How to Fuel an Ultra — with Guidance from Leading Sports Nutritionists

When you run or walk for many hours in an ultramarathon, your body needs a steady supply of energy — mainly carbohydrate, plus some protein and fat — along with fluids and electrolytes. If you don't get that balance right, you risk bonking, gut issues, or being forced to slow dramatically. The good news: with the right preparation and strategy, fuelling an ultra is a learnable skill.

What is CHO?

Throughout this guide, you'll see the term CHO, which stands for carbohydrate — the body's primary source of fast energy during endurance events. CHO intake is measured in grams per hour (g/hr). Managing this carefully helps you stay fuelled without upsetting your stomach. The right CHO target is not one-size-fits-all — it depends on your pace, body weight, and how well your gut has been trained to absorb carbohydrates under race conditions.

The Science Behind Ultra Fuelling

This article draws on the work of several leading sports nutritionists:

• Dr Asker Jeukendrup — Sports scientist, founder of Mysportscience.com, and researcher into carbohydrate metabolism and endurance fuelling.
• Paul Booth — UK-based sports nutritionist specialising in endurance performance.
• Renee McGregor — Registered Sports & Eating Disorder Dietitian, author of Training Food and The Female Athlete.
• Nigel Mitchell — Former Performance Nutritionist for Team Sky and British Cycling.

A core principle running through all their work: what you eat on race day should never be a surprise to your body. Every element of your plan — from gels to real food to caffeine — must be rehearsed in training.

Pre-Race Nutrition: The Night Before and Race Morning

How you fuel in the 24 hours before the start line matters enormously. Arriving with well-stocked glycogen stores gives you a meaningful advantage, particularly in the opening miles when pace is higher and CHO demands are greatest.

The Night Before: Carbohydrate Loading

Aim for 7–10 g of carbohydrate per kilogram of body weight across the full day before your race — not just at dinner. For a 70 kg athlete, that's 490–700 g CHO. Spread it across meals: porridge with banana at breakfast, pasta or rice at lunch, a carbohydrate-rich evening meal. Keep fat, fibre, and protein moderate to avoid digestive discomfort. This is not the night for a large, rich restaurant meal or anything new.

Good options: white pasta or rice, bread, potatoes, bananas, fruit juice, sports drinks. Avoid high-fibre grains, legumes, and heavy sauces.

Race Morning: Easy-Digest Fuelling

Eat your last proper meal 2–3 hours before the start, targeting 1–4 g CHO per kilogram of body weight. For a 70 kg athlete that's 70–280 g — a wide range reflecting individual tolerance. Classic options: porridge with honey, white toast with jam or peanut butter, or a rice-based meal. If your stomach is nervous on race mornings, stick to the lower end or choose a liquid meal such as a smoothie or sports drink.

In the final 15–30 minutes before the start, a small gel or banana (~20–30 g CHO) is fine for most runners if needed.

Gut Training: The Most Underrated Race Preparation

No matter how sound your nutrition plan looks on paper, your gut has to be able to execute it on the day. The gastrointestinal system is trainable — but only if you put in the work before the race.

Start conservatively in training: 30–40 g CHO/hr on long runs, increasing by 10 g/hr every 2–3 weeks as tolerance builds. Over 8–12 weeks, well-trained runners can adapt to absorb up to 90 g CHO/hr using multiple carbohydrate sources (glucose + fructose combinations). Practise at intensities that match your race effort — gut stress is intensity-dependent, and fuelling that feels easy on an easy jog can become very uncomfortable at race pace. The golden rule: nothing new on race day.

Key Takeaways from the Experts

• Train your gut — Progressively practise your race-day fuelling in training to improve carbohydrate absorption.
• Use multiple carbohydrate sources — Combining glucose and fructose (found in most sports gels, Coke and many bars) improves absorption and reduces gut distress at higher intake rates.
• Match CHO to intensity — Faster runners working at higher effort need more CHO; walkers and slower runners can rely more on fat oxidation and need less per hour.
• Add some protein and fat — Small amounts of protein can help reduce muscle damage across longer efforts (McGregor, Mitchell).
• Hydrate smartly — 400–800 ml fluid/hr and 300–600 mg sodium per hour, adjusted for conditions.
• Practice everything — Test all products, quantities, and timing in training. Your body's response is personal.

THE LAP — Example Race Context

THE LAP is a 47-mile / 75 km ultra trail race around Lake Windermere in the Lake District. It features 5 fully stocked feed stations, 1 halfway drop bag station at Troutbeck, and food including water, cordial, electrolyte drink, Coke, energy bars and gels, salty snacks, and hot pizza at Troutbeck. The aid station layout makes consistent fuelling achievable — but you still need a plan. Know roughly what you need at each stop before you arrive. Fatigue and decision fatigue both set in; removing guesswork saves energy.

How Much to Eat and Drink

Practical guidelines for a 70 kg athlete. Note that CHO targets should reflect actual effort level — 60–90 g/hr suits faster runners at moderate-to-high intensity. If you are walking or moving at a very slow pace, fat oxidation is significantly higher and your CHO needs drop considerably; 30–50 g/hr is more realistic and reduces the risk of gut overload.

Example Hour-by-Hour Plan (Steady Runner)

• 0:00 – Start: Sip electrolyte drink (~25 g CHO). Resist pushing hard early — glycogen stores are full, no need to rush.
• Every 30–45 min: Eat 20–30 g CHO (gel, chew, bar, or small real food). Eat to schedule, not hunger.
• At Aid Stations: Refill bottles, take one CHO source (~20–30 g), assess gut and adjust.
• Halfway (Troutbeck): Eat a small meal (pizza slice or savoury snack), refill bottles, adjust salt and electrolytes. A good point to introduce caffeine if you haven't yet.
• Final 10 miles: Increase small, frequent CHO doses. Coke at later aid stations delivers both caffeine and fast-acting carbohydrate and is typically very well tolerated late in a race.

Caffeine Strategy: Use It Wisely

Caffeine is one of the most well-researched performance aids in endurance sport. The evidence-based dose is 3–6 mg per kg of body weight — for a 70 kg athlete, that's 210–420 mg. But timing matters as much as quantity.

Don't start on caffeine. Using it from mile one when you feel fresh provides minimal benefit and burns through your tolerance early — so it delivers far less when you actually need it in the second half. Hold caffeine back until after Troutbeck (roughly the halfway mark), then use caffeinated gels, Coke at aid stations, or caffeine chews in the later miles when fatigue and focus start to slip.

Be aware: caffeine can accelerate gastric motility (it may make you need the toilet more urgently). Don't introduce high doses suddenly if you haven't trained with it. Practical sources: caffeinated gels (~25–75 mg per gel), Coke at aid stations (~35 mg per 330 ml), caffeine chews or tablets.

When GI Issues Strike: How to Respond

GI problems — nausea, bloating, cramps — are common in ultras even among well-prepared athletes. Respond calmly and systematically.

• Slow down first. Reducing pace diverts more blood back to the gut and often resolves nausea within minutes. Even a short walk can make a real difference.
• Sip plain water. Stop gels and sugary drinks temporarily. Small sips of plain water help settle the stomach; large gulps can worsen bloating.
• Move to bland, savoury foods. Sweet fatigue — especially from gels — is a common trigger for nausea in the second half. Shift to pretzels, crackers, salted potato, or broth. The savoury options at THE LAP's aid stations (including pizza at Troutbeck) serve exactly this purpose.
• Return to fuelling gradually. Once symptoms ease, resume slowly with small, bland foods before reintroducing drinks or gels. Don't try to compensate for lost intake all at once — that usually triggers a second wave.

Hydration & Electrolyte Notes

• Fluids: 400–800 ml/hour — move toward the upper end in warm conditions or at higher intensity.
• Electrolytes: 300–600 mg sodium/hour. Look for white marks on skin or kit as signs of heavy salt loss.
• Monitor: Cramping, bloating, or swelling of hands and feet can indicate under- or over-hydration. Drink to thirst and ensure electrolyte intake keeps pace with fluid intake.
• Don't over-drink: Hyponatraemia (low blood sodium from too much plain water) is a real risk in slower ultras. Never drink beyond thirst, and always pair fluids with electrolytes.

Race-Day Tips

• Mix sweet and savoury throughout to avoid flavour fatigue — plan this into your drop bag and aid station strategy.
• Eat early — don't wait for hunger. By the time hunger signals arrive, you may already be behind on energy (Jeukendrup).
• Plan for the weather — heat and humidity significantly increase fluid and electrolyte needs. Adjust targets on the day.
• Reserve caffeine for the second half of the race. Coke at later aid stations is a reliable, well-tolerated option.
• Pack variety in your Troutbeck drop bag — quick carbs, solids, savoury options, and something you genuinely look forward to eating late in a race.
• Label your drop bag clearly and know exactly what is in it before race morning.

Summary

Fuelling an ultramarathon is about strategy, practice, and personal awareness — not guesswork. Arrive at the start with full glycogen stores from a well-executed carb load. Fuel consistently during the race at targets that match your actual pace and intensity — remembering that walkers need less CHO per hour than faster runners. Use caffeine strategically in the second half. Train your gut thoroughly in the weeks before so nothing on race day is a surprise. And when GI issues arise: slow down, sip water, move to savoury, return to fuelling gradually. Apply the science, and keep your energy steady across every mile of THE LAP.

References

• Jeukendrup, A. Mysportscience.com — Fuelling for Endurance Sports.
• Booth, P. — UK-based sports nutritionist specialising in endurance performance.
• McGregor, R. (2019). Training Food. Bloomsbury Publishing.
• Mitchell, N. — Former Performance Nutritionist for Team Sky and British Cycling.
• For peer-reviewed evidence on carbohydrate intake and gut training in endurance sport, see the International Journal of Sport Nutrition and Exercise Metabolism and Mysportscience.com.