Most DNFs in 100-mile races are stomach problems disguised as something else. The runner stops eating, then stops drinking, then sits at an aid station for 40 minutes feeling sorry for themselves, then drops. The whole cascade started 3 hours earlier with one missed gel. This guide is about preventing that.
The base equation
Calories per hour for an ultrarunner sits in a fairly narrow band:
- Untrained gut, 130 lbs: 180–220 cal/hr
- Trained gut, 150 lbs: 240–280 cal/hr
- Trained gut, 180 lbs: 280–320 cal/hr
- Trained gut, 200 lbs: 300–360 cal/hr
"Trained gut" is the operative phrase. Your stomach has to learn to absorb 250+ calories an hour while running. That training takes weeks of long runs eating at race pace. Don't show up to a 100-miler having only ever eaten 150 cal/hr in training — you'll find out at hour seven that you can't process more.
The mix: gel vs real food vs liquid
The most resilient strategy is a three-source mix:
- 1/3 gels (Maurten 100, Spring, SiS, Huma) — fast carbs, easy to swallow at speed
- 1/3 real food (boiled potatoes, PB&J, pretzels, broth) — savory variety, slower-burning
- 1/3 liquid (Tailwind, Skratch, UCAN) — hydration + carbs in one bottle
The reason for the split: at hour 8, sweet flavors stop working. The body craves salty/savory. If you've trained on only gels, you're now in a bad spot 50 miles into your race. The runner with PB&J in their drop bag and a thermos of broth at the next aid station has a major advantage.
Sodium math
Sodium loss varies wildly between runners. A heavy-sweater can lose 1,500 mg per hour in heat. A light-sweater might lose 400. The conservative target for most ultrarunners in mild conditions is 500–700 mg/hr; in heat (>80°F), 800–1,200 mg/hr.
Sodium sources, by mg per serving:
- LMNT — 1,000 mg sodium per packet
- SaltStick FastChews — 200 mg per chew
- Skratch Hyper Hydration — 1,720 mg per packet
- Tailwind Endurance Fuel — 310 mg per scoop (also 200 cal of carbs)
- Most gels — 50–100 mg sodium per gel
The math: if your target is 700 mg/hr and you take 2 gels (150 mg total) and one bottle of Tailwind (310 mg) per hour, you're at ~460. Add 1 SaltStick chew (200 mg) and you're at the target. In heat, double it.
The bonk window
Every ultra has a bonk window between hours 4 and 10. Glycogen is depleted from racing pace, the gut is stressed, the brain is tired, and your fueling discipline drops. Most runners who DNF made the critical mistake here.
The defense: set a watch alarm every 30 minutes. When it beeps, you eat — even if you're not hungry. Especially if you're not hungry. The "I'm not hungry" feeling at hour 7 is your stomach failing to communicate, not telling you the truth. Eating preempts the bonk; once you've bonked, you can't eat your way out of it.
A working hourly plan, by hour
For a 24-hour 100-miler, 250 cal/hr target, mild conditions:
| Hour | What to take | Cal | Sodium |
|---|---|---|---|
| 0–2 | Bottle Tailwind + 1 gel/hr | ~290 | ~410 |
| 2–6 | Bottle Tailwind + alternating gel/chew | ~270 | ~510 |
| 6–10 | 1 gel + 1 PB&J quarter + bottle Skratch | ~260 | ~600 |
| 10–16 | Real food + broth at aid + bottle | ~280 | ~700 |
| 16–22 | Salty food + small gels + flat coke | ~240 | ~600 |
| 22–24 | Whatever stays down | — | — |
By hour 16, your body wants warm, salty, and simple. Aid station broth, a quarter of a grilled cheese, a few salt-and-vinegar chips. The plan stops being a plan and becomes "what stays down."
Heat, cold, altitude — adjustments
- Heat (>80°F): double sodium, halve solid food, lean heavy on liquid + electrolyte mix.
- Cold (<40°F): switch to warm calories at aid stations. Soup, hot chocolate. Cold gels become impossible to eat.
- Altitude (>8,000 ft): reduce solid food, increase liquid. Gut absorption slows at altitude — gels and Tailwind are kinder to a stressed stomach.
The "stop eating" mistake
The most common late-race mistake: nausea sets in, runner stops eating, runner walks for 20 miles, runner DNFs. The fix is counterintuitive: when nausea hits, don't stop fueling. Switch sources. If gels nauseate you, switch to Coke. If Coke fails, try ginger ale or Sprite at aid stations. If liquid fails, try a salty cracker. Something almost always works. The mistake is taking "I can't eat anything" as a permanent statement.
Train your gut
Long runs are not just leg-training; they're stomach-training. Eat at race pace and frequency on every long run from week 9 of your training plan. Use the same gels, same flasks, same flavors you'll use on race day. The gut adapts the same way the legs do — slowly, and only if you train it.
Most importantly: the day before your race is not the time to try a new gel flavor. Bring exactly what you've trained on. The gut has zero tolerance for novelty under stress.