Gut training for ultras: how to absorb more carbs and avoid GI distress

Written by James Frost, founder of Flaming Phoenix and the maker of Phoenix Bars. Updated October 2026.

Quick answer: Gut training means practising race-day eating and drinking on long runs, building the amounts gradually, so your stomach copes on race day. In one trial, two weeks of daily one-hour runs taking 90g of carbohydrate an hour cut runners' gut symptoms and signs of poor absorption. Start at least four weeks before your race, at about 30g an hour.

This page is the training side: what the studies found, how much carbohydrate to train towards, and a week-by-week plan. What to eat on the day itself, hour by hour, is in the first ultramarathon nutrition guide, and fuelling across all long events is in our ultra-endurance and expedition nutrition guide.

What is gut training?

It is deliberate practice of eating and drinking while you exercise, at rising amounts, so your digestive system adapts to the load you plan to give it on race day. The idea was set out by sports scientist Asker Jeukendrup in a 2017 review in Sports Medicine, Training the Gut for Athletes.

His review describes three kinds of change. The stomach can empty faster. Feelings of fullness and discomfort at a given intake can fall. And the gut can absorb more, because a high-carbohydrate diet increases the number and activity of SGLT1, the transporter that carries glucose across the wall of the small intestine. Fructose uses a different transporter, GLUT5, which is why mixed sugars can be absorbed faster than glucose alone.

How common are stomach problems in ultras?

Very. The International Society of Sports Nutrition's 2019 position stand on ultramarathons gives a conservative estimate of 30 to 50% of runners having gut problems in an ultra, with other reports of 70 to 80%. In one group of 272 runners at the Western States 100, 96% had symptoms at some point and 44% said they hurt their performance; nausea changed the race plan of 60%.

The ISSN names a high intake of carbohydrate, especially very concentrated drinks and gels, as the main dietary cause, and links symptoms to the fall in gut blood flow that comes with dehydration and a rising core temperature. It also advises avoiding anti-inflammatory painkillers such as ibuprofen during races. Jeukendrup adds that the causes are partly genetic and made worse by heat and dehydration. Gut training works on the part you can change: how much your gut can handle at race pace.

Can you really train your gut?

Yes. The evidence is stronger than for most nutrition ideas, though much of it is short and small. The table summarises three studies; the first two are as described in Jeukendrup's review, the third from its own abstract (Miall and colleagues, 2018).

Jeukendrup's review extrapolates from the animal work that raising dietary carbohydrate from 40% to 70% of calories could double the transporters in about two weeks, while noting that no human study has yet mapped the timeline. Taken with the 2018 trial, the working answer is that a few weeks of regular practice makes a difference you can feel.

One myth to drop: "train low, race high", the habit of doing every long run on little or no fuel and then eating a lot on race day. The gut adapts to what you regularly ask of it. Jeukendrup's advice to athletes who restrict carbohydrate in training is to include some high-carbohydrate days, so the gut keeps its capacity.

How much carbohydrate should you train towards?

Train towards the amount you plan to eat in the race, which for most ultras is 30 to 60g an hour. Glucose on its own tops out at about 60g an hour, because its transporter saturates; Jeukendrup's review recommends about 90g an hour of mixed glucose and fructose for exercise lasting two hours or more. That ceiling suits marathons, triathlons and cycling.

Ultras are slower and much longer. The ISSN suggests 30 to 50g of carbohydrate an hour within 150 to 400 calories, plus 5 to 10g of protein, and considers 90g unrealistic for most runners in races over six hours. Pick your target from your race and your training. The ISSN range suits most runners, but finishers in one 100-mile study reported in the same paper averaged about 66g an hour, so a well-trained gut can go higher.

Your everyday diet matters as well, because the review links higher daily carbohydrate to more transporters. The British Dietetic Association's sport and exercise fact sheet suggests 5 to 8g of carbohydrate per kg of body weight a day for 2 to 3 hours of intense training on 5 or 6 days a week, and 8 to 10g for 3 to 6 hours a day. For a 70kg runner, that is 350 to 560g, or 560 to 700g on the biggest weeks.

A gut training plan, week by week

This is the progression I would suggest for the ten weeks before a race, stepping up by about 10g an hour every fortnight on your long runs. It is a framework built on the studies above, not a tested protocol, so slow it down if symptoms rise.

Make the practice as close to the race as you can: same effort, same foods, same pockets, and, if your race starts at 6am or runs through the night, some long runs that do the same. If you only have four weeks, start at 30 to 40g and step up each week. Practising drinking matters as much as eating; Jeukendrup's review notes that comfort with large volumes of fluid also improves with practice. Doing this consistently is also your best insurance against hitting the wall late in a race.

What should you practise with?

Everything you will eat on the day, including the real food for the later hours. Start with gels, chews and drinks, then work in solid food from our list of the best foods for ultra running, savoury options and soft high calorie foods, because those are what you will want after four or five hours. Our guides to energy gel alternatives and easy to digest high calorie foods have more ideas. The ISSN supports gut training and, for runners with persistent trouble, trying lower-FODMAP foods.

Little and often works better than big hits. Kerry, a triathlete, described her method: "My fuelling strategy is to take a bite size portion every 15 minutes". Timing matters too, and that is the kind of thing to find out in training, not at a start line.

How do Phoenix Bars fit gut training?

Full disclosure: I make these. Phoenix Bars carry up to 557 calories in only 120g, with up to 66g of carbohydrate and up to 19g of protein, so they cover the carbohydrate and the protein the ISSN suggests in one food. A quarter of a bar every 30 minutes is about 30 to 33g of carbohydrate an hour; add 500ml of an isotonic sports drink (about 31g, using the BDA's figure of 50g per 800ml) and you are at 61 to 64g. Stirred into water, a bar makes a soft porridge to rehearse for a late aid station; high calorie porridge and how to use Phoenix Bars explain how. If they are in your race plan, they need to be in your long runs first. To compare them with other high calorie bars by carbohydrate, see the best energy bars for ultra running.

When is gut training not the answer?

When the symptoms are not only about running. If you get stomach pain, sickness or changed bowel habits away from exercise too, if symptoms are getting worse as you build up instead of better, or if you ever see blood, stop pushing your intake and speak to your GP before carrying on.

Gut training for ultras: common questions

How long does it take to train your gut? Two weeks of daily practice reduced symptoms in the 2018 trial by Miall and colleagues, and mice nearly doubled their glucose transporters in two weeks on a high-carbohydrate diet. Cox's cyclists ate high carbohydrate for 28 days. Give yourself at least four weeks before a race, and ten if you can.

Can you train your gut to take more fluid? Yes. Jeukendrup's review cites work in which comfort with large volumes improved with practice, and suggests training to drink larger volumes. The ISSN's range for ultras is 450 to 750ml an hour, drinking to thirst, so practise near the volume you will drink in the race.

Does gut training help in the heat? It helps, but heat works against you: the ISSN links gut symptoms to the fall in gut blood flow that comes with a rising core temperature and dehydration. Do some of your practice in warm weather, keep drinking, and lean on fluids and small, salty, soft food. Our guide to fuelling the Marathon des Sables covers desert heat.

Do you need to gut train for a 50km? If you will be out for five hours or more and plan to eat 30 to 50g of carbohydrate an hour, yes: at least your last four or five long runs should rehearse the plan. For a shorter, faster race you can get away with less, but nothing on race day should be new.

Should you eat less if your stomach is sensitive? Usually not. Eating too little is its own problem in an ultra. Start lower, around 30g an hour, build more slowly, use familiar foods and try lower-FODMAP options; and if symptoms happen away from running too, see your GP first.

Sources: Jeukendrup, Training the Gut for Athletes, Sports Medicine (2017), full text; PubMed abstract of the same review; Miall and colleagues, Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption (2018); ISSN position stand on ultra-marathon nutrition (2019); British Dietetic Association: sport and exercise nutrition.