How to Fuel for a 100km Run: Complete Guide

Table of Contents

• How to Fuel for 100km Run: The Basics That Actually Matter
• Carbohydrate Intake Per Hour: Finding Your Number
• Best Energy Gels for Ultramarathon Running
  • Gels vs Chews vs Drink Mix: What to Carry
• Ultra Marathon Hydration Strategy: Sodium, Sweat Rate and Fluid Intake
• How to Avoid Bonking in Ultra Running
  • Gut Distress Management: The Troubleshooting Guide Most Guides Skip
  • Night-Time Fuelling: When Your Appetite Quietly Goes on Strike
  • Weather Adjustments: Heat, Cold, and Everything In Between
• Race Day Nutrition Plan Template for 100km
  • Real Food vs Engineered Fuel: When to Switch
  • Checkpoint Strategy: The 90-Second Audit
• Frequently Asked Questions

Last Updated: October 4, 2026

How to Fuel for 100km Run: The Basics That Actually Matter

Let's be honest: 100km is a long way. Somewhere past 60km, your body stops negotiating and starts issuing demands, and fuelling is how you answer them.

The core principle is simple. Fuel for 100km is really about matching what your gut can absorb to what your muscles are burning, hour after hour. The Sports Dietitians Australia guidance on endurance fuelling is a sensible starting point if you want the clinical version.

Carbohydrate Intake Per Hour: Finding Your Number

Most runners settle between 60 and 90 grams of carbohydrate per hour for ultra distances, but your number depends on gut tolerance, pace and conditions. Start lower and build:

1. Work out your expected finish time and multiply by your target hourly carbs.
2. Test that intake on long training runs, not race day.
3. Increase by small increments every few weeks rather than jumping to a high number.
4. Log how you felt at hours three and six, not just at the end.

The most common mistake is copying an elite's numbers from a podcast. That athlete has spent years training their gut. You have not. Yet.

Training Week	Target Carbs/Hour	Fuel Format	What to Watch
Weeks 1-4	40-50g	Gels plus water	Stomach comfort
Weeks 5-8	50-70g	Gels, chews, drink mix	Energy stability
Weeks 9-12	70-90g	Drink mix plus gels	Absorption, sloshing
Race week	Your tested number	Nothing new	Stick to the plan

Best Energy Gels for Ultramarathon Running

The best energy gel for an ultramarathon is one your stomach has already met, offers enough carbs per serve to matter, and fits your carry system. Flavour fatigue is real over 100km, so variety helps.

A 30g gel means a lot of wrappers over a long day, which is why many ultra runners move to larger formats. The Precision Fuel & Hydration PF 90 Energy Gel carries 90g of carbs in a single resealable pouch, effectively three servings in one.

Gels vs Chews vs Drink Mix: What to Carry

Each format solves a different problem, and most successful 100km runners carry a mix.

• Gels: Fast, precise, easy to count. Best when you need carbs now and your stomach is settled.
• Chews: Good for breaking up the monotony. Slower to absorb.
• Drink mix: Carbs and fluid together, which suits hot conditions. Maurten Drink Mix 320 puts 80g of carbs into 500ml and uses hydrogel technology to reduce sloshing at high intensity.

Ultra Marathon Hydration Strategy: Sodium, Sweat Rate and Fluid Intake

An ultra marathon hydration strategy should match fluid and sodium to your personal sweat rate, not a generic rule.

Weigh yourself before and after a one-hour run in similar conditions. Every kilogram lost is roughly a litre of fluid, your baseline intake rate, and far more useful than any chart.

Sodium matters just as much. Sweat sodium varies enormously between individuals, and replacing fluid without electrolytes can leave you flat and headachy. Precision Fuel & Hydration PH 1000 Electrolyte Tablets deliver 1,000mg of sodium per litre when mixed as directed, with a mild citrus flavour designed to reduce flavour fatigue.

For a deeper look at how hydration affects endurance performance, the Australian Institute of Sport hydration guidance is worth reading before your next long run.

How to Avoid Bonking in Ultra Running

To avoid bonking in ultra running, keep carbohydrate intake steady rather than waiting until you feel empty. Bonking isn't sudden, it's the bill arriving for a debt you built over the previous two hours.

The signs are familiar: irritability, foggy thinking, heavy legs, and a sudden inability to do maths. By then you're behind. The fix is boring and effective.

• Eat on a schedule, not on feel, for the first half.
• Never let a full hour pass without carbs once you're past 90 minutes.
• Carry a small emergency gel for the moment things turn.

Gut Distress Management: The Troubleshooting Guide Most Guides Skip

Here's the bit nobody puts in the brochure. Most 100km runners don't pull out because their legs quit, they pull out because their gut did.

First, work out which problem you've got. They feel similar and need opposite fixes.

• Sloshing and fullness: usually too much fluid or too concentrated a mix sitting in the stomach. Fix: dilute your drink, slow the intake, and switch to small sips of plain water for 20-30 minutes.
• Nausea without fullness: often too much sugar hitting an empty-ish gut, or cumulative carb overload. Fix: back off the gels, take a few salt tabs with water, and try something bland and savoury, a plain boiled potato, a bit of white bread, a few salty crackers.
• Bloating and gas: frequently fibre, fat, or dairy from aid-station food, or a sudden switch to real food your gut hasn't met. Fix: keep race-day food boring and tested. Save the adventurous stuff for the pub afterwards.

The golden rule of a dodgy gut: never make two changes at once.

Gut training is the real fix. Your stomach is trainable, just like your legs.

Night-Time Fuelling: When Your Appetite Quietly Goes on Strike

Somewhere around 2am, something odd happens. You're not hungry. You're not thirsty. You're just... done with food. And that's exactly when your body still needs fuel, because you've got another 30km of dark, cold trail ahead. Appetite suppression at night is real and catches out first-timers constantly. The runners who finish strong planned for it.

• Set watch alarms. Every 30-45 minutes, eat something. Don't trust feel, because feel is asleep.
• Switch to easy-to-open, easy-to-swallow fuel. Cold gels turn into bricks. Chews go rock-hard. Drink mix and softer options win at night.
• Keep fuel warm. Tuck gels in a pocket near your body, not an outside pouch where they'll freeze your hands and your enthusiasm.

Weather Adjustments: Heat, Cold, and Everything In Between

A 100km race rarely serves up one set of conditions all day. You might start in 8°C drizzle and finish in 28°C sun. Your fuelling needs to flex with it.

In the heat:

• Fluid needs climb, but gut tolerance often drops. Lean toward drink mix rather than heavy solids.
• Increase sodium. You're losing more of it, and plain water alone can leave you flat and headachy.
• Chill your bottles and use ice at checkpoints. Cold fluid is easier to get down when you're hot.

In the cold:

• You'll drink less without realising it, because you don't feel thirsty. Sip deliberately at every checkpoint.
• Cold can slow gastric emptying, so favour smaller, more frequent intake over big meals.
• Warm fluids, broth, tea, even warm water, are easier to stomach and genuinely lift morale.

In the wet and windy:

Precision Fuel & Hydration PF 90 Energy Gel – 90g Carbs →

• Everything gets harder to open. Pre-open wrappers or decant into soft flasks before the race.
• Keep a dry layer and dry fuel. Wet hands and wet wrappers are a special kind of misery.

A checkpoint strategy that includes a quick fuel audit, what have I actually consumed in the last hour, and what does the next hour need?, is worth more than any single product.

Race Day Nutrition Plan Template for 100km

Use this as a starting template, then adjust it to your tested numbers and the course profile.

First 30km: Settle in. Take carbs from the first hour, even though you feel fine.

30-60km: This is where the plan earns its keep.

60-85km: Expect a low patch. It usually passes.

85-100km: Light, fast carbs. Nothing heavy. Get it in and keep moving.

Real Food vs Engineered Fuel: When to Switch

Here's the honest truth most gel brands won't tell you: by hour eight, the thought of another sweet, sticky gel can make you genuinely queasy.

The fix isn't to abandon engineered fuel. It's to know when to bring real food into the mix.

Engineered fuel (gels, chews, drink mix) wins when:

• You need precise, fast carbs with minimal gut work.
• You're moving well and don't want to slow down to chew.
• Conditions are hot and you want carbs and fluid together.

Real food wins when:

• Sweet flavours have become unbearable.
• You need something to chew, the mechanical act of eating can settle a queasy stomach.
• You're at an aid station and can afford 2-3 minutes.

The classic ultra real-food options that actually work:

• Boiled potatoes rolled in salt, the unofficial national dish of ultra running.
• Plain white bread or a simple sandwich. Nothing fancy, nothing fibrous.
• Rice, either plain or in small savoury balls.

The trade-off to respect: real food takes longer to digest, delivers carbs more slowly, and is harder to portion precisely.

Checkpoint Strategy: The 90-Second Audit

Aid stations are where good fuelling plans go to die. You arrive, you're overwhelmed by choice, you grab something random, and you leave having consumed 40g of carbs when you needed 80g.

Here's a 90-second routine that keeps you on track:

1. Before you arrive, know what you need. Check your watch or plan. Behind on carbs? Fluid? Sodium?
2. Fill your bottles first. Fluid and electrolytes are the hardest things to catch up on later.
3. Grab one specific food item that matches your plan, not whatever looks nicest.
4. Eat something small while you're standing there. A few crackers, half a banana, a cup of broth. It's free time.
5. Do a quick mental tally. What have I had in the last hour? What's the next hour look like?

If you're crewed, brief your crew on this routine beforehand. A good crew member who knows your plan can hand you exactly what you need before you've even asked.

Frequently Asked Questions

How many grams of carbohydrates per hour do I need for a 100km run?

Most runners target 60 to 90 grams of carbohydrate per hour, but the ceiling depends on your gut. Dextrose and fructose use separate absorption pathways, so blends let you process more without backing up. Start at 60 grams per hour on long training runs and build up over several weeks. Tailwind High Carb Fuel delivers 90 grams per serving for athletes who have trained their gut to handle it, while Precision Fuel & Hydration PF 90 Energy Gel packs 90 grams into one resealable pouch.

What are the best energy gels for ultramarathon running?

The best gel is the one your stomach tolerates at hour eight, not hour one. Look for a mix of glucose and fructose, mild flavours to reduce flavour fatigue, and a format that suits your pockets. Precision Fuel & Hydration PF 90 Energy Gel carries 90 grams of carbohydrate in a resealable pouch, which cuts down on wrapper faff. Test any gel on a long training run before race day, because a gel that works at 20km can behave very differently at 80km.

How do I avoid gastrointestinal distress during a 100km race?

Gut distress usually comes from too much, too fast, or too unfamiliar. Practise your exact race-day fuel on long runs so your gut adapts to absorbing carbohydrate while running. Keep fluid intake steady rather than gulping litres at aid stations, and use hypotonic options like Precision Fuel & Hydration PH 1000 Electrolyte Tablets, which mix at 1,000mg sodium per litre with an osmolarity of 74 mOsmol/L for faster absorption. If your stomach turns, ease back on solids and lean on liquid carbs for a stretch.

Should I rely on real food or energy gels for an ultramarathon?

Both have a place. Gels and drink mixes are easy to absorb and simple to carry, which matters when your jaw is tired and your pockets are full. Real food can break up flavour fatigue and feel more satisfying late in a race, but it digests slower. Many runners run liquid and gel fuel early, then add small amounts of real food at checkpoints. Skratch Labs Super High Carb Drink Mix uses Cluster Dextrin, a complex carbohydrate that digests steadily, sitting somewhere between the two.

How do I create a nutrition plan for a 100km event?

Work backwards from your goal intake. Decide your carbohydrate target per hour, then map it against aid station spacing so you know what to carry and what to grab. Write it as a simple table: time, kilometres, carbs, fluid, sodium. Include a night-time fuelling plan if you will run through darkness, and adjust for heat or cold. Test the full plan on back-to-back long runs before race day, and treat it as a draft you refine rather than a fixed script.

How should I fuel before a long run or a 100km race?

Eat a familiar, carbohydrate-focused meal two to three hours before the start, then top up with 15 to 30 grams of carbohydrate in the final hour. Keep fat and fibre low so your stomach empties quickly. On race morning, sip fluid rather than drinking large volumes at once. The goal is to start with full glycogen stores and a calm stomach, not a heavy breakfast you will regret at kilometre 10.