A Practical Guide for Endurance Runners
Ultrarunning pushes the body into territory that most sports never reach. Whether it’s a 50km trail race or a 100km ultramarathon, performance depends on more than fitness alone. Nutrition becomes one of the most important factors determining whether you finish strong or struggle through the final hours.
Many experienced runners say that ultras are eating competitions with some running in between — and there is truth in that. When the body is working for 10, 15, or even 20 hours, fueling strategy becomes essential.
This guide explains the principles behind effective ultramarathon fueling and how to avoid the common mistakes that derail many races.
Why Fueling Matters in Ultras
During endurance exercise, the body primarily relies on carbohydrates and fat for energy. The challenge is that carbohydrate stores in the body are limited.
Muscle glycogen stores typically provide around 1,500–2,000 calories, which can be depleted after approximately 90–120 minutes of sustained exercise.
In a marathon this is challenging.
In a 100km ultramarathon, it’s critical.
Without adequate fueling, runners experience what is often called “bonking” or “hitting the wall”, where blood glucose drops and energy levels collapse.
Proper fueling keeps:
- blood glucose stable
- muscles working efficiently
- cognitive function sharp
- fatigue manageable
The Golden Rule of Ultramarathon Nutrition
One of the most widely accepted guidelines is:
Consume 60–90 grams of carbohydrate per hour.
This can come from multiple sources such as:
- energy gels
- sports drinks
- energy bars
- real food
Elite athletes often aim toward the higher end of the range, but this requires training the gut to tolerate larger carbohydrate intake.
Start Fueling Early
One of the most common mistakes in ultrarunning is waiting until you feel hungry to eat.
By that point, energy levels may already be dropping.
Instead, aim to start fueling within the first 30–45 minutes of the race.
A good rhythm is to eat small amounts every 20–30 minutes, rather than large amounts infrequently.
This keeps energy supply steady and reduces the risk of stomach upset.
Gels vs Real Food
Ultrarunners often experiment with different fuel sources. What works for one athlete may not work for another.
Energy Gels
Energy gels are popular because they provide:
- rapid carbohydrate absorption
- consistent dosing
- easy portability
However, relying solely on gels for many hours can lead to flavour fatigue and stomach discomfort.
Real Food
Many ultrarunners prefer to mix in real food options such as:
- bananas
- boiled potatoes
- sandwiches
- rice cakes
- flapjacks
Real food provides variety and can feel more satisfying during long races.
A combination of gels and real food is often the most practical approach.
Hydration Strategy
Hydration is equally important during ultrarunning.
Fluid requirements vary depending on conditions, but many runners aim for approximately:
400–750 ml of fluid per hour.
Sweat rates differ significantly between individuals, so this should ideally be tested during training.
Electrolytes Matter
When sweating for many hours, runners lose important electrolytes such as sodium, potassium, and magnesium.
Replacing these helps prevent issues such as:
- muscle cramps
- nausea
- fatigue
- hyponatremia (dangerously low sodium levels)
Electrolytes can be consumed through:
- sports drinks
- electrolyte tablets
- salty foods such as crisps or salty snacks.
Training Your Gut
Fueling for ultras is not something to experiment with on race day.
Just like muscles adapt to training stress, the digestive system can adapt to higher carbohydrate intake.
During long training runs, practice:
- the same gels or foods you plan to use in the race
- the same hydration strategy
- the same fueling intervals.
This allows you to identify problems early and refine your approach.
Common Fueling Mistakes
1. Eating Too Little
Many runners underestimate how much energy they need. Under-fueling leads to progressive fatigue.
2. Eating Too Much Too Quickly
Large meals during a race can overwhelm the digestive system and cause nausea.
Small, frequent fueling is usually more effective.
3. Ignoring Electrolytes
Drinking large amounts of plain water without electrolytes can dilute blood sodium levels.
Balanced electrolyte intake is essential.
4. Trying New Foods on Race Day
Race day is not the time for experimentation. Stick to what you have practiced during training.
Example Fueling Plan for a 100km Race
Every runner will adapt their plan slightly, but a simple strategy might look like this:
Every hour:
- 1 energy gel (25g carbohydrate)
- 500 ml sports drink (30g carbohydrate)
- small snack such as banana or flapjack (20–30g carbohydrate)
This provides approximately 75–90g of carbohydrate per hour, which is within the optimal fueling range.
The Psychological Side of Fueling
During ultras, appetite often disappears as fatigue builds.
Yet fueling remains critical.
Many experienced ultrarunners follow the rule:
“Eat before you feel like you need to.”
Even when you don’t feel hungry, small amounts of food can keep energy levels stable and prevent a sudden crash.
Final Thoughts
Ultrarunning rewards preparation and patience. Fitness alone is rarely enough to carry a runner through a 100km race.
The athletes who perform best are often those who have tested their fueling strategy extensively during training.
Successful ultramarathon nutrition comes down to:
- consistent carbohydrate intake
- steady hydration
- electrolyte balance
- practice during training.
Because in an ultramarathon, the difference between finishing strong and struggling through the final miles is often decided not by the legs — but by what you put into your body.