Ultra running nutrition is fundamentally a problem of managing extreme energy deficits while keeping a stressed digestive system functional enough to absorb what you put into it. Daily energy shortfalls during ultra-endurance events can reach roughly 7,000 calories, and no runner fully replaces what they burn. The goal is not perfect energy balance but rather minimizing the gap enough to maintain pace, avoid medical emergencies, and recover afterward. What makes this harder than fueling a marathon is that the gut itself becomes compromised the longer you run, turning food intake into a negotiation between what you need and what your body can tolerate.
How Much You Actually Need to Take In
The International Society of Sports Nutrition recommends that ultra runners aim for 150 to 400 calories per hour during racing, with about 30 to 50 grams of carbohydrate and 5 to 10 grams of protein per hour from a variety of calorie-dense foods.1PubMed Central. International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing That is a wide range, deliberately so, because a 55-kilogram runner doing a flat 50-miler has different needs from a 90-kilogram runner grinding through a mountain 100. The point is that there is a floor below which your race falls apart. Data from a 100-mile mountain footrace showed that finishers consumed significantly more calories, carbohydrate, fat, and sodium per hour than runners who dropped out, and the difference was already apparent in the first 48 kilometers.2Taylor & Francis Online / Journal of the American College of Nutrition. Race diet of finishers and non-finishers in a 100 mile (161 km) mountain footrace Early under-eating is one of the most common race-ending mistakes.
Even with diligent fueling, an energy deficit of thousands of calories per day is essentially unavoidable in events lasting six hours or more. That deficit leads to measurable losses of both fat and muscle mass over the course of events like multi-day running races or 24-hour competitions.3PubMed Central. Nutrition in Ultra-Endurance: State of the Art The practical takeaway is that you should start eating early and eat consistently. Waiting until you feel depleted usually means you are already behind and your gut has started shutting down.
Carbohydrate Is Still the Primary Fuel
For daily training, the evidence supports a moderate-to-high carbohydrate intake of around 5 to 8 grams per kilogram of body weight per day to prevent chronic glycogen depletion.4PubMed Central. International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing During racing, the 30 to 50 grams of carbohydrate per hour target is lower than what elite marathon runners consume because ultra runners are moving at a lower intensity and relying more heavily on fat oxidation. Still, carbohydrate remains the dominant fuel when you need to push harder on a climb or maintain pace in the final hours.
An important detail about carbohydrate intake during exercise: mixing glucose with fructose lets you absorb and burn more total carbohydrate than glucose alone. Research on ultra-endurance cycling found that a glucose-fructose mixture raised peak exogenous carbohydrate oxidation to about 1.4 grams per minute, compared with roughly 1.2 grams per minute from glucose alone, and increased the percentage of ingested carbohydrate that was actually used for fuel.5PubMed. Exogenous carbohydrate oxidation during ultraendurance exercise This is why most modern sports drinks and gels use dual-sugar formulas. For a practical standpoint, if you are relying on a single carbohydrate source at higher intake rates, you are leaving absorption capacity on the table and raising your risk of gut distress.
Why Your Gut Becomes the Bottleneck
Gastrointestinal problems are the single most common medical complaint in ultra running. The underlying biology is straightforward: when you exercise hard, your body diverts blood away from the digestive organs and toward working muscles. A systematic review found that exercise lasting two hours or more at moderate intensity is enough to cause measurable intestinal injury, increased gut permeability, and impaired absorption, regardless of how fit you are.6PubMed. Systematic review: exercise-induced gastrointestinal syndrome-implications for health and intestinal disease Heat and dehydration make things worse. When you are already somewhat dehydrated, the reduction in blood flow to the gut is even more pronounced, further compromising its ability to absorb nutrients and maintain its barrier function.7PubMed Central. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations
This creates a vicious cycle: you need calories and fluid to keep running, but the act of running compromises your ability to take them in. By the late stages of a 100-miler, many runners can barely tolerate anything beyond sips of broth or small bites of bland food. The runners who fare best are typically those who have trained their gut to handle intake during exercise and who eat and drink early before the system starts breaking down.
Training the Gut
The digestive system is more adaptable than most runners realize. Practicing your race nutrition during training, sometimes called “gut training,” can improve gastric emptying, reduce feelings of fullness, and increase the intestine’s capacity to absorb carbohydrate. A high-carbohydrate diet over time increases the density and activity of the specific transporters in the intestinal lining that absorb glucose, allowing greater carbohydrate delivery during exercise.8PubMed Central. Training the Gut for Athletes These adaptations appear to be nutrient-specific, meaning you should practice with the actual foods and drinks you plan to use on race day.
The timeline for meaningful adaptation is shorter than you might expect. A two-week gut-training protocol where runners consumed carbohydrate during exercise sessions reduced gastrointestinal symptoms by roughly 60 percent, improved blood glucose availability, and enhanced running performance compared with a placebo.9PubMed. Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance Two weeks is not a long commitment. If you are doing nothing else to prepare nutritionally, practicing eating and drinking during your long runs is probably the single highest-return investment you can make.
Hydration Without Overhydration
Dehydration degrades performance and worsens gut problems, but overdrinking poses its own serious risk. Exercise-associated hyponatremia, a dangerous drop in blood sodium concentration, was first described in ultra runners in the mid-1980s and remains one of the most feared medical complications of ultra-distance events. Mild cases produce vague symptoms, but severe hyponatremia can cause brain swelling, respiratory failure, and death.10PubMed Central. Exercise-Associated Hyponatremia in Endurance and Ultra-Endurance Performance-Aspects of Sex, Race Location, Ambient Temperature, Sports Discipline, and Length of Performance: A Narrative Review The main driver is drinking far more than you lose in sweat, diluting blood sodium to dangerous levels.
The long-running debate over planned drinking schedules versus simply drinking to thirst has largely settled into a context-dependent answer. For shorter, cooler, lower-intensity efforts, drinking when you are thirsty is generally safe and adequate. For ultra-distance events, especially in the heat, a more structured approach has advantages. Planned fluid intake helps preserve blood volume and reduce cardiovascular strain during prolonged exercise, but rigid schedules can backfire if they lead to overdrinking.11PubMed Central. Hydration Strategies in Ultra-Endurance Running: A Narrative Review of Programmed Versus Thirst-Driven Approaches The most practical advice is to know your approximate sweat rate (weigh yourself before and after a training run of similar intensity and conditions), use that to set a rough intake target, and adjust based on thirst and conditions on race day.12PubMed Central. Drinking Strategies: Planned Drinking Versus Drinking to Thirst
One common assumption is that taking extra sodium tablets will prevent cramping and hyponatremia. A study of ultramarathon runners found that sodium intake rates were similar between runners who developed hyponatremia and those who did not, and between those who cramped and those who did not.13PubMed Central. Sodium Intake During an Ultramarathon Does Not Prevent Muscle Cramping, Dehydration, Hyponatremia, or Nausea Sodium supplementation is not worthless, but it is not the simple protective factor many runners treat it as. Fluid volume is a bigger lever than sodium dose for avoiding hyponatremia.
The Low-Carb and Ketogenic Debate
Because ultra runners operate at lower intensities where fat oxidation provides a large share of fuel, the idea of adapting to a high-fat, low-carbohydrate diet has genuine physiological logic behind it. Keto-adapted ultra runners burn dramatically more fat during exercise: one study found peak fat oxidation was about 2.3 times higher in low-carb athletes compared with high-carb athletes, and fat contributed roughly 88 percent of their energy during submaximal running versus 56 percent in the high-carb group.14PubMed. Metabolic characteristics of keto-adapted ultra-endurance runners Interestingly, resting muscle glycogen levels and the degree of glycogen depletion after three hours of running were the same in both groups, suggesting the body still maintains glycogen stores even on a very low-carb diet.
Despite these metabolic differences, the performance evidence is not persuasive. A systematic review examining both ketogenic diets and ketone supplements in endurance runners found no benefits for aerobic performance compared with controls.15PubMed Central. The Effects of Ketogenic Diets and Ketone Supplements on the Aerobic Performance of Endurance Runners: A Systematic Review One study of recreational runners found that a high-fat diet did not impair 5K performance at high intensity either, so the relationship is not as simple as “carbs good, fat bad.”16PubMed Central. High Rates of Fat Oxidation Induced by a Low-Carbohydrate, High-Fat Diet, Do Not Impair 5-km Running Performance in Competitive Recreational Athletes The nuance the ISSN position stand captures well is that deliberately limiting carbohydrate before certain low-intensity training sessions may enhance fat-burning capacity and mitochondrial function, but this comes at the cost of performance during high-intensity work.17PubMed Central. International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing For most ultra runners, a periodized approach that includes adequate carbohydrate most of the time, with occasional low-carb training sessions, is a more evidence-based strategy than committing fully to either extreme.
As for exogenous ketone supplements, they are a growing product category, marketed as an alternative fuel source. Various forms exist, including ketone esters, ketone salts, and medium-chain triglyceride oil.18PubMed Central. The science of ketogenic supplements for athletes: boosting endurance, efficiency, and energy metabolism They can raise blood ketone levels without dietary restriction, and ketone bodies do serve as signaling molecules that may modulate inflammation and recovery.19PubMed Central. Acute and Intermittent Exogenous Ketosis to Support Recovery From Exercise and Adaptations to Exercise Training: A Narrative Review Whether that translates into real-world performance gains for ultra runners remains unproven.
Protein Gets Overlooked Mid-Race
Ultra running causes significant muscle damage, and protein plays a clear role in both limiting that damage and recovering from it. The ISSN recommends daily protein intakes of about 1.6 grams per kilogram of body weight for maintaining lean mass and supporting recovery, with amounts up to 2.5 grams per kilogram potentially warranted during heavy training blocks.20PubMed Central. International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing Survey data from ultra runners preparing for 50K and 100K events suggest most are hitting these targets during training, with median intakes ranging from about 1.6 to 2.1 grams per kilogram per day depending on the training phase and race distance.21PubMed Central. The Diet Quality of Ultramarathon Runners Taking Part in an Australian Event: A Cross-Sectional Explorative Study
Where protein gets neglected is during racing itself. The 5 to 10 grams per hour recommendation during a race is easy to forget when you are focused on getting enough carbohydrate. Real food at aid stations, things like nut butters, cheese, turkey wraps, or recovery drinks, can provide this. Many runners find that savory, protein-containing foods actually sit better than another gel as the hours pile up and sweet flavors become nauseating.
When the Race Lasts Days
Multi-day stage races introduce an additional constraint: you often carry all your own food, and every gram of pack weight matters. Runners in self-sufficient multi-stage events tend to prioritize weight savings over caloric density, and the result is an energy deficit that no amount of planning fully closes. Research on multi-stage ultra runners found that even with carbohydrate intakes around 4.3 grams per kilogram per day, runners entered a state of ketosis that persisted throughout the event. Even those who managed much higher carbohydrate intakes, up to 9 grams per kilogram per day, could not avoid ketosis.22PubMed. Carbohydrate intake and ketosis in self-sufficient multi-stage ultramarathon runners This suggests that competing in ketosis is simply part of the multi-day experience, and runners preparing for these events would benefit from some degree of metabolic flexibility training beforehand.
Sex Differences in Fuel Use
Women and men burn fuel differently during prolonged exercise, and these differences have practical implications for nutrition planning. Compared with men, women rely less on carbohydrate oxidation and more on fat at the same relative exercise intensity, use less liver and muscle glycogen, and may have a greater capacity to use the fat stored within muscle fibers.23PubMed. Sex-based differences in endurance exercise muscle metabolism: impact on exercise and nutritional strategies to optimize health and performance in women These differences are largely mediated by estrogen, which means they can shift across the menstrual cycle and with hormonal contraceptive use.
This metabolic profile sounds like a built-in advantage for ultra running, and to some extent it is. Women show greater fatigue resistance and adopt more even pacing strategies during endurance running.24PubMed. Sex Differences in Endurance Running But the picture is not one-sided. Women also tend to have lower oxygen-carrying capacity, a higher prevalence of gastrointestinal distress during races, and sex-hormone-related effects on cellular function and injury risk that can offset those metabolic advantages.25PubMed. Do Sex Differences in Physiology Confer a Female Advantage in Ultra-Endurance Sport? Nutritionally, the practical implication is that women may not need quite as aggressive a carbohydrate strategy as men at the same relative intensity but should pay extra attention to GI tolerance and iron status.
NSAIDs and Kidney Risk
Popping ibuprofen before or during an ultra is extremely common and genuinely dangerous. Ultra-endurance exercise already stresses the kidneys through dehydration, muscle breakdown, and reduced blood flow to the organs. Adding non-steroidal anti-inflammatory drugs compounds that stress. There is strong evidence linking NSAID use during ultra events to increased risk and severity of acute kidney injury, including a more serious form of damage called acute tubular necrosis.26PubMed Central. Ultra-Endurance Participation and Acute Kidney Injury: A Narrative Review Kidney injury during ultras is interconnected with other pathologies runners face, including rhabdomyolysis (muscle tissue breakdown releasing proteins into the blood), hyponatremia, and GI damage. Adequate hydration and fueling reduce the risk of all of these. If you need pain management during a race, acetaminophen is a safer alternative, though no drug is without risk during this level of exertion.
Caffeine as a Tool
Caffeine is the most well-supported ergogenic substance in endurance sport, and it takes on special relevance in ultra running because of sleep deprivation. Races lasting 24 hours or more force runners to push through the night, and cognitive decline from sleeplessness becomes as much of a limiter as muscular fatigue. A consensus supports the strategic use of caffeine during ultra events to sustain performance and offset the safety risks of sleep deprivation.27Frontiers in Neurology. Sleep in marathon and ultramarathon runners: a brief narrative review The key word is strategic: many experienced ultra runners save their caffeine for the nighttime hours rather than dosing early, reserving its alertness-boosting effects for when they are needed most. High doses on an already-stressed stomach can backfire, so timing and quantity matter.
What Happens to Your Gut Microbiome
Beyond the acute digestive distress during a race, ultra running appears to cause rapid changes in gut microbial activity. After an ultra-endurance triathlon, researchers found significant shifts in the metabolic output of gut bacteria, including drops in short-chain fatty acids like butyrate and certain bile acids, even though the overall composition of the microbial community did not change dramatically.28PubMed. Ultra-Endurance triathlon competition shifts fecal metabolome independent of changes to microbiome composition Butyrate is an important fuel for the cells lining the colon, and reduced butyrate-producing bacteria have been observed in ultramarathon runners more broadly.29Scientific Reports. Alterations in intestinal microbiota in ultramarathon runners
A case study of an elite ultramarathon runner documented some of the most dramatic microbial shifts in the exercise literature, with certain bacterial genera exploding in abundance within just two hours of finishing a race.30PubMed Central. Rapid gut microbiome changes in a world-class ultramarathon runner What these changes mean for long-term health and recovery is still an open question. But the emerging picture suggests that the gut microbiome is not just a passive bystander during ultra running; it is actively responding to the metabolic stress, and its post-race state may influence how quickly the gut barrier repairs itself and how well you absorb nutrients during recovery. This is an area where the science is young, but it provides one more reason to prioritize gut-friendly nutrition and fiber intake in the days and weeks surrounding a race.
High-Altitude Races
Mountain ultras held above about 2,500 meters introduce additional nutritional challenges. Appetite suppression at altitude is well documented and can make it even harder to hit calorie targets. Access to fresh food is limited, environmental cold increases caloric demands, and physiological changes at altitude alter how efficiently you metabolize different fuels.31Frontiers in Sports and Active Living. Nutrition, hydration and supplementation considerations for mountaineers in high-altitude conditions: a narrative review Runners preparing for high-altitude ultras should plan for smaller, more frequent feedings of highly palatable, calorie-dense foods and account for the reality that they will likely eat even less than planned. Liquid calories become especially valuable when solid food is unappealing.