Ultra Fueling
Why Marathon Nutrition Rules Don't Apply

A different game

Marathon fueling has a relatively simple brief: take in 60-90 grams of carbohydrate per hour for 2 to 5 hours, practice it in training, execute on race day. It works. But ultramarathons — 50K, 100K, 100 miles — are a fundamentally different metabolic challenge. You're not sustaining a high burn rate for a few hours. You're managing energy intake across 8 to 30+ hours of continuous exercise, often through the night, often over mountains, often while everything in your stomach sounds disgusting.

And yet most ultra runners under-fuel dramatically. Costa et al. analyzed nutritional intake during ultra-endurance events and found that athletes typically consumed only 30-50 g/hr of carbohydrate — well below the rates shown to optimize performance.[1] That's not a small gap. It's a 50-75% shortfall from what the evidence supports.

The International Society of Sports Nutrition published a comprehensive position stand on ultra-endurance nutrition in 2019, authored by Tiller et al., which laid out the unique demands: the lower relative intensity, the role of fat oxidation, the inevitability of GI distress, and the psychological dimension of eating when you don't want to eat.[2] If there's one paper to read before your first ultra, it's that one.

Fueling targets for ultras

The headline number hasn't changed much from marathons: 60-90 g/hr of carbohydrate is the minimum target, and trained athletes with adapted guts can push to 120 g/hr. Podlogar and Wallis's 2022 review in Sports Medicine updated the practical recommendations based on dual-transport carbohydrate research.[3]

But here's where ultras diverge from marathons: intensity matters enormously. A marathon is typically run at 70-85% VO2max. A 100-miler might average 40-55% VO2max, especially over mountainous terrain. At those lower intensities, your body's fat oxidation rate is substantially higher — you're getting more energy from fat and less from carbohydrate per minute. The crossover concept still applies, but the crossover point shifts in your favor.

This doesn't mean you can skip the carbs. Even at lower intensities, carbohydrate availability improves performance, maintains cognitive function, and prevents the cascade of hormonal disruption that comes with prolonged glycogen depletion. But it does mean that a 100-mile runner doesn't need to hit 90 g/hr with the same urgency as a marathoner. The window is wider.

Hearris et al. demonstrated in 2022 that trained athletes can oxidize exogenous carbohydrate at rates consistent with 120 g/hr intake, and that the delivery format doesn't matter — gels, drink mix, chews, or a combination all produced comparable oxidation rates.[4] Tour de France riders like Pogačar now routinely hit 100-120g/hr during mountain stages using glucose-fructose products. For ultra runners, the takeaway is that total intake matters more than the form — which opens the door to real food without sacrificing absorption.

Distance	Typical intensity	Carb target	Duration
Marathon	70-85% VO2max	60-90 g/hr	2-5 hrs
50K	60-75% VO2max	60-90 g/hr	4-8 hrs
100K	50-65% VO2max	50-80 g/hr	8-15 hrs
100 miles	40-55% VO2max	40-80 g/hr	18-30+ hrs

Real food vs gels

One of the most common questions in ultra nutrition: should you eat "real food" or stick with engineered sports nutrition? The research answer is refreshingly simple: it doesn't matter much, as long as the total carbohydrate intake is adequate.

Pfeiffer et al. analyzed the nutritional practices of 221 endurance athletes during competition and found no significant difference in GI symptom incidence between those consuming predominantly "real food" and those relying on sports products.[5] The GI problems that plague ultra runners are driven more by exercise intensity, dehydration, and individual tolerance than by whether the calories come from a gel packet or a potato.

Stuempfle and Hoffman's landmark study of nutrition at the Western States 100-Mile Endurance Run found that solid food was commonly consumed and well tolerated by finishers. Importantly, dietary variety was associated with better compliance — athletes who had more food options available ate more consistently throughout the race.[6]

Practical real food options

The ultra community has collectively field-tested an enormous range of foods. The best options share a few traits: they're calorie-dense, easy to chew while moving, tolerable when warm, and don't require utensils. Common choices include:

Rice balls (onigiri) — easy to eat, customizable with salt/soy sauce. Boiled potatoes with salt — a Western States staple. PB&J sandwiches cut into quarters. Bananas — nature's gel, roughly 25g of carbs each. Dates and dried fruit — extremely calorie-dense. Broth — not high in calories but warm, salty, and psychologically restorative at mile 70.

Sports nutrition products still have a role, particularly during higher intensity sections — big climbs, race-pace surges, or the final push where you need rapid carbohydrate delivery without the mechanical work of chewing. The smart approach is a mix: gels and drink mix for carb density and convenience, real food for palatability, variety, and the psychological boost of eating something that actually tastes good.

The GI problem

Gastrointestinal distress isn't a risk factor in ultramarathons. It's a near-certainty. Stuempfle and Hoffman's study at the Western States 100 found that approximately 96% of runners experienced some form of GI symptoms during the race. Nausea was the most common complaint, affecting over 60% of participants.[6]

This isn't just uncomfortable — it's race-ending. Hoffman and Fogard analyzed factors related to successful completion of 161 km ultramarathons and found that nausea and vomiting were the single strongest predictors of DNF.[7] Not fitness. Not muscle fatigue. Not blisters. The stomach. If you can keep eating, you can probably keep moving. If you can't keep eating, you're in serious trouble.

Why it happens

Costa et al. identified the primary mechanisms driving exercise-associated GI distress: splanchnic hypoperfusion (blood flow diverted away from the gut to working muscles), dehydration, NSAID use, and heat stress.[8] During prolonged exercise, up to 80% of blood flow can be redirected away from the GI tract. Your gut becomes ischemic — starved of oxygen and unable to function properly. The intestinal lining becomes more permeable, absorption slows, and anything sitting in your stomach just... sits there.

NSAIDs (ibuprofen, naproxen) make this dramatically worse. They further reduce splanchnic blood flow and damage the gut lining directly. Despite this, NSAID use during ultramarathons is alarmingly common. If you take one thing from this section: don't take ibuprofen during your ultra.

Gut training works

The good news: your gut adapts. Cox et al. showed that 28 days of high-carb feeding during training significantly increased exogenous carbohydrate oxidation rates — intestinal transporters upregulate in response to repeated demand.[9] Miall et al. demonstrated that even two weeks of gut training reduced GI symptoms and carbohydrate malabsorption during exercise.[10]

Fat adaptation: the evidence

The appeal of fat adaptation for ultra runners is intuitive: if you could train your body to burn more fat, you'd need fewer carbohydrates, and the fueling problem largely dissolves. You've got 30,000+ calories of stored fat. Why not use them?

The research has been thorough — and the answer is clear. Burke et al.'s 2017 SUPERNOVA study put elite race walkers on a low-carbohydrate, high-fat (LCHF) diet for 3.5 weeks. Fat oxidation increased, as expected. But the diet impaired exercise economy — athletes required more oxygen per unit of work — and it negated the performance benefits of intensified training.[11] They burned more fat but went slower.

The follow-up SUPERNOVA 2 study (Burke et al. 2021) asked an important question: what if you fat-adapt first and then restore carbohydrate availability for racing? The finding was damning. Even after glycogen restoration, the impairment in carbohydrate oxidation persisted.[12] Fat adaptation had durably downregulated the carbohydrate oxidation pathways. You can't have it both ways.

In 2024, the International Society of Sports Nutrition published a position stand specifically addressing ketogenic diets and endurance performance. Leaf et al. concluded that ketogenic diets do not improve and may impair endurance performance.[13] The evidence is about as settled as nutrition science gets.

The nuanced position

There is a more moderate approach — "train low, compete high" — where athletes periodically train with low carbohydrate availability to stimulate mitochondrial adaptations, then restore full carbohydrate availability for key sessions and races. This has some support for enhancing metabolic flexibility. But this is not the same as chronic keto or LCHF dieting. It's strategic periodization, not ideology.

Hydration: less is more

The biggest hydration risk in ultra-endurance events is not dehydration. It's overhydration. Almond et al.'s landmark study of Boston Marathon runners found that 13% of finishers were hyponatremic — dangerously low blood sodium caused by drinking more fluid than they were losing.[14] Three runners in that study had critical hyponatremia. One died.

The current evidence-based recommendation is simple: drink to thirst. Your thirst mechanism is remarkably well-calibrated for regulating fluid balance during prolonged exercise. Overriding it by drinking on a fixed schedule — the old "drink before you're thirsty" advice — is how people end up hyponatremic.

Sodium: it's complicated

Sodium supplementation is ubiquitous in ultramarathon culture — salt capsules at every aid station, electrolyte tabs dissolved in bottles. The evidence for this is weaker than most runners assume. The primary cause of exercise-associated hyponatremia is excess fluid intake relative to losses, not sodium depletion per se. Taking salt pills while chugging water doesn't fix the problem.

That said, sodium losses over 20-30 hours are real and can be substantial — some athletes lose 1-2 grams of sodium per hour in sweat. The fix is matching sodium intake to sodium loss, not simply drinking more water. Salty foods (broth, pretzels, salted potatoes) and electrolyte drinks contribute meaningfully here.

A practical range for fluid intake during an ultra is 400-800 ml per hour, adjusted by conditions, effort, and thirst. In hot conditions, err toward the higher end. In cool conditions or at lower intensities (hiking uphills), you may need much less. Monitor urine color — pale yellow is the target. Clear means you're overdoing it.

Triathlon: fuel the bike, save the run

Long-course triathlon (70.3 and Ironman) presents a unique fueling challenge: you need to manage nutrition across three disciplines and 4-17 hours of racing. The bike leg is where the fueling war is won or lost.

Rowlands and Houltham studied Ironman athletes and found that those using glucose-fructose at higher intake rates on the bike had faster overall race times.[15] This wasn't just correlation — the mechanism is direct. Aggressive bike fueling protects glycogen stores for the marathon that follows.

The bike-to-run transition

Here's what happens physiologically: 4-6 hours of cycling depletes glycogen in your Type I (slow-twitch) muscle fibers — the same fibers you rely on most heavily for running. When you start the run, your body recruits a partially different fiber population, but overall glycogen status is already compromised. If you under-fueled the bike, you're starting the marathon portion in a glycogen hole you can't climb out of.

The bike is where you eat; the run is where you survive on what you ate.

Practical targets for the Ironman bike leg: 80-120 g/hr of carbohydrate, primarily from glucose-fructose drink mix and gels. The bike position allows your GI tract to function more effectively than running (less mechanical jostling), so this is your window to be aggressive. On the run, most athletes drop to 40-60 g/hr as GI tolerance decreases. Flat cola, gels, and whatever you can keep down become the priority.

Ironman leg	Carb target	Strategy
Swim (3.8 km)	n/a	Pre-race nutrition only
Bike (180 km)	80-120 g/hr	Drink mix + gels, aggressive intake
Run (42.2 km)	40-60 g/hr	Gels, cola, whatever stays down

Building your ultra fueling plan

All of this research distills into a few practical principles for race day. Here's how to put it together.

Set your hourly targets

Based on your event distance and expected intensity, set a carbohydrate target per hour. For a 50K, treat it like a long marathon — 60-90 g/hr. For 100K and beyond, plan for 50-80 g/hr as your baseline, with the understanding that intake will fluctuate. Some hours you'll hit 80g. Some hours you'll struggle to get down 30g. That's normal. Aim for consistent average intake, not perfect hourly compliance.

Plan for food variety

This is the most underrated element of ultra nutrition planning. Taste fatigue is real. A food that tastes incredible at hour 3 may be revolting at hour 15. Plan a roster of 5-8 different food options across sweet, savory, salty, and liquid categories. Your drop bags and crew should have options from each category at every major checkpoint.

Aid station strategy

Know what the race provides and plan around it. Most ultras stock basics — PB&J, fruit, chips, broth, cola, water, electrolyte drink. Use these as supplements to your own nutrition, not your primary plan. Carry enough between aid stations to hit your hourly target without relying on what's on the table.

Crew and drop bag planning

If your race allows crew or drop bags, this is your secret weapon. Pack each drop bag with: your primary fuel (gels/mix), 2-3 real food options, a backup food you haven't been eating (for when everything else sounds terrible), and a caffeinated option for the later stages. Label everything by checkpoint name, not number — at hour 20, you won't remember what "Aid 7" means.

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