Ultra Marathon Nutrition: What to Eat During a 100-Miler

Nutrition is the fourth discipline of ultrarunning. You can have the fitness, the mental toughness, and the perfect pacing strategy, but if your nutrition plan falls apart, so does your race. In events lasting 20 to 30 or more hours, the runners who cross the finish line are almost always the ones who managed to keep eating when everything inside them was saying to stop.

A 100-mile race demands somewhere between 8,000 and 12,000 calories. You cannot possibly replace all of them during the event. The goal of race-day nutrition is not to break even on calories but to slow the rate of depletion enough that you can keep moving forward. This guide covers exactly how to do that.

The Calorie Math

A 150-pound runner burns roughly 100 calories per mile. Over 100 miles, that is 10,000 calories of expenditure. Your body stores about 2,000 calories of glycogen and can access a virtually unlimited supply of fat. The problem is that fat oxidation cannot keep up with the energy demands of running. You need to supplement with exogenous carbohydrates to maintain your pace.

Research shows that the human gut can absorb 60 to 90 grams of carbohydrate per hour during exercise when using a mix of glucose and fructose. That translates to 240 to 360 calories per hour. In practice, most ultrarunners find a comfortable intake range of 200 to 300 calories per hour, with the lower end during the night when appetite suppression is strongest.

Macros That Matter

During a 100-miler, carbohydrates are king. They are the most rapidly available fuel source and the easiest to digest during exercise. Aim for 80 to 90 percent of your race-day calories to come from carbs. The remaining 10 to 20 percent can come from fat and protein, mostly from real food consumed at aid stations.

Protein becomes more important in the later stages of very long ultras. After 15 or more hours of running, your body begins breaking down muscle protein for fuel, a process called gluconeogenesis. Small amounts of protein, 5 to 10 grams per hour in the second half of the race, can help mitigate this. Good sources include nut butters, cheese, turkey wraps, and protein-fortified drinks.

Fat is not a practical fuel source during the race itself because it is slow to digest and can cause GI distress at higher intensities. However, in the very late stages when pace slows to walking, some runners tolerate fattier foods like avocado, cheese, or potato chips. Listen to your body and eat what you can tolerate.

Sodium Strategy

Hyponatremia, a dangerous drop in blood sodium levels, is one of the most serious medical risks in ultrarunning. It occurs when you drink too much water without replacing sodium. The symptoms, confusion, nausea, and swelling, can mimic dehydration, leading runners to drink even more water and making the problem worse.

Most ultrarunners need 300 to 600 mg of sodium per hour, with higher intake in hot conditions. Salt capsules such as SaltStick or LMNT packets are the most convenient delivery method. Some runners prefer salty real food like broth, pickles, pretzels, or potato chips. The key is consistency. Take sodium on a schedule, not just when you feel crampy.

• Consume 300 to 600 mg sodium per hour, more in heat
• Use salt capsules, electrolyte drinks, or salty foods
• Drink to thirst rather than on a fixed volume schedule
• Watch for signs of hyponatremia: confusion, nausea, puffy hands
• Weigh yourself at aid stations if possible. Weight gain during a race is a red flag for overhydration.

Caffeine Timing

Caffeine is the most effective legal performance aid in ultrarunning. It reduces perceived exertion, improves alertness, and can genuinely pull you out of a low point. But timing is everything. If you use caffeine too early, it loses its effectiveness when you need it most.

Save your first caffeine dose for the second half of the race. If you start in the morning, hold off on caffeine until sunset or until your first significant low point. Then use it strategically: 50 to 100 mg every 2 to 3 hours. Sources include caffeinated gels, cola, coffee at aid stations, or caffeine pills. Avoid taking caffeine within 3 to 4 hours of when you hope to sleep at a crew station. If you are not planning to sleep, deploy caffeine freely through the night.

Real Food vs. Gels

In a marathon, gels are sufficient. In a 100-miler, you need real food. No one can consume gels for 24 or more hours without developing severe nausea and palate fatigue. The most successful ultra nutritional strategies blend processed fuel for the early miles with real food for the later stages.

In the first 10 to 15 hours, gels, chews, and sports drink work well because your intensity is higher and your gut can handle simple sugars. As the race progresses and your pace slows, switch to real food: boiled potatoes with salt, PB&J sandwiches, quesadillas, rice balls, fruit, soup, and whatever sounds appetizing. Aid stations at most 100-milers offer a remarkable spread.

• Hours 0 to 10: Gels, chews, sports drink, bars
• Hours 10 to 20: Mix of gels and real food. PB&J, potatoes, fruit.
• Hours 20 and beyond: Whatever you can tolerate. Broth, cola, ramen, sandwiches.
• Always have a backup plan. If your stomach rejects your primary fuel, know what alternatives work.

Training Your Gut

Your gastrointestinal system is trainable, just like your cardiovascular system. If you never practice eating during runs, your stomach will rebel on race day. If you consistently practice fueling during training runs, your body adapts by increasing gastric emptying rate, improving nutrient absorption, and reducing GI distress.

Start gut training early in your build. On every run over 90 minutes, practice your fueling plan. Eat on a schedule. Use the same products you plan to use on race day. Try different foods to identify what works and what does not. Keep a fueling log to track what you ate, how much, and how your stomach responded.

By the time you reach your peak long runs, your race-day nutrition plan should be dialed in. You should know your calorie target per hour, your sodium needs, your preferred fuel sources for each stage of the race, and your backup options. Leave nothing to chance.

Race-Day Nutrition Schedule

A sample nutrition schedule for a 100-miler with an expected finish time of 24 hours might look like the following. Adjust quantities based on your body weight, conditions, and personal tolerance.

1. 1Pre-race (3 hours before start): 400 to 600 calories. Oatmeal, bagel with peanut butter, banana. Familiar foods only.
2. 2Hours 0 to 6: 250 calories per hour from gels and sports drink. 1 salt capsule every 45 minutes. Drink to thirst.
3. 3Hours 6 to 12: 250 calories per hour, transitioning to real food. PB&J, potatoes, fruit. Continue salt capsules.
4. 4Hours 12 to 18: 200 to 250 calories per hour. Whatever sounds good. Introduce caffeine. Broth and warm food if cold.
5. 5Hours 18 to 24: 150 to 200 calories per hour. Cola, broth, whatever stays down. Continue caffeine every 2 to 3 hours.

The Most Common Nutrition Mistakes

1. 1Not eating enough in the first half. By the time you feel the deficit, it is too late to catch up.
2. 2Overdrinking water without electrolytes. This leads to hyponatremia, not better hydration.
3. 3Relying only on gels. You will develop palate fatigue and nausea by hour 12.
4. 4Skipping calories when nauseous. Nausea in ultras is usually from underfueling, not overfueling. Try broth, ginger, or small sips of cola.
5. 5Not testing race nutrition in training. Your gut needs to be trained just like your legs.
6. 6Ignoring temperature. Hot conditions demand more sodium and fluid. Cold conditions suppress appetite but you still need calories.
7. 7Using caffeine too early. Save it for when you truly need the boost, typically after sunset.