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Why GI Distress Ruins More Races Than Injuries—And How to Bulletproof Your Gut
Jonah Rosner
June 05, 2025
Why GI Distress Ruins More Races Than Injuries—And How to Bulletproof Your Gut
Estimated read time: 4.1 minutes (about as long as it takes your gut to tell you that pre-race donut was a no go). 🚨
Hey Performance Nerds! Jonah here. 🤓
Ever watched 20 weeks of training evaporate because your stomach staged a revolt at mile 18? Join the club—population: every endurance athlete who's ever existed.
GI distress causes 3x more DNFs than injuries, yet 90% of athletes never train their gut.
But here’s the good news: your gut is trainable.
Today we're breaking down:
🌟 Why 90% of athletes neglect the most fixable race-day failure
🏋️️ How to train your gut like your legs—with progression and overload
📅 Two proven protocols (8-week and 10-week) to bulletproof your gut
📊 The exact carb targets, timing, and fuel to use
💬 Your training success is my priority.
If you’re hitting a wall, feeling off-track, or just have a question (seriously—no question is too small!), hit reply. I read every message and I’m genuinely here to help you keep crushing it.
🧬 Performance Sponsor:
SiS is now the official sponsor of Marathon Science. Their performance products—like fueling bars, hydration mixes, and recovery supplements—are trusted by pros and grounded in real sport science.
🎯 Get 15% off any SiS product at the Feed with code JONAH15 at checkout
🗓️ Valid to June 15th | One use per customer
🧠 The Science: Tolerance vs. Absorption (And Why Keto Athletes Struggle)
Studies show up to 50%–90% of endurance athletes experience GI distress that kills performance.
Meanwhile, my dog Augie can inhale a tennis ball and feel fine. Dogs clearly have elite-level gut genetics. (Augie: 1, humans: 0.) 🐶
Here's what gut training actually does: It expands your stomach's comfort zone, so you can take in more fuel without the nausea, bloating, or sloshing. You're not changing absorption (that's enzyme-limited), you're improving tolerance.
Why keto wrecks your gut: Low-carb diets down-regulate carb transporters in your gut. So when race day comes? Your GI system freaks out because it's not used to processing carbs at speed.
If you're racing long, you need carbs. And your gut needs practice using them.
🏆 Gut Training Methods That Actually Work
🔄 Progressive Overload (The Cornerstone)
Start with 45g carbs in the final 30 minutes of long runs. Once comfortable, add another 30g earlier. Build to your target hourly rate, then exceed it slightly to build buffer capacity.
💧 Volume Loading (Stomach Expansion 101)
Like competitive eaters, you can train your gut with fluid. High-volume hydration (within reason) can stretch the stomach and accelerate gastric emptying—which helps reduce race-day sloshing.
🍽️ Post-Meal Training (The Contrarian Move)
Train occasionally on a full stomach during easy runs. This helps your body tolerate race-day fullness when fueling every 30-40 minutes.
Don’t do this every run—but once a week? Game changer.
🔧 Protocol 1: The 8-Week Gut Builder
Best for: Athletes new to fueling or with GI sensitivity history.
The Setup: Choose your longest weekly run (2+ hrs) and a moderate session (60–90 min) for gut practice.
Fuel: Use 1:0.8 glucose-to-fructose gels for max absorption.
Golden Rules:
✅ Only progress if the previous week felt fine
✅ Practice your exact race-day products
✅ Fuel earlier over time to mimic race timing
🗓 Protocol 2: 10-Week Elite Gut Tolerance Plan
Best for: Athletes targeting 90g/hour intake with iron stomach reliability.
The Strategy: Overreach with fuel to create a tolerance buffer, then return to race-day targets. Practice 2x per week.
Fuel: Same 1:0.8 glucose-to-fructose ratio for max uptake.
Pro Tips:
Practice in race-like conditions (heat, pace, terrain)
Hydrate: 500–700ml/hr alongside carbs
Advance only when previous load feels smooth
Exceed your target: You might max out at 90g/hr, but training at 100–110g builds a buffer so race-day stress doesn’t wreck your gut.
💪 Event-Specific Adjustments
⏱️ Half Marathon: 30–45g/hr (3–5 weeks gut training usually sufficient)
⛳ Marathon: 60–90g/hr (8–10 weeks recommended)
♿ Ultra/IRONMAN: 90+g/hr (add 2–4 weeks to advanced protocol)
🎯 Practical Summary:
🔄 Start with 15–30g in final 30 min of long runs
✅ Progress weekly: add 10–15g if previous week felt fine
⛽ Use glucose:fructose 1:0.8 ratio products
🌊 Hydration reality check: 2% dehydration cuts carb absorption by 20-30%. Aim for 500-700ml/hour
⏰ Practice 1–2x/week minimum 6-10 weeks pre race
🏃♂️ On race day, start fueling within the first 20 minutes
🚨 Shoe Hack to Reduce Your Injury Risk?
Did you miss my post about how shoe rotations might help with reducing injury risk? You can find it below!
I won’t lie. These videos take me a while to make. If you find it helpful, share it on your story or with a friend. It helps me a ton!
Are You a True Running Nerd? Prove it.. 🧐
Welcome to the prove you’re a nerd section. Each week, I ask a question about a common running science myth.
Answer correctly, and you’ll be entered into a weekly raffle to win a package of Jonah’s favorite supplements.
Why can cyclists often handle way more training volume than runners without breaking down? 🚴♂️🧠 |
Last Week’s Results: Lactate Truth! ⚡️🦵🧂
Y’all brought the heat! 🔥 Over 470 of you weighed in on which supplement truly moves the needle in all-out efforts like 5Ks and HYROX—and most of you nailed it like a final sled push on race day.
Correct Answer:
B – Sodium bicarbonate 🧁 ✅
Sodium bicarb works by buffering hydrogen ions—the acidic byproducts that build up when you're working above your lactate threshold. Translation? Less burn, more output. It won’t make you faster on its own, but it can delay that dreaded wall in events where muscle acidosis shuts down performance.
Vote Breakdown:
🐟 A. Fish oil – 22 votes
🧁 B. Sodium bicarbonate – 312 votes ✅
🥤 C. Beetroot juice – 126 votes
💅 D. Collagen peptides – 14 votes
Let’s bust some myths:
Beetroot juice is helpful—mainly for aerobic events (like a tempo run or long steady-state ride) due to its effect on nitric oxide.
Takeaway Tip:
If you’re racing short and hard, like a 5K or HYROX, test sodium bicarb in training. Dose it properly (0.2–0.3g/kg body weight about 60–90 min before), and expect some GI experimentation. 💣
Science is spicy. So is baking soda. 🧪🧁
Nerdy Finds of the Week 📚🧑🔬
This section includes my favorite research, podcasts or books about running/lifting science.
Paper: Influence of Exercise Heat Acclimation Protocol Characteristics on Adaptation Kinetics: A Quantitative Review With Bayesian Meta-Regressions
🔬 Core Finding
Just 8 days of exercise-heat acclimation cuts heart rate, cools your core, boosts plasma volume—and massively improves endurance.
📊 Key Research Points
After 8 days of heat training:
Time to exhaustion went up 49%
VO₂-style fitness tests improved by 14%
Time trial performance improved by 3%
Resting heart rate dropped 5 bpm, and 17 bpm during exercise
Core temperature dropped 0.19°C at rest, and 0.43°C after exercise
Plasma volume grew by 5.6%, total blood volume by 2.9%
Body used 87 mL less oxygen per minute during exercise
Sweating increased by 163 mL/hour
Sweat got less salty—20 mmol/L less sodium
Adding extra heat exposure:
Each extra day ➔ +1.9 g hemoglobin, more sweat, cooler skin
Each extra 15 minutes ➔ lower heart rate, more plasma volume
Training in 5°C hotter temps ➔ even lower heart rate, cooler core and skin
🛠️ Practical Applications
⚡ 8+ days of heat training at ~39°C ➔ Big endurance gains and lower heart rate
⏱️ Add 1 more day ➔ Slightly better cooling, more sweat, and more oxygen-carrying blood (Hbmass +1.9 g)
⏱️ Add 15 minutes per session ➔ More plasma volume (+0.4%) and slightly lower heart rate
⚡ Train in hotter conditions (+5°C) ➔ Even more cooling and lower heart rate during exercise
🍽️ Sweat becomes less salty ➔ Helps your body cool better through evaporation
🚫 No major benefit from fancy protocols (like controlled hyperthermia) ➔ Simple heat training works just as well
⚡ Higher starting heart rate? ➔ You’ll see bigger drops after heat training
⚡ Less fit athletes ➔ See bigger improvements in exercise efficiency
🍽️ More humid conditions (higher Pa) ➔ Bigger sweat increase and blood volume gains
🚫 Only a small boost (3%) in time trials ➔ Heat training helps more in long, fixed-pace efforts
🎯 Bottom Line
Heat training works—but not all heat is equal.
Tailor your protocol: more days, more heat, longer sessions = bigger gains.
Recovery and environmental details matter as much as the sessions themselves.
Don’t forget: You + Science = AWESOMENESS 😎
Yours in science,
Jonah
P.S. - We have a crew of 15,870+ nerds here who are running FAST using science.
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