Gut training is the training of your digestive system: teaching your stomach and intestine to tolerate, then absorb, more energy while you run, cycle, or swim. Studies measure a clear reduction in digestive problems after two weeks of regular practice. This guide explains why it works, which levers to use, and how to build your progression over six weeks.
Between 30 and 50% of endurance athletes experience digestive problems in competition, and the proportion rises to as much as 90% in some ultra events. Nausea, bloating, cramps, an urgent need to go: these are some of the main reasons why a well-prepared race ends badly. The good news is that your digestive tract responds to training like your legs do. By regularly giving it work during exercise, you make it more tolerant and more efficient.
I have run the Mulebar shop since 2008, and the subject comes up in every discipline, from the 10K to long-distance triathlon. This guide brings together what science has demonstrated, what it still leaves open, and above all how to apply it in practical terms, week after week. Each technical concept is explained in one simple sentence, and each recommendation becomes an action. The sources are dated and collected at the end of the article.
What exactly is gut training?
Gut training (literally “training the gut”) refers to all the practices that accustom your digestive system to functioning during exercise: eating and drinking while running, gradually increasing the amount of carbohydrates consumed per hour, and then repeating your race strategy exactly during training. The term became established in 2017 with the landmark article by researcher Asker Jeukendrup in the journal Sports Medicine, which brought together evidence showing that the stomach, intestine, and their sugar transporters adapt when regularly challenged. Before that, runners knew it instinctively: you “learn to eat” during a race. Since then, it has been documented.
In practical terms, gut training is based on a simple idea: whatever you ask of your gut on race day, you should have asked of it dozens of times beforehand. A runner who always trains without consuming anything tolerates 60 g of carbohydrates per hour poorly on race day; after a few weeks of eating during long runs, they assimilate more with fewer problems. It is a component of training, just like pace sessions, much more than a diet or a product.
Gut training concerns everyone as soon as exercise exceeds about an hour and a half: marathon runners, trail runners, cyclists, triathletes, as well as half-marathon runners who want to take a gel without grimacing and beginners discovering long runs. The longer, hotter, or more intense your event, the greater its impact on the result.
Why your stomach gives out during exercise
To understand what we are training, we first need to understand what goes wrong. Four mechanisms build up during prolonged exercise, and each has a countermeasure.
Blood goes to the muscles
At rest, your digestive system receives a generous share of the circulating blood. During exercise, the body redistributes its priorities: the working muscles and the skin, which releases heat, receive more, while the digestive organs receive less. Measurements show that blood flow to the intestine can drop by up to 80% during intense exercise. Scientists call this splanchnic ischemia: “ischemia” means a lack of blood flow, while “splanchnic” refers to the area of the viscera, that is, the stomach, intestine, and nearby organs.
An intestine with reduced blood flow works at a slower pace. Its cells receive less oxygen, absorb sugars and water less effectively, and the wall temporarily becomes more permeable, meaning it lets through substances that it usually retains. This general slowdown lays the groundwork for nausea and cramps, especially when food is added on top of it.
The stomach empties more slowly and the intestine absorbs less
Gastric emptying is the speed at which your stomach transfers its contents to the intestine. At high intensity, it slows down. What you swallow stays in your stomach longer, causing a feeling of heaviness, reflux, and nausea. A drink with a very high sugar concentration slows emptying even further because the stomach waits for the contents to be diluted before letting them pass.
Then, in the intestine, sugars enter the bloodstream thanks to “transporters,” specialized kinds of doors in the intestinal wall. Each type of door has a limited capacity per hour. When you swallow more sugar than the doors can let through, the excess continues along the intestine, draws in water through a simple balancing effect (osmosis), and ferments further down: bloating, gas, diarrhea. This is called malabsorption, and it is precisely one of the issues that gut training improves.
The jolts, heat, and stress make things worse
Running literally jolts the contents of your stomach with every stride, which explains why runners suffer more from “lower-GI” problems (cramps, urgent bowel movements) than cyclists, whose bodies are supported by the bike. Heat makes everything worse: the hotter it is, the more blood is sent to the skin to cool you down, leaving less for the intestines, and the dehydration that sets in further slows stomach emptying.
Competition stress also plays a role. The nervous system that prepares the body for exercise (the sympathetic nervous system, the “action” system) naturally slows digestion. Pre-race anxiety, starting too fast, and the excitement of race day therefore add to the physical strain. That is why a nutrition plan that works very well during an easy outing can cause problems in a race if you have not tested it at race pace.
Exercise-induced gastrointestinal syndrome, in one sentence
Researchers have grouped all of this under one name: exercise-induced gastrointestinal syndrome. The systematic review by Ricardo Costa and his team, published in 2017, shows that disruptions become significant after about two hours of exercise at 60% of VO2max, that is, at a perfectly ordinary endurance intensity. VO2max is the maximum amount of oxygen your body can use; 60% corresponds roughly to a pace at which you can still speak in short sentences.
Remember the key point: beyond two hours, your digestive system is under strain, whatever you do. Gut training helps expand your room for maneuver under these conditions; the strain itself is part of the game.
Symptoms and their frequency
Digestive problems during exercise fall into two groups. “Upper” symptoms come from the stomach and esophagus: reflux, heartburn, nausea, bloating, the urge to vomit, and the feeling that nothing is going down. “Lower” symptoms come from the intestines: abdominal cramps, gas, an urgent need to go, diarrhea, and sometimes bleeding after very long events. Running tends to cause lower-GI problems, while cycling and swimming tend to cause upper-GI problems, and triathlon sometimes gives you both.
In terms of frequency, the figures vary depending on the discipline, distance, and how the question is asked. The review by Erick de Oliveira, Roberto Burini, and Asker Jeukendrup published in 2014 places the range between 30 and 90% of participants depending on the events; competition studies often report 30 to 50% for marathons and triathlons, and the upper end of the range for ultras. Over these very long distances, digestive problems are among the leading causes of withdrawal and underperformance.
| Category | Typical symptoms | Sports most affected |
|---|---|---|
| Upper (stomach, esophagus) | Reflux, heartburn, nausea, bloating, vomiting | Cycling, swimming, uphill trail running |
| Lower (intestine) | Cramps, gas, urgent need to go, diarrhea | Running, marathon, downhill trail running |
| Systemic (whole body) | Dizziness, general queasiness, aversion to sweet foods | Ultras, intense heat |
One reassuring point: the vast majority of these symptoms are harmless and disappear in the hours after finishing. They cost minutes and enjoyment, but rarely health. That is precisely what makes them trainable.
What worsens the problems
Athletes differ in their susceptibility to digestive problems, and many aggravating factors depend on you. Knowing them makes it possible to remove obstacles before even beginning the protocol.
The first factor is intensity combined with duration: the faster you go for longer, the more blood leaves the intestine. Next come heat and dehydration, which slow stomach emptying and thicken the blood. The third factor, often underestimated, is what you have eaten in the hours and days beforehand: a meal rich in fiber, fat, or protein just before the start stays in the stomach for a long time and ferments in the intestine.
| Factor | What it does | What you can do |
|---|---|---|
| Intensity and duration | Less blood reaches the intestine after 2 hours | Start at the planned pace; test your nutrition at race pace |
| Heat and dehydration | Slowed gastric emptying, reduced absorption | Drink regularly, prepare for the heat, reduce solid food |
| Meal beforehand too rich | Fiber, fat, and protein slow digestion | Easily digestible meal 2 to 3 hours beforehand; reduce fiber during the preceding 24 to 72 hours |
| Drink that is too concentrated | The stomach retains the contents to dilute them | Follow the indicated dilution; drink water with gels |
| Anti-inflammatory drugs (ibuprofen and similar medications) | Worsen intestinal wall damage during exercise | Avoid them on days of long efforts unless medically advised |
| Competition stress | The nervous system slows digestion | Repeat the plan under conditions close to those of the race |
| High-dose caffeine | May speed up transit in sensitive people | Test during training, increase gradually |
Two factors deserve a little more attention. Nonsteroidal anti-inflammatory drugs, or “NSAIDs” such as ibuprofen, are sometimes taken before a race to prevent pain: a 2012 study showed that they worsen exercise-induced damage to the intestinal lining, making them a false friend for your stomach. FODMAPs, which will be discussed later, are a group of sugars found in certain common foods (onion, garlic, wheat, apple, legumes, milk) that ferment easily in the intestine: in sensitive people, reducing their intake in the days before an event decreases symptoms.
Finally, women report exercise-related digestive problems more often than men in several surveys, although science still does not fully explain why; gut training studies include few women, a limitation acknowledged by the 2023 systematic review. The principles remain the same; progression simply benefits from being more cautious.
What is really trainable, and what remains to be proven
Gut training has a solid scientific foundation, provided we are precise about what it improves. First demonstrated adaptation: gastric emptying. As early as 1991, a study showed that three days of a very high-glucose diet (400 g per day) accelerated stomach emptying for this substance. In other words, the stomach quickly learns to let through what it frequently receives. Second adaptation: tolerance to volume. A stomach that is regularly challenged during exercise signals less quickly that it is “full,” reducing heaviness and nausea.
Third adaptation, and the most debated: sugar transporters. The gateways that let glucose through (called SGLT1) and those that let fructose through (called GLUT5) become more numerous and more active when the diet is rich in carbohydrates. This mechanism has been demonstrated in animals and is consistent with what is measured in runners: after two weeks of gut training, the proportion of ingested carbohydrates that end up poorly absorbed drops significantly. The exact link between the two in humans remains to be clarified, but the practical result has been measured.
Here is the result. The systematic review by Martinez and colleagues, published in Sports Medicine in 2023, brought together eight gut training studies, with protocols lasting 4 to 28 days. After two weeks, digestive discomfort decreases by 47 to 48%, carbohydrate malabsorption by 45 to 54%, and nausea by 59 to 79%. The two seminal trials by Costa's team, in 2017 and 2018, had runners run for two consecutive weeks while consuming 90 g of carbohydrates per hour: fewer symptoms, less malabsorption, and more glucose available in the blood compared with the control group.
Here are the current limitations of the evidence, which should be stated just as clearly. Markers of the intestinal barrier (proteins that signal a damaged lining) remain unchanged after gut training: the intestine tolerates and absorbs better, while its lining retains the same markers. Data on women are scarce, effects beyond four weeks are poorly studied, and each research team uses its own protocol. This guide therefore bases its progression on what has been proven and says so when it extrapolates.
The seven gut training strategies
Jeukendrup describes five strategies in his 2017 article; practice and more recent research add two more. Each strategy acts on a specific mechanism discussed above, and it is their combination that produces the result.
Training on a full stomach
The first strategy is to run, cycle, or swim with something in your stomach, to get it used to working while you move. This means having a meal or snack rich in easily digestible carbohydrates one to two hours before an easy session, then gradually moving the intake closer to the start over the following weeks. In this way, the body learns to empty the stomach despite the effort and quiet the feeling of fullness.
Start with the easiest sessions: an hour-long jog, an endurance cycling outing, or a relaxed swimming session. The test meal should resemble what you will eat on race morning: bread, rice, a banana, compote, a little jam, and very little fat and fiber. If an energy bar or cake is on the plan for race day, this is the time to incorporate it. The goal of this strategy is achieved when you can set off thirty to sixty minutes after eating without thinking about it.
Drinking properly during exercise
Drinking during exercise is training in itself: the stomach must accept volume while being shaken, and the intestine must absorb water despite reduced blood flow. The guidelines used on Mulebar's sports pages and in most guides are 500 to 700 ml per hour, adjusted for heat and your sweat rate, always in small sips rather than large gulps. Drinking water after each gel or bar also forms part of the method: it dilutes the sugar in the stomach and speeds its passage into the intestine.
The concentration of what you drink matters just as much as the quantity. A drink is considered isotonic when its particle concentration (its osmolality) is close to that of blood: it leaves the stomach quickly. A drink that is too sugary, known as hypertonic, is more concentrated than blood: the stomach retains it to dilute it, and the intestine draws in water to balance it, which promotes bloating and diarrhea. Following the dilution indicated for a sports drink, and drinking plain water with gels, therefore helps protect gastric emptying. To choose, the comparative guide to sports drinks details the differences between hydration drinks and energy drinks.
Increasing carbohydrates per hour, step by step
This is the heart of gut training: gradually increasing the amount of carbohydrates you consume each hour of exercise so that your gut learns to absorb them. The scientific guidelines, formalized by Jeukendrup in 2014 and adopted in the American College of Sports Medicine's 2016 position stand, depend on the duration of the effort. For up to one hour, water is often enough. For one to two hours, small amounts (up to 30 g per hour) help you maintain your pace. For two to three hours, 60 g per hour is the classic target. Beyond two and a half hours, the literature goes up to 90 g per hour, provided you mix two types of sugar; I'll come back to that just below.
| Duration of the effort | Carbohydrates per hour (guideline) | What this represents | Source |
|---|---|---|---|
| Less than 1 h | Optional, a little if the intensity is high | A few sips of sports drink | Jeukendrup 2014 |
| 1 to 2 h | Up to 30 g | One gel, or one bar, or half a bottle of sports drink | Jeukendrup 2014; ACSM 2016 |
| 2 to 3 h | Around 60 g | Two gels, or one gel and one compote, or one bar and one bottle | Jeukendrup 2014; ACSM 2016 |
| More than 2 h 30 min | Up to 90 g of glucose + fructose | Three gels, or a mix of gel, compote, and drink | Jeukendrup 2014; ISSN 2019 |
| Cyclists and highly trained athletes | 90 to 120 g for some professionals, after extensive gut training | Not suitable for most athletes | Podlogar et Wallis 2022 |
Why these limits? Because the glucose pathway (SGLT1) allows a maximum of approximately 60 g of glucose per hour to pass through. Beyond that, the sugar accumulates in the intestine. The 90 g per hour only becomes possible by opening a second pathway, that of fructose, which adds approximately 30 g per hour of capacity. The 100 to 120 g per hour observed in professional cyclists results from intensive gut training over months and very specific conditions; it is included here to indicate the upper end of the scale, as a reference.
Combining glucose and fructose
Glucose is the “basic” sugar that fuels your energy, the one your muscles use directly. Fructose is the sugar found in fruit and honey, and it uses a different intestinal pathway. By combining the two, you use both pathways at the same time and can absorb more carbohydrates per hour with less risk of malabsorption. This is the principle of “multiple-transportable carbohydrates,” demonstrated in Jeukendrup's research and now incorporated into most sports-fueling products in the form of syrup and sugar blends.
For a long time, the recommended ratio was two parts glucose to one part fructose (2:1). More recent research, including Rowlands' 2015 critical review and the systematic review by Mlinaric and Mohorko published in 2025, shows that at high intakes (above 70 to 90 g per hour), a more balanced ratio, between 1:0.8 and 1:1, is better tolerated and oxidized more efficiently. For you, this means one simple thing: read the ingredient list of your products. A blend of rice syrup (glucose) and fruit purée or juice (fructose) naturally provides both pathways; this is the case with compotes and fruit bars. During an effort lasting less than two hours, the ratio is a secondary concern; it matters once you are aiming for 60 g per hour or more.
Vary the textures: bar, compote, gel, drink
The stomach gets tired. After two or three hours, sweet foods become sickening, and chewing becomes difficult as intensity increases. Alternating textures is therefore a strategy in its own right: solid (bar) when the terrain and pace allow it, semi-liquid (compote) and gel when intensity increases, and liquid (sports drink) when the stomach refuses everything else or when it is hot. Each texture also has its own digestion rate: liquids leave the stomach first, solids last.
Gut training must therefore include all the textures you will use in the race, so your stomach becomes familiar with each one at every pace. Mulebar's product guidelines provide a basis: about one bar every hour on flat terrain, one gel every 45 minutes taken in three servings using the resealable cap, one compote per hour in three or four servings, and water after every bite. A word on “hydrogel” gels, presented as easier to digest thanks to a texture that gels in the stomach: the 2025 systematic review concludes that their benefit over a properly formulated conventional carbohydrate product remains to be demonstrated. The choice of texture remains a matter of personal tolerance, to be developed through training.
Sodium, the discreet ally of absorption
Sodium (the main component of salt) plays a direct role in carbohydrate absorption: the glucose transporter SGLT1 takes in glucose and sodium in the same movement - both pass through the gate together. A sports drink containing sodium therefore helps the intestine absorb both sugar and water. This is also why plain water, consumed in large quantities without anything else, can eventually cause problems during very long events.
The guidelines used in Mulebar's trail guide and in expert positions on ultra-endurance events range from 300 to 600mg of sodium per hour when you sweat heavily, adjusted for the heat and your sweat (sweat that stings your eyes or leaves white marks indicates high losses). Gut training includes this intake: test your sports drink at the exact dilution you will use in the race and observe whether the water and sugar “go down” more easily. Salty foods also come back into play from the second hour onward, when sweet foods begin to become tiring.
Repeat your race plan in training
The final lever is the simplest and most often overlooked: repeat your exact race nutrition strategy in training, using the same products, the same quantities, at the same times, and at a pace close to that of race day. This is the explicit recommendation of the joint position of the American College of Sports Medicine, the Academy of Nutrition and Dietetics, and Dietitians of Canada, published in 2016, and it is the basis of the rule you know: nothing new on race day.
Repeating also means reproducing the constraints: running at the race time, in similar heat conditions, with the planned carrying equipment, and taking your products in the order and at the pace specified in the plan. Each repetition is an opportunity to adjust a flavor, texture, or timing. After a few outings, the plan becomes automatic, and that is exactly what you want at the thirtieth kilometer.
Your plate and the 72 hours before
Gut training takes place during exercise, but what you eat during the rest of the week determines how your gut responds. Three points matter.
A daily diet rich in carbohydrates
The gut's sugar transporters adapt to what they receive. A daily diet rich in carbohydrates increases their capacity, and the 1991 study on gastric emptying shows that the stomach itself adapts within a few days to a high glucose intake. An endurance athlete who eats very few carbohydrates every day therefore starts with a gut that is less prepared to absorb them during a race, regardless of their gut-training protocol.
It is accessible to everyone: starchy foods at every meal (rice, pasta, bread, potatoes, oats), fruit, and a gradual increase in the proportion of carbohydrates during the two days before a long event. The carbohydrate-loading guidelines used on the blog, 8 to 10g of carbohydrates per kilogram of body weight during the final 48 hours, are explained in detail in the article on the importance of glycogen and in the guide to avoiding the marathon wall. Glycogen is the form in which your muscles and liver store sugar: the fuller your reserves are at the start, the less work your gut has to do during exercise.
The 24 to 72 hours before: fiber, fat, FODMAPs
In the two to three days before a race, the goal is a calm gut. This means reducing fiber (raw vegetables, whole grains, legumes), fats, and very large meals, all of which slow digestion and ferment in the gut. The pre-race meal, meanwhile, should be eaten two to three hours before the start and remain simple and easy to digest; the article what to eat before sport provides the details.
FODMAPs deserve a paragraph. This English acronym refers to a family of short-chain sugars (fructans, lactose, excess fructose, polyols) that the gut absorbs poorly and that bacteria ferment, producing gas and drawing in water. They are found in wheat, onions, garlic, legumes, apples, pears, milk, and sweeteners ending in "-ol". A 2019 study in runners showed that a low-FODMAP diet for six days reduced exercise-related digestive symptoms. The 2025 systematic review confirms its effectiveness while warning that this diet is restrictive and, if prolonged, impoverishes the diet. The best practice is therefore to reserve it for the few days before an important event, for people who know they are sensitive, and to return to a varied diet afterward.
Probiotics, supplements, and other promises
Probiotics (live bacteria intended to improve the gut flora), glutamine (an amino acid presented as protective of the intestinal lining), and various "digestion" supplements are regularly offered to endurance athletes. The state of the evidence is as follows: the 2025 systematic review finds that results for probiotics are mixed, with some studies showing positive effects and others none, depending on the strains and protocols. Gut training itself remains irreplaceable.
My position as a shopkeeper is simple: I would rather see you invest your weeks in the protocol below than in a product that promises to solve the problem for you. If you want to try a probiotic, do so outside competition periods, over several weeks, and assess it based on how you feel. Supplements have their place for other needs (protein on high-volume days, magnesium in case of a deficiency); for digestion during exercise, training remains the foundation.
The six-week gut training protocol
Studies measure effects after as little as two weeks of daily practice: in the landmark trials, runners ate during exercise every day. Six weeks at a rate of one to two sessions per week is the version compatible with normal life: a deliberate extrapolation that builds progressively without overwhelming your gut or disrupting your training. This protocol organizes the levers discussed above into three two-week stages. It is a pragmatic progression built on proven benchmarks, to be adapted to how you feel: everyone progresses at their own pace, and the time frames given are study-based benchmarks, never a guarantee.
Before starting: prerequisites
Gut training is incorporated into your long runs, those lasting more than an hour. If you do not have one in your week, start by adding one: it will serve as your gut training session, and it is in any case the session that matters most for an endurance event. Set your goal (a race, a distance, or simply "eating without grimacing on a long run") and decide which products you will use on race day, so you can incorporate them from the first week. A notebook, a note on your phone, or the tracking app you already use is enough to record after each session what you took and how it went.
Two precautions. If you suffer from digestive problems at rest, persistent pain, blood in your stool, or unexplained weight loss, consult a doctor before starting any protocol: gut training is intended for healthy intestines that struggle under exertion, and nothing more. If you are in the middle of training and your race is in two weeks, start directly at the stage corresponding to your current habits rather than starting over from the beginning: two weeks done properly already produce a measurable effect.
Weeks 1 and 2: establish the habit
The goal of this first stage: eat and drink regularly while moving, without aiming for large quantities. On every outing lasting more than an hour, take something every 20 to 30 minutes and aim for 30 to 40g of carbohydrates per hour - for example, a gel (about 27g of carbohydrates for a 37g Mulebar gel) taken in three portions, a bar eaten in two or three spaced-out bites, or a bottle of sports drink sipped in small amounts. Drink water after each intake. At the same time, on one or two easy runs that week, set out one to one and a half hours after a small carbohydrate-rich meal: this is the "full stomach" lever.
These two weeks also serve as a laboratory: test the flavors, identify the texture that suits you best at an easy pace, and rate your digestive comfort from 0 (nothing to report) to 10 (session ruined) after each outing. If everything goes well from the first week, resist the urge to skip steps: consistency in taking them matters more than quantity at this stage, because it is what gets your stomach used to emptying while you are moving.
Weeks 3 and 4: increase the load
Second stage: reach 50 to 60g of carbohydrates per hour on your long runs, the classic target for two- to three-hour efforts. In practice, two gels per hour (in six small intakes), or one gel and a compote, or a bar and a bottle of sports drink. Alternate textures from one intake to the next, and introduce something savory (a savory-tasting product or a sports drink) from the second hour to anticipate getting tired of sweet flavors. If you run two long runs per week, both become gut-training sessions; otherwise, add a specific pace session during which you take at least one gel, to teach your stomach to work when the intensity rises.
This is the stage at which science identifies the first measurable effects: Costa and Miall's trials show that, after two weeks of repeated intakes, there are fewer symptoms and fewer poorly absorbed carbohydrates than in the control group. You may feel this as a calmer stomach at the end of a run, or simply as intakes that happen without thinking about them. If a session goes badly (comfort score of 5 or more), drop down a level the next time instead of insisting: adaptation is built through successful sessions.
Weeks 5 and 6: simulate the race
Third stage: replicate the exact race-day plan. Same products, same quantities, same intake times, identical carrying equipment, and a pace close to race pace for part of the run. If your event lasts more than two and a half hours and your tolerance allows it, this is the time to test the 70 to 90g-per-hour range with a glucose and fructose blend, for example three gels per hour or a mix of gel, compote, and drink. If your race lasts less than two hours, stay around 30 to 60g: going higher would add strain without benefit.
Carry out at least two full simulations over these two weeks, including one at the actual race times and, if possible, in similar heat conditions. The last long run before the race is your dress rehearsal: nothing unfamiliar that day. During the tapering week before the event, keep one or two intakes during your short sessions to maintain the habit, without aiming for volume.
| Weeks | Target carbohydrates per hour | Frequency and sessions | Key practices | Scientific guidelines |
|---|---|---|---|---|
| 1 and 2 | 30 to 40g | All runs longer than 1h; 1 to 2 “full stomach” easy runs | Take something every 20 to 30 min, drink water after each intake, comfort score from 0 to 10 | Mulebar guidelines (sports pages); Jeukendrup 2014 (1 to 2h: up to 30 g/h) |
| 3 and 4 | 50 to 60g | Long runs + one pace session with at least one gel | Alternate textures, add something savory from the second hour, and drop down a level if the score reaches 5 | Costa 2017 and Miall 2018 (effects after 2 weeks); Jeukendrup 2014 (2 to 3 h: 60 g/h) |
| 5 and 6 | Exact race plan; 70 to 90g of glucose + fructose if the event exceeds 2h30 | Two complete simulations, one of them at the actual times | Same products, same times, same equipment; dress rehearsal on the last long outing | ACSM 2016 (repeat the race strategy); Rowlands 2015 and Mlinaric 2025 (ratio close to 1:1 at high intakes) |
Measuring your progress without fooling yourself
Your digestive comfort score from 0 to 10 after each session is your measuring instrument. It is subjective, but that is exactly what studies use, in a more detailed form, to measure symptoms. Also record the amount you actually consume per hour, the conditions (heat, elevation gain, intensity), and the time of your last meal. After three weeks, you will see a trend; after six, you will know what your intestines tolerate and under which conditions.
The progression rule is simple: move up one level when two consecutive sessions receive a score of 3 or lower; stay at the same level when the score is 4; move back down when it reaches 5 or higher. This rule is better than a fixed schedule because adaptation varies from person to person and from one period to another (a week of extreme heat or fatigue can raise the score even when you have done nothing wrong). The figures from the studies, a reduction of about half in symptoms over two weeks, are group averages: your curve will be your own.
Troubleshooting during a session
Even when well prepared, your stomach will have bad days. The steps that follow are common sense, consistent with the mechanisms described above, and should also be tested in training.
For nausea or a heavy feeling in your stomach, slow your pace for a few minutes (blood returns to the intestines), space out the next intake, and switch to liquids. For bloating or a rumbling stomach, drink plain water in small sips to dilute what is in the intestines, and reduce the dose of the next intake rather than skipping it. For abdominal cramps, slow down considerably, breathe slowly from your belly, and resume fueling with small amounts of liquid once the pain subsides. For an urgent need to go, stop if possible, then set off again more gently with liquids. In all cases, note what happened: a bad session well documented is a lesson.
After the goal: maintaining it year-round
Gut training is a habit rather than a one-off preparation. The literature says little about how quickly the gut “unlearns” when you stop challenging it, but the mechanism described above (transporters and gastric emptying adapting to what they receive) suggests that tolerance needs to be maintained. The simplest approach is to keep one weekly long session all year round during which you eat and drink as you would in a race. You maintain your gut, test new flavors without pressure, and your next training cycle starts at level 2 rather than level 1.
This is also, in my view, the healthiest way to look at things: eating during exercise is part of training, just like drinking or warming up, all year round. We eat real, simple food all year round, and race day becomes just another long run, with a race number.
Gut training for your sport
The mechanisms and protocol are common to all endurance disciplines. What changes is the dominant constraint: jostling when running, position and duration on the bike, transitions in triathlon, elevation gain and duration in trail running. Here is how to adjust.
Road: from 10K to marathon
For 10K races and half marathons, the effort rarely lasts more than two hours: one or two servings and hydration are enough, and gut training is mainly about being able to take a gel at a sustained pace without side stitches or reflux. Practice taking it during your specific-pace sessions, in addition to your long run, because this is the intensity at which the stomach is most reluctant. The guidelines for each distance are detailed in the 10K nutrition tips and for the half marathon.
For marathons, gut training becomes crucial: the target is around 60 g of carbohydrates per hour, up to 90 g for runners who run for more than three hours and tolerate it well. Taking the gel in three portions to smooth out the arrival of sugar, drinking water at every aid station, and carbohydrate loading during the 48 hours beforehand are the three habits to repeat. The running nutrition page brings together the guidelines for each distance, and marathon hydration has its own dedicated guide. Also think about the jostling: when running, lower gastrointestinal issues are the most common, which is why it is useful to limit fiber and FODMAPs during the 72 hours beforehand.
Trail and ultra
Trail running adds duration, elevation gain, and often heat or altitude. Uphill, the stomach is compressed and the heart beats fast: liquids and semi-liquids go down more easily; on smooth, calm sections, solid foods become practical again. Beyond six hours, take in fuel every 20 to 30 minutes in liquid or semi-liquid form, and salty foods become essential to counteract an aversion to sweet flavors. A trail runner's gut training therefore includes practicing eating uphill, drinking downhill, and alternating sweet and savory foods over the duration; the trail nutrition guide and the trail nutrition page explain these guidelines in detail.
For ultra events, the position statement of the International Society of Sports Nutrition published in 2019 explicitly recommends gut training during preparation and emphasizes adjusting intake to the conditions: heat and altitude reduce appetite and tolerance, while the review by Costa and colleagues published in 2020 on events in hot environments shows that heat stress amplifies digestive disturbances. During these events, also allow yourself the comfort of aid-station foods (soup, bread) after six to eight hours of effort, while maintaining a regular carbohydrate intake. And prepare your gut for the heat just as you prepare your legs: a few fuelled training sessions in hot conditions are better than discovering it on race day.
Cycling and mountain biking
Cycling is the discipline best suited to gut training: your body is supported, your stomach is barely jolted, and your hands are free to eat. This explains why professional cyclists reach intakes of 90 to 120g per hour after extensive gut training. For you, the key is different: start eating within the first twenty minutes, because hunger strikes after the energy deficit has already built up, and learn to take a bar or gel when the pace picks up or the terrain gets rough on a mountain bike.
Gut training on the bike therefore relies on consistency (one intake every 20 to 30 minutes during a two- to four-hour ride) and variety (bars on flat sections, gels and purées when the intensity rises, and sports drinks to “eat while drinking” over long distances). Dry air and duration make you forget to drink, especially when mountain biking: your bottle is part of your training. Energy gels for cycling have their own article, and the cycling and mountain biking nutrition page provides guidelines based on ride duration.
Triathlon
Triathlon combines three constraints. In the swim, you swallow water and your stomach may already be tied in knots; on the bike, it is the best time to eat, and this is where the subsequent run is decided; on foot, after two or three hours of cycling, the stomach is both full and shaken, hence the frequent nausea after the second transition. A triathlete's gut training consists of learning to eat regularly on the bike (bars when stable, gels and drink when intensity rises) and to split intake on the run into small amounts of liquid or gel, smoothing out intake rather than making up for a shortfall all at once.
Bike-to-run sessions are your ideal gut training sessions: use them to reproduce your race nutrition, including the transition. Over long distances, many triathletes aim for 60 to 90g per hour on the bike, then reduce this slightly on the run depending on tolerance. The article on triathlon nutrition and the triathlon nutrition page provide detailed guidelines for each format, from sprint to Ironman.
At home: turbo trainer and treadmill, the ideal laboratory
Indoor sessions are underestimated for gut training. On a turbo trainer or treadmill, everything is controlled: the intensity, temperature, timing, access to products and water, and the toilet is ten meters away. It is the perfect place to test a new product, a higher dose, or a more concentrated drink without risking ruining a session. The heat of an enclosed room even partly reproduces the conditions of a summer race.
Use these sessions for the tests that concern you: a first attempt at 60g per hour, taking nutrition at high intensity, or trying a new flavor. Record your comfort rating as you would for an outdoor session, then transfer what worked to outdoor training. In winter, when long sessions become shorter, one fueled indoor session per week is enough to maintain the tolerance you have built up.
Women, masters, beginners
Gut training studies have mainly included young, trained men, a limitation acknowledged by the 2023 systematic review. Surveys show that women more often report exercise-related digestive issues; the proposed mechanisms (hormones, intestinal sensitivity, differences in stomach volume) remain to be confirmed. In practice, the levers are the same, with more cautious progression and particular attention to phases of the cycle when digestion is more temperamental. The article on sports nutrition for women discusses these specificities.
For master athletes (over 50) and beginners, the logic is the same: start from where you are, progress step by step, and give each step time to settle in. A beginner discovering long outings has every interest in getting used to eating from their first one-hour outings, in small quantities, rather than waiting until the eve of their first race to start.
Misconceptions and limitations
Gut training has its myths. Here are three of them, plus one limitation you should know.
“Running fasted also trains the stomach”
Fasted or “low-carbohydrate” training (scientists’ “train low”) has a specific purpose: encouraging the muscles to use fat more efficiently and triggering certain cellular adaptations. The theoretical framework developed by Impey and colleagues, published in 2018 under the name “fuel for the work required,” proposes modulating carbohydrate intake according to the session: little for short, easy sessions, and plenty for long or intense sessions. This framework is compatible with gut training, provided that long sessions remain fueled.
Running fasted therefore trains your muscles; your gut, on the other hand, is trained on fueled outings. An athlete who did all their outings without consuming anything would show up on race day with an unaccustomed stomach, regardless of their level. The solution is periodization: short fasted runs if you wish, and long outings where you eat as you would during a race. The two approaches serve different purposes and complement each other.
“90 g per hour is for everyone”
90 g of carbohydrates per hour is a studied upper limit for efforts lasting more than two and a half hours, among athletes who have trained their gut. For a two-hour outing, 60 g is more than enough; for a half marathon, a single gel can make the difference. Aiming higher than necessary adds digestive strain without improving performance, and this is one of the most common causes of problems among runners discovering the higher recommendations.
The right question is: how many carbohydrates do I need for the duration and intensity of my effort, and how much can my gut tolerate today? The answer is built through training, using the step-up table above and the comfort log. Elite figures serve as a benchmark; your goal is your own tolerance.
“Gels cause diarrhea”
A gel is a carbohydrate concentrate, most often a mixture of syrups and sugars with a little water. Swallowed all at once, without water, at full intensity, it reaches the stomach in a highly concentrated form: the stomach holds it back, the intestine draws in water to dilute it, and symptoms follow. The problem comes much more from how you take it than from the product itself. The energy gel guide explains its composition and use.
The gel is taken in three servings over 45 minutes, with water with each bite, and alternated with other textures over the course of the activity. An intestine trained to receive gels this way tolerates them; an intestine encountering three at once on race day reacts like any organ that is overloaded. What is true is that fructose tolerance varies from person to person: if fruit-rich products bother you even during training, choose products in which glucose predominates and reserve fructose for your highest intakes.
When to seek medical advice
Gut training is intended for healthy digestive systems that suffer under the strain of exercise. Some signs go beyond this scope and call for medical advice: blood in the stool or vomiting blood, abdominal pain that persists at rest, unexplained weight loss, digestive symptoms occurring outside exercise, unusual fatigue, or anemia. These signs may have causes unrelated to sport, and only a doctor can investigate them.
Likewise, if your exercise-related symptoms remain severe after six weeks of a properly followed protocol, a sports dietitian or gastroenterologist can investigate an intolerance (to fructose or lactose) or a particular sensitivity. Gut training is a training tool; it complements medical monitoring when necessary, and the doctor has the final say.
And where does Mulebar fit into all this?
Gut training is the most concrete reason behind the products I have been selling since 2008: an intestine trains with what it is regularly given, and on race day it tolerates what it knows. My bars are made with real ingredients cooked slowly, my fruit purées combine rice syrup and fruit purée, my gels are taken in three servings thanks to their resealable cap, and all three are made in France. These are products designed to be eaten every week during training, and that is exactly what the protocol requires.
One thing must be clear: training is what builds tolerance, and a product, including mine, supports this process when it is easy to digest, enjoyable to eat after three hours, and consistent from one box to the next. For the protocol, here is one way to distribute the textures: energy bars and energy fruit purées to establish the habit in weeks 1 and 2, energy gels and sports drinks to increase the load in weeks 3 and 4, then the exact mix from your race plan in weeks 5 and 6. To test the flavors without committing to a box, the gel discovery pack and the bar pack are available for that.
If you are unsure about the quantities, textures, or intake schedule for your sport and your next race, the online assessment asks you the right questions and suggests a starting plan in just a few minutes.
TAKE MY NUTRITION ASSESSMENT →
Frequently asked questions
How long does it take to train your gut?
Studies measure effects after two weeks of repeated intake during exercise: an approximately 50% reduction in digestive discomfort and carbohydrate malabsorption, according to the 2023 systematic review. These figures are group averages, and each person progresses at their own pace.
The six-week protocol in this article allows time to increase level by level, at a rate of one to two sessions per week, without rushing the gut. The effects beyond four weeks have been little studied, but the adaptation mechanism encourages maintaining the habit year-round.
How many carbohydrates per hour should you aim for?
It depends on the duration of the effort: up to 30g per hour for efforts lasting between one and two hours, around 60g between two and three hours, and up to 90g beyond two and a half hours, provided you mix glucose and fructose. These guidelines come from Jeukendrup's work (2014) and the position statements of professional societies.
The amount you can tolerate today is the real starting point. The protocol gradually increases from 30 to 40g per hour up to your race plan, and the comfort rating after each session indicates when to move to the next level.
Should you do gut training during every session?
One to two sessions per week are enough, ideally the long runs and one session at a specific pace. The other sessions can remain easy or be done fasted if that is your choice, as part of a carbohydrate periodization strategy.
What matters is consistency over several weeks: the stomach and intestinal transporters adapt to what they receive frequently, much more than to an occasional intake.
Why mix glucose and fructose?
Glucose and fructose enter the bloodstream through two different gateways in the intestinal wall. By using both, you absorb more carbohydrates per hour with less risk of malabsorption: this is what makes intakes of 90 g per hour possible.
The historical ratio is two parts glucose to one part fructose; recent research shows that at high intakes, a ratio close to one to one is better tolerated. Below 60 g per hour, the question is secondary.
What should I do if I feel nauseous or have cramps during a race?
Slow down for a few minutes to let blood return to the intestine, switch to liquid in small sips of water, and space out or reduce the next intake. Cramps ease when breathing slowly through the belly at a reduced pace.
Then note what happened (product, dose, time, pace, heat): this log is what allows you to adjust the plan next time.
Does gut training protect the intestine?
It improves tolerance and absorption: fewer symptoms and fewer poorly absorbed carbohydrates. The markers of the intestinal barrier, however, remain unchanged in the available studies, which means that gut training makes the intestine more efficient under exertion rather than "fortifying" it.
Signs such as blood in the stool, pain at rest, or unexplained weight loss fall outside the scope of gut training and call for medical advice.
Can gut training be done on an indoor trainer or treadmill?
Yes, and it is even the ideal place for testing: intensity, temperature, and access to products are controlled, and a failed session costs little. Test higher doses or new products there before taking them outdoors.
In winter, one indoor session fueled per week is enough to maintain the tolerance acquired during the season.
Sources
- Jeukendrup AE. Training the Gut for Athletes. Sports Medicine, 2017 ; 47 (Suppl 1) : 101-110. https://link.springer.com/article/10.1007/s40279-017-0690-6
- Martinez IG, Mika AS, Biesiekierski JR, Costa RJS. The Effect of Gut-Training and Feeding-Challenge on Markers of Gastrointestinal Status in Response to Endurance Exercise: A Systematic Literature Review. Sports Medicine, 2023 ; 53 : 1175-1200. https://link.springer.com/article/10.1007/s40279-023-01841-0
- Mlinaric J, Mohorko N. Nutritional strategies for minimizing gastrointestinal symptoms during endurance exercise: systematic review of the literature. Journal of the International Society of Sports Nutrition, July 2025 ; 22 (1). https://www.tandfonline.com/doi/full/10.1080/15502783.2025.2529910
- Costa RJS, Miall A, Khoo A, et al. Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Applied Physiology, Nutrition, and Metabolism, 2017 ; 42 (5) : 547-557. https://cdnsciencepub.com/doi/10.1139/apnm-2016-0453
- Miall A, Khoo A, Rauch C, et al. Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption. Scandinavian Journal of Medicine & Science in Sports, 2018 ; 28 (2) : 630-640. https://onlinelibrary.wiley.com/doi/10.1111/sms.12912
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- Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise, 2016 ; 48 (3) : 543-568. https://pubmed.ncbi.nlm.nih.gov/26891166/
- Tiller NB, Roberts JD, Beasley L, et al. International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing. Journal of the International Society of Sports Nutrition, 2019 ; 16 : 50. https://link.springer.com/article/10.1186/s12970-019-0312-9
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- Costa RJS, Gaskell SK, McCubbin AJ, Snipe RMJ. Exertional-heat stress-associated gastrointestinal perturbations during Olympic sports: Management strategies for athletes preparing and competing in the 2020 Tokyo Olympic Games. Temperature, 2020 ; 7 (1) : 58-88. https://pmc.ncbi.nlm.nih.gov/articles/PMC7053925/
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- Mulebar. Nutrition pages for Running, Trail, Cycling/Mountain Biking, and Triathlon, and the guide “Trail Nutrition and Eating” (10/08/2025), hydration, sodium, and progression guidelines consulted on 10/06/2026. https://mulebar.com/pages/nutrition-running-performance-bio-mulebar ; https://mulebar.com/pages/nutrition-trail-naturelle-et-efficace-mulebar ; https://mulebar.com/pages/nutrition-cyclisme-sans-chimie-mulebar ; https://mulebar.com/pages/nutrition-triathlon-multi-distance-mulebar ; https://mulebar.com/blogs/news/nutrition-trail
- Mulebar. Product sheets (37 g energy gels, 444 g gel refill, fruit purées, bars): ingredients and usage advice consulted on 10/06/2026. https://mulebar.com/collections/gels-energetiques ; https://mulebar.com/collections/compotes-energetiques ; https://mulebar.com/collections/barres-energetiques
Article published on February 12, 2026, updated on October 6, 2026.
