This page turns the 24-hour fueling evidence into numbers you can write on a crew sheet. For the race format itself, start with the 24-hour race guide; for the general question of what ultrarunners eat, see what ultrarunners eat during a race.
The Calorie Math
Only one study has measured intake and expenditure through an actual 24-hour race. Costa and colleagues followed 25 runners who covered 122–208 km. They spent about 55 MJ (≈13,100 kcal) and ate about 20 MJ (≈4,800 kcal), a gap of roughly 8,400 kcal. They averaged 37 ± 24 g of carbohydrate and 378 ± 164 mL of water per hour. Every runner finished in energy deficit, 90% had ketones in their urine afterwards, and only one met the standard carbohydrate benchmark for endurance exercise.
You can check that against distance. The International Society of Sports Nutrition (ISSN) ultramarathon position stand puts the cost of running at 47.8–71.7 kcal per km; its worked example is a 70 kg runner spending about 9,900 kcal over 161 km.
| Line | Number | Basis |
|---|---|---|
| Spent in a measured 24-hour race | ≈55 MJ ≈ 13,100 kcal | Costa 2014, 25 runners, 122–208 km |
| Eaten in the same race | ≈20 MJ ≈ 4,800 kcal | Costa 2014 |
| Energy cost per km | 47.8–71.7 kcal | ISSN position stand |
| Cost of 160 km | 7,600–11,500 kcal | 160 × 47.8 to 160 × 71.7 |
| Eaten at 250 kcal per hour | 6,000 kcal | 24 × 250 |
| Deficit at 160 km | 1,600–5,500 kcal | Cost minus intake |
So the answer to "how many calories does a 24-hour race burn" is roughly 8,000–13,000 kcal. To estimate your own, multiply your target distance in km by 48–72. You cannot eat that much, and you do not need to: a deficit of a few thousand kilocalories is normal. The goal is to keep intake steady enough that you do not slow down for lack of fuel.
How Much Carbohydrate per Hour
Carbohydrate is the number to plan around, because it is what runs out. The ISSN position stand gives a racing range of 150–400 kcal, 30–50 g carbohydrate and 5–10 g protein per hour. We use four tiers built from that range and from the intake studies:
| Tier | Per hour | Evidence |
|---|---|---|
| Floor | At least 30 g carbohydrate and 200 kcal | In a 16-runner Western States study, finishers ate 4.6 kcal/kg/h and non-finishers 2.5, about 320 vs 175 kcal/h at 70 kg (Stuempfle 2011) |
| Default | 50 g | Top of the ISSN 30–50 g/h range; what to plan without deliberate gut training |
| Trained gut, racing for distance | 60–75 g from glucose–fructose products | The ISSN cites 100-mile finishers at 65.8 ± 27 g/h; three elite 100-milers averaged 71 ± 20 g/h and 333 kcal/h (Stellingwerff 2016) |
| Ceiling | 90 g | The ISSN says up to 90 g/h "may be unrealistic" in races over 6 hours; Costa 2019 calls tolerance in ultras questionable |
If you have not trained your gut, plan on 50 g. If you have, using the progression in our gut-training guide, plan on 60–75 g. The answers page on what ultrarunners eat explains where the higher 90 g figure comes from.
Hour-by-Hour Fueling Plan
This is our recommendation, built from the tiers above. Write it on the crew sheet as four bands rather than 24 separate rows.
| Hours | Carbohydrate per hour | Energy per hour | What it looks like | Band total |
|---|---|---|---|---|
| 0–4 | 50 g | 200–250 kcal | Drink mix and gels, a few bites of banana or rice; no caffeine | 200 g, 900 kcal |
| 4–10 | 40–60 g (plan 50) | 250–275 kcal | Add one solid each hour (half a sandwich, potatoes, rice) and 5–10 g protein; still no caffeine | 300 g, 1,560 kcal |
| 10–18 (night) | 40–50 g (plan 45) | 225–250 kcal | Warm savory food such as rice or potatoes in broth; caffeine about 50 mg per hour | 360 g, 1,920 kcal |
| 18–24 | Floor: at least 30 g | About 200 kcal | Whatever stays down: cola, sports drink, gels, soft food | 180 g, 1,200 kcal |
| Whole race | About 43 g average | About 233 kcal average | About 17% more carbohydrate per hour than the measured 24-hour field ate | About 1,040 g, 5,600 kcal |
Scaling up for a trained gut. Run hours 0–10 at 60–75 g per hour and the night at 50–60 g, and keep the 30 g floor at the end. That raises the total to about 1,200–1,400 g of carbohydrate and 6,100–7,100 kcal. Do not plan 90 g per hour for 24 hours.
If you fall behind, go back to the band. Do not double the next hour to catch up; a large bolus is a common way to start vomiting.
Real Food vs Gels
Use both, in that order. Products are the easiest way to reach 50 g per hour early: the elite 100-mile runners in Stellingwerff's study took 93% of their calories from commercial products. Real food earns its place later, for variety and because, as the ISSN notes, runners prefer savory food as races get longer. Our default is products for most of the carbohydrate through hour 10, then a growing share of real food overnight.
Carbohydrate per Portion
Approximate values; check your own labels.
- Gel: 20–25 g per sachet
- Sports drink at 6%: 30 g per 500 mL
- Cola: about 27 g per 250 mL
- Medium banana: about 27 g
- Boiled potatoes: about 20 g per 100 g
- Cooked white rice: about 28 g per 100 g
- Pretzels: about 24 g per 30 g, with sodium
- Half a jam sandwich: about 20 g
A 50 g hour is therefore one gel plus 500 mL of sports drink, or a banana plus 100 g of rice. Test every item, in these portions, on long training days.
Hydration and Electrolytes
Drink to thirst.For most runners that comes to about 400–750 mL per hour; the ISSN gives 450–750 mL per hour. The 24-hour runners in Costa's study drank only 378 mL per hour (9.1 L in total), yet their plasma volume rose by about 10%, so they were not dehydrated.
- Sodium: about 300–600 mg per hour from drink and food by default, and 600–1,000 mg per hour in heat or if you are a known salty sweater.
- Body mass: expect to lose some. Modelling for a 68.8 kg runner over 161 km shows a loss of 1.9–5.0% is needed just to keep body water steady (Hoffman 2018). Gaining weight during the race means you are overdrinking.
- Salt does not cancel overdrinking. The hyponatremia consensus statement identifies drinking beyond losses as the main cause of exercise-associated hyponatremia.
The hydration and sodium guide has the sweat-rate test, the sodium arithmetic and the fluid warning signs.
When Your Stomach Shuts Down
At Western States, 96% of runners reported gastrointestinal symptoms, and 35.6% of non-finishers named them as a reason for dropping out, nausea above all (Stuempfle 2015). Heat, pace, concentrated carbohydrate, large boluses and unfamiliar ingredients all contribute (Costa 2017). The fix is to shrink the dose, not to stop.
- Walk until the nausea eases.
- Drop to the floor of 30 g per hour in small doses: about 7–8 g every 15 minutes, such as 120 mL of a 6% sports drink or 75 mL of cola.
- Switch texture to the option that has worked in training: sips if solids will not go down, bland bites if sweet drinks will not.
- Return to your planned band once two or three small doses have stayed down.
Caffeine Strategy
The ISSN advises aiming caffeine at the later stages of an ultra, when sleep deprivation sets in, and using small repeat doses of 1–2 mg per kg (70–140 mg at 70 kg) rather than repeated 4–6 mg/kg boluses (280–420 mg). It describes about 50 mg per hour overnight as likely to be well tolerated. The ISSN caffeine position stand finds 3–6 mg/kg effective and reports frequent side effects at about 9 mg/kg.
- Hours 0–10: none.
- Hours 10–18: about 50 mg per hour, or 400 mg across the night.
- Hours 18–24: keep to about 50 mg per hour, or take one 1–2 mg/kg dose every two hours if that suits you better.
- Total: roughly 600–800 mg at 70 kg, below the 912 mg the elite 100-milers reported per race (Stellingwerff 2016).
Test the exact products in late training runs, and add coffee, cola, gels and tablets to the total. Skip caffeine if it causes palpitations, tremor or worse nausea, and check with a clinician first if you are pregnant, have a heart condition, or take interacting medication.
Night Nutrition
Hours 10–18 are where most 24-hour plans fail, because sweet products become hard to face. Plan the night band in advance:
- Warm and savory: rice in broth, noodle soup, or salted potatoes, which also carry sodium.
- 45 g per hour: for example 100 g of rice in broth plus 150 mL of cola, or 200 g of salted potatoes plus a few sips of sports drink.
- Protein: 5–10 g per hour from milk, yogurt, or a sandwich with ham or cheese, if it sits well.
- Caffeine: about 50 mg per hour, as above.
Common Mistakes
- Starting low: falling below 30 g and 200 kcal per hour in the first hours is how a deficit becomes a slowdown.
- Chasing missed calories: doubling the next hour causes more trouble than the missed hour did.
- Planning 90 g per hour untrained: use 50 g unless you have trained your gut to 60–75 g.
- One texture or flavor: carry sweet, savory, liquid and solid options you have tested.
- Forcing fluid or salt: drink to thirst; weight gain means overdrinking, and salt does not make that safe.
- Caffeine too early: save it for hour 10 onward.
Sources
- Costa, R.J. et al. (2014) — Perturbed energy balance and hydration status in ultra-endurance runners during a 24 h ultra-marathon. British Journal of Nutrition, 112(3), 428–437.
- Tiller, N.B. et al. (2019) — International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing. Journal of the International Society of Sports Nutrition, 16(1), 50.
- Stuempfle, K.J. et al. (2011) — Race diet of finishers and non-finishers in a 100 mile (161 km) mountain footrace. Journal of the American College of Nutrition, 30(6), 529–535.
- Stellingwerff, T. (2016) — Competition Nutrition Practices of Elite Ultramarathon Runners. International Journal of Sport Nutrition and Exercise Metabolism, 26(1), 93–99.
- Costa, R.J.S. et al. (2019) — Nutrition for Ultramarathon Running: Trail, Track, and Road. International Journal of Sport Nutrition and Exercise Metabolism, 29(2), 130–140.
- Stuempfle, K.J. & Hoffman, M.D. (2015) — Gastrointestinal distress is common during a 161-km ultramarathon. Journal of Sports Sciences, 33(17), 1814–1821.
- Costa, R.J.S. et al. (2017) — Systematic review: exercise-induced gastrointestinal syndrome-implications for health and intestinal disease. Alimentary Pharmacology & Therapeutics, 46(3), 246–265.
- Hew-Butler, T. et al. (2015) — Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 25(4), 303–320.
- Hoffman, M.D. et al. (2018) — Considerations in the Use of Body Mass Change to Estimate Change in Hydration Status During a 161-Kilometer Ultramarathon Running Competition. Sports Medicine, 48(2), 243–250.
- Guest, N.S. et al. (2021) — International society of sports nutrition position stand: caffeine and exercise performance. Journal of the International Society of Sports Nutrition, 18(1), 1.
Frequently Asked Questions
How many calories do you burn in a 24-hour race?
Roughly 8,000–13,000 kcal. The only 24-hour race study measured about 55 MJ (≈13,100 kcal) spent by 25 runners covering 122–208 km, against about 20 MJ (≈4,800 kcal) eaten. The ISSN position stand puts the energy cost of running at 47.8–71.7 kcal per km, which is 7,600–11,500 kcal over 160 km. Eating 250 kcal an hour supplies 6,000 kcal, so a deficit of 1,600–5,500 kcal is normal and does not need to be closed.
How many grams of carbohydrate per hour should you eat in a 24-hour race?
Plan on 50 g per hour as the default. Never drop below a floor of 30 g and 200 kcal per hour: in a Western States study, finishers ate 4.6 kcal/kg/h and non-finishers 2.5. With a trained gut and glucose–fructose products, 60–75 g per hour is realistic; three elite 100-mile runners averaged 71 g per hour. Treat 90 g per hour as a ceiling, which the ISSN calls possibly unrealistic beyond six hours.
Should I eat gels or real food during a 24-hour race?
Both. Drink mix and gels are the easiest way to hit 50 g per hour in the first hours; elite 100-mile runners took 93% of their calories from commercial products. From hour 4, add one solid portion and 5–10 g of protein each hour, and switch to warm savory food overnight, when runners tend to prefer savory food.
What do you do when nothing sounds good during an ultra?
Walk, and drop to the floor of 30 g carbohydrate per hour taken in small doses: about 7–8 g every 15 minutes, such as 120 mL of a 6% sports drink or 75 mL of cola. Go back to your planned band once it stays down. Repeated vomiting or being unable to keep fluid down means race medical staff, not more food.
How much should I drink per hour in a 24-hour race?
Drink to thirst. For most runners that comes to about 400–750 mL per hour; the ISSN position stand gives 450–750 mL per hour, and runners in the only 24-hour intake study averaged 378 mL per hour. Take about 300–600 mg of sodium per hour, or 600–1,000 mg in heat or if you are a salty sweater. Gaining weight during the race means you are overdrinking, and salt does not make that safe.