The Runner’s Complete Guide to Fueling on the Run

Keep reading for the detail, because the number is only half of it. There is a specific reason a single gel type stops working past a certain rate, there is a rule in the older version of this guide that I have softened, and most of what runners blame on gels turns out to be caused by something else entirely.

Fueling through sports drinks, gels, chews, or real food is a big part of getting the most out of a long run or race. It also jump-starts your recovery afterwards. As a certified running and nutrition coach, I tell my coaching clients that what they take in on the run can make or break the day, and it sets the tone for how they feel in the hours and days that follow.

Fueling is also individual. Your training level, your metabolism, your genetics, the intensity you run at, and the weather all change how much fuel you need. Get any of those wrong and a plan that worked last month falls apart.

What actually happens when you run out of fuel

When you run, your body draws energy from two main places: fat stores, and glycogen stored in your muscles and liver. Carbohydrate that you eat gets stored as glycogen for later.

You carry a lot of fat and not much glycogen. A trained runner stores somewhere around 400 to 500 grams of glycogen in total, which is roughly 1,600 to 2,000 calories. At marathon effort you can burn through that in about two hours.

That is what bonking is. It is not weakness or bad pacing. It is your liver and muscle glycogen hitting empty while your brain and legs still need it. Fat is an almost unlimited fuel source, but it cannot be converted quickly enough to hold marathon pace on its own.

Fueling during the run does not top your tank back up. It slows the rate at which you drain it, and it keeps your blood glucose high enough that your brain stays happy. That distinction matters, because it explains why starting early beats catching up later.

How much carbohydrate do you actually need?

The older version of this guide suggested about 25 grams of carbohydrate every 30 to 45 minutes, which works out to roughly 33 to 50 grams an hour. That was reasonable advice at the time. The research has moved, and for longer efforts that target is now on the low side.

Current guidance scales with how long you will be out there.^1,2

A few things worth knowing about that table.

These targets are independent of body weight and training status.¹ A 200-pound runner and a 130-pound runner have similar absorption ceilings. This surprises people, but the limit is in your intestine, not your legs.

They should be adjusted downward when you are running easy. If your absolute intensity is low, you are burning less carbohydrate per hour, so you need less.¹

And the top of the range is a ceiling, not a goal. Most recreational marathoners do very well at 45 to 60 g/h.

Why 90 grams an hour needs two kinds of sugar

This is the single most useful piece of fueling science most runners have never heard.

Glucose is absorbed from your intestine by a transporter called SGLT1. That transporter saturates at roughly 60 grams per hour. Once it is full, extra glucose does not get absorbed. It sits in your gut, pulls water in with it, and gives you the sloshing, bloated, urgent feeling that ruins races.

Fructose uses a completely different transporter, GLUT5. So if part of your fuel is fructose, you open a second lane. Combining glucose with fructose, usually in something like a 1:0.8 or 2:1 ratio, lets total absorption reach roughly 90 grams per hour.¹

These are the products built specifically around that dual-source ratio, and they are what I would look at if you are targeting the higher end for a marathon or ultra.

Both deliver about 80 grams of carbohydrate in a single bottle, which simplifies the arithmetic enormously. They are also more expensive than mixing your own, so treat them as race-day tools rather than everyday training fuel.

Do you need 120 grams an hour?

You have probably seen elite fueling numbers creeping up. There is real research behind it, and real caveats.

In one randomized trial, 20 elite runners took 120, 90 or 60 g/h during a mountain marathon, and the 120 g/h group showed lower markers of exercise-induced muscle damage afterwards.⁴ A companion study found similar advantages for neuromuscular function.⁵

Note the detail that matters: those runners had previously undertaken nutritional and gut training.⁴ They spent months building that tolerance. Attempting 120 g/h off the couch is how you end up in a portable toilet at mile 18.

For the vast majority of runners, including most sub-3 marathoners, 60 to 90 g/h is the useful range.

When to start fueling

You do not need fuel for runs under 60 to 90 minutes. Once you go past that, you will notice how run down you get without it. For most of us that is somewhere between 5 and 10 miles into a run.

• Half marathon and below: figure out at what point in a run you tend to get tired or hungry. Then start fueling a mile or two before that point.
• Marathon and ultra: fuel from early on, especially if you will be out longer than four hours. You will reach a point where you cannot absorb fuel as fast as you burn it, so the idea is to start replacing carbohydrate from the beginning rather than digging a hole first.
• Set a timer. Every 30 minutes, whether or not you feel like it. Thirst and hunger are terrible guides once you are working hard.

Types of fuel and what they are good for

Energy gels

Gels are the default for a reason: predictable dosing, easy to carry, quick to take. These are the ones I keep coming back to.

I like Honey Stinger because they are organic, made with real honey, and contain electrolytes. Clif Shot is another of my go-to gels simply because it is easy to find and often handed out at races, and it comes in enough flavors that if you dislike one there are plenty more to try. GU is the one that started the category and uses a maltodextrin and fructose blend, which puts it in the dual-source group.

Once you narrow down the type of fuel, only test one kind at a time. If you bought a few different brands, try them on different days so you know how each one sits. Especially the caffeinated versus caffeine-free versions. Race expos and running stores are great places to pick up samples cheaply.

Drink mixes and hydration

Tailwind is popular with ultra runners because it puts calories and electrolytes in one bottle and tends to sit lightly. Skratch uses real fruit and keeps the sugar concentration low, which some sensitive stomachs prefer. Nuun is electrolytes with almost no carbohydrate, so treat it as hydration rather than fuel and pair it with gels if the run is long.

Chews and blocks

Chews suit runners who would rather nibble than swallow a whole gel at once. The salted watermelon version carries roughly double the sodium, which is worth knowing if you are a heavy or salty sweater.

Real food and cheaper options

Plenty of runners fuel on regular food: gummy bears, Starburst, honey sticks, dried fruit, boiled potatoes with salt for ultras. Just avoid high-fiber options. Fig Newtons would be perfect if the fiber did not get so many people, and bananas cause the same trouble.

I like Combos pretzels, especially the peanut butter filled ones, because they bring carbohydrate, salt and a bit of protein. The problem is that I do not like carrying them, so I stash them in the car or at a water stop.

One thing to note: not all fuels are equal. Some contain caffeine, some do not, and sodium content varies a lot between products. Read the labels rather than assuming.

One category I would skip is BCAAs. They are marketed for endurance but do very little for fueling, and the carbohydrate is what you are actually there for.

Hydration and sodium

Fuel and fluid are separate problems that interact, and the older version of this guide sent you elsewhere for the fluid half. Here are the essentials.

Drink to thirst as your default. For most runners in most conditions, thirst is a reasonable guide, and it protects you from the bigger risk in both directions.

Do not force fluids. Drinking far more than you lose dilutes your blood sodium and can cause exercise-associated hyponatremia, which is genuinely dangerous and has killed runners at major marathons. International consensus guidance exists specifically on preventing and treating it.⁶ If you finish a race heavier than you started, you drank too much.

Sodium matters more as duration and heat go up. Sweat sodium concentration varies enormously between individuals. If you finish long runs with salt crust on your face and your kit, you are on the salty end and should look for higher-sodium products.

Our guide on how to stay hydrated when you run goes deeper on electrolytes and what they actually do.

Caffeine

Caffeine is one of the few genuinely well-supported performance aids, and a lot of gels contain it.

The International Society of Sports Nutrition position stand reports consistent performance benefits at 3 to 6 mg per kilogram of body weight, with aerobic endurance showing the most consistent moderate-to-large benefits. Very high doses around 9 mg/kg bring side effects without extra benefit, and the usual timing is about 60 minutes before exercise.⁷

For a 155-pound runner, 3 to 6 mg/kg works out to roughly 210 to 420 mg. A caffeinated gel typically has 25 to 100 mg, so they add up quickly across a marathon. Count what you are taking, including your pre-race coffee.

Our article on caffeine and running performance covers the details. And if you are sensitive to it, over-consuming caffeine can leave you jittery or cause GI trouble, which is the last thing you want mid-run.

Help, gels upset my stomach

This is the most common objection I hear, and you are far from alone. Studies suggest that 30 to 50 percent of endurance athletes experience gastrointestinal complaints.⁸

Most symptoms are mild and not a health risk, but they will ruin your day. The causes fall into three groups: physiological, mechanical and nutritional.⁸ Here is the practical version.

Causes related to gels and drinks

• Concentration. Gels and carbohydrate drinks are concentrated to deliver energy fast. Too concentrated, and they draw water into your gut, causing bloating and discomfort.
• Single-source overload. Going past about 60 g/h on glucose alone leaves unabsorbed carbohydrate sitting in your intestine.¹ This is a genuinely common cause and the fix is switching to a dual-source product, not eating less.
• Specific ingredients. You might be sensitive to fructose, artificial sweeteners or thickeners. Experiment to identify the trigger.
• Volume and speed. Too much too quickly overwhelms your digestive system. Take smaller, more frequent amounts.

Causes that have nothing to do with your gels

• Dehydration. During intense exercise, and especially when you are already low on fluid, blood flow to the gut is reduced. That is believed to be one of the main contributors to GI symptoms.⁸
• Your pre-run meal. High fiber, fat or protein before a run is hard to digest during one. Our guide to what to eat before a run has plenty of options.
• Under-eating. Too little fuel leads to low blood sugar, which brings its own nausea.
• Intensity. Hard efforts divert blood away from the gut. The same fueling plan that works on an easy long run can fail at race pace.
• Underlying conditions. IBS and lactose intolerance can be aggravated by exercise. If symptoms persist, see a doctor.

Train your gut, not just your legs

This is the part that turns fueling from a gamble into a skill, and it is the biggest gap in the original version of this guide.

Your ability to absorb carbohydrate during exercise is trainable. Repeatedly taking carbohydrate during training increases intestinal transporter capacity and improves comfort at higher intake rates.³ That is why elite fueling studies specify that participants completed gut training first.⁴

How to do it:

1. Start where you are. If 30 g/h currently feels fine, begin there.
2. Add roughly 10 g/h every couple of weeks on your long runs, not on every run.
3. Use your race products. Gut training is product-specific enough that practicing with something you will not use on race day wastes part of the benefit.
4. Practice at race pace at least a few times. Fueling that works at easy effort can fail at goal pace.
5. Give it 6 to 10 weeks. This is a training block, not a race-week fix.

How to actually take a gel

• Take gels with water. Gels are concentrated and need fluid to dilute them. Follow the packet instructions on how much.
• Be careful stacking a gel on top of a sports drink. The older version of this guide said never to mix the two. The principle behind that is right, which is that you can easily end up with too much sugar and too little water in your gut at once. But the absolute version is too strict, since plenty of runners successfully combine drink mix and gels on long efforts. What matters is your total carbohydrate per hour, whether it comes from multiple transportable sources, and whether you are taking in enough fluid alongside it. Practice the combination before you race it.
• Check the course map for water stop locations so you can plan to take fuel just before or at them.
• Pay attention to how you feel afterwards. You should notice a slight lift shortly after. Never take more than the packaging recommends in one go.

Pre-race fueling

For runs longer than an hour, eat a light meal or snack of roughly 400 to 500 calories, one to three hours beforehand, weighted toward carbohydrate and light on fiber and fat. This tops off the liver glycogen you burned overnight.

Skipping breakfast before an endurance race can be disastrous. Whatever you choose, practice it. Race morning is not the time for an unfamiliar bagel.

Post-run refueling

Get carbohydrate and protein in reasonably soon after finishing. The classic advice is a 30 to 90 minute window, and the older version of this guide leaned on that hard.

The nuance now: how urgently this matters depends on when you next run. If you have another session within about eight hours, refuel aggressively and immediately. If your next run is tomorrow, your total intake across the day matters more than hitting a narrow window.²

Either way, eating soon after a long run is a good habit, and it usually makes you feel better regardless of what glycogen resynthesis is doing.

Do not let race nerves wreck the plan

Nerves can kill your appetite on race morning. Skipping your planned fueling because you do not feel like it is how runners end up crashing at 18 and then feeling too sick to take anything.

Do everything the same on race day as you did in training. Your body expects it.

Underfueling is the bigger risk

Most of the fueling problems I see are not runners taking too much. They are runners taking far too little, across weeks and months rather than in a single race.

Persistent low energy availability, where intake does not cover training plus basic function, has consequences well beyond a slow race: impaired bone health, hormonal and metabolic disruption, and reduced performance. The IOC published a consensus statement on Relative Energy Deficiency in Sport covering this in detail.⁹

If you are constantly exhausted, losing performance despite training well, getting injured repeatedly, or you have lost your period, that is worth taking to a doctor or sports dietitian.

What about fat adaptation and low-carb running?

There is a real process, sometimes called fat adaptation, where you train your body to rely more on fat. Achieving it takes months of very low carbohydrate intake.

It does what it claims metabolically, but there is a cost. When elite race walkers followed a ketogenic low-carbohydrate high-fat diet for three weeks alongside intensified training, fat oxidation rose sharply, but exercise economy got worse, meaning they needed more oxygen to hold the same speed. Performance was impaired compared with high-carbohydrate groups, despite an improvement in peak aerobic capacity.¹⁰

For runners chasing a time goal, high carbohydrate availability around hard sessions and races remains the better-supported approach.¹¹

Building your own fueling plan

Here is how I would put it together for a first marathon, as a worked example.

1. Pick a target. Say 60 g/h for a four-hour marathon, so roughly 240 g total.
2. Choose your delivery. Three gels an hour is a lot to swallow. Two gels plus a bottle of drink mix per hour is easier.
3. Check the carbohydrate sources. If you are at or above 60 g/h, make sure the products list two carbohydrate types.
4. Schedule it. Write the actual clock times on your hand or your watch. Every 30 minutes, no negotiating.
5. Rehearse it twice on long runs at goal pace, using the same products, in similar conditions.
6. Have a backup. Know what the course provides in case you lose a bottle, and practice with it once.

Frequently asked questions about endurance fueling (FAQ)

How many carbs per hour should a runner take?

For efforts of 1 to 2.5 hours, 30 to 60 grams per hour. Beyond about 2.5 hours, up to roughly 90 grams per hour, provided the carbohydrate comes from multiple transportable sources such as a glucose and fructose blend.¹

Do I need to fuel on a 10 mile run?

If it takes you longer than about 75 to 90 minutes, yes. Most runners covering 10 miles will benefit from taking something around the hour mark, both for the run itself and for recovery afterwards.

Why do gels make me sick?

Usually concentration, rate, or dehydration rather than the gel itself. GI complaints affect 30 to 50 percent of endurance athletes, and reduced blood flow to the gut during hard or dehydrated running is a major contributor.⁸ Try taking gels with more water, spreading intake out, and switching to a dual-source product if you are pushing past 60 g/h.¹

What is the difference between a sports drink and an electrolyte tablet?

Sports drinks provide both carbohydrate and electrolytes. Electrolyte tablets like Nuun provide almost no carbohydrate. Tablets hydrate; they do not fuel. On a long run you generally need both.

Should I use caffeinated gels?

They work, within limits. Benefits show up at 3 to 6 mg per kilogram of body weight, and higher doses mainly add side effects.⁷ Count your total across the race, including pre-race coffee, and never try caffeinated gels for the first time on race day.

Can I train my stomach to handle more fuel?

Yes, and you should. Regularly taking carbohydrate during training increases your capacity to absorb it and improves comfort at higher intakes.³ Build up over 6 to 10 weeks using your race-day products.

What should I eat right after a long run?

Carbohydrate plus some protein, reasonably soon. If you are running again within about eight hours, prioritize speed. Otherwise your total daily intake matters more than hitting a precise window.²

What I tell my runners

Fueling is a skill, not a purchase. The runners who nail it are not the ones with the most expensive gels. They are the ones who picked a plan, practiced it on long runs until it was boring, and then executed exactly the same plan on race day.

Start earlier than feels necessary, take more than feels necessary, and practice both.

When it comes to fueling there are a lot of moving parts, so I recommend working with a coach who understands both your running and your nutrition. A sports dietitian is another great option, particularly if you suspect an underlying gut issue.

Happy running, and practice your fueling.

Sources

1. Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 2014. PubMed 24791914
2. Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise, 2016. PubMed 26891166
3. Jeukendrup AE. Training the Gut for Athletes. Sports Medicine, 2017. PubMed 28332114
4. Viribay A, Arribalzaga S, Mielgo-Ayuso J, Castañeda-Babarro A, Seco-Calvo J, Urdampilleta A. Effects of 120 g/h of Carbohydrates Intake during a Mountain Marathon on Exercise-Induced Muscle Damage in Elite Runners. Nutrients, 2020. PubMed 32403259
5. Urdampilleta A, Arribalzaga S, Viribay A, Castañeda-Babarro A, Seco-Calvo J, Mielgo-Ayuso J. Effects of 120 vs. 60 and 90 g/h Carbohydrate Intake during a Trail Marathon on Neuromuscular Function and High Intensity Run Capacity Recovery. Nutrients, 2020. PubMed 32679728
6. Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference. British Journal of Sports Medicine, 2015. PubMed 26227507
7. Guest NS, VanDusseldorp TA, Nelson MT, et al. International Society of Sports Nutrition position stand: caffeine and exercise performance. Journal of the International Society of Sports Nutrition, 2021. PubMed 33388079
8. de Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine, 2014. PubMed 24791919
9. Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine, 2023. PubMed 37752011
10. Burke LM, Ross ML, Garvican-Lewis LA, et al. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. The Journal of Physiology, 2017. PubMed 28012184
11. Podlogar T, Wallis GA. New Horizons in Carbohydrate Research and Application for Endurance Athletes. Sports Medicine, 2022. PubMed 36173597