If you've ever hit a wall at mile 40 of a hundred-miler, you already know something is wrong with the standard fueling playbook. Sports medicine professionals are increasingly vocal about a pattern they see repeated across ultra-trail events: athletes who rely almost entirely on gels, chews, and sugary sports drinks, then pay for it in the second half of the race with nausea, cramping, and a performance collapse that no amount of mental toughness can fully compensate for.
The problem isn't carbohydrates. Carbohydrates remain essential for endurance performance. The problem is the near-total exclusion of everything else.
Why Sugar-Only Fueling Becomes a Liability After Hour Four
For efforts under two hours, simple carbohydrates are well-matched to what the body demands. You're working at a pace that favors glycolytic metabolism, your gut is still cooperative, and the volume of food you need is manageable. Ultra-trail running breaks all three of those assumptions.
Events lasting six, twelve, or twenty-four hours involve sustained low-to-moderate intensity punctuated by high-effort climbs. The physiological demand shifts substantially. Your body's stored glycogen, roughly 400 to 500 grams when fully topped off, can only carry you so far. Attempting to replace every calorie with simple sugars at the intake rates required for long efforts creates a predictable cascade of problems.
First, the gastrointestinal system. At race intensity, blood is redirected away from the gut toward working muscles. This significantly reduces the absorptive capacity of your small intestine. Flooding a compromised system with high concentrations of fructose and glucose, especially from multiple gel formats consumed back-to-back, is a well-documented trigger for bloating, cramping, and vomiting. Research on ultramarathon runners has consistently found GI distress to be the primary reason athletes slow down or drop out, with some studies placing it as a limiting factor in more than 90% of DNFs at events beyond 100 kilometers.
Second, the blood sugar dynamics. Simple sugars spike insulin, which pulls glucose into cells rapidly. Without slower-digesting food in the mix, you're managing a constant cycle of spikes and troughs. Late in a race, when stress hormones are elevated and your ability to monitor your own cognitive state is diminished, a blood sugar trough hits harder and recovers more slowly.
The Shift Toward Mixed-Substrate Fueling
Sports nutrition science has moved away from the idea that carbohydrate is the only fuel worth managing during endurance events. Fat adaptation strategies, which involve training the body to oxidize fat more efficiently at race pace, have accumulated meaningful evidence over the past decade, particularly for efforts in the ultra-endurance range.
Fat adaptation doesn't mean going carbohydrate-free on race day. It means arriving at the start line with a metabolic system that can draw from both fat stores and ingested carbohydrates, reducing your dependency on any single source. Athletes with well-developed fat oxidation capacity can sustain aerobic output at lower carbohydrate intake rates, giving the gut a better chance to manage what it's receiving. The research on very low-carbohydrate and ketogenic approaches for endurance athletes remains nuanced. For a broader picture of how low-carb metabolic strategies are being studied across health contexts, Keto and Cancer: New Studies Cut Through the Hype offers useful perspective on what the emerging evidence does and doesn't show.
Mixed-substrate fueling in practice means your race nutrition deliberately includes sources that digest at different rates, draw on different metabolic pathways, and support gut function through varied texture and composition. It's not radical. It's closer to how humans have always eaten during prolonged physical effort.
What a Practical Race-Day Fueling Plan Looks Like
The goal is building a nutrition strategy that front-loads fast-absorbing carbohydrates when you need them most, specifically at the start and during high-intensity sections, while integrating slower whole-food sources during sustained flat or downhill segments when gut blood flow is more adequate.
Here's how that breaks down practically:
- Rapid carbohydrates (gels, chews, sports drink): Best used during climbs, in the early hours, and when you need a quick energy correction. Limit yourself to one gel per 30 to 45 minutes maximum and avoid stacking multiple high-fructose products simultaneously.
- Whole-food sources: Rice balls, boiled potatoes with salt, banana halves, oat-based bars, and nut butter sachets digest more slowly, provide a broader nutrient profile, and are far easier on the gut at low-to-moderate intensity. Aid station food at most major ultras exists for a reason. Use it.
- Protein: For events exceeding six hours, a modest protein intake of 10 to 15 grams per hour helps attenuate muscle breakdown and, according to some research, may reduce perceived fatigue in the later stages. Savory broth, eggs, and small amounts of meat or cheese are all appropriate aid station choices.
- Electrolytes: Sodium is the critical variable. Hyponatremia, low blood sodium caused by drinking too much plain water while sweating out electrolytes, is a genuine medical emergency in ultra events. Sodium intake targets of 500 to 1000 milligrams per hour are commonly cited for hot, high-sweat conditions. Don't rely on sugary sports drinks alone to cover this.
- Fat sources: Small amounts of fat slow gastric emptying and extend the energy availability of a meal or snack. Nuts, cheese, or nut butter consumed alongside carbohydrates at aid stations can help stabilize blood sugar between high-effort sections.
The Amateur Athlete Problem
The all-sugar fueling pattern is especially pronounced among amateur and recreational ultra runners, and the reasons aren't hard to identify. Gel and sugar-based nutrition products are heavily marketed, easy to carry, and positioned at every race expo and sports retailer as the obvious solution to endurance fueling. The global sports nutrition market exceeded $50 billion in 2023, with gels and chews representing a significant and fast-growing segment.
Amateur athletes often build their race-day strategy around what they see elite runners carry, without accounting for two important differences. First, elite athletes have typically spent years adapting their gut to high intake rates through deliberate training. Second, elite athletes in many cases work with sports dietitians who design protocols far more sophisticated than "take a gel every 45 minutes."
If you're self-coaching your nutrition, the same principle applies as in self-coaching your training. Generic protocols carry real limitations. Understanding your own metabolic response, your sweat rate, your gut tolerance at race intensity, and your energy expenditure at different grades requires systematic data collection, not guesswork. The approach mirrors what applies to other fitness tracking contexts, as explored in How to Actually Use Your Fitness Tracker to Get Stronger. Tools and data are only useful if you know what you're trying to measure and why.
Training the Gut Is as Important as Training the Legs
One point that sports medicine practitioners consistently raise is that race-day nutrition is not a standalone decision. Your gut's ability to absorb carbohydrates, tolerate fats and proteins at exercise intensity, and handle volume is trainable, and it requires deliberate practice across your long training runs.
Showing up to a 100-mile race having only ever trained with water and the occasional gel is functionally equivalent to showing up undertrained in your legs. Your gut hasn't been exposed to the volume or variety it will need to handle. Athletes who practice their full race-day nutrition strategy, including whole foods, electrolyte products, and calorie targets, during long training efforts arrive at the start line with a functional system, not a guessing game.
This also applies to supplement use. If you're considering adding anything to your protocol, such as electrolyte capsules, amino acid products, or gut-health supplements, understanding what you're actually taking matters. How to Pick a Supplement That Actually Does What It Claims outlines a practical framework for evaluating evidence before spending money on products with questionable benefit.
The Broader Performance Picture
Ultra-trail running doesn't exist in a metabolic vacuum. Your overall nutritional status, your body composition, your strength foundation, and your recovery capacity all shape how well your fueling strategy works on race day.
Strength training, for instance, directly influences how efficiently your muscles use available fuel. Stronger legs produce more power per stride at a lower metabolic cost, which matters enormously when you're 70 miles in and every calorie counts. The research on resistance training and longevity, covered in Lifting Is No Longer Just About Looks. It's About Living Longer., underscores that the adaptations from strength work extend well beyond aesthetics into fundamental metabolic health.
Sleep, too, affects both gut function and glucose metabolism in ways that compound over a race week. Poor sleep in the days before an event impairs insulin sensitivity and increases cortisol, both of which make blood sugar management harder when you're also dealing with race-day stress. More Sleep, Healthier Heart: What the Data Shows contextualizes why sleep quality isn't peripheral to your performance preparation.
The Takeaway
Simple sugars are a tool, not a strategy. For ultra-trail events, relying on them exclusively is a documented risk factor for GI failure, blood sugar instability, and late-race collapse. A fueling plan that incorporates whole foods, protein, fat, and electrolytes alongside rapid carbohydrates is not more complicated. It's more complete.
The athletes who finish strong at mile 80 aren't the ones who found a better gel. They're the ones who treated nutrition as a discipline, trained it systematically, and built a gut that could deliver energy when the body needed it most.