Ultramarathon GI Issues: Causes and Prevention of Nausea, Vomiting, and Diarrhea

Prevalence of GI Issues in Ultramarathon Runners

According to international trail running research:

• 60-90% of ultramarathon runners experience at least one GI symptom
• 30-40% have their performance affected
• 10-20% DNF as a result

Common symptoms: nausea, vomiting, bloating, diarrhea, and gastroesophageal reflux.

The 5 Main Causes of GI Issues

1. Reduced Splanchnic Blood Flow

During exercise, blood is shunted to the muscles, and GI blood flow can drop by 60-80%. Over prolonged periods, this causes intestinal mucosal hypoxia and increased permeability (commonly known as leaky gut), triggering inflammation and discomfort.

2. Mechanical Vibration

The repetitive impact of running causes food in the stomach to slosh up and down, especially on downhill sections, stimulating the vomiting center.

3. Osmotic Imbalance

Consuming overly concentrated energy gels (>8% sugar concentration) causes water to remain in the stomach, slowing gastric emptying and leading to bloating and nausea.

4. Dehydration

When dehydrated to 2-3% of body weight, gastric emptying rate is cut in half. Food lingers and ferments, increasing gas and discomfort.

5. Psychological Stress

Pre-race anxiety elevates cortisol and adrenaline, causing abnormal gastric acid secretion + slowed gastric emptying.

Symptom Severity Classification

Mild (Does Not Affect Running Pace)

• Mild nausea, burping, bloating
• Food can still be taken in and reaches the stomach

Moderate (Affects Fueling)

• Strong nausea, no desire to eat anything
• Even trying to drink water causes queasiness

Severe (Affects Running Pace)

• Vomiting
• Abdominal pain, diarrhea
• Must walk

Emergency (Requires Dropping Out and Seeking Medical Attention)

• Continuous vomiting with inability to rehydrate
• Bloody stool, severe abdominal pain
• Dehydration + electrolyte imbalance
• Altered consciousness

Prevention Strategies

Prevention 1: Pre-Race Diet

Time Point	Recommendation
3 days before	Low-residue diet (fewer vegetables, less high-fiber food)
1 day before	High-carb but low-fat, low-fiber
4 hours before	Main meal of 300-400 kcal, low-fat low-fiber
1 hour before	Small light snack (banana, bread)
15 minutes before	Small sips of water or sports drink

Avoid: spicy food, fried food, raw vegetables, coffee, alcohol, and excessive dairy.

Prevention 2: Fueling Formula

• Keep sugar concentration at 6-8%
• Glucose + fructose in a 2:1 ratio (higher absorption ceiling)
• Pair every 1g of carbs with 8-10ml of water
• Diversify (sweet → salty → liquid → solid)

Prevention 3: Gut Training

Start “practice eating” 6-8 weeks before the race:

• Start at 40g of carbs per hour
• Increase by 10g each week
• Reach the target amount (e.g., 90g) before race day
• Test tolerance for various foods

Prevention 4: Avoid New Things

Never try new foods on race day:

• New energy gel brands
• New flavors
• Sports drinks you’ve never had
• Unfamiliar local foods (especially at international races)

Prevention 5: Mental Relaxation

• Pre-race nervousness can trigger GI issues
• Deep breathing, light stretching, chatting with companions
• Don’t stay up late, don’t get overly excited

In-Race First Aid

Mild GI Management

1. Walk for 5-10 minutes: reduce impact
2. Sip ginger candy / peppermint candy
3. Switch to cola (de-fizzed): sugar + caffeine + stomach relaxation
4. Switch to salty foods: miso soup, potato chips, crackers

Moderate GI Management

1. Stop running completely: sit down at an aid station for 5-10 minutes
2. Drink water in small, frequent amounts (50-80ml at a time)
3. Eat simple foods: white bread, potatoes
4. Ginger candy / probiotic tablets

Severe GI Management

1. Don’t rush to eat after vomiting: rest for 30 minutes
2. Electrolyte replacement (sports drink diluted by half)
3. Wipe face with cold water, cool down
4. Seriously consider dropping out

Useful In-Race Remedies

• Probiotic tablets (more effective if started 1-2 weeks before the race)
• Ginger candy, peppermint candy, chewing gum: stimulate saliva, relax the stomach
• Cola (de-fizzed): the classic rescue drink
• TUMS / antacid tablets: neutralize stomach acid
• IV saline (race medical staff): for severe dehydration

Post-Race Recovery

Day 0

• Don’t force yourself to eat
• Sip water + electrolytes
• Probiotic drinks

Day 1-2

• Simple, gentle foods: congee, bread, white toast
• Avoid fried food, spicy food, raw vegetables

Day 3-7

• Gradually return to a normal diet
• Increase vegetable and fiber intake
• Supplement protein to repair tissue

When to See a Doctor?

• Vomiting persists within 48 hours after the race
• Bloody stool
• Severe abdominal pain that won’t subside
• Dark urine, very low urine output (severe dehydration)
• Abnormal heart rate, dizziness, fainting

At-Risk Groups

The following runners are more prone to GI issues:

• Those who suddenly increase training volume
• Highly anxious individuals
• Those with allergies (especially FODMAP sensitivity)
• Those with a cold or sleep deprivation before the race
• Women affected by their menstrual cycle

Conclusion

Ultramarathon GI issues are not a “willpower problem” — they are a physiological mechanism. Through pre-race dietary control, long-term gut training, diversified fueling, and mental relaxation, the incidence rate can be greatly reduced. Once it occurs, early intervention is more likely to save your race than toughing it out.

Related Reading

• Handling Nausea in Ultramarathons: On-the-Spot Responses and Fueling Adjustments for GI Discomfort
• Gastrointestinal Issues in Ultramarathons: Prevention and Treatment of GI Discomfort During Prolonged Exercise
• Post-Exercise GI Problems: How to Avoid Stomach Discomfort When Cycling and Running
• GI Distress During Running: A Complete Analysis of Causes and Prevention