Gut Training for Runners: What Works and What Doesn’t

*This article was produced with sponsorship from DM Mitsui Sugar Co., Ltd.

The target for full marathon fueling is 60–90 g of carbohydrate per hour. But when they try to take that much, many runners get bloating, pain, or diarrhea. Between 30% and 50% of endurance athletes experience GI symptoms during exercise※1.

A well-known countermeasure for this problem of stomach trouble when you fuel more is gut training (training the gut to tolerate carbohydrates). It is a method of repeatedly taking carbohydrate during training to get your gut used to fueling.

In short, studies of runners show that continuing for about two weeks reduces abdominal discomfort and carbohydrate malabsorption during exercise※2※3※4. On the other hand, there are no human data showing that the upper limit of how much the gut can absorb goes up. This article separates what gut training can and cannot do.

For how many grams per hour to aim for, see How Many Carbs Per Hour for a Marathon?. For how to take gels (with water, and the carbohydrate blend), see Marathon Gel Strategy.

Why Did the Idea of “Training the Gut” Appear?

The benefit of carbohydrate fueling is determined not by how much you put in your mouth but by how fast your gut can absorb it. A 2000 review concluded that the upper limit of exogenous carbohydrate (carbohydrate taken in from outside) that the body can use during exercise is 60–66 g per hour, and that the limit lies not in the muscles but in the gut (or liver)※5.

With glucose alone, this limit is set by saturation of the transporter (SGLT1). Adding fructose engages another transporter and raises the limit to 90 g per hour or more※6. See Marathon Gel Strategy for details.

The problem is that when you actually try to take this much, stomach issues become likely. This is called exercise-induced gastrointestinal syndrome (EIGS). When exercise intensity exceeds 60% of maximal oxygen uptake and continues for two hours or more, gut damage, delayed gastric emptying, and malabsorption become more likely※7.

The main causes are reduced blood flow to the internal organs during exercise, along with the actions of the autonomic nervous system and hormones. Gut training is the idea of not writing this off as “just my constitution,” but of getting the gut used to fueling so that the symptoms ease.

However, the root cause, reduced blood flow to the internal organs during exercise, does not go away even with gut training.

What Is Gut Training?

What Do You Actually Do?

The definition is simple: deliberately taking carbohydrate repeatedly during training※8. On long runs and key workouts, take the same gels and drinks as on race day, in the same amount and at the same timing.

Continue this for several weeks to get your gut used to being fed. This idea was presented in a 2011 review as “the gut may be trainable”※9.

Effects Confirmed in Humans

In a study of runners (25 participants)※3, two weeks of repeated intake reduced GI symptoms during exercise by about 60% and improved a marker of carbohydrate malabsorption※2※3.

A systematic review that collected eight studies also found that in studies with two weeks of repeated intake, abdominal discomfort fell by an average of 47% and carbohydrate malabsorption fell by 45–54%. However, one study found no change in malabsorption※4.

Put another way, what you can reliably expect from gut training is being able to take your target amount without upsetting your stomach.

What Has Not Been Confirmed in Humans

On the other hand, the same analysis found no change in gastric emptying speed, gut damage, or intestinal permeability※4. No human study has directly measured whether the upper limit of what the gut can absorb (maximum g/h) goes up. Even in the study of runners mentioned above, the rate at which carbohydrate was oxidized (burned) during exercise did not increase※3.

Reviews explain that more of the carbohydrate transporter (SGLT1) increases absorption capacity※8. But the evidence for this comes from animal studies. When mice were fed a high-carbohydrate diet for 10 days, the amount of SGLT1 protein increased, but recovery of glycogen after exercise did not※10.

In an experiment where rats trained five days a week for six weeks, SGLT1 rose in the group that trained six hours a day but not in the group that trained one hour a day※11. This means adaptation appears to need a fairly sizable dose of stimulus.

It is known that SGLT1 in the human gut changes with circumstances. For example, people with type 2 diabetes had gut SGLT1 more than four times that of healthy people※12. But that is a change tied to the metabolic state of diabetes, and it does not show that SGLT1 rose when runners kept fueling.

How to Do Gut Training

When to Start and How Long

Symptoms eased in two weeks in the studies※2, but to build up to your target amount step by step, starting 6–10 weeks before the race is realistic. Fit it into long runs and race-pace key workouts.

That is because animal studies show that short training sessions are unlikely to trigger gut adaptation※11. Do it in training close to race intensity and duration, not as an add-on to an easy jog (→ Long Run Training: Optimal Pace, Distance & Timing).

How to Increase the Amount

Start at 30–40 g per hour, which is one to one and a half gels. Increase by 10–20 g every one to two weeks, and work up to being able to handle 60–90 g in training.

The amounts and time frame for increases are a practical guideline based on the advice in a review※8 to start small and build gradually. No study has tested these exact numbers.

It is important to use the same product, concentration, and timing (every 20–30 minutes) as on race day※8※13.

What to Do When It Isn’t Working

If stomach trouble appears, don’t push through. Drop back one step and start again. Some people see no improvement even after several weeks.

In a study of top athletes, increasing carbohydrate above current guidelines and combining a high-carbohydrate diet with gut training did not produce effects beyond the control group. Individual differences were very large, and in race walkers, upper GI symptoms in the second half of exercise actually increased※14.

If it does not seem to suit you, first review your carbohydrate blend (a product containing both glucose and fructose) and whether you take water with it. These two have quicker effects (→ Marathon Gel Strategy).

Summary

Gut training is the finishing touch of a fueling strategy. First set your target amount with How Many Carbs Per Hour for a Marathon?, sort out your carbohydrate blend and water with Marathon Gel Strategy, and then spend the weeks before the race getting your gut used to it. Built in this order, your plan can reduce both the late-race slowdown and stomach trouble.

References

※1 de Oliveira EP, Burini RC, Jeukendrup A (2014) “Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations” Sports Med

※2 Miall A, Khoo A, Rauch C, et al. (2018) “Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption” Scand J Med Sci Sports

※3 Costa RJS, Miall A, Khoo A, Rauch C, Snipe R, Camões-Costa V, Gibson P (2017) “Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance” Appl Physiol Nutr Metab

※4 Martinez IG, Mika AS, Biesiekierski JR, Costa RJS (2023) “The Effect of Gut-Training and Feeding-Challenge on Markers of Gastrointestinal Status in Response to Endurance Exercise: A Systematic Literature Review” Sports Med

※5 Jeukendrup AE, Jentjens R (2000) “Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research” Sports Med

※6 Fuchs CJ, Gonzalez JT, van Loon LJC (2019) “Fructose co-ingestion to increase carbohydrate availability in athletes” J Physiol

※7 Costa RJS, Snipe RMJ, Kitic CM, Gibson PR (2017) “Systematic review: exercise-induced gastrointestinal syndrome-implications for health and intestinal disease” Aliment Pharmacol Ther

※8 Jeukendrup AE (2017) “Training the Gut for Athletes” Sports Med

※9 Jeukendrup AE, McLaughlin J (2011) “Carbohydrate ingestion during exercise: effects on performance, training adaptations and trainability of the gut” Nestlé Nutr Inst Workshop Ser

※10 Takahashi Y, Matsunaga Y, Yoshida H, Shinya T, Sakaguchi R, Hatta H (2021) “High Carbohydrate Diet Increased Glucose Transporter Protein Levels in Jejunum but Did Not Lead to Enhanced Post-Exercise Skeletal Muscle Glycogen Recovery” Nutrients

※11 Kondo S, Fukazawa A, Karasawa T, Terada S (2019) “Effects of long-term exercise training for different durations on pancreatic amylase activity and intestinal glucose transporter content in rats” Physiol Rep

※12 Dyer J, Wood IS, Palejwala A, Ellis A, Shirazi-Beechey SP (2002) “Expression of monosaccharide transporters in intestine of diabetic humans” Am J Physiol Gastrointest Liver Physiol

※13 Burke LM, Jeukendrup AE, Jones AM, Mooses M (2019) “Contemporary Nutrition Strategies to Optimize Performance in Distance Runners and Race Walkers” Int J Sport Nutr Exerc Metab

※14 King AJ, Etxebarria N, Ross ML, 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