Mental Strength vs Fuelling: What Really Limits You in Ultras?

Ultra running is often described as a test of mental toughness, but is that really what limits performance?

When things fall apart late in an ultra, it’s easy to blame mindset. The negative thoughts creep in, motivation drops, and suddenly it feels like a mental battle just to keep moving. But what if that “mental fatigue” isn’t purely mental at all?

In reality, the line between mental strength and fuelling is far more connected than most runners realise. From energy availability to decision-making and pacing, your physiology plays a powerful role in how strong, or how flat, you feel.

This article breaks down what limits you in ultras and why the strongest athletes aren’t just mentally tough, they’re fuelling smart.

Ultras Are Hard, Mentally & Physically
What is “Mental Toughness”
When “Mental Fatigue” Is Actually Low Fuel
Fuelling Is a Mental Skill
What Really Limits You Late in an Ultra
What To Aim For: Carbohydrate, Fluids & Electrolytes

Ultras Are Hard, Mentally and Physically

Ultras are tough. Long hours, changing conditions and the build-up of fatigue mean you’re constantly managing discomfort. Mental strength plays a huge role, staying focused, problem-solving, and continuing to move forward when things get hard. But that mental load doesn’t exist in isolation. It’s influenced by how well you’re fuelling, hydrating and pacing from the very beginning.

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Fatigue in ultra-endurance events is multifactorial, with both central (brain) and peripheral (muscle) components. As energy availability drops, particularly carbohydrate, the brain increases perceived effort and reduces motor drive¹,². This means what feels like a “mental battle” is often the result of physiological strain, not just willpower.

What is “Mental Toughness”

Mental toughness in ultras isn’t just about pushing through pain. It’s about making good decisions over and over again. It’s sticking to your pacing plan early when you feel good, eating when you don’t feel like it, and staying consistent when things start to unravel. It’s less about big heroic moments, and more about small, disciplined actions repeated across hours.

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Cognitive function, attention, decision-making, and emotional regulation, declines with fatigue, dehydration, and low energy availability. Athletes who maintain fuelling strategies are better able to preserve executive function, which directly impacts pacing, nutrition choices, and overall race execution³.

When “Mental Fatigue” Is Actually Low Fuel

That sudden drop in motivation, irritability, or “I just can’t do this” feeling often gets labelled as a mental low. But more often than not, it’s a fuelling issue. When intake falls behind, everything feels harder, physically and mentally. Recognising this early and responding with fuel can shift how you feel surprisingly quickly.

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Low carbohydrate availability reduces blood glucose levels, which the brain relies on as a primary fuel source. This can impair neurotransmitter function, increase perceived exertion and reduce motivation¹,². Studies consistently show that carbohydrate intake during endurance exercise helps maintain both physical performance and cognitive output²,³.

Fuelling Is a Mental Skill

Fuelling in an ultra isn’t passive, it requires intention. Eating when you’re not hungry, drinking in cold conditions, or continuing to take in carbs when flavours become unappealing all require mental effort. The athletes who do this well aren’t just physically prepared, they’ve trained themselves to follow through on their plan regardless of how they feel.

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Gut tolerance and carbohydrate oxidation rates are trainable. Repeated exposure to higher carbohydrate intakes during training can improve gastric emptying and absorption, allowing athletes to sustain higher energy availability during racing³. However, adherence to these strategies under fatigue relies heavily on behavioural consistency and cognitive control.

What Really Limits You Late in an Ultra

Late in an ultra, performance rarely falls apart because of one single factor. It’s usually a combination, small fuelling errors, slight dehydration, pacing mismatches, all adding up. This is where mental strength matters most, not to override the body, but to recognise what’s happening and respond early. The goal isn’t to push through anything, it’s to stay on top of it.

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Progressive glycogen depletion, cumulative energy deficit, and fluid-electrolyte imbalances contribute to reduced muscle function and increased central fatigue. As these stressors compound, the brain acts as a regulator, increasing perceived effort to protect the body¹⁻³. Athletes who maintain energy intake and hydration are better able to delay this protective response and sustain performance.

What To Aim For: Carbohydrate, Fluids & Electrolytes

When it comes to ultras, consistency beats perfection. Having a simple target to work towards can help take the guesswork out of fuelling, especially when decision-making gets harder late in the race.

Carbohydrate → Aim for 60-120g of carbohydrate per hour¹⁻⁴

Start at the lower end if you’re newer to fuelling.
Work up to higher intakes with gut training.
Use a mix of sources (gels, drink mix, real food) to improve tolerance.

Fluids → 600mL to 1.0L per hour (and up to around 1.2L/hr in hot conditions)⁵

Hot/humid, aim for the higher end.
Cooler conditions, you may need less.
Avoid both over- and under-drinking. Drink to a plan, not just thirst.

Sodium → Sodium needs vary with sweat rate, sweat sodium and environmental conditions. Most athletes benefit from around 300-800mg per hour, while hot conditions or heavy sweaters may require 1000-1500mg/hr⁶⁻⁸.

Sodium can come from drinks, capsules, or foods.
Replacing sodium alongside fluids helps maintain hydration and plasma sodium, but individual strategies are recommended⁹,¹⁰.

Ash Miller
Dietitian and Nutritionist (Masters)
Bachelor of Physical and Health Education
Instagram: @ashthomo_nutrition

References:

Jeukendrup AE. Carbohydrate intake during exercise and performance. Nutrition. 2004;20(7–8):669–677.
Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and athletic performance. J Acad Nutr Diet. 2016;116(3):501–528.
Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011;29(sup1):S17–S27.
Viribay A, Arribalzaga S, Mielgo-Ayuso J. Effects of ingesting 120 g/h of carbohydrates on exercise performance and neuromuscular fatigue during a mountain marathon: a randomized trial. Nutrients. 2020;12(5):1367.
McCartney D, Desbrow B, Irwin C. The effect of fluid intake following dehydration on subsequent athletic and cognitive performance: a systematic review and meta‑analysis. Sports Med Open. 2017;3(1):13.
McCubbin AJ. Modelling sodium requirements of athletes across a variety of exercise scenarios: identifying when to test and target, or season to taste. Eur J Sport Sci. 2023;23(6):992–1000.
McCubbin AJ. Sodium intake for athletes before, during and after exercise: review and recommendations. Perform Nutr. 2025;1:11.
Baker LB, Cotter JD, Cheuvront SN, Kenefick RW. Variation in sweat sodium concentration. Exp Physiol. 2022;107(7):675–689.
Baker LB, Dougherty KA, Kuklski TR, Chow M, Kenney WL. Sweat rate and sweat sodium: variability and testing. Sports Med. 2017;47 Suppl 1:79–98.
Australian Institute of Sport. Sports Supplement Framework: Sports Drinks & Electrolyte Supplements. AIS; updated guideline.

Disclaimer:

The content in this blog is for general information only and is not a substitute for professional medical advice, diagnosis, or treatment. Always speak with your doctor or allied health team before changing your diet, exercise, or taking supplements, especially if you have a health condition or take medication. Please use this information as a guide only. Aid Station doesn't take responsibility for individual outcomes.