Gut Training for Endurance Athletes: The 2025 Science

Gut Training for Endurance Athletes – I get asked a lot about this! Gut training is the deliberate, progressive practice of fueling at race-target carbohydrate rates during training. Costa et al. 2025 — the SDA/USSF Joint Position Statement on exercise-induced gastrointestinal syndrome (EIGS) — graded it Grade I evidence. Two weeks is the minimum effective dose. Eight to twelve weeks is what actually wins races. Probiotics, glutamine, and chronic LCHF do not work and were graded against. Here is the 10-week build I run with my pros.

By the time my pros line up at Unbound or SBT GRVL, every variable I can control is locked in — watts, sleep, travel, sodium plan. The one most amateurs leave to chance until race week is the gut.   If you bonked at hour three of your last A-race, your stomach was not the problem. **Your protocol was.**

The 2025 Joint Position Statement on EIGS from Sports Dietitians Australia and the Ultra Sports Science Foundation — Costa et al., *Sports Medicine* 55:1097–1134 — graded every published gut intervention by evidence tier. Three things actually work. Three popular ones do not. And if you race over 90 minutes at intensity, the gut you bring to the start line is a trainable variable, not a fixed trait.   The 2023 Martinez systematic review, which Costa et al. 2025 leans on heavily, found a 47% reduction in gut discomfort and 54% less carbohydrate malabsorption after just two weeks of structured 30–90g/hr fueling during training. That is not a marginal gain. That is a primary intervention.

What is gut training?

Gut training is the deliberate, progressive practice of consuming carbohydrate during exercise at increasing rates to drive specific physiological adaptations in your gastrointestinal tract. It is a multi-week protocol — not a race-week shortcut — and it sits beside intervals and strength work in the periodization of competitive endurance preparation.  The mistake most amateurs make is thinking gut training fits inside the taper. It does not. Two weeks is the floor for measurable adaptation. Eight to twelve weeks is what actually prepares you for race day.   If your A-race is more than three hours long, you need a minimum of ten weeks of gut training. Two to three hours of racing? Six to eight weeks is a reasonable floor. Start sooner than you think you need to.

Why your gut adapts:

SGLT1 and GLUT5: Two intestinal transporters control how much carbohydrate gets from your gel into your bloodstream.  

SGLT1: sodium-dependent glucose transporter 1 — absorbs glucose and maltodextrin. It saturates at approximately **60g of glucose per hour**. Above that, excess glucose backs up in your gut and drives the bloating, cramping, and urgency that wreck races.

GLUT5: absorbs fructose through a completely separate pathway. Because glucose and fructose use different transporters, combining them allows total carbohydrate absorption to climb past 90g/hr — and into the 120g/hr territory documented in Hearris et al. 2022, with peak oxidation rates of 1.56–1.66 g/min. Repeated exposure to high-carbohydrate fueling **upregulates SGLT1 density and activity** in the intestine. Measurable changes appear after just a few days of dietary stimulus. Train it. It adapts. If your current gel only lists maltodextrin or dextrose with no fructose or sucrose on the label, you have already hit your ceiling no matter how many you take.

What the 2025 position paper actually graded

Costa et al. 2025 ranked every intervention from Grade I (strongest evidence) through Grade IV (weakest). Three earned **Grade I** as performance-enhancing and gut-protective:

1. **Carbohydrate during exercise (60–90 g/hr).** Triggers postprandial hyperemia — increased blood flow to the gut — which blunts the splanchnic ischemia that drives EIGS. Snipe et al. 2017, in 35°C heat: the 90 g/hr group cut gut barrier injury markers roughly in half compared to water alone.

2. **Two-week gut training.** Cuts carbohydrate malabsorption from clinical levels to nearly zero (Costa et al. 2017; Miall et al. 2018). The Martinez 2023 systematic review confirmed 47% less gut discomfort and 54% less malabsorption after just two weeks at 30–90 g/hr.

3. **Low-FODMAP eating 24–48 hours pre-race.** Reduces GI symptom severity by roughly 50% (Lis et al. 2018). Covered in detail in [my pre-race FODMAP article](#). Three earned a clear “do not bother” verdict.

Probiotics.** No effect on barrier injury, endotoxemia, inflammation, or symptoms across single-strain and multi-strain protocols, doses up to 10¹¹ CFU/day, durations up to 12 weeks.

Glutamine.** Findings inconsistent and clinically irrelevant in trained athletes.

Chronic LCHF / ketogenic diets.** McKay et al. 2023: six days of LCHF in elite race walkers raised gut barrier injury (I-FABP), endotoxin binding protein, and CD14 — with no symptom benefit. You stack two insults, you get two insults.

The 10-week gut training build I run with my pros

Apply this on every training session over 75 minutes. Shorter sessions don’t require active gut training. Progress weekly — the point is progressive overload, not aggressive overload.

Phase | Weeks | Carb target | Focus

Foundation | 1–2 | 30–45 g/hr | Establish habit, measure baseline. Mild bloating is normal.

Build 1 | 3–4 | 45–60 g/hr | Push toward the single-transporter ceiling. Introduce dual-carb sources.

Build 2 | 5–6 | 60–75 g/hr | Break through SGLT1 ceiling using glucose + fructose.

Race Sim | 7–8 | 75–90 g/hr | Race-day products, race-day timing, race-day intensity.

Refinement | 9–10 | 75–120 g/hr | No new products.

Race day = practiced repeat.

For the 90 g/hr gel-and-drink stack, body-weight-scaled targets, and the full 10-week build with product recommendations (Maurten, SiS Beta Fuel, Precision Fuel PF30/PF90, real food alternatives), download my free Around-Training Carbs Guide Here.

Five gut training mistakes that will set you back

1. Starting too close to race day. Two weeks is the minimum, not the goal. Three weeks out leaves you adapting during taper.

2. Glucose-only products. No fructose on the label = a hard 60 g/hr ceiling, regardless of how many gels you take.

3. Not training in race conditions. GI tolerance drops in heat, at high intensity, and under stress. Costa et al. 2025 lists hot conditions among the strongest exacerbators of EIGS. Train where you’ll race.

4. Introducing new products on race day. Ever. Your race-day plan is a list of things your gut already knows.

5. Confusing tolerance for ceiling.** Comfortable at 60 g/hr is a signal to build, not to stop.

What gut training will and won’t do

After 8–10 weeks of consistent practice, you can expect significantly reduced bloating and nausea on long sessions, the ability to take in more carbs per hour without symptoms, more stable energy in the back half of long efforts, fewer “bonk” moments, and a fueling protocol you have actually proven in race-like conditions.   It will not compensate for chronic underfueling. It will not eliminate symptoms if hydration falls apart. It will not work if you start two weeks before race day and expect full adaptation.

FAQ

How long does gut training take to see results?

Measurable changes in carbohydrate malabsorption appear after about 2 weeks (Miall et al. 2018; Martinez et al. 2023). For an A-race, plan 8–12 weeks of structured progression.

What is the maximum safe carbohydrate intake per hour?

Grade I evidence supports 60–90 g/hr. Hearris et al. 2022 documented 120 g/hr in trained cyclists across 3 hours at 95% of lactate threshold with minimal GI distress — but only with adapted guts.

Do I need glucose plus fructose, or is glucose alone fine?

Glucose alone caps you at ~60 g/hr because of SGLT1 saturation. Above 60 g/hr you need a 2:1 glucose:fructose ratio (or 1:0.8 maltodextrin:fructose for higher rates).

Does coffee or caffeine count toward gut training?

No. Caffeine doesn’t train carbohydrate absorption, and high doses can worsen lower GI symptoms. Test your race-day caffeine plan in training, never on race day.

Can I gut-train on the bike but not on the run?

Some sport specificity matters because of mechanical jostling. Triathletes should practice gut tolerance in both modalities, especially the run.

Are probiotics worth taking for gut symptoms?

Costa et al. 2025: no. Across every probiotic study reviewed, there was no measurable effect on gut barrier, endotoxemia, inflammation, or symptoms in endurance athletes.

The bottom line

Costa et al. 2025 made the message plain: train the gut, fuel during exercise, skip the supplement aisle. Probiotics and glutamine are noise. Chronic LCHF stacks the deck against you. The athletes I see PR are not the ones with the most expensive supplement stack. They are the ones who practiced their fueling for 14 days straight.   When you are 8 weeks out from your A-race and ready to translate the build into actual race-day execution — hourly targets, product combinations, the timing protocol that prevents the hour-three bonk, download my free Carbs for Racing Guide HERE. It picks up where this article leaves off.

Interesting in doing a deeper dive? Apply to become a part of the Nutritional Revolution athlete roster here!

Listen to our latest podcast episodes with researchers and elite and professional athletes here: www.nutritional-revolution.com/podcasts.

Sources

Costa RJS et al. (2025). SDA/USSF Joint Position Statement on Exercise-Induced Gastrointestinal Syndrome. *Sports Medicine* 55:1097–1134. – Costa RJS et al. (2017). Systematic review: exercise-induced gastrointestinal syndrome. *Alimentary Pharmacology and Therapeutics* 46:246–265. [doi.org/10.1111/apt.14157](https://doi.org/10.1111/apt.14157) – Miall A et al. (2018). Two-week gut challenge protocol reduces carbohydrate malabsorption. *Scandinavian Journal of Medicine and Science in Sports* 28(2):630–640. – Snipe RMJ et al. (2017). Carbohydrate and protein intake during heat stress and gut barrier injury. *Applied Physiology, Nutrition, and Metabolism* 42(12):1283–1292. – Martinez IG et al. (2023). The Effect of Gut-Training and Feeding-Challenge on Markers of Gastrointestinal Status. *Sports Medicine* 53(6):1175–1200. [doi.org/10.1007/s40279-023-01841-0](https://doi.org/10.1007/s40279-023-01841-0) – Hearris MA et al. (2022). 13C-glucose-fructose labeling reveals comparable exogenous CHO oxidation when consuming 120 g/h. *Journal of Applied Physiology* 132(6):1394–1406. [doi.org/10.1152/japplphysiol.00091.2022](https://doi.org/10.1152/japplphysiol.00091.2022) – Jeukendrup AE (2017). Training the Gut for Athletes. *Sports Medicine* 47(Suppl 1):S101–S110. [doi.org/10.1007/s40279-017-0690-6](https://doi.org/10.1007/s40279-017-0690-6) – McKay AKA et al. (2023). LCHF and exercise-induced gut barrier injury. *International Journal of Sport Nutrition and Exercise Metabolism* 33(6):305–315. – Lis DM et al. (2018). Low FODMAP diet reduces exercise-associated GI symptoms. *Medicine and Science in Sports and Exercise* 50(1):116–123.