The Science of Race-Day Fuelling: How Many Calories Do You Actually Need Per Hour?

“Take in 250 calories per hour.”

You’ve heard this advice. It’s in race guides, forum posts, and well-meaning articles across the internet. And for a decent chunk of the running population, it’s wrong — sometimes dangerously so.

The reality is that caloric needs during an ultra are highly individual, dependent on body weight, exercise intensity, duration, environmental conditions, and your gut’s trained capacity to absorb fuel while working hard. A 60kg runner shuffling a mountain 100-miler has vastly different needs from an 85kg runner racing a flat 50km.

Let’s break down the actual science so you can calculate your number — not someone else’s.

Why Fuelling Matters More Than You Think

During prolonged exercise, your body draws from two primary fuel sources: stored glycogen (in muscles and liver) and fat. Even the leanest runner carries enough stored fat for days of exercise. The problem is that fat oxidation is slow — it can only fuel low-to-moderate intensity effort.

As intensity increases, your body relies more heavily on carbohydrate (glycogen). And here’s the critical number: most runners store approximately 1,500–2,000 calories of glycogen. At ultra intensity, you burn through somewhere between 400–800 calories per hour depending on your size and pace.

Do the maths. Without external fuelling, you’ll bonk somewhere between 2–5 hours into a race. For anything beyond a marathon distance, race-day nutrition isn’t optional — it’s the difference between finishing and not finishing.

The Caloric Expenditure Formula

Your hourly caloric expenditure during running can be estimated using a well-validated formula:

Calories burned per hour ≈ Body weight (kg) × distance covered per hour (km) × 1.0–1.05

This is a simplification, but it’s remarkably accurate for steady-state running. For a 75kg runner covering 8km per hour (a typical 100km mountain ultra pace), that’s:

75 × 8 × 1.02 = 612 calories per hour

For a 60kg runner at 6km/h on a mountain course:

60 × 6 × 1.02 = 367 calories per hour

For an 85kg runner at 10km/h on a flat course:

85 × 10 × 1.02 = 867 calories per hour

You can see the range. And you can see why “250 calories per hour” is woefully inadequate for many runners.

How Much Can You Replace?

Here’s the nuance: you don’t need to replace 100% of calories burned. Your body can draw from fat stores to make up a portion of the energy deficit. Research suggests that for events lasting 4+ hours, you should aim to replace 30–50% of calories burned through ingestion.

Using our 75kg runner at 8km/h:

612 cal/hr burned × 0.35–0.45 = 215–275 cal/hr target intake

But here’s where it gets interesting. Recent research from Dr. Asker Jeukendrup and others has shown that trained guts can absorb significantly more carbohydrate than previously thought — up to 90–120g of carbohydrate per hour (360–480 calories from carbs alone) when using multiple transportable carbohydrates (glucose + fructose blends).

This means your practical intake target is likely between 200–400 calories per hour, depending on:

Your body weight — larger runners need more
Your intensity — faster = higher carb demand
Race duration — longer = higher total deficit = more important to fuel
Your gut’s trained capacity — untrained guts max out at 40–60g carbs/hr

The Carbohydrate Hierarchy

Not all calories are equal during a race. Carbohydrates are king — they’re the fastest-absorbed fuel source and the primary currency your muscles want during exercise.

Here’s the tiered approach:

Tier 1: 60–90g Carbs Per Hour (Gold Standard)

For races over 3 hours, current sports science consensus recommends 60–90g of carbohydrate per hour using a 2:1 glucose-to-fructose ratio. This maximises absorption by utilising two different intestinal transport pathways.

At 4 calories per gram, that’s 240–360 calories per hour from carbohydrate alone.

Products designed for this:

Maurten (hydrogel formulation)
Precision Fuel & Hydration
SiS Beta Fuel
Neversecond C90
Homemade carb drinks using maltodextrin + fructose

Tier 2: 40–60g Carbs Per Hour (Moderate)

If your gut hasn’t been specifically trained, or if you’re running at lower intensities where GI stress is more likely, 40–60g per hour is a realistic starting point. This equates to roughly one gel every 25–30 minutes, or a concentrated carb drink sipped consistently.

Tier 3: Supplementary Real Food

For ultras lasting 10+ hours, many runners benefit from incorporating solid food alongside their carb drink or gel strategy. The psychological relief of “real food” matters, and the slower digestion can provide a steadier energy stream.

Good options:

Rice balls with salt (easy to digest, calorie-dense)
Boiled baby potatoes dipped in salt
Pretzels (salt + carbs)
PB&J sandwiches cut into small squares
Banana segments

The key is to use real food as a supplement to your primary carb strategy, not a replacement for it.

Calculating Your Personal Target

Here’s the step-by-step process:

Step 1: Estimate hourly caloric expenditure

Body weight (kg) × estimated pace (km/hr) × 1.02

Step 2: Calculate replacement target

Hourly expenditure × 0.35 (conservative) to 0.45 (aggressive)

Step 3: Cross-check against gut capacity

If you haven’t specifically gut-trained: cap at 60g carbs/hr (~240 cal) If you have gut-trained consistently: target 70–90g carbs/hr (~280–360 cal)

Step 4: Plan your fuelling products

Map your target to specific products with known calorie/carb content

Example: 75kg Runner Racing UTA 100

Estimated pace: 7.5 km/hr average (including climbs and aid station time)
Hourly expenditure: 75 × 7.5 × 1.02 = 574 cal/hr
Replacement target (40%): 230 cal/hr
Gut-trained capacity: 80g carbs/hr = 320 cal/hr
Practical target: 250–300 cal/hr

Fuelling plan:

500ml carb drink per hour (Maurten 320 = 320 cal in 500ml → sip 400ml = ~260 cal)
1 gel every 60–90 minutes as top-up (~100 cal each)
Real food at aid stations from 50km onwards (rice, potatoes)
Total: ~280–350 cal/hr

Gut Training: The Missing Piece

Your gut is a trainable organ. The enzymes that break down carbohydrate, the transporters that absorb it, and the stomach’s tolerance for fuel during exercise — all of these adapt with consistent practice.

The Protocol

Start 8–12 weeks before your target race
Practice fuelling on every long run — no “fasted long runs” in this period
Begin at 40g carbs/hr if you haven’t trained your gut before
Increase by 10g per week until you reach your target intake
Use your race-day products — train with exactly what you’ll use on race day
Practice at race intensity — walking pace gut capacity ≠ running pace gut capacity

Common Gut Training Mistakes

Starting too high. If you jump straight to 90g/hr without building up, your GI system will revolt. Nausea, cramping, and worse. Be patient.
Only training on long runs. Practice on medium runs too. The more your gut is exposed to fuel during exercise, the faster it adapts.
Ignoring hydration. Concentrated carbohydrate without adequate water = gastric distress. Always match fuel intake with fluid.
Switching products on race day. Your gut has adapted to specific products. Race day is not the time to try something new because it was free at the expo.

Electrolytes: The Other Half

Caloric intake without electrolyte management is an incomplete strategy. Sodium is the primary electrolyte lost in sweat, and maintaining sodium balance prevents hyponatraemia (dangerously low blood sodium from overhydration) and helps with fluid absorption.

General guidelines:

300–500mg sodium per hour for moderate conditions
500–700mg sodium per hour for hot conditions or heavy sweaters
1,000mg+ per hour in extreme heat for heavy sweaters

Most gels contain negligible sodium. Most sports drinks contain some but not enough. Consider dedicated electrolyte supplements (SaltStick, Precision Hydration) as part of your fuelling plan.

Putting It All Together

Race-day fuelling isn’t about following someone else’s plan. It’s about understanding the science, calculating your personal numbers, training your gut, and then executing with discipline on race day.

The runners who nail their nutrition don’t wing it. They have a plan, they’ve practised it, and they execute it by the clock — not by feel. Because by the time you feel hungry in an ultra, you’re already behind.

Here’s your action checklist:

Calculate your personal caloric target using the formula above
Choose your primary fuel source (carb drink, gels, or a combination)
Map out exact timing and quantities for every hour of your target race
Begin gut training 8–12 weeks out with progressive carb loading
Practise your complete fuelling plan on at least 3–4 long training runs
Include electrolyte strategy as a non-negotiable part of the plan
Set a timer on race day — eat by the clock, not by feel

Your nutrition plan is your race plan. Train it like you train your legs.

Ready to put this into practice? Generate a personalised race-day nutrition plan with RacePlan AI → Enter your weight, pace, and race details to get a customised fuelling strategy with specific timing and quantities.