Running 10 – Eating on long runs: train your gut like your legs

At kilometer 27 of the marathon, three portable toilets stand right at the roadside. In front of each one, a line of people shifts from foot to foot. You stand at the end of a line, clutching your stomach. Ten minutes ago, at the aid station, you were handed two passion-fruit energy gels from the sponsor and tore open and ate both at once so you would have “enough to finish”. Your stomach has been cramping ever since. Through four months of the training plan, you carried only a bottle of plain water on every long run, and some days you even went out before breakfast to get used to it.

Your legs were trained very carefully. Long runs built up gradually, there were recovery weeks, and there were strength sessions. Your stomach and gut, on the other hand, only had to take in food mid-run on race morning, for the first time, and it was something they had never met before.

Four months of training the legs, not one session for the gut

Stomach pain, nausea and having to find a toilet mid-route are more common than people think. The book ACSM’s Advanced Exercise Physiology from the American College of Sports Medicine notes that 20–50% of endurance athletes have digestive problems while running (cited secondhand in the book; we have not been able to check the original study). In trail races (running on hill and mountain paths) and ultras (races longer than a marathon), it is even more marked. The review by Viljoen and colleagues (2021) found that the most common illness on race day was digestive, mainly nausea and vomiting, more common even than metabolic and heart problems.

Two things make the stomach more likely to act up. The first is anti-inflammatory painkillers such as ibuprofen. The same book notes that this group of drugs makes digestive problems while running worse. A tablet taken “in advance so the legs won’t ache” can therefore contribute to stomach cramps mid-route. The second is food your gut has never had to handle while running.

Running plans often have columns for kilometers, pace and rest days, but no line for eating. Yet swallowing a gel, a banana or a ball of sticky rice while your whole body bounces with every stride is a skill, and skills have to be practiced.

The watch on your wrist decides whether you need to eat

How much carbohydrate (starches and sugars) you need to take in depends on how long you are out on the course. According to Stellingwerff and Cox (2014) and the International Society of Sports Nutrition (ISSN; Kerksick and colleagues, 2017; Tiller and colleagues, 2019):

• Under 1 hour: no need to eat while running;
• Over 1 hour: 30–60 g of carbohydrate per hour;
• Over 2–2.5 hours: this can go up to 90 g per hour, using gels or drinks that contain both glucose and fructose;
• Women and smaller people: 30–50 g per hour usually suits better.

All of the amounts above have to be adjusted to what each person’s stomach can tolerate. No single number is right for everyone.

To know how much you are taking in, read the label. Gels, sports drinks and energy bars all list the grams of carbohydrate per pack or per bottle. Divide your hourly target by the grams in one pack and you know how many packs you need per hour. Bananas, bread and sticky rice have no label, which is all the more reason to bring them on long runs to learn how much you can eat while your stomach stays calm.

Two sugars, two routes

There is a reason the 90 g mark comes with the condition “containing both glucose and fructose”. The gut absorbs each type of sugar through its own route, and a single source of sugar can only be burned for energy (oxidized) by the body at up to about 60 g per hour. To go past that mark, you combine two sugars that use two different routes; the technical literature calls this a multiple transportable carbohydrate mix.

Gel and drink labels usually list the ingredients. Check whether there is both glucose and fructose before thinking about going above 60 g per hour. The review most often cited on this topic is by Jeukendrup (2014). The author has ties to a sports drink company, so here it is used only alongside the ISSN recommendations.

The 90 g mark is for races longer than 2–2.5 hours, and for people who have already eaten at lower amounts over many training sessions with a settled stomach. If you are running your first marathon and your stomach is not yet used to eating on the run, stay within 30–60 g for now.

The 120 g figure in running groups

In recent years, 120 g of carbohydrate per hour has been promoted quite widely. The data for this amount come from exactly two small trials. Viribay and colleagues (2020) studied 20 people and found that two blood markers of muscle damage, CK and LDH, were lower after 24 hours. Urdampilleta and colleagues (2020) studied 26 people and found that neuromuscular function was better preserved. The participants in both studies were men, elite mountain runners, who had already trained their gut beforehand.

Those two studies measured muscle enzymes and neuromuscular function, that is, surrogate outcomes: stand-in measurements rather than actual running results. Neither study measured race performance, so it is not known whether eating 120 g per hour helps you finish sooner. The second study also shows signs of a link to a manufacturer, which we have not been able to verify clearly.

The clinic does not recommend 120 g for recreational runners. If it is used at all, it is only for top-level, professional athletes with their own nutrition supervision. Recreational runners differ from those participants in almost every way, from sex and level to whether their gut has been trained or not.

Every long run is an eating practice

Gut training sounds strange, but it is simple to do. On your weekend long run, bring exactly the gel, the drink and the food you plan to use on race day, and eat on exactly the schedule you plan to follow that day. At home, write a few lines: what you ate, when, how many grams, and how your stomach felt. If your stomach was calm, keep it; if it cramped, change the food or reduce the amount. Only once your stomach is used to the lower amount do you increase it gradually in later sessions.

The gel in the opening scene was what the sponsor handed out at the station. Before the race, you can ask the organizers which gels and drinks the aid stations will offer and practice with exactly those, or carry enough of your own familiar food. On race day, try nothing new, including what the aid stations hand out.

The evidence for gut training is still thin. King and colleagues (2022) had athletes eat a high-carbohydrate diet with gut training for two weeks, and digestive symptoms and performance changed only slightly. Ryan and colleagues (2023) pooled 7 studies with a total of 105 people and found that gut training and a low-FODMAP diet looked promising in ultra runners, but not enough to draw conclusions. FODMAP is the umbrella name for a group of sugars and fibers that ferment easily in the gut. If every long run gives you stomach pain and you want to try cutting out this group, ask a doctor first, because broad restrictions can easily lead to eating too little.

Even so, the advice to practice eating on long runs stands. In our view, it is the only way to find out which foods your stomach can handle before you have to rely on them at kilometer 27. Two weeks is too short. It is best to start from the very first long runs of your training plan.

A ball of sticky rice at the ultra aid station

In an ultra, runners are out on the course for ten hours, sometimes more than a day. The ISSN (Tiller and colleagues, 2019) suggests taking in 150–400 kcal per hour, including 30–50 g of carbohydrate and 5–10 g of protein. Real food is preferred, and the longer the race, the more savory foods should be chosen. The hourly carbohydrate amount here is lower than the 90 g mark above; in return, there is added protein and energy from real food.

Travelers in Vietnam used to carry a ball of rice and a small packet of sesame salt on long journeys. The rice ball was filling, the sesame salt was savory, and both fitted easily into a cloth bundle. That custom fits the advice for ultra aid stations today: a ball of sticky rice, a piece of banh mi (Vietnamese sandwich), a few pieces of sweet potato. A stomach tired of sweet things after many hours of running still has something to eat.

That said, the sticky rice ball also has to be tested beforehand, on long runs in the mountains, just as the gel is tested on city roads.

The rice pot in the days before the race

The energy you eat while running is only half the story. The other half lies in your everyday meals. The ISSN (Kerksick and colleagues, 2017; Tiller and colleagues, 2019) suggests that people training for an ultra eat about 5–8 g of carbohydrate per kg of body weight per day, and 8–12 g/kg on the days when the body’s stores need to be filled before a race. A person weighing 60 kg would therefore need about 300–480 g of carbohydrate a day during training, and 480–720 g on loading days. Every meal needs rice, noodles, sweet potatoes or bread, plus snacks. This g/kg range is often attributed to the ACSM; we have not been able to check the original, so here it is given according to the ISSN only.

The runner in the opening scene did some runs before breakfast so the body would get used to burning fat, the same idea as the keto diet (very little carbohydrate, a lot of fat) and fasted running. The clinic does not recommend keto for trail or ultra runners. The meta-analyses by Gejl and Nybo (2021), Koerich and colleagues (2022) and Cao and colleagues (2021) found that eating little carbohydrate did not improve endurance performance. In the first week, performance even dropped. The body does burn more fat, but this does not turn into better running, while bone markers get worse. In race walkers, fewer than 4 weeks on keto made these markers worse (a study by Heikura, cited in a book). A 2026 review leans toward alternating low-carbohydrate phases, but that review only counted the studies for and against, without pooling the data.

The Spleen governs the muscles and the four limbs

Traditional medicine assigns the muscles and the limbs to the Spleen: “the Spleen governs the flesh and the four limbs” (脾主肌肉、四肢, pí zhǔ jī ròu, sì zhī). The traditional healer (lương y) Nguyen Huu Duc taught that whether the muscles are full or wasted is the physical side of the Spleen, while the strength of the arms and legs is the qi side of the Spleen. Eating and digestion are also the work of the Spleen and Stomach (the Spleen together with the Stomach, which here means the stomach itself).The running legs and the stomach that takes in food therefore sit under one and the same organ.

From this we infer one thing that no study has measured: people whose Spleen and Stomach are weak to begin with, who normally digest slowly and often feel bloated, are more likely to feel unwell when they have to eat during a long run. People like this need more patience when practicing eating on the run, starting with small amounts.

At the clinic, the doctor asks in detail what you eat and when during long runs, how your stomach reacts, and what medicines you are taking. The doctor first checks for any signs that call for lab tests or referral to a specialist, before discussing any medicine.

Some people have long-lasting fatigue, slow recovery, a pale complexion and a poor appetite after a heavy training block. After a complete blood count and ferritin (a measure of the iron stored in the body) have been done, the doctor may place them in the pattern of qi and blood deficiency (气血虚 / 氣血虛, qì xuè xū; the body is short of both strength and the blood that nourishes it) and prescribe a decoction in ready-to-drink 50 ml pouches aimed at supplementing qi and blood, along the lines of Danggui Buxue Tang plus Si Jun Zi Tang (当归补血汤加四君子汤 / 當歸補血湯加四君子湯, Chinese Angelica Blood-Supplementing Decoction plus Four Gentlemen Decoction). If you are low in iron, iron is supplemented based on the lab results; the pouches cannot replace iron. And out on the course you still have to eat your gels and sticky rice; the pouches cannot do that part for you.

The front thigh muscle speaks for the small intestine

Traditional medicine groups the muscles and the stomach under the Spleen, while Applied Kinesiology (AK, a way of testing the strength of each muscle by hand to read the state of the body) links them in its own way. Goodheart found that a quadriceps testing weak often goes along with a poorly working small intestine; stimulating the neurolymphatic reflex of the small intestine, that is, the AK reflex point linked to the small intestine, makes the quadriceps strong again. The abdominal muscles also belong to the small intestine group (Walther, Synopsis, p. 14, 313, 317).

According to Gangemi, the small intestine absorbs almost all of the water, faster or slower depending on the salt and sugar that come with it. Stress messengers such as norepinephrine also change the electrolytes inside the gut. When the small intestine absorbs water poorly, the quadriceps and abdominal muscles work below threshold, meaning they test weak, sometimes bringing back, hip or knee pain that few people would connect to the gut (ICAK 2009–2010, p. 155–157).

Take the example of a runner who comes in with stomach cramps and tired knees in the second half of long runs. On muscle testing, his quadriceps is working below threshold. The doctor has him taste, one after another, in the sequence Gangemi describes: a sip of water, a pinch of salt, salt mixed with water right in the mouth, a little sugar, then sugar water. The step at which the muscle becomes strong again suggests what the gut may need in order to hold on to water. Gangemi also found that with a solution of more than 8% carbohydrate (more than 8 g of sugar in 100 ml of water), the gut absorbs water poorly, and fructose on its own easily causes cramps. Only at the end comes the gel planned for race day.

A muscle getting stronger when sugar is tasted is not necessarily a good sign: when blood sugar is low, tasting sugar also makes the muscle stronger (Walther, Basic Procedures, p. 46). So on the day of your appointment, eat breakfast as usual. Depending on how the muscle responds, the doctor stimulates the small intestine reflex again, adjusts how you mix your bottle for the next long run, and then retests the quadriceps. This check cannot set doses; the grams per hour still follow the amounts above.

When to see a doctor

You should see a doctor when:

• Every long run brings stomach pain, nausea or a dash for the toilet, even though you have practiced eating starting from small amounts;
• You have long-lasting fatigue, a poor appetite and slow recovery after a heavy training block;
• You are planning to switch to eating very little carbohydrate or to fasted running to prepare for a race.

During a race, someone who is vomiting a lot, has diarrhea or is dehydrated needs the race medical team first, to replace fluids and electrolytes. But if someone is vomiting with a headache and confusion and has already drunk a great deal of water, the aid station should not pour them any more; take them straight to the medical team.

Next year, if you sign up for that race again, the waist pack on your Sunday long runs should hold exactly the gel you will use on race day, along with a small notebook for recording when you eat. At kilometer 27, chances are you will run straight past the row of toilets without stopping. And if you have trained for a full season and your stomach still acts up, bring the gels, the drinks, your supplements and the medicines you are taking in your bag when you come in. Book a consultation.

Worth keeping

• For runs under 1 hour, there is no need to eat on the way;
• For runs over 1 hour, aim for 30–60 g of carbohydrate per hour, using the grams printed on the label;
• To go up toward 90 g per hour in races over 2–2.5 hours, choose products with both glucose and fructose and practice over many sessions first;
• On every long run before the race, bring exactly the food you plan to use on race day, eat on schedule, and note how your stomach felt;
• On race day, try nothing new, including the gels handed out at the aid stations;
• In ultras, favor real food, including savory food, plus 5–10 g of protein per hour;
• Do not use keto or fasted running to prepare for a long race.

References

Sources in Vietnamese

• Nguyễn Hữu Đức. Bài giảng Tạng Tỳ. T7.

Sources in English

• ACSM’s Advanced Exercise Physiology. p. 377.
• Austin K, Seebohar B. 2011. Performance Nutrition: Applying the Science of Nutrient Timing. Champaign (IL): Human Kinetics. p. 123–125.
• Cao J, Lei S, Wang X, Cheng S. 2021. The Effect of a Ketogenic Low-Carbohydrate, High-Fat Diet on Aerobic Capacity and Exercise Performance in Endurance Athletes: A Systematic Review and Meta-Analysis. Nutrients. 13(8). doi:10.3390/nu13082896. PMID: 34445057.
• Gangemi SC. 2009–2010. Advanced (De)Hydration Assessment: When Water Alone is Not Enough. ICAK-USA Collected Papers 2009–2010. p. 155–157.
• Gawelczyk M, Kaszuba M, Zając A, Maszczyk A. 2026. Effects of Low-Carbohydrate and Ketogenic Diets on Aerobic Performance in Trained Athletes: A Systematic Review and Meta-Analysis. Nutrients. 18(5). doi:10.3390/nu18050740. PMID: 41829910.
• Gejl KD, Nybo L. 2021. Performance effects of periodized carbohydrate restriction in endurance trained athletes – a systematic review and meta-analysis. J Int Soc Sports Nutr. 18(1):37. doi:10.1186/s12970-021-00435-3. PMID: 34001184.
• Jeukendrup A. 2014. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med. 44 Suppl 1(Suppl 1):S25-33. doi:10.1007/s40279-014-0148-z. PMID: 24791914 (ghi chú: tác giả có liên hệ với hãng đồ uống thể thao).
• Kerksick CM, Arent S, Schoenfeld BJ, Stout JR, Campbell B, Wilborn CD, et al. 2017. International society of sports nutrition position stand: nutrient timing. J Int Soc Sports Nutr. 14:33. doi:10.1186/s12970-017-0189-4. PMID: 28919842.
• King AJ, Etxebarria N, Ross ML, Garvican-Lewis L, Heikura IA, McKay AKA, et al. 2022. Short-Term Very High Carbohydrate Diet and Gut-Training Have Minor Effects on Gastrointestinal Status and Performance in Highly Trained Endurance Athletes. Nutrients. 14(9). doi:10.3390/nu14091929. PMID: 35565896.
• Koerich ACC, Borszcz FK, Thives Mello A, de Lucas RD, Hansen F. 2022. Effects of the ketogenic diet on performance and body composition in athletes and trained adults: a systematic review and Bayesian multivariate multilevel meta-analysis and meta-regression. Crit Rev Food Sci Nutr. 2023;63(32):11399-11424. doi:10.1080/10408398.2022.2090894. PMID: 35757868.
• Michaud TC. Injury Free Running: Course Workbook. Coursera course: Running Mechanics and Injury Prevention Strategies. p. 201.
• Ryan T, Daly E, Ryan L. 2023. Exploring the Nutrition Strategies Employed by Ultra-Endurance Athletes to Alleviate Exercise-Induced Gastrointestinal Symptoms-A Systematic Review. Nutrients. 15(20). doi:10.3390/nu15204330. PMID: 37892406.
• Stellingwerff T, Cox GR. 2014. Systematic review: Carbohydrate supplementation on exercise performance or capacity of varying durations. Appl Physiol Nutr Metab. 39(9):998-1011. doi:10.1139/apnm-2014-0027. PMID: 24951297.
• Tiller NB, Roberts JD, Beasley L, Chapman S, Pinto JM, Smith L, et al. 2019. International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing. J Int Soc Sports Nutr. 16(1):50. doi:10.1186/s12970-019-0312-9. PMID: 31699159.
• Urdampilleta A, Arribalzaga S, Viribay A, Castañeda-Babarro A, Seco-Calvo J, Mielgo-Ayuso J. 2020. 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. 12(7). doi:10.3390/nu12072094. PMID: 32679728.
• Viljoen CT, Janse van Rensburg DC, Verhagen E, van Mechelen W, Tomás R, Schoeman M, et al. 2021. Epidemiology of Injury and Illness Among Trail Runners: A Systematic Review. Sports Med. 51(5):917-943. doi:10.1007/s40279-020-01418-1. PMID: 33538997.
• Viribay A, Arribalzaga S, Mielgo-Ayuso J, Castañeda-Babarro A, Seco-Calvo J, Urdampilleta A. 2020. Effects of 120 g/h of Carbohydrates Intake during a Mountain Marathon on Exercise-Induced Muscle Damage in Elite Runners. Nutrients. 12(5). doi:10.3390/nu12051367. PMID: 32403259.
• Walther DS. 1981. Applied Kinesiology. Vol. I: Basic Procedures and Muscle Testing. Pueblo (CO): Systems DC. p. 46.
• Walther DS. 2000. Applied Kinesiology: Synopsis. 2nd ed. Pueblo (CO): Systems DC. p. 14, 313, 317.

Sources in Chinese

• 李冀, 连建伟, 主编. 方剂学. 第 174 页.
• 王新华, 主编. 中医药学高级丛书 · 中医基础理论. 第 219、225、239、402、413 页.