.
• Weeks 3–4: 45 g/hour. Add a second type of fuel, such as a gel plus a banana.
• Weeks 5–6: 60 g/hour. Use a glucose–fructose product for at least half.
• If you're racing longer than 2.5 hours: build gradually toward 75–90 g/hour over the following weeks.

If you get cramping, nausea or urgent bathroom stops, drop back one step for a week before progressing.

What 30 grams of carbohydrate looks like

Check labels, but here are typical amounts:

• One standard energy gel: 20–30 g
• One medium banana: about 27 g
• Two Medjool dates: about 36 g
• 500 ml of a typical 6% sports drink: 30 g
• One tablespoon of honey: about 17 g
• A small boiled potato with salt (150 g): about 25 g

Drink to thirst – often 400–800 ml per hour depending on heat and sweat rate – and add sodium on hot or very long days.

Sample 2-hour long run

At an easy 6:15/km (10:03/mile) pace, about 19 km (12 miles):

• 0:30 – one gel (25 g) with a few sips of water
• 0:50 – half a banana (14 g)
• 1:10 – one gel (25 g)
• 1:30 – two dates (36 g)
• 1:50 – a few mouthfuls of sports drink (10 g)

That totals about 110 grams across the run – roughly 55 g/hour. It's the same rhythm you'll use on race day.

Fuelling during the run is only one piece. The days before matter too: see our guides to modern carb loading and the night-before-race meal.

Find your next long race on Runtra

Ready to put your fuelling plan to work? Browse marathons for a 3–5-hour test, step up to ultramarathons, or search for trail ultras near you.

FAQ

How many carbs per hour should I eat on a long run?

For runs of 1–2 hours, aim for about 30 grams per hour. For 2–3 hours, work up to 60 grams per hour. For races over 2.5 hours, trained runners can go up to 90 grams per hour using a glucose–fructose mix.

Do I need gels for a 10K or a half marathon?

For a 10K, no – your glycogen stores are enough, though a sports-drink mouth rinse may help slightly. For a half marathon that takes you more than about 75–90 minutes, one or two gels can help you hold pace in the final kilometres.

Is eating sugar on runs bad if I'm trying to lose weight?

Fuelling a long run doesn't undo weight loss, because the carbohydrate is burned during the run. Under-fuelling often backfires: you run slower, recover worse and tend to overeat later. Keep added sugar low in daily meals, not during training.

Can I fuel with real food instead of gels?

Yes. Bananas, dates, honey, boiled potatoes and rice balls all work well, especially at the slower pace of ultras. Gels are simply easier to carry and digest at faster paces. Practise with whatever you plan to use on race day.

Why does my stomach hurt when I take gels?

The usual causes are taking too much at once, not drinking enough water with it, or a gut that isn't used to fuel during exercise. Space intake every 20–30 minutes and train your gut gradually. If symptoms are severe or include blood, see a doctor.