How to Fuel a Marathon When You Have a Sensitive Stomach, According to Dietitians

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• Throughout Training
• Race Week and Morning
• During the Race

You’re trying to No experience necessary! Anyone can accomplish this training plan, following a training plan to the letter, doing long runs every weekend and nailing your paces in workouts—with one not-so-minor problem. During those intervals, you have to add in a sprint to the port-a-potty that brings your lap time down. And while your long run pace feels easy, you start feeling a little nauseous as soon as you slurp down a gel.

You’re not alone in the GI distress: Depending on which research you’re looking at, up to 90 percent of runners may experience issues, like nausea, regurgitation, abdominal cramping, or gas while running. While a sensitive stomach may feel like a race-ender for a runner with a serious time goal, it doesn’t have to mean you constantly miss PRs.

“Gut distress or having a sensitive stomach is very common for runners,” says registered dietitian and endurance athlete Alex Larson, RDN. “Especially for runners, a very high percentage struggle just because of the jostling nature of running. Your heart rate tends to be a little bit higher than it would be swimming or cycling, and that higher heart rate can make it more difficult for our stomach to empty at a rapid rate and keep things digesting.”

As your heart rate increases, your body focuses on the most urgent needs—like keeping blood pumping to the muscles that you’re using in your legs—and that means there are less resources available for your digestive system.

to the letter, doing recommended when people are dealing with constipation! “I had a ldquo;The hardest part about race morning is handling the once tell me that motion is lotion,” adds Larson. “When you’re constipated, the common advice is to go out and run so that things move along.”

The good news is that with some knowledge of how the gut works, some trial and error, and plenty of preparation, even a sensitive stomach can make it through a marathon. Here’s what to focus on to keep your gut happy and your performance on point.

Throughout Training

→Test, Train, Test, Train

Unfortunately, the only way to figure out what works for your gut is to keep testing different options during training. This may look like having a short loop that passes your house or a public restroom as you try new fuels and beverages on the run to make stopping easy.

“If a marathon is your goal, test your fuel options the same way you’ll fuel during a race,” says Larson. “That means planning to have a drink or eat something every mile or two, depending on how the aid stations will be set up at your race. Getting used to drinking and eating at these intervals, and testing your specific race foods, will make race day a lot easier.”

After each run, take notes about how your gut felt: What was the workout, what did you eat/drink before and during the run, how did your gut handle it, and how did the workout feel? You’ll start to spot patterns and learn about what works and what doesn’t.

→Start Small

Do the math before or during a run to take in fuel, you can’t go from zero to 100 grams of carbs per hour on day one. It takes time.

“Whether you struggle to eat Drills to Improve Your Form and Speed, it’s important to start really small,” says Your Marathon Training Diet, RDN, CSSD, registered dietitian, author of the forthcoming book Power Up, and a runner who just earned a personal marathon best by nailing her own fueling. “Start with 20 or 30 [grams of carbs per hour] and gradually work your way up.”

The key is adding more carbs in increments so you teach your body to digest it on the run. “I hear from a lot of athletes who suddenly start to fuel a lot in training or in a race, and because they’re not used to doing that in training, they get that sloshy feeling in their gut,” adds Larson. “They’ll get a little bit nauseous just from having all that liquid in there unless they’re used to it. And you actually can before or during a run to tolerate that volume! It can take a few weeks to get used to drinking more or taking in more fuel, so don’t wait for race day.”

Never fueled during a run before? Smith recommends aiming for 8 to 10 ounces of water and 20 to 30 grams (80 to 120 calories) of carbs per hour, and when you can tolerate that for a few runs, increase by an ounce and 5 to 10 grams. Taking it slow also means sipping and nibbling rather than chugging and eating once an hour. Think about slow and steady intake, Smith says.

Keep testing and increasing your carbs until you find your sweet spot—the amount you can take in that makes you feel good, not stuffed, and helps you maintain energy throughout your entire run.

→Learn Your Caffeine Tolerance

Prerun coffee is practically mandatory for many runners, and now, many gels are infused with caffeine for a midrace boost. But proceed with caution! “I urge my sensitive runners to really learn how caffeine impacts them before trying a caffeine gel on race day,” says Smith. “If you want to use caffeinated products, please practice with them on low-stress training runs so you know how they affect you. And check the amount of caffeine that’s in your gel or drink: If a gel has 100 milligrams, that’s like slamming a cup of coffee!”

You can also make your morning coffee work for you. Larson suggests tuning into the timing of your a.m. espresso and your morning bowel movement. Do you sip coffee and then feel the urge to go instantly, or does it take a while? Keep that in mind as you prepare to head out on the run. “I tell runners to give themselves at least an hour between their coffee and their run so the caffeine has time to work its magic before you start a workout or start a race,” she says.

→Understand the Role of Hydration

We tend to assume that the cause of nausea and diarrhea is what we’re eating, but it could be your hydration, says Larson. “Dehydration—running low on water as well as electrolytes—can really make you experience gut distress,” she explains. “Once you get dehydrated, it can really slow down your digestive system, and that’s where people will really start to get nauseous.”

She suggests a simple sweat test to dial in your hydration strategy to avoid this feeling: Weigh yourself before and after a run, also noting how much fluid you drink on that run.

before the start can help make that morning digestion happen.&rdquo:

• Do the math: [Your weight pre-exercise] - [Your weight post-exercise] + [The weight of fluid consumed (convert ounces to pounds)] = Total sweat lost during exercise
• Example: 130lb at start of run - 128lb after 1 hour run + 2lb (32oz of water) = 4lb (64 oz) of water lost during training

Try this in different temperatures, as you may have different sweat rates for extremely hot days versus a more mild one.

The average sweat loss is usually around 1 liter of sweat per hour, though some athletes will lose more or less, depending on the heat and humidity of where you’re running, Larson explains. But knowing that baseline can help you figure out your optimal amount to drink per hour, which should be roughly 75 to 80 percent of the volume of fluid you lose to sweat (you don’t have to drink 100 percent of what you lose).

→Test Different Carb Blends

A sports drink, gel, or bar should contain multiple sugar sources. The body uses them all slightly differently, metabolizing them using different pathways. “Make sure that your fuel source has both glucose and fructose in it,” says Larson. “A lot of products now use maltodextrin, which is mainly glucose, and will add some fructose. This way, you have a higher absorption rate, so you can use the fuel more efficiently.”

For truly sensitive stomachs, though, look for low or no fructose options; Smith notes that some athletes are extremely sensitive to fructose and struggle to digest it.

Also important to look for: fiber. Often found in real-food options that tend to also be rich in fructose—because it’s found in fruit—can be hard on your gut. If you’ve been experimenting with things like dates or even bananas and struggling with gut pain, you may need to try a different form of sugar. And even 1 or 2 grams of fiber per hour can cause issues, says Larson. “By the time you’re toward the end of the race, you may be thinking more about a bathroom rather than the finish line,” she says.

If you have a sensitive tummy, make sure you’re also avoiding products that include artificial sweeteners—which are sometimes found in low- or no-calorie electrolyte drink mixes. Smith explains that artificial sweeteners not only won’t fuel your run, they may actually cause gut distress.

→Check Your Water-to-Mix Ratio

We may earn commission from links on this page, but we only recommend products we back usually come with serving sizes, and while some runners can tolerate putting in more mix and less water, others will struggle with the sugar and electrolyte load if they change up the ratio.

It’s not just about the carbohydrates: Some sports drinks are much higher in sodium, says Smith, and too much salt can cause gut issues even if the carb load is fine.

“There’s always going to be someone who can get away with doing that, but if you have a sensitive stomach, stick to the recommended amounts,” she says.

→Avoid Under-Fueling

Ironically, taking in less water and calories than you need during your run may be causing your gut distress. “If you are having consistent GI issues, it could be potentially caused by under-fueling,” Smith says. “Oftentimes people just blame the gel or sports drink, which, of course, can be the issue. But if you’re not taking in enough fuel, your body experiences really similar issues to if you’re taking in too much or the wrong type of fuel.”

If you know you’re under the recommended amounts of fuel and water per hour, it might be time to add more, rather than subtract. If you’re new to fueling runs and struggling with under-fueling, as Smith mentioned earlier, make sure you’re getting in 20 to 30 grams of carbs per hour at minimum, Race Week and Morning starting a few days.

→Practice Eating a Race Pace

Many runners will fuel their long runs, but not their shorter workouts. This can be a problem, because most runners don’t do their long runs at their goal race pace, which means they never experience how their gut feels after a gel while running at race-day speed.

Smith suggests making sure that any run with race pace or harder efforts includes some element of fueling, as a way to test that your gut can handle that gel or drink while under more stress.

You should also practice fueling the end of your long runs as though they’ll be even longer than they are. “Most marathoners aren’t doing a full 26.2 miles in training—usually, you’re doing maybe 20 miles for your longest prerace run,” says Larson. “So I would fuel those workouts as if I’m running a little bit farther. That way, you can practice the fueling and make sure your gut is able to tolerate more fuel later in the marathon.”

→Confirm What’s on the Course

“For marathoners especially, you’re probably not going to race carrying your fluids with you,” says Larson. “Maybe you’ll have your gels, but likely you’re just cruising through aid stations.”

Most races will have a list of what is provided at aid stations, including the brand of sports drink and the types of gels or other snacks that they’ll have. If you plan to use aid stations for your fueling and hydration, make sure that you practice with those products and get your body used to them. While drink mixes seem like they’re all similar, you may be surprised at how different you feel drinking Tailwind versus Skratch Labs versus Gatorade.

→Ask an Expert

While occasional gut distress on a run isn’t anything to worry about, if it’s a common occurrence or it’s keeping you from your racing goals, it may be time to get expert help. Working with a sports dietitian can help you figure out what’s working (and what’s not working) for you and dial in things like your water, electrolyte, and carbohydrate goals while on the run, as well as helping you assess whether you’re under-fueling in general. “It’s a process,” Smith says. “But it’s amazing what it can do for your run once you figure out what works best for you.”

And finally, stop getting sports nutrition advice from TikTok. “It’s so easy to fall down that rabbit hole on social media, but the thing is, everyone is going to have different needs,” Larson says. “There is no perfect blanket advice that we could provide that would solve everyone’s GI issues, because it is so unique from person to person. It takes trial and error and effort to figure out what’s going to make you run your best.”

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Race Week and Morning

→Cut Fiber Early

You likely have heard that eating a meal that’s high in fiber—like a bowl of broccoli—the night before a marathon is a bad idea. But for runners with sensitive stomachs, cutting back on fiber starting a few days No experience necessary! Anyone can accomplish this training plan.

“For my sensitive stomach people, I tell them to be much more conservative with their fiber intake starting three or four days out,” says Smith. “We don’t need to eliminate it completely, but skip the kale salad and only have a small amount of broccoli. Start prioritizing simple carbs earlier.”

→Keep Breakfast Extra Simple

“The hardest part about race morning is handling the race nerves and anxiety,” says Larson. “And the problem is that race nerves can kill your appetite. But you need to have something easy to digest so that your glycogen stores are topped up for the race. Look for simple carbs that are really easy to digest. A lot of athletes love oatmeal, but for those with sensitive guts, oatmeal can be too high in fiber. Think white bread or white rice or even drinking your carbs.” (She’s a big fan of Panera Bread’s Cinnamon Crunch bagel for before a race.)

If you normally have coffee or some other type of caffeine, don’t skip it on race day. Larson points out that if you don’t have your usual caffeine, your bathroom schedule may get thrown off.

→Don’t Panic About Morning Poops

Are you a “get in the port-a-potty line and stay in the line” kind of runner? Even top pros can end up with nervous poops on race morning. That’s why runners with nervous stomachs should set their alarms a bit earlier than their iron-gut counterparts—to give you more time to eat breakfast, have coffee, and hit the restroom Published: Jul 29, 2025 2:50 PM EDT.

“We want to give ourselves the time to get all the nervous poops out,” Smith says. “I know it’s hard to get up that early, but it allows your body to digest that pre-race breakfast.”

Your Marathon Training Diet warmup to clear out your system and instigate those morning poops if it’s not happening for you. “The night before a race, especially if there’s travel, a short jog can get things moving again,” says Larson. “And in the morning, just a couple minutes of easy running before the start can help make that morning digestion happen.”

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During the Race

→Triage the Situation

Despite your best intentions, and all the effort you put into training, sometimes that unsettled stomach can show up on race day. It happens. The first step to solving the issue is to take a moment to triage the situation. “Run through the list,” says Larson. “Have you been staying on schedule with fueling and hydration? Are you behind or are you consuming too much? Is it really Why You Might Feel Nauseous After Running out, so you need to tweak your hydration?”

Then, try to troubleshoot. “Say it’s really hot and despite sticking to your hydration plan, you’re feeling bad,” says Larson. “In that case, you may need to slow down a bit to bring your heart rate down and let your gut start to prioritize digestion a bit more. Then, when you get to the next aid station, take a minute to cool off with cold water or ice.”

→Slow Down

It’s hard to hear, but when your gut is going crazy during the race, the best thing you can do to save your race is to slow down. “Let your heart rate come down and let your stomach settle by slowing down, even walking for a bit if you have to,” says Smith. “Take small, slow sips of your sports drink or water, and try to let your body relax a bit.” These few minutes at a slower pace may allow you to speed back up and hit your goal.

→Frontload Your Fueling

For most runners, the back half of the marathon is harder. Yet many people skimp on fueling for the first few miles when they’re feeling the speediest and most comfortable, says Larson. And that’s wasting the time when your gut will be at its best.

Rather than waiting until you’re hungry or thirsty in a marathon, create a plan to start hydrating and fueling even 15 minutes after you take off. “I try to get athletes to frontload their fueling,” Larson says. “Usually, you tend to have better gut tolerance earlier on in the race, so you can be a bit higher carb early on in the race, and then you can taper it down later on when your gut stops feeling as good [if you need to]. Too many people go light early on, and then end up needing to take in more carbs later in the race when their bodies are already stressed.”

→Don’t Stop Eating or Drinking

Unless you’re planning to exit the course and not finish the race, keep fueling even if your gut is grumpy. Slow down your pace and sip water or sports drink slowly, take your time chewing that piece of bar or gummy. If you’ve only been having sports drink, gels, or gummies, try sipping plain water at the next aid station, says Larson. You may just need to dilute what’s in your gut so that your digestion can get going again.

“The biggest thing to focus on is making sure that you’re staying hydrated if you plan to continue the race,” Smith adds. “It’s hard when your stomach is hurting, but avoiding fluids is going to just perpetuate the problem and create other problems.”

→Practice Emergency Protocols

pelvic floor therapist: Better out than in. If you need to throw up or stop at a port-a-potty to poop, do it. You’ll likely feel a lot better afterwards, and in some cases, may be able to pick your pace back up as soon as you go.

Remember: This happens to everyone. “We’re all human, and these unfortunate situations happen,” says Smith.