Lifelong runner Dan DeBaun learned something new during his eighth marathon, in 2023: Endurance events, in addition to being physically and mentally challenging, are effectively eating competitions.
Around mile 15 of that marathon, DeBaun bonked. His body simply ran out of fuel. “I was cramping up and miserable,” the Minnesota-based public-relations manager recalls.
Shaking, lightheaded, and barely able to move his legs, DeBaun questioned whether he could make it another mile. “It went horribly,” he recalls. He managed to complete the 26.2-mile race, thanks to sheer willpower and extra calories from sports drinks provided on the course.
“I never wanted to feel that way again,” DeBaun says.
After the race, he realized his training needed to include a nutrition strategy. What he ate before and during a race was going to be key to his performance.
Along with putting in the miles, DeBaun spent the training months before his next marathon testing and retesting his fuel intake. He began by experimenting with the timing of his intra-race fueling. “Instead of taking some carbohydrates every 45 to 50 minutes like I had been doing, I tried every 20 to 25 minutes and also every 30 minutes.”
When race day came, he felt like he had found his sweet spot. “I settled on every 25 to 30 minutes. More often upset my stomach; any longer [and] I didn’t have as much energy.”
By replenishing those all-important carbs more frequently, DeBaun was ultimately providing his body with more fuel over the length of a race.
His fueling strategy was a game-changer. Running his ninth marathon — Grandma’s Marathon in Duluth, Minn. — DeBaun recorded his second-fastest time ever: 3:23.
“I learned that in prior races, I was underfueled,” he says. It was a mistake he vowed never to repeat.
Once upon a time, the popular approach to race-day nutrition was simple: Ingest some combination of carbs and caffeine. Many endurance athletes relied on carbo-loading the night before an event and then downing an espresso or energy drink for a caffeine boost off the starting line.
During the race, they did their best to drink plenty of water — or, as a secret weapon, gulp flat Coca-Cola or eat jelly beans. There was little science to it but lots of superstition.
“Figuring out your race-day nutrition strategy is as important as what shoe you’re going to wear,” says Olympian Carrie Tollefson, an elite middle-distance runner.
Today, sports nutrition and endurance fueling are at the cutting edge of performance. Sure, new technology, refined technique, and better training all play a role in winning races and breaking records, as well as in simply crossing the finish line. But there’s a new level of understanding about the essentials of fueling before, during, and after an event.
“Figuring out your race-day nutrition strategy is as important as what shoe you’re going to wear,” says Olympian Carrie Tollefson, an elite middle-distance runner.
In bicycle racing, for instance — from the one-day “classic” races to the Tour de France — a revolution in nutrition science is helping racers eat more fuel during competition. That development is a big reason these events are claiming more record-setting performances than ever before: By eating better, riders are able to sustain these elevated levels hour after hour, day after day — and recover more quickly to do it all over again.
There’s a simple explanation for this, says Ashley Koff, RD: “Without usable energy, you can’t perform at your best.”
From Food to Fuel
Your body uses food as an energy source by breaking down macronutrients (carbohydrates, proteins, and fats) into smaller molecules — namely glucose, amino acids, and fatty acids. It then uses them to produce adenosine triphosphate (ATP), the body’s primary energy currency.
There are several ways your body turns what you eat into what you do. Known as your metabolic energy systems, these pathways all switch on during physical exercise — but their roles depend on the available energy and specific demands of your chosen activity, explains Mike Thomson, CSCS, USATF, a Life Time running and triathlon coach.
Once you begin intense activity, it takes just thousandths of a second for the adenosine triphosphate-creatine phosphate system, or phosphagen system, to kick in. The ATP stored in the muscles can fuel only about six to 10 seconds of serious effort. This system might engage at the start of a race for quick acceleration, but its utility is short-lived.
The glycolytic system provides energy for high-intensity, short-duration bursts of activity. It can produce ATP rapidly without oxygen, using glucose as fuel. It kicks in for short bursts lasting about 10 to 75 seconds — great for picking up speed to overtake a competitor.
It’s the third energy system — the oxidative system — that is most critical for endurance athletes who need to maintain a relatively moderate intensity for a longer duration. This system uses oxygen to generate ATP. The oxidative system is also called the aerobic system.
“Think of it as the body’s slow-burning furnace,” designed to provide the body with most of its energy, says Thomson. This is the system you want your nutrition plan to stoke. (Learn more about the body’s energy systems at “All About Your Metabolic Energy Systems.”)
Mind Your Gut
Because the body breaks down glucose, fatty acids, and amino acids to produce ATP, it might seem intuitive to fuel and refuel for an endurance event with a balanced blend of carbs, fats, and protein. But not so fast.
“Food as fuel is not the same as nutrition for your health,” says Koff, founder of The Better Nutrition Program. While consuming optimal amounts of macro- and micronutrients is imperative for overall wellness, the unique needs of a body under duress can make getting them from certain healthy foods hard to stomach.
During intense or prolonged activity, the body diverts blood to the muscles, lungs, heart, and brain, and away from the digestive tract, which may impair digestion and drastically alter gastrointestinal transit time. And hormonal changes during intense and long-duration exercise can tamp down hunger cues and diminish appetite. As a result, it’s common for endurance athletes to experience nausea, diarrhea, abdominal pain, and vomiting.
But digestive upset is not a foregone conclusion, says Koff. She suggests avoiding high-fiber, high-fat, and high-protein foods and drink — all of which are more filling and as a result more difficult to digest — immediately before or during exercise. And limit these nutrients the day leading up to an event.
Thomson recommends avoiding unfamiliar foods, drinks, and nutritional aids the day of — or even in the week leading up to — your race.
So, what should you consume to fuel your performance?
Prerace: Build a Strong Foundation
It’s vital to make sure you’re topped up with fluids, electrolytes, and carbohydrates before you begin your activity.
“Trying to make up for a deficit if you start a bit dehydrated or energy depleted by eating and drinking during the activity itself is definitely leaving it too late,” writes sports scientist Andy Blow, BSc, in an article on his training website, Precision Fuel and Hydration. In other words, you can’t play catch-up during heavy activity.
Follow these tips in the days leading up to your event.
Carbo-Load With Care
We primarily store glycogen in our skeletal muscle and liver. An adult can store about 100 grams of glycogen in their liver and approximately 15 to 25 grams of glycogen per kilogram of muscle mass in their skeletal muscle (total storage varies by muscle mass).
On average, adults can store about 600 grams of total glycogen — and you want those stores to be topped off before race day. Rather than eat as much pasta and bread as possible the night before an event, take a few days to shift your carb-to-fat ratio, Blow advises. Your overall caloric intake likely won’t change; rather, the proportion of your calories from carbs gradually increases while that from fat declines.
This doesn’t have to be drastic. An extra spoonful or two of oatmeal or rice (or other carb-rich foods that you enjoy and tolerate) at each meal can go a long way while sparing your digestive tract from an abrupt change, notes Koff. (Learn more about carbo-loading at “What Is Carbohydrate Loading?“)
Hydrate Holistically
Like carbo-loading, sufficient hydration is a gradual process and involves more than just chugging plain water before toeing the start line. Samantha McKinney, RD, CPT, recommends the following best practices for hydration throughout your training cycle and in the days leading up to your event.
- Aim to drink at least half your body weight in ounces of water per day, plus an additional 16 ounces per hour of exercise.
- Add electrolytes around workouts. Sodium, potassium, chloride, calcium, and magnesium support muscle and nerve function.
- Avoid drinks and electrolyte products with artificial colors, flavors, or sweeteners.
- Limit excess — or avoid — caffeine and alcohol.
Plan Your Prerace Meal
It’s paramount to test any preworkout nutritional strategy during your training cycle and to avoid mixing it up the day of your event.
With that important note, here’s what experts suggest for your first race-day meal:
- One to two hours before your event, consume a meal of easily digestible, low-fiber carbohydrates (70 to 80 percent of the meal’s calories) with low to moderate levels of fat (less than 15 percent of calories) and protein (10 to 12 percent).
- Additionally, drink 16 ounces of water with electrolytes. (For preworkout food ideas, visit “12 Quick Preworkout and Postworkout Snacks.”)
With ample glycogen and hydration stores, you’ll be set to perform your best when the race starts.
During the Race: Stay Charged Up
Your in-race strategy is to keep your tank from sputtering to zero. The two-pronged focus for most athletes is on carbohydrates and hydration.
The exact amount, form, and timing of carbs and hydration will be unique to you, and it may take a fair amount of trial and error to ascertain those details. Still, there are general guidelines to help you determine what works best. These are largely based on how long and how hard you’re working.
The exact amount, form, and timing of carbs and hydration will be unique to you, and it may take a fair amount of trial and error to ascertain those details.
“Time and intensity dictate the fuel,” says Thomson. “For example, if you and I did five minutes of extremely hard effort, we would essentially fire the same substrate. But if we ran a marathon, my time might be three hours and five minutes, and yours might be longer or shorter, impacting our [nutrition] needs.”
Top Off the Carbs
Nailing the right amount of energy is a Goldilocks affair. Inadequate carbohydrate intake can lead to glycogen depletion, which occurs when your body runs out of sugar to burn. Many endurance athletes are familiar with this phenomenon, often called “bonking” or “hitting the wall”: The lack of fuel causes fogginess and extreme fatigue and typically leads to compromised performance.
When you hit the wall, it’s not just your muscles that run out of fuel. Your brain, too, runs on glucose and requires a steady supply for proper functioning. Without that supply, you may experience brain fog and loss of coordination and other cognitive skills, says Myles Spar, MD, an expert in personalized performance medicine and national director and vice president of medical services at AndHealth.
Glycogen depletion can also lead to digestive upset and impaired recovery after an event. In rare cases, it can cause exertional rhabdomyolysis — a potentially fatal condition in which muscle fibers break down and toxic compounds enter the bloodstream.
Overfueling can also cause problems, notably digestive distress: Excess sugar stimulates the gut to release water and electrolytes, which can loosen bowel movements. Other signs that you’ve ingested too much include lethargy, stomachache, cramping, and vomiting.
The optimal carbohydrate intake ranges from 40 to 90 grams per hour, says Thomson, but this can depend on the length of the event:
- For endurance efforts of 60 to 150 minutes, you may want to stick to the lower end of this range and ingest 40 to 60 grams of carbs per hour.
- For longer endurance or ultraendurance efforts lasting four to six hours or more, you may benefit from edging closer to 90 grams of carbs per hour. (It may take rigorous training to ingest more than 90 grams per hour, explains Blow, who notes that this approach isn’t necessary for most exercisers.)
Again, finding the right range and timing for you requires practice. Take advantage of your training to determine what works best for you. You might find that 80 grams per hour is your sweet spot but only when divided into 40-gram increments every half hour.
Your experiment should include liquid carbs, solid carbs, and gels. They are all good options — and you can mix and match these as desired and tolerated.
A small banana, for instance, contains about 23 grams of carbs — about the same as a serving of many sports gels and chews. Sports-drink mixes can support hydration and replenish energy stores; super-high-carb mixes can provide as much as 100 grams of easy-to-digest carbohydrates.
In general, Spar recommends trying to avoid carb sources containing high-fructose corn syrup and artificial flavors, preservatives, and dyes.
Maintain Fluid Intake
“Hydration is about much more than water intake,” McKinney says. Your needs will depend on numerous factors. These include temperature, humidity, and altitude; your stress levels; and how well hydrated you were when you woke up on the day of your event, among others.
If you sufficiently hydrate prior to your event and anticipate a lower-intensity effort or one lasting less than 90 minutes, you might only need to sip small amounts of an electrolyte drink. Blow notes that elite athletes competing in high-intensity aerobic events have benefited from rinsing their mouths with a carb-containing electrolyte drink and spitting it out. You can experiment with these methods during training.
Sweat loss can become more significant between 90 minutes and two hours into a race, making it important to rehydrate. As a general target, McKinney recommends drinking about 16 ounces of water with electrolytes per hour of physical activity. This helps replenish the minerals lost through sweating. Some elite athletes choose to measure their sweat rate and adjust their intake to account for sweat loss, she says. (Blow has instructions for testing this here.)
Drinking too much water is less common than drinking too little, but the consequences of extreme overhydration can be as dangerous as those associated with dehydration. Overhydration can dilute sodium levels in the blood, which can cause hyponatremia (an electrolyte imbalance) and water intoxication (a rare phenomenon that occurs when water intake exceeds the amount of water excreted by the kidneys).
For athletes who exercise for hours or even days on end, maintaining a precise blood chemistry can be extremely important. Look for electrolyte products containing sodium, potassium, phosphorous, calcium, and magnesium to make the most of every sip. (Learn more about the importance of electrolytes here and at “Everything You Need to Know About Hydration.”)
Consider Proteins and Fats
By the time you start your event, protein and fat will likely take a back seat to carbs — at least until the race is over. That’s because fat and protein take longer to digest, says Koff. They offer less potential as a quick fuel source and they may cause digestive distress.
But that doesn’t mean you should completely avoid either macronutrient. Test out what works best for your body several times before the actual race or event day.
Protein can help minimize potential muscle damage, a normal consequence of prolonged, intense activity. The International Society of Sports Nutrition recommends consuming about 0.25 grams of protein per kilogram of body weight — about 0.11 grams per pound — per hour of intense endurance exercise when taken along with carbohydrates. For a 160-pound individual, that’s almost 18 grams of protein, roughly equivalent to three large eggs or ¾ cup of Greek yogurt.
For more convenient options, look for — or make your own — bars, gels, or chews that contain protein. (Steer clear of additives, like sugar alcohols, to ensure digestibility.) Plant-based athletes can find options containing protein sources such as chia seeds, nuts, and pea protein. (For more on fuel for plant-based athletes, see “The Plant-Powered Athlete.”)
If you carry a hydration pack, you can mix in protein or amino-acid powders, which offer the muscle-protective properties of protein alongside water, electrolytes, and carbs. Clear protein powders are a good option if a thick shake is unappetizing or difficult to transport.
Unlike protein, fat doesn’t offer much benefit during an endurance event. “Fat doesn’t turn over into fuel well in the middle of a workout, and, ultimately, it may slow digestion and could cause GI issues,” Thomson says.
Think of fat as an incidental part of fuel, and pick something that you tolerate well. This might include relatively small amounts of fat in, say, dates stuffed with a bit of nut butter or a pat of butter on some sweet potato. Again, spread with a light hand and practice ingesting it during training.
Post Race: Keep an Eye to Recovery
With the finish line behind you, the focus of your fueling shifts from performance to recovery. Exercise breaks down your body’s tissues, and good postworkout nutrition can repair this damage and support your body’s regenerative powers. This helps not only to minimize postevent fatigue and soreness but also to rebuild your body stronger than before so it can tackle your next endeavor.
Immediately following your event, continue to hydrate with water supplemented with electrolytes and amino acids. You might not have an appetite at this stage, but if you can stomach a protein shake or some other digestible source of protein and carbs, it may be beneficial. (Find more tips on managing impaired hunger after exercise.)
An hour or two after completing your effort, aim to eat a meal of solid food that includes complex carbohydrates, nonstarchy vegetables, and at least 20 grams of protein.
Further support your recovery by engaging in light movement (stretching and foam rolling are great options), doing breathwork or meditation, and catching some z’s.
(For even more nutrition tips to boost your recovery, visit “A Simplified Guide to Pre- and Post-Workout Nutrition.”)
Practice Makes Perfect
It can’t be overemphasized: Make intra-event fueling part of your training. “It’s important to train like you race and race like you train,” Thomson insists.
The number of training sessions needed to ensure a successful approach varies. One study on endurance runners, published in Applied Physiology, Nutrition, and Metabolism, found that two weeks of gut training resulted in improved gastrointestinal symptoms.
Other studies and experts say a gut-training protocol takes four to 10 weeks. “I recommend taking at least a month to adapt to nutrition changes,” says Spar.
During each training session, pay attention to your body’s signals to adjust what, how much, and when you eat. Feeling lightheaded or dizzy can signal dehydration, Spar notes. Craving salt can be an early sign that you’re running low on sodium. Stomach pain and muscle cramps are reliable indications of low electrolytes.
And feeling a lack of energy could be a sign of carbohydrate deficiency. If you crave sugar immediately after a workout, you likely didn’t eat enough carbs before or during your routine.
Learn from your missteps and your successes. Take note of what works for you and rehearse that winning recipe. By race day, you’ll be optimally fueled and ready to go.
This article originally appeared as “Endurance Fuel” in the March/April 2025 issue of Experience Life. Additional reporting by senior editor Maggie Fazeli Fard.
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