Nutrition series - Part 1
How Many Carbs Do You Actually Need for a Marathon or Ultramarathon?
Endurance athletes hear numbers like 60, 90, and 120 grams per hour. But a three-hour marathon and a twenty-hour 100-miler are very different efforts. Carbohydrate intake should be considered alongside how long and how hard you expect to run, rather than distance alone.
What does the research say? Controlled endurance studies consistently show that carbohydrate can improve performance during prolonged exercise, especially when racing hard for several hours. Research in ultramarathon runners shows that longer events often use lower hourly carbohydrate intakes because intensity, food preferences, and digestive tolerance change over time. The ranges in this guide are practical, research-informed starting points, not a universal prescription.
First: Think in Hours, Not Kilometres
Your body does not know what distance is printed on your bib. It knows how hard and how long you have been working, how much carbohydrate is available, and what your stomach can tolerate. Start with your target finishing time.
Carbohydrate becomes particularly valuable during harder endurance exercise lasting roughly 1-4 hours. Beyond four hours the benefit remains, but intensity generally decreases and fat contributes more energy. Longer does not automatically mean more carbohydrate per hour.
A Simple Carb Calculator
| Target race time | Practical starting range |
|---|---|
| Under 1.5 h | 30-60 g/h |
| 1.5-4 h | 60-90 g/h |
| 4-8 h | 50-70 g/h |
| 8-12 h | 40-60 g/h |
| 12-24 h | 30-50 g/h |
These are practical research-informed starting ranges, not proven cut-off points.
Now Adjust for Effort
- Easy, all-day effort: use the lower end of the range.
- Steady race effort: use the middle of the range.
- Hard race effort: use the upper end of the range.
The harder you run, the more heavily your body relies on carbohydrate. As a race becomes extremely long and pace decreases, fat oxidation contributes more energy.
Examples
- 3-hour marathon: 60-90 g/h. A hard effort might target 80 g/h, or 40 g every 30 minutes.
- 4.5-hour marathon: 55-65 g/h. At 60 g/h, total planned carbohydrate is 270 g.
- Fast 50 km: 65-70 g/h, or roughly 35 g every 30 minutes.
- 100 km in 10 hours: around 50 g/h, or 25 g every 30 minutes.
- 20-hour 100-miler: 35-50 g/h. At 45 g/h, that is 22.5 g every 30 minutes.
Total carbohydrate increases with duration. Carbohydrate per hour does not necessarily need to.
What Do Real 24-Hour Runners Eat?
At the 2019 24-Hour World Championships, researchers followed elite runners covering 193-272 km on a flat circuit. The athletes averaged 62.2 g carbohydrate per hour, but individual intake ranged from approximately 14 to 105 g/h. Despite substantial food intake, runners still finished with a large energy deficit.
You are not trying to replace every calorie you burn. During very long races, completely matching energy expenditure is usually unrealistic. The goal is to provide enough carbohydrate and energy to support performance without overwhelming the digestive system.
Why Do Ultra Recommendations Sometimes Say Only 30-50 g/h?
The International Society of Sports Nutrition ultramarathon position stand recommends approximately 30-50 g carbohydrate per hour, alongside roughly 150-400 kcal per hour for single-stage ultramarathons. It also emphasizes individual tolerance, food variety, and the increasing preference for savoury foods as races become longer.
A 15-24 hour ultramarathon is a very different metabolic and digestive problem from racing hard for three hours.
Does That Mean 90 g/h Is Wrong?
No. It means context matters. A meta-analysis covering 96 studies found carbohydrate ingestion improves prolonged endurance performance. Intakes around 80-100 g/h produced a larger effect than 60-80 g/h in the analysed studies, while intakes above 100 g/h were not clearly superior.
Much of that evidence comes from shorter, higher-intensity, frequently cycling-based laboratory exercise. The performance effect became smaller once exercise exceeded roughly four hours. So 90 g/h can make excellent sense for one athlete and be unnecessary for another.
Calculate Your Own Plan
- Estimate your finishing time. For example: 100 km in 11 hours.
- Pick the duration range. Eleven hours suggests 40-60 g/h.
- Choose according to effort. Easy: 40-45 g/h; steady: around 50 g/h; hard: 55-60 g/h.
- Calculate total carbohydrate. At 50 g/h for 11 hours, plan 550 g.
- Break it into manageable portions. 50 g/h is 25 g every 30 minutes.
Now you have something you can actually build a race plan around.
Your Stomach Gets a Vote
No calculator can predict how much carbohydrate you can comfortably absorb while running. Test your planned intake during long runs, increasing gradually when appropriate. Do not make race morning the first meeting between your stomach and a new fueling strategy.
The Bottom Line
Start with three questions: How many hours will I be running? How hard will I be running? How much carbohydrate can my gut comfortably tolerate?
Carbohydrate per hour x target race hours = planned race carbohydrate.
Harder marathon and short-ultra racing commonly moves toward 60-90 g/h. As races extend toward 8, 12, 20, or 24 hours and intensity decreases, 30-60 g/h is often a reasonable starting point, while some elite athletes successfully consume more. The goal is not to win a competition for most gels consumed. The goal is to keep running.
Research Behind This Guide
- Bourdas et al. (2021), a meta-analysis of carbohydrate solution intake during prolonged exercise.
- Tiller et al. (2019), the ISSN position stand for single-stage ultramarathon nutrition.
- Lavoue et al. (2020), analysis of food and fluid intake at the 24-hour World Championship.
- Knechtle et al. (2026), a narrative review of 24-hour ultramarathon running.
Important: the duration-and-effort ranges are a practical synthesis, not a clinical or performance-prediction formula. Training status, temperature, body size, pace, previous nutrition, gastrointestinal tolerance, and race conditions can change what works best.