Stomach issues during exercise: causes, solutions, prevention
TL;DR. 30 to 50% of endurance athletes suffer from stomach complaints during a race. The 5 main causes: excessive osmolarity, fat or fiber in your diet, dehydration, an untrained gut, or too high a total dose. Live solution: lower the dose, drink water, reduce intensity. Prevention: 2:1 mix, low-fat/low-fiber products, gut training (see cluster 3).
Stomach complaints are the silent killer of a good race. Cramps at kilometer 30 of your marathon. Diarrhea halfway through your gravel race. Nausea in the final of your triathlon. In this article, I list the 5 real causes and give you a protocol to never suffer from them again. Do you want to understand the broader fueling context first? Read my pillar guide on carbohydrates during sports.
How often does this occur? The figures
Pfeiffer and colleagues investigated GI complaints in 221 athletes across various disciplines in 2012. The results:
- Marathon runners: 4 to 32% report severe complaints
- Cyclists: 10 to 24%
- Triathletes (long distance): 30 to 45%
- Ultra-runners: up to 60% report GI issues
De Oliveira and colleagues confirmed this prevalence in 2014 and linked it directly to fueling strategy and hydration choices. GI complaints are therefore not bad luck, but the result of mistakes that you can avoid.
The 5 real causes of stomach complaints
1. Excessive osmolarity of your sports drink or gel
Osmolarity is the concentration of dissolved substances in your drink or gel. Above 350 mOsm/kg, the contents pull water into your gut instead of away from it. Result: cramps, diarrhea, bloating. Many people mix their sports drinks too strongly ("a double scoop is double the energy") and trigger this exact problem.
The fix: stick to 6 to 8% carbohydrates in your sports drink. In 500ml, that is 30 to 40g of carbs. No more. Taste often means too concentrated.
2. Fat and fiber in your sports nutrition
Fat slows down your gastric emptying. Fiber does too, plus it ferments in your large intestine and causes gas and cramps. Sports bars with more than 2g of fat or more than 3g of fiber per serving are asking for trouble during high-intensity exercise. Muesli bars, mixed nuts, or "high fiber" bars have no place in your fueling.
For sports nutrition during exercise, choose products that are low in fat (below 2g) and low in fiber (below 2g) per serving. Want to understand this more deeply? Read why Eagle is different from classic fruit candy.
3. Dehydration
A loss of more than 2% of body weight in fluid significantly slows down gastric emptying. You get the feeling that your food is not "settling." That is literally true: your gut needs water to transport carbs via SGLT1. No water = no transport.
The fix: 500 to 750ml of fluid per hour during exercise, spread out in small sips. Not 1 liter at once halfway through. Continuously consuming small doses of 100 to 150ml every 15 minutes works better.
4. Untrained gut above your transport capacity
This is the main cause that Pfeiffer 2012 highlighted. Athletes who fuel above their trained ceiling experience complaints. If you take 90g per hour tomorrow while your gut is at 50g per hour, you are going to feel it. It is not the carbs that are at fault, but the mismatch with your adaptation.
The fix: build up gradually with a protocol. My 6-week gut training protocol will get you structurally to 90g per hour without complaints.
5. Too high intensity for your fueling strategy
Above 75 to 80% of VO2max, your blood flow to your gut is significantly reduced. The blood goes to your working muscles; your gut becomes secondary. If you then want to digest 90g per hour, the system stutters. This is why sprinters do not take nutrition during a 60-minute criterium, but marathon runners do.
The fix: scale your fueling goal to your intensity. At a race pace above 85% HRmax, 60g per hour is more realistic than 90g per hour. Better to be stably digested than ambitious and broken down.
Live solutions: you have complaints during your session, what now?
Complaints during a ride or run? Do not panic. Follow this order:
- Immediately reduce your intensity by 10 to 15%. Return blood to your gut.
- Drink water (not sports drink) for 10 to 15 minutes to lower osmolarity.
- Temporarily stop carbs. 20 to 30 minutes of rest to let your gut recover.
- Resume with a low dose. Half a bar plus water. Slowly build back up to your original goal.
- Skip gels if they were the trigger. Bars and sports drinks are usually milder on your gut.
Persistent cramps or diarrhea? Stop. No story is worth harming your health for one race or training session.
Prevention: the 5-check system
Before every race or long ride, I run through my 5 checks. Don't forget:
| Check | What to check | Threshold |
|---|---|---|
| 1. Sports drink concentration | % carbs in your bottle | 6-8% max |
| 2. Fat per bar | Label | <2g per serving |
| 3. Fiber per bar | Label | <3g per serving |
| 4. Hydration per hour | Bottle volume / pace | 500-750ml per hour |
| 5. Training match | Race dose already tested? | Min 3 race sims |
Why 2:1 standard causes fewer complaints than 1:1
The ratio of your glucose-fructose mix partly determines how well your stomach tolerates it. A 1:1 ratio puts a relatively heavier load on the fructose channel and increases the risk of fructose malabsorption. A 2:1 mix distributes transport better and therefore causes fewer complaints at the same total dose.
Pure fructose sources (some honey gels, fruit pulp without additives) can specifically cause problems at high doses because they overload a single channel. The 2:1 ratio is scientifically recognized as the standard for athletes who want to fuel over long periods.
Scientific sources
- de Oliveira EP, Burini RC, Jeukendrup AE. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine, 2014. doi:10.1007/s40279-014-0153-2
- Pfeiffer B et al. Nutritional intake and gastrointestinal problems during competitive endurance events. Medicine and Science in Sports and Exercise, 2012. doi:10.1249/MSS.0b013e31822dc809
- Jeukendrup AE. Periodized Nutrition for Athletes. Sports Medicine, 2017. doi:10.1007/s40279-017-0694-2
Frequently asked questions about stomach complaints during exercise
How do I know if it is my nutrition or my body?
Test your nutrition during 3 workouts at race intensity. If you consistently experience complaints on exactly the same products, it is the nutrition. Varying complaints with varying nutrition point more to individual sensitivity or stress.
Can stress cause stomach complaints during a race?
Yes, to a high degree. Race stress activates your sympathetic nervous system and reduces blood flow to your gut. Many athletes with "race-day diarrhea" never have this during training. The fix: breathing techniques before the start and using familiar nutrition.
Which gel or bar is the mildest for my stomach?
Products with low osmolarity (check the label: below 350 mOsm/kg), low fat (<2g), low fiber (<2g), and without sorbitol/mannitol/aspartame. Those are the objective criteria.
Can I take medication for cramps during a race?
Some ultra-athletes use antidiarrheal medication (loperamide) preventively, but this is controversial and can cause other problems. Better: resolve your cause via nutrition and gut training, not via medication. Consult a sports physician regarding your situation.
Do probiotics work to prevent stomach complaints?
Limited evidence. Some studies show slight improvement with chronic use of specific strains (Lactobacillus rhamnosus, Bifidobacterium longum). It is not a replacement for gut training, at most an addition.
What if I get diarrhea every time during long rides?
Three suspects: excessive osmolarity (dilute your drink), too much fructose (choose a 2:1 product), or intolerance to specific ingredients (sorbitol, soy protein, lactose). Eliminate one source per training session and test systematically.
How long does it take for my gut to recover after complaints?
Mild complaints 24 to 48 hours. Severe diarrhea or vomiting: 3 to 7 days of carefully building up with light nutrition. Do not eat fiber-rich food, fat, or spicy food for the first 24 hours after complaints. Water plus salt plus light carbohydrates (white rice, banana, toast) as a basis.
Bars with low fat and low fiber, 2:1 ratio
For athletes who want to fuel over long periods. No artificial sweeteners, no sorbitol, no fat bombs. Functional and mild.
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