Triathlon
Why Your Stomach Upsets During Hard Efforts (And What to Do)
That painful cramp, nausea, or runner's trots during a hard effort isn't normal. Learn the science behind GI distress during exercise and proven strategies to fix it.
Quick answer
GI distress during exercise stems from reduced blood flow to the gut, improper fueling timing, or gut sensitivity — fix it by training your gut with gradually increasing carbs, practicing race-day nutrition in training, and timing fuel 30+ minutes before hard efforts.
You know the feeling. You're cruising along in a workout or race, feeling strong — then suddenly your stomach churns, you feel nauseous, or worse. For many athletes, GI distress during exercise is frustratingly common.
But here's the truth: an upset stomach during hard efforts isn't something you just have to accept. It's a solvable problem with the right approach.
Let's break down why your gut rebels during exercise and exactly what to do about it.
The Science Behind Exercise-Induced GI Distress
During intense exercise, your body prioritizes blood flow to working muscles and away from the digestive system. This reduced blood flow — called splanchnic hypoperfusion — can damage gut lining and increase intestinal permeability.
When you consume carbohydrates during exercise, especially in large amounts, the gut has to work harder to digest and absorb them. Combined with reduced blood flow, this creates a perfect storm for discomfort.
Common Causes of Stomach Problems During Exercise
- Fueling too close to the start of hard efforts
- Consuming osmotically active drinks (high sugar, artificial sweeteners)
- Dehydration or overhydration
- High-intensity efforts exceeding your gut's tolerance threshold
- Using new products on race day
- Undertraining your gut for the carb loads you need
Quick Fixes for GI Distress
If you're mid-training block and struggling with stomach issues, try these immediate adjustments:
- Reduce carb intake during exercise by 20–30% temporarily
- Space fueling 30+ minutes before hard efforts
- Avoid caffeine and artificial sweeteners during exercise
- Sip fluids steadily rather than chugging large volumes
- Choose simpler carb sources (glucose vs. fructose blends)
- Slow down the pace until your gut adapts
Long-Term Solution: Train Your Gut
The most effective way to eliminate GI distress is to systematically train your gut to handle higher carb loads.
Just like your cardiovascular system adapts to training, your digestive system can become more efficient at absorbing and processing fuel.
The 6-Week Gut Training Protocol
| Week | Carb Rate | Focus |
|---|---|---|
| Week 1 | 30 g/hr | Establish baseline, practice timing |
| Week 2 | 40 g/hr | Increase by 10 g/hr |
| Week 3 | 50 g/hr | Add complexity (gels + drink) |
| Week 4 | 60 g/hr | Test race-day products |
| Week 5 | 70 g/hr | Push toward race targets |
| Week 6 | 80–90 g/hr | Final race-pace adaptation |
The key is consistency: practice your race fueling in training at least once per week so your gut learns to process fuel efficiently.
Best Foods and Products for Sensitive Stomachs
Not all fueling products are created equal for gut sensitivity. Consider these options:
- Single-transportable carbs (glucose-only) are easier to absorb than multiple-transportable blends
- Liquid calories (drinks, smoothies) often digest faster than solid foods
- Small, frequent doses are gentler than large boluses
- Avoid sugar alcohols (sorbitol, xylitol) which draw water into the gut
When to Seek Medical Help
Some symptoms warrant medical evaluation:
- Persistent diarrhea or bloody stools
- Severe cramping unrelated to fueling timing
- Weight loss or inability to maintain race weight
- Symptoms that don't improve with gut training
These could indicate underlying conditions like IBS, gastroesophageal reflux, or other medical issues requiring professional treatment.
A Simple 6-Week Gut-Training Progression
The gut is trainable like any other system, and the protocol is straightforward enough to run yourself. Start where you are comfortable — say 30g of carbs per hour on long runs — and add roughly 10g per hour every week or two, always using the exact products you intend to race on. Over six to twelve weeks most athletes can build from 30g toward the 80-90g per hour that fuels a strong marathon or 70.3, while the gut adapts by upregulating the transporters that absorb carbohydrate. The keys are to progress gradually, rehearse race-day foods rather than novelties, and never debut a new fueling level on race day.
Auto-calculating that weekly carb progression and spacing it around your hard efforts is something MAVR can manage from your race goal, so you are not tracking it on paper. But the progression itself is simple to follow on your own — the discipline of nudging the number up week by week is what actually trains the gut.
Train your gut smarter with AI-powered fueling guidance.
Frequently Asked Questions (FAQ)
Why do I always get diarrhea during hard runs?
Runner's trots is caused by reduced blood flow to the gut, increased intestinal motility from jostling, and sometimes fueling timing. Training your gut and adjusting caffeine intake can help significantly.
Should I stop eating during exercise if my stomach hurts?
Not necessarily. Reduce carb intake temporarily and focus on hydration. Once symptoms ease, resume small, frequent fuel doses. Persistent severe pain warrants stopping and medical evaluation.
What is the best fuel for sensitive stomachs?
Simple glucose-based drinks, diluted fruit juice, or commercial products with single-transportable carbs work best. Avoid artificial sweeteners and high-fructose blends if you have a sensitive gut.
Can running cause stomach ulcers?
Exercise itself doesn't cause ulcers, but intense training can exacerbate existing gastritis. If you have persistent upper GI issues, consult a healthcare provider.
How long does gut training take?
Most athletes see meaningful improvement within 4–6 weeks of consistent practice. Full tolerance for race-level fueling (80–90 g/hr) typically requires 8–12 weeks of systematic training.
Written by
MAVR Sports Nutrition Team
The MAVR sports-nutrition team translates peer-reviewed endurance research (Jeukendrup, Burke, the ISSN) into daily fueling for runners and triathletes.