Why Your Stomach Upsets During Hard Efforts (And What to Do)

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.

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.

• 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

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

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 key is consistency: practice your race fueling in training at least once per week so your gut learns to process fuel efficiently.

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

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.

MAVR takes the guesswork out of gut training with smart, personalized guidance.

• Auto-calculates your gut-training carb progression based on race goals
• Spaces fueling timing to avoid overlap with hard efforts
• Suggests gut-friendly products and combinations
• Adapts recommendations as your tolerance improves