Carbohydrates During Exercise: The Complete Fueling Guide

Carbohydrates are your main fuel during exercise. Not fat. Not protein. Sugars. At race intensity, your body burns 60 to 90% of its energy from glycogen, and that supply is limited. Too little refueling? You'll bonk. Too much? Your gut will go on strike. In this guide, I'll summarize the complete science, the figures, the mistakes, and the practical application so you know exactly how to fuel during exercise. Are you new to fueling on the bike? First, check out my complete guide to energy bars for cycling for the basics.

Why carbohydrates are your primary fuel

Your body runs on two tanks. Fat and carbohydrates. During low-intensity exercise, you can primarily burn fat, but as intensity increases to tempo, threshold, or race level, your system switches to glycogen as the dominant fuel. Glycogen is the stored form of sugar in your muscles and liver.

The numbers don't lie. A trained endurance athlete has approximately 400 to 500 grams of glycogen in their muscles and about 80 to 110 grams in their liver. Together, this is enough for about 90 to 120 minutes of serious work. After that, it's depleted.

What you feel then isn't a headwind. It's an empty tank. Power drops, focus disappears, and the finale becomes about survival instead of performance. You don't make a difference by training harder on that day. You do by supplying extra carbs externally during exercise to make your glycogen stores last longer and keep your brain functioning at normal levels.

Sports science is crystal clear on this: during moderate to high intensity exercise, carbohydrates are the dominant fuel, and supplementation during exercise is necessary for optimal performance. That's not an opinion. That's physiology.

How many carbohydrates per hour? 60 to 90 grams is the sweet spot

The scientific consensus is clear. The IOC consensus by Burke and colleagues (2011) and Jeukendrup's personalized model (2014) set the framework: 30g per hour is sufficient for efforts of 1 to 2 hours, 60g per hour for 2 to 2.5 hours, and up to 90g per hour for efforts exceeding 2.5 hours. Research by King and colleagues from 2019 tested different doses over 3 hours of cycling and found that 90g per hour maximized time trial performance. Going higher (100g per hour) did not improve anything and increased muscle glycogen utilization instead of sparing it.

Currell and Jeukendrup already found in 2008 that a glucose-fructose mix of 1.8g per minute (108g per hour) improved time trial performance by 8% versus glucose alone and by 19% versus water. That's a huge difference for one hour of intense cycling. The most recent confirmation comes from Rowe in 2022: 90g per hour in a 2:1 ratio improved 5km time trial performance by 7.6% and significantly reduced gastrointestinal discomfort.

The reality on the ground is miles away from that recommendation. Cox and colleagues (2010) measured an average intake of only 25g per hour in elite triathletes during competitions. Two-thirds of the analyzed athletes stayed below the 60g per hour recommended for their event duration. The biggest fueling mistake is therefore not "too much" but "too little." Practically, this means: even if you consciously train your fueling, you often still fall short of your optimum. So your intake should be systematically and deliberately higher.

The hype around 120g per hour and more? For the majority of athletes, this is not immediately relevant. Above 90g per hour, there's a separate body of evidence pointing to a more fructose-rich ratio (1:0.8), coupled with strict gut training. I'll come back to that later in this guide. First, let's cover the basics.

For each session type, it looks practically like this:

The 2:1 glucose-fructose ratio: the true physiological optimum

This is the core of the story. Glucose and fructose are two different sugars, and your gut processes them via two separate transport channels. Glucose enters through one channel (SGLT1), fructose through another (GLUT5). The glucose channel becomes saturated around 60g per hour. Beyond that point, you cannot absorb more glucose, no matter how much you ingest at once.

The review by Fuchs and colleagues (2019) solidified this mechanistically: glucose absorption via SGLT1 plateaus around 1.2 grams per minute. By adding fructose via GLUT5, total carbohydrate oxidation increases to 1.75 grams per minute. This is precisely why the 2:1 ratio (60g glucose + 30g fructose = 90g total) is the proven optimum for those aiming for high fueling targets.

Jeukendrup and Moseley mechanistically demonstrated in 2010 that a 2:1 glucose-fructose solution significantly accelerates gastric emptying and fluid absorption compared to glucose alone. In other words: you get fuel into your system faster and in greater quantities, without sugars remaining in your gut to ferment and cause stomach upset.

This is not a marketing story. It is the physiological foundation of every Eagle fruit energy bar of 25g (~20g carbs in a 2:1 ratio). No gimmick. But solid engineering.

What about systems that mix 4, 5, or 6 carbohydrate sources?

Some sports nutrition brands use mixtures of four to six different carbohydrates such as dextrose, maltodextrin, sucrose, isomaltulose, glucose, and fructose. The underlying idea comes directly from the practice of WorldTour cycling teams: by combining sources with different digestion rates, you get a flatter energy curve and fewer peaks or dips in blood sugar.

This principle is supported by practice and is not just marketing. Riders who have been testing what their bodies can handle at race pace for years know what they are talking about. For those who need to perform consistently for hours, a flatter curve is valuable.

But physiologically, the nuance is stark: there are only two transport channels in your gut. The glucose channel (SGLT1) and the fructose channel (GLUT5). Whether you mix two sources or six. Dextrose, maltodextrin, and sucrose all burden the glucose channel. Sucrose is chemically just glucose plus fructose. Isomaltulose is a slower-absorbing variant of glucose-fructose. They are all variations on the same two routes.

Full is full. If your glucose channel is saturated around 60g per hour, it doesn't matter if those 60g come from one source or four sources. The extra carbohydrates you ingest beyond your transport capacity will remain in your gut, ferment, and cause stomach or bowel problems. A 6-source mix therefore does not increase your total absorption capacity above the 90g per hour that a well-dosed 2:1 mix provides. What it does is vary the absorption rate within those routes. Practical utility, no extra capacity.

There is currently no strong RCT evidence that a 4 to 6-source mix delivers measurably better performance than a well-dosed 2:1 glucose-fructose mix at the same total intake. My choice is simple: a fruit energy bar in a 2:1 ratio, the scientifically best-supported optimum, combined with a sports drink that follows the same 2:1 logic. Variation in texture and format (solid bar plus drink) naturally creates variation in absorption rate within the system. Simplicity that works.

Before you go: pre-race fueling and carb loading

Fueling during sports doesn't start at minute 1. It starts 24 to 48 hours earlier. For efforts longer than 90 minutes, pre-race carb loading has proven effective. The idea is simple: you fill your glycogen stores to the maximum so you start with a full tank.

The practical guideline:

• 36-48h before the race: 8 to 10g carbs per kg body weight per day. For a 70 kg rider, that's 560 to 700g carbs per day. Pasta, rice, bread, fruit, potatoes.
• 3 to 4 hours before start: a carbohydrate-rich meal of 1 to 4g carbs per kg. Low in fat and fiber. Oatmeal, white rice with honey, pancakes with banana.
• 30 to 60 min before start: small snack or gel possible (~30g carbs). Avoid excess here, as it can cause an insulin spike that will penalize you in reverse during the first 30 minutes.
• Hydration: 500 ml water or sports drink in the 2 hours before start. Urinate before you leave.

Most important rule: never experiment with new nutrition on race day. What you test for a race, you test weeks earlier in training. If your gut doesn't know something, it will learn it the worst possible way.

Timing: start early and keep refueling

The biggest fueling mistake? Starting too late. Waiting until you feel hungry or a dip in energy is waiting until it's too late. By then, your glycogen is already depleted and your gut cannot catch up.

The practical rule: start your first intake within the first 30 minutes of your exercise. Then every 30 minutes thereafter. For a goal of 60g per hour, that means about 30g every 30 minutes. For 90g per hour, that's 45g every 30 minutes.

Translated into practice:

• Minute 20-30: first sip of sports drink and/or first bite of an Eagle bar
• Every 30 minutes: supplement according to your hourly goal
• Finale (last 30 min): keep some fuel in reserve, possibly a gel for the final peak

A trick that works: set an alarm on your sports watch every 15 or 20 minutes. This forces you to take something, even if you don't feel hungry. In the heat of battle, everyone forgets to eat. The alarm is dumb but effective.

Drink, gel, or bar? The form matters less than you think

Scientifically, drink, gel, and solid bar perform almost identically, provided the product is low in fat, fiber, and protein. These three components slow down gastric emptying and digestion, which is precisely what you don't want during exercise. Eagle bars are designed for low fat and fiber content so they function as fuel rather than feeling like a snack.

Practically, the context does make a difference. For calmer sections, a long climb, or a planned fueling stop, a solid bar is ideal. You chew, you taste, you feel you're eating something, and that helps mentally. In the thick of the battle (sprint, technical singletrack, intense head-to-head seconds), a gel is more practical because you don't need to chew. And a sports drink continuously delivers small doses without you having to think about it.

That's precisely why my approach doesn't choose one format, but a system. Want to delve deeper into the difference between bars and gels? Read my comparison energy bar vs. energy gel: when to use what. And if you're wondering why more and more cyclists are swapping their classic gels for clean label fruit bars: that's explained here.

Why fruit works as a carrier for your carbs

A question that often arises: is a natural carbohydrate source (fruit) as effective as a synthetic sports nutrition product (gel or chew)? The answer from science is clear. Too and colleagues in 2012 compared raisins with commercial sports chews in trained runners during 80 minutes of submaximal exercise followed by a 5km time trial. Result: identical performance, identical blood glucose, no difference in gastrointestinal complaints. The raisins even showed a lower insulin peak and better fat oxidation.

Conclusion: fruit as a carrier for carbohydrates is functionally equivalent to synthetic gels and chews. At comparable doses, the body works with it just as well. And you get natural micronutrients, polyphenols, and fiber in microdoses. Without artificial colors, flavors, or scents.

For Eagle, this is no surprise, but an important scientific foundation. A fruit energy bar with ~50% fruit pulp, a 2:1 glucose-fructose ratio, and no artificial additives delivers exactly what you need. The same absorption kinetics as a synthetic product, with natural taste and a clean label as a bonus. Related reading: how Eagle differs from classic fruit candy in terms of functional composition.

The Eagle fueling framework: a system instead of separate bites

The biggest misconception among many athletes is that they view fueling as "taking a bar when I feel like it." That doesn't work. Fueling is a system that starts before the session and continues throughout every phase. My fixed framework for 2+ hour efforts:

• 6-8% carb sports drink as the main source: 500 to 800 ml per hour provides 30 to 60g carbs and keeps you hydrated. Constant supply, little thought required.
• 1 to 2 Eagle bars per hour as a solid component: each bar provides ~20g carbs in a 2:1 ratio. Solid food keeps your stomach busy and mentally signals that you're fueling.
• Possibly 1 gel in the final stages: for the very last push or at high intensity where chewing is no longer possible.

Goal: 60 to 90g carbs per hour through the combination. Not through bars alone, not through drink alone. The system wins.

An Eagle fruit energy bar fits perfectly here. 25g format. ~20g carbs in 2:1. Soft and chewable, not a brick. Vegan, gluten-free, and without artificial colors, flavors, or scents. Produced in a certified Belgian production facility (BRCGS Food Safety, ACS). Functionally designed, not as a snack but as fuel during exercise. Want inspiration for your next ride? Check out this fuel for your bike ride.

By sport type: tips for cycling, running, triathlon, and MTB

The fueling principles remain the same, but practical execution varies greatly by sport. What works on the bike may not necessarily work during a marathon.

Cycling (road)

The ideal sport for solid food. You're seated, your hands are free, chewing is not an issue. Water bottles in your cages, bars in your back pockets. My standard for a 3 to 4-hour ride: 2 bottles of sports drink (~750 ml/hour), 1 to 2 Eagle bars per hour, and 1 gel in reserve for the final push or a tough climb. Start fueling as soon as the neutralization ends, not just on the first climb.

Running (road and marathon)

The impact of each stride causes more movement in your gut. This increases the risk of gastrointestinal issues with high carb intakes. Gels and concentrated liquids are generally more convenient than large pieces of solid food. For a half marathon, 30 to 45g per hour is sufficient; for a full marathon, aim for 60 to 90g per hour via gels or a combination of sports drink and small portions of solid food (an Eagle bar cut into pieces works well). Drink small sips at each aid station.

Triathlon

Three sports, three strategies. During swimming, you can't ingest anything. On the bike, your big fueling moment is: as much solid food and sports drink as possible. Aim for 90g per hour if your gut can handle it. During running, your strategy automatically shifts to gel-focus and concentrated drink, as solid food is harder to digest there. Bottom line: you refuel most on the bike.

Mountain bike and gravel

Rough terrain, technical sections, hands often on the handlebars. Eat and drink in the calmer sections, not in a rock garden. A soft flask with concentrated sports drink in your backpack often works better than bottles. For longer gravel events, an Eagle bar fits in a top-tube bag that you can easily open and consume in 2 to 3 bites.

Adapting to the weather: warm, cold, and altitude

Climate changes your fueling strategy. Not drastically, but measurably.

Warm weather (above 25°C)

Heat directs more blood to your skin to cool down and less to your gut. This lowers your carb tolerance. In the heat, lower your carb goal by 10 to 20% and increase your fluid intake to 800 or even 1000 ml per hour. Add more sodium (500-700 mg per liter of sports drink). Solid food can be heavier, so prefer colder bottles and possibly more gels instead of bars.

Cold weather (below 5°C)

In the cold, your fingers stiffen, and opening a gel or getting a bar out becomes annoying. Furthermore, your thirst signal decreases in the cold, causing you to often drink too little and secretly dehydrate. Solution: concentrated carbohydrate drinks in an insulating bottle. High carb density in a small fluid volume, and you prevent your drink from freezing. Eagle bars still work, but pack them in an inner pocket where your body heat keeps them soft.

Altitude (above 2000m)

At altitude, you burn up to 20% more energy for the same effort. Your appetite can decrease at the same time, which is a dangerous combination. Aim at the upper end of your fueling range (rather 90g than 60g per hour). Drink extra because you lose more fluid through your breathing. Plan small, frequent portions instead of large chunks.

Caffeine as a performance booster

Caffeine is one of the most thoroughly researched legal performance enhancers in sports. Spriet showed in 2014 that low doses of 1.5 to 3 mg per kilogram of body weight are sufficient for a measurable performance effect. The ISSN position stand from 2021 places the sweet spot between 3 and 6 mg/kg for most athletes. For a 70 kg rider, that is 210 to 420 mg.

How caffeine works: it blocks adenosine receptors in your brain, making you feel less fatigued for the same effort. It also lowers your perception of pain and effort. Bonus: caffeine can slightly stimulate fat oxidation, which saves extra glycogen in long events.

Practical strategy: 30 to 60 minutes before your hardest hour, or as microdosing spread during long efforts (50 mg every 1-2 hours). Eagle BOOST provides 50 mg of caffeine per bar, comparable to half an espresso, embedded in a 2:1 carb matrix. Not intended to charge your ride for long, but to enter the decisive phase sharply.

Do you want the full science behind caffeine and sports? Read my in-depth article: caffeine, sport and performance: the science behind every watt.

Hydration and electrolytes: the other half of fueling

Carbs without fluid is a recipe for a gut crash. Your gut needs water to transport absorbed sugars to your bloodstream. Concentrated drink without sufficient water even draws fluid from your blood into your gut (osmotic pull), causing you to dehydrate even faster.

The guideline:

• Fluid intake: 500 to 800 ml per hour. In the heat: 800 to 1000 ml.
• Sports drink concentration: 6 to 8% carbs. This is called isotonic and is the optimal balance between carb intake and gastric emptying.
• Sodium: 300 to 700 mg per liter of sports drink. In heat or for heavy sweaters, aim higher.
• Potassium, magnesium, calcium: less important during, especially relevant after exercise.

For competitions longer than 3 hours, in the heat, or for heavy sweaters, it pays to choose an endurance bar with extra sodium baked in. No loose salt packet, no separate tablet, just incorporated into your solid food.

Above 90 grams per hour: why 1:0.8 is becoming the new reference point

In recent years, a second ratio has been gaining traction: 1:0.8 glucose-fructose. Not 1:1, not 2:1, but deliberately slightly more fructose-heavy than glucose. Why?

O'Brien and Rowlands published a comparative study in 2011 (in the American Journal of Physiology) in which they compared several fructose-maltodextrin ratios. At intakes around 108g per hour, the 1:0.8 ratio performed significantly better than the classic 2:1: higher exogenous carbohydrate oxidation, fewer stomach complaints, and better time trial performance.

Rowlands and colleagues (2015) built on this with a critical review. Conclusion: for athletes who want to process more than 90g per hour structurally, the 1:0.8 mix provides the best balance between fuel supply and gut comfort. Urdampilleta and colleagues (2020) confirmed this principle in a more recent RCT: a fructose-richer mix increases total carb oxidation and dampens the cortisol response to heavy exertion. The most recent narrative review, Podlogar and Wallis (2022), describes 1:0.8 as the current standard for elite endurance fueling up to 120g per hour.

The physiological logic is simple. Glucose remains capped at its ~1.2g per minute via SGLT1. If you want to exceed that, the extra fuel must enter via GLUT5, and that channel works best with relatively high fructose supply. 1:0.8 gives that fructose pathway more room.

For the majority of athletes who want to reach the optimum of 60-90g per hour, 2:1 remains the proven reference point and a fruit energy bar in 2:1 ratio is the right workhorse. Those who want to go to 100g+ consciously move to 1:0.8 and invest in gut training. Not the other way around. If you want to delve deeper into this discussion, I will come back to it in a separate cluster blog.

The 7 most common fueling mistakes

Most performance problems in endurance sports are fueling problems. Not training problems. Avoid these seven classics and you will instantly raise your level.

1. Starting too late. Your first intake should be within 30 minutes of starting. Waiting until you feel hungry or a dip is wasted.
2. Too much at once. A 30g gel at one moment is OK, but emptying 60g in 10 minutes is asking for gut problems. Always spread in small doses.
3. Relying solely on solid food. Without sports drink, you won't reach your hourly goal and you'll miss out on hydration. Drink is the base, bars are the supplement.
4. Unknown products on race day. What you've never eaten in training, you don't eat on race day. Taste, texture, gut reaction: everything is unknown and everything can be disappointing.
5. No gut training. 90g per hour is achievable, but your gut needs to learn it. Train your fueling protocol weeks before your big goal.
6. Pure glucose without fructose. Above 60g per hour from glucose alone is physically impossible to fully absorb. Always use a 2:1 mix if you want to reach 60g+.
7. Forgetting hydration. Carbs and water go together. Drink at least 500 ml per hour, even in the cold. Thirst is a late signal.

Stomach complaints during exercise: causes and solutions

The review by de Oliveira and colleagues from 2014 showed that 30 to 90% of endurance athletes experience gastrointestinal complaints during an event. For Ironman distances, this even rises to 93%, with 31% reporting severe symptoms. This is not a marginal phenomenon, but one of the biggest performance inhibitors in endurance sports.

Pfeiffer and colleagues (2012) measured 221 athletes in marathons, Ironman triathlons, and cycling races and observed something interesting: higher carbohydrate intake correlated with both more GI complaints and faster finish times. In other words, those who want to perform hard must fuel high, but that high intake costs your gut more. That is no reason to refuel less. It is THE reason to train your gut beforehand.

The main causes can usually be traced back to four categories:

• Too high sugar concentration: drinks with more than 10% carbs draw fluid into your gut and cause bloating or cramps.
• Too much fat, fiber, or protein: these slow down gastric emptying and remain in your system during exercise. Avoid them during.
• Dehydration: less fluid means less blood flow to your gut and less capacity to transport sugars.
• One source of carbohydrates above 60g per hour: saturation of the glucose channel leads to residual sugars and fermentation.

What to do if the complaints do strike? Immediately reduce your carb intake by 30-50% and switch to pure water or very diluted sports drink for 15 to 30 minutes. Avoid solid food during that period. Only when the complaints subside, restart cautiously with small doses. In severe cases, pausing or stopping is always better than pushing through and ruining the whole day.

Prevention remains the best cure: 2:1 glucose-fructose mix, spread intake, sufficient water, and a trained gut.

Calculate your personal fueling plan

Finally, concrete. Here's how to calculate your own fueling plan for a specific ride or race in 4 steps.

Step 1. How long is the effort?
Under 75 minutes = water is sufficient. Between 75 min and 2h = 30g/hour. Between 2h and 3h = 60g/hour. Above 3h = 90g/hour.

Step 2. What is the intensity?
At high intensity (race, threshold pace) aim at the upper end of your range. At a calm endurance pace, at the lower end. A 3-hour race can therefore require 90g/hour, a calm 3-hour ride only 60g/hour.

Step 3. Distribute over drink, bars and possibly gel.
Sports drink typically provides 30-60g per hour via 500-800 ml. A 25g Eagle bar provides ~20g carbs. A gel provides 20-25g.

Step 4. Plan timing.
Rotate bottles every 20 minutes with a few sips. Bars every hour (or in 2 pieces every 30 min). Gel only at the end or during intense phases.

Example 1. Cyclosportive 4 hours at pace (70 kg rider).
Goal: 70g carbs per hour. Plan: 800 ml sports drink per hour (6% concentration = 48g carbs) + 1 Eagle bar per hour (20g carbs) = 68g/hour. Total for 4 hours: ~272g carbs. Add 1 gel in the last hour for the finale = +25g, total ~297g.

Example 2. Marathon 3h30 (60 kg runner).
Goal: 60g carbs per hour. Plan: 1 gel every 30 minutes (6 gels = 150g) + sports drink at each aid station (~30g total) + possibly 1 Eagle bar at km 25 for mental boost and soft carbs (20g). Total ~200g. Hydration: small sips at each station.

Example 3. Half-Ironman (75 kg triathlete).
Bike (2h30): 90g per hour via 1 bottle of sports drink/hour (45g) + 2 Eagle bars/hour (40g) = 85g/hour, total ~210g. Run (1h45): 60g per hour via gels and sports drink, total ~105g. Total for 4h15 of effort: ~315g carbs.

Gut training: practice your fueling, don't just practice your legs

A final, often underestimated point. Your gut is a muscle that can be trained. Athletes who try 90g per hour for the first time often experience stomach complaints. Not because the product is bad, but because their gut is not used to the absorption. Research shows that 2 weeks of consistent practice with high carb intakes during training literally upregulates the SGLT1 transporters. Your gut physically gets better at its job.

Jeukendrup described this in 2017 under the heading of "periodized nutrition": you progressively adapt your carb intake to your training goal. It is exactly the same principle as training your legs on a climb. First build up, then test, then apply on race day. Anyone who wants to process 90g+ per hour on race day must teach their gut to work at that pace 4 to 6 weeks before the event.

The approach: start with 30-60g per hour, gradually build up during training rides, and test what your gut can handle. Race day is never the time to try something new. Once you know your personal limit, you can move further and refine your product choice to achieve your goals.

A practical training protocol over 6 weeks:

• Week 1-2: 60g per hour during your long endurance training sessions. Combination of sports drink + 1 Eagle bar per hour.
• Week 3-4: increase to 75g per hour. Add a second bar or a gel.
• Week 5-6: aim for 90g per hour. Test your race day protocol at actual race pace during a long training ride.
• Race day: only use what you tested in week 5-6. No experiments.

Closing

Carbohydrates during exercise revolve around four things: enough, on time, in the right form, and in the right proportion. 60 to 90g per hour. From minute 30. In a 2:1 ratio via sports drink plus 1 to 2 solid bars. Possibly caffeine in the final. Nothing more, nothing less. No wellness, no marketing fluff, just fuel that works.

No gel. No brick. Fuel during exercise. Fuel your flight.

Choose your fuel