Gut Training: The Organ Nobody Trains

Earlier this week we talked about the real problem behind race-day stomach distress: it's not bad luck, it's an untrained gut. Today we get into the how.

I train legs, lungs, and mind for months. But the organ that decides whether all of that actually reaches the finish line — the digestive tract — almost never shows up in the plan. And it's exactly the same kind of adaptable tissue as everything else in the body: expose it to progressive load, and it adapts. Surprise it on race day, and it fails.

What Gut Adaptation Actually Is (and Why It's Trainable)

The small intestine absorbs carbohydrate through specific transporters — mainly SGLT1 for glucose and GLUT5 for fructose. These transporters aren't fixed in place; their density and activity respond to repeated exposure to intake during exercise. Consuming carbohydrate consistently during long training sessions improves gastric tolerance, reduces fullness, nausea, and urgency, and allows you to absorb more grams per hour without distress.

This lines up with the general principle of physiological adaptation to repeated stress that we see in other systems of the trained body — the same "controlled exposure → adaptation → greater tolerance" mechanism documented in exercise biomarker research more broadly [PMID:41922043]. The gut isn't an exception to the rules of training. It's just another tissue following the same logic.

The good news: you don't need an exotic protocol. You need repetition with intention — the same approach you'd use for marathon pace.

Week-by-Week Protocol: Dose Progression

Here's a general framework I use with runners from 10K through marathon. Adjust based on individual tolerance and race goal.

Weeks 1–2: Baseline

• 20–30g of carbohydrate per hour on runs of 60+ minutes.
• One single source type (gel or drink — don't mix variables yet).
• Log tolerance before increasing the dose.

Weeks 3–4: Progression

• 40–50g/hour on long runs (90+ minutes).
• Introduce the exact source you'll use on race day — if the plan is Brand X gel, practice with Brand X gel, not something else.
• Start pairing it with moderate intensity (some km at race pace) to simulate the drop in splanchnic blood flow that happens at competitive effort.

Weeks 5–6: Specificity

• 60–90g/hour if the goal is marathon distance or anything requiring that range (glucose:fructose blends at roughly 2:1 improve total absorption versus glucose alone, because they use different transporters and don't compete with each other).
• Practice at your actual race-day start time and, if possible, in similar heat.
• Include at least one trial with warm water or heat exposure — thermal stress reduces intestinal blood flow, and that's a variable you need to test, not assume away.

Week 7 and Taper:

• Reduce volume but keep 1–2 sessions with the full race-nutrition protocol intact.
• Nothing new gets introduced. No gel, drink, or bar that hasn't been tested at least 3–4 times in training.

Why Gels Fail on Race Day

It's almost never the product. It's the context.

First-time use on race day. The classic mistake: a runner tries a new gel at kilometer 25 because "they handed it out at the expo" or "everyone uses it." The gut has had zero prior exposure. It's a near-perfect recipe for cramping or diarrhea.

Too much, too fast. Jumping from 0 to 90g/hour without progression overwhelms absorption capacity. Unabsorbed carbohydrate sits in the gut, pulls in water osmotically, and produces bloating, urgency, and diarrhea.

Heat. When ambient temperature rises, the body redirects blood flow to the skin to dissipate heat, pulling it away from the gut. This reduces digestive capacity exactly when you need it most. If you only trained your nutrition in cool weather, a hot race day is a brand-new variable your gut has never met.

High intensity. The higher the percentage of VO2max, the lower the splanchnic perfusion. That's why the same gel that sits fine during an easy jog can cause distress at race pace — the stomach literally gets less blood to process what it's handed. This connects to what we already know about how prolonged, high-demand exercise stresses physiological systems well beyond the musculoskeletal one [PMID:40667749]. The body under sustained load prioritizes resources, and the gut doesn't always win that competition.

The combination of these four variables — novelty, dose, heat, and intensity — explains the overwhelming majority of gastric crises in races. None of them get solved on race day. All of them get solved in the weeks before.

What to Track in Your Nutrition Log

You don't need a complex app. A notebook or a note on your phone with four columns is enough:

1. Dose — grams of carbohydrate per hour, and the exact source (brand, flavor).
2. Timing — exact minute of intake relative to the start of the run.
3. Context — pace, temperature, terrain, fasted or post-breakfast.
4. Symptoms — a simple 0 (nothing) to 3 (significant) scale: fullness, nausea, urgency, reflux, cramping.

After 4–6 logged sessions, the pattern becomes obvious. There's almost always a clear ceiling dose, a timing window that works best, and a source your body tolerates better than others. That's race intelligence — not a guess.

An Important Note on Digestive Health

This protocol assumes a healthy digestive system that simply isn't conditioned for the stress of prolonged exercise. If gastrointestinal symptoms are persistent, severe, include bleeding, pain outside the context of exercise, or fail to improve with gradual dose progression, that's no longer a training issue — it's a signal to see a gastroenterologist or sports medicine physician. Endurance exercise places real demands on multiple physiological systems, and distinguishing between "untrained gut" and "underlying medical condition" isn't something a nutrition plan can or should sort out on its own [PMID:38227551].

The Coach's Perspective

What I repeat most to my athletes: race nutrition isn't last-minute logistics — it's a skill trained the same way you'd train marathon pace or a long run. The athlete who arrives at the start line having practiced their carbohydrate protocol fifteen or twenty times has a quiet edge over the one who tried it twice under comfortable conditions and hopes it holds up under pressure, heat, and accumulated fatigue.

It's not glamorous. It's boring repetition during long training sessions. But that boring repetition is, very often, the difference between finishing strong and finishing in the portable toilet at kilometer 30.

References

• Siebers M, Bizjak DA. Exercise biomarkers. Adv Clin Chem. 2026. [PMID:41922043]
• Couvert A, Goumy L. Effects of a Cycling versus Running HIIT Program on Fat Mass Loss and Gut Microbiota Composition in Men with Overweight/Obesity. Med Sci Sports Exerc. 2024. [PMID:38233990]
• Frandsen J, Aaroe M. Cardiac Effects of Prolonged Endurance Exercise in Young and Older Athletes. Scand J Med Sci Sports. 2025. [PMID:40667749]
• Dayer MJ, Green I. Mortality during marathons: a narrative review of the literature. BMJ Open Sport Exerc Med. 2019. [PMID:31321073]
• Durante A. Marathon not sprint: fatigue and early symptoms detected with sympathetic dysfunction in patients with heart failure. Eur J Cardiovasc Nurs. 2024. [PMID:38227551]