Stomach issues during exercise: causes, solutions, prevention
TL;DR. 30 to 50% of endurance athletes suffer from stomach problems during competition. The 5 main causes: too high osmolarity, fat or fiber in your nutrition, dehydration, untrained gut, or too high total dose. Live solution: reduce dose, drink extra water, reduce intensity. Prevention: 2:1 mix, low-fat/fiber products, gut training (see cluster 3).
Stomach problems 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 provide you with a protocol to never suffer from them again. Do you first want to understand the broader fueling context? Read my pillar guide on carbohydrates during exercise.
How common is this? The numbers
In 2012, Pfeiffer and colleagues investigated GI complaints in 221 athletes across various disciplines. 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 directly linked them to fueling strategy and hydration choices. GI complaints are not bad luck but the result of preventable mistakes.
The 5 real causes of stomach problems
1. Too high 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 draw water into your gut instead of out of it. Result: cramps, diarrhea, bloating. Many people mix their sports drink too strong ("double scoop is double energy") and precisely trigger this problem.
The fix: stick to 6 to 8% carbohydrates in your sports drink. For 500ml, that's 30 to 40g carbs. No more. Tasty 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, causing 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. Muesli bars, mixed nuts, or "high fiber" bars do not belong in your fueling.
Eagle bars are deliberately low in fat (under 2g) and low in fiber (under 2g) per 25g serving. Not because fat and fiber are bad (they are essential outside of sports), but because they cause friction during exercise. Do you want to understand this more deeply? Read why Eagle is different from classic fruit candy.
3. Dehydration
Loss of more than 2% of body weight in fluid significantly slows down your gastric emptying. You get the feeling that your nutrition is not "settling". That's 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 over small sips. Not 1 liter all at once halfway through. Continuous small doses of 100 to 150ml every 15 minutes work better.
4. Untrained gut above your transport capacity
This is the main cause Pfeiffer 2012 highlighted. Athletes who fuel above their trained ceiling get complaints. If you take 90g per hour tomorrow while your gut is used to 50g per hour, you will feel it. The carbs are not wrong, but the mismatch with your adaptation is.
The fix: gradually build up with a protocol. My 6-week gut training protocol will structurally get you to 90g per hour without complaints.
5. Too high intensity for your fueling strategy
Above 75 to 80% VO2max, your blood flow to your gut significantly decreases. Blood goes to your working muscles, your gut becomes secondary. If you still want to digest 90g per hour, the system falters. This is why sprinters don't take nutrition during a 60-minute criterium and marathon runners do.
The fix: scale your fueling goal to your intensity. At race-pace above 85% HRmax, 60g per hour is more realistic than 90g per hour. Better stably digested than ambitious and broken down.
Live solutions: you have complaints during your session, what now?
Complaints during a ride or run? Don't panic. Follow this order:
- Immediately reduce your intensity by 10 to 15%. Give your gut blood back.
- Drink water (not sports drink) for 10 to 15 minutes to lower osmolarity.
- Temporarily stop carbs. 20 to 30 minutes break to let your gut recover.
- Resume with a low dose. Half an Eagle 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 for your gut.
Persistent cramps or diarrhea? Stop. No story is worth harming your health for one race or training.
Prevention: the 5-checks system
Before each race or long ride, I go 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 simulations |
Why 2:1 usually causes fewer complaints than 1:1
The ratio of your glucose-fructose mix directly influences your GI tolerance. A 1:1 ratio disproportionately stresses the fructose channel and increases the chance 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 mixing) can cause problems specifically at high doses because they overload one channel. Eagle's 2:1 ratio is precisely calibrated to balance both routes.
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 problems during exercise
How do I know if it's my nutrition or my body?
Test your nutrition during 3 training sessions at race intensity. If you consistently get complaints from the exact same products, it's due to the nutrition. Varying complaints with varying nutrition indicate more individual sensitivity or stress.
Can stress cause stomach problems during a race?
Yes, to a great extent. Race stress activates your sympathetic nervous system and reduces blood flow to your gut. Many athletes with "race-day diarrhea" never experience 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. Eagle bars meet all these criteria.
Can I take medication for cramps during a race?
Some ultra-athletes use anti-diarrhea medication (loperamide) preventively, but this is controversial and can cause other problems. Better: solve your cause through nutrition and gut training, not through medication. Consult a sports physician for your situation.
Do probiotics work to prevent stomach problems?
Limited evidence. Some studies show slight improvement with chronic use of specific strains (Lactobacillus rhamnosus, Bifidobacterium longum). It is not a substitute for gut training, at most a supplement.
What if I get diarrhea every time during long rides?
Three suspects: too high osmolarity (dilute your drink), too much fructose (choose a 2:1 product), or intolerance to specific ingredients (sorbitol, soy protein, milk sugar). Eliminate one source per training 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: gradually build up with light food for 3 to 7 days. For the first 24 hours after complaints, avoid high-fiber foods, fat, or spicy food. Water plus salt plus light carbohydrates (white rice, banana, toast) as a base.
Eagle bars: low-fat, low-fiber, 2:1 ratio
Deliberately designed to minimize GI complaints. No artificial sweeteners, no sorbitol, no fat blocks. Functional and mild.
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