Complete 100K Ultramarathon Fueling Plan: Hourly Calorie, Sodium, and Solid-Liquid Switching Science

Why Is Ultramarathon Fueling Harder Than Marathon Fueling?

A marathon finishes within 3–4 hours, and one gastric emptying cycle is enough; but a 100K ultramarathon often takes 10–14 hours. Once your fueling rhythm falls apart, the consequences range from hitting the wall to hypoglycemic fainting or gastric stasis that ends your race entirely.

According to post-race surveys by the International Trail Running Association (ITRA), about 40% of ultramarathon runners drop out mid-race due to “gastrointestinal issues”—a higher rate than muscle cramps.

The 4 Core Principles of Ultramarathon Fueling

Principle 1: Fixed Caloric Intake Per Hour

• Body weight 60–70kg: 200–250 kcal per hour
• Body weight 70–80kg: 250–300 kcal per hour
• Body weight >80kg: 300–350 kcal per hour

Principle 2: Carbohydrate-Dominant, with Protein and Fat as Support

• First 8 hours: 85% carbs, 10% protein, 5% fat
• After 8 hours: 70% carbs, 15% protein, 15% fat (introduce solid foods)

Principle 3: Sodium Intake Based on Sweat Volume

• Normal sweat-salt loss: 500–700mg sodium per hour
• High-salt sweat profile: 800–1200mg sodium per hour

Principle 4: Alternate Liquids and Solids to Avoid Gastric Stasis from a Single Form

Hour-by-Hour Fueling Schedule for a 100K Ultramarathon

Using a 70kg runner targeting a 12-hour finish as an example:

Total: approximately 2800–3000 kcal + 6500–7500mg sodium

The “3-Phase Strategy” for Switching Between Solids and Liquids

Phase 1: First 3 Hours – Liquid-Dominant

The body is in a high-metabolic state with rapid gastric emptying. 85% of fueling comes from energy gels and sports drinks, avoiding solid foods that add gastrointestinal burden.

Phase 2: 3–7 Hours – Semi-Solids + Hot Food

Gastric emptying slows, and liquid fueling begins to create a “waterlogged feeling.” Switch to rice balls, energy bars, savory congee, and other semi-solids plus hot food to stimulate gastric motility.

Phase 3: After 7 Hours – Easily Absorbed Liquids + Caffeine

The stomach may be nearing shutdown. Switch back to liquids that are low-residue and easily absorbed, and use caffeine (3–6mg/kg) to combat central fatigue.

The Relationship Between Sodium and Cramps

Signs of Insufficient Sodium Intake (<500mg/h)

• Tingling sensation in fingers and corners of the mouth
• Recurring mild cramps
• Confusion and unexplained fatigue

Signs of Excessive Sodium Intake (>1500mg/h)

• Increased thirst and dry tongue
• Wanting to drink large amounts of water without relief
• Abnormally elevated heart rate

4 Practical Ultramarathon Fueling Tips

1. “Gut Training” 14 Days Before the Race

Simulate your race-day fueling rhythm during 3 sessions of 30K training to train the stomach to adapt to high-frequency fueling, reducing the risk of gastrointestinal shutdown on race day.

2. Fixed Routine at Every Aid Station

Set a “must-do at every aid station” routine: use the restroom, drink 200ml of water, eat half a solid food item, and tuck 1 pack of salt tablets into your pocket.

3. 30-Minute Alarm Reminder

In the middle of an ultramarathon, the brain is fatigued. Set your watch to vibrate every 30 minutes as a reminder to “eat now,” so you never miss a feeding.

4. Carry 2 Forms of Fuel as a Backup

Getting sick of solids or feeling nauseated by energy gels is common. Carry at least 2 flavors and 2 textures so you can switch on the spot.

Conclusion

An ultramarathon is not a race of “running fast” but a contest of “fueling right.” Precise hourly calories, sodium, and solid-liquid switching, combined with 14 days of gut training, are the keys to finishing a 100K. Remember this phrase: “Glycogen is the engine, sodium is the radiator, and caffeine is the backup battery”—all three are indispensable.

Related Reading

• Ultramarathon Fueling Strategies: Energy and Electrolyte Rhythm from 50K to 100K
• Trail 100K Fueling Strategies: Energy, Hydration, and Gastric Acid Management for Races Over 12 Hours
• Ultramarathon Fueling Strategies: The Ratio of Solid Food vs. Liquid Fueling in 100km Races
• Ultramarathon Fueling Strategies: Timing Management of Solid Food, Fluids, and Electrolytes