.webp)
Gastrointestinal problems are the most common performance limitation in endurance sports. Not lactate. Not VO2 max. Not even the legs. The intestine.
As developers of performance nutrition, we learned one thing early on: The problem is rarely the product. It is the lack of understanding of what really happens in the body under competition stress.
This item is not a product recommendation. It is an attempt to explain gastrointestinal problems in endurance sports from the perspective that we as developers must take on - systemically, multifactorially, and with the knowledge that even the best formulation is only one component.
The uncomfortable truth: Better products don't solve every problem
When an athlete comes to us with GI distress, the first question is usually: “Which product should I take?”
The honest answer? The choice of product often plays a smaller role than you think.
That is not false modesty. It is realism.
Of course, product formulation makes a difference. Osmolality, consistency, carbohydrate ratios - all of this influences how well a product is tolerated. But when the intestines switch into stress mode under competition stress, even isotonic, perfectly formulated products can cause problems.
Because the limiting factor isn't the product. It is the state of the intestines under stress.
{{cta}}
What really happens in competition: The sympathetic nervous system takes over
Training at 150 watts. It's all relaxed. You absorb 80g of carbohydrates per hour. No problems
Competition at 150 watts. Same performance. Same product. But: nausea. bloating. cramps.
What happened? The sympathetic system.
During competition - even with the same wattage - your sympathetic nervous system is more active. Adrenaline rises. Cortisol rises. Your body switches to a state that could simply be called “Fight or Flight.”
That means:
- Blood flow is redistributed: Away from the intestines, towards the muscles
- Gastric emptying slows: Food stays in the stomach longer
- Bowel motility becomes uncontrolled: Either too fast (diarrhea) or too slowly (bloating)
It's not a dysfunction. That is physiology. Your body prioritizes muscle work over digestion. Evolutionarily, this makes sense - when you flee from a predator, energy intake is secondary.
But there is no predator in Ironman. But your body still reacts that way.
Sport-specific realities: cycling is not running
When we develop products, we must understand that “endurance sport” is not a homogeneous category.
Biking:
- Stable sitting position
- No impact load
- Relatively constant blood flow
- sequel: Highest tolerance for carbohydrate intake (100-140g/h possible)
Run:
- Vertical tremors with every step (2-3x body weight)
- Stomach contents are “shaken”
- Difficult blood flow due to higher heart rate with the same performance
- sequel: Significantly lower tolerance (60-90g/h realistic)
Triathlon:
- Cumulative load over 8-16 hours
- Change of load type (wheel → run = critical transition)
- Intestine already tired from cycling
- sequel: Strategy must be differentiated (70% supply on bike, 30% when running)
As product developers, we cannot “formulate away” these biomechanical differences. We can only ensure that our products work in the best possible way under the respective conditions.
But the reality remains: A gel that works well on the bike at 120g/h can cause problems when running at 70g/h. Not because the product is bad - but because the conditions are fundamentally different.
The four levels of a functioning nutrition strategy
When we think about better about competition nutrition, we do it in four levels. Everyone must vote. If one is missing, the system breaks down.
Level 1: Product formulation
That is our job. We have control over this.
What we pay attention to:
- osmolality: Isotonic or slightly hypotensive formulations empty out of the stomach more quickly
- Transporter systems: 2:1 glucose:fructose ratio uses SGLT1 and GLUT5 in parallel (higher absorption capacity)
- Dosability: Flexible concentration in the bottle allows individual adjustment - not everyone tolerates the same dose per sip
- consistency: Liquid solutions (carb mix in the bottle) are taken up differently than gels - both have their place
- additives: Any ingredient that is not performance-relevant is a potential source of error
But: Even the perfect formulation can't prevent GI distress if the other levels aren't right.
Level 2: Individual absorption capacity
This is where things get interesting and complex. The ability to absorb carbohydrates under load is trainable. It's like a muscle.
Two athletes, same product, same conditions:
- Athlete A: 60g/h maximum, above → nausea
- Athlete B: 110g/h without problems
The difference? Athlete B has systematically trained his intestines. About 8-12 weeks. With targeted high-carb training sessions.
Most athletes only test nutritional strategies 4-6 weeks before the competition. It's too late. The intestine needs months to develop structural adaptations: More transporter expression, enlarged villi, optimized gastric emptying.
Level 3: Timing and Dosing
Even the best product won't work if it's used incorrectly.
Common mistakes:
- Too much at once: 60g spread over 10 minutes instead of 30 minutes
- Too little water: Highly concentrated supply without sufficient liquid leads to osmotic stress
- Wrong time: carbohydrate intake during an increase (high intensity = poor gastric emptying)
- Wrong dosage form: Some athletes tolerate liquid carb drinks better, others solid gels - that's individual
Level 4: Competition reality vs. training
That is the level that is most underrated. No problem when training at 200 watts, zone 2, relaxed → 90g/h. In competition at 200 watts, nervous, adrenaline → 70g/h lead to nausea.
Why Competition stress activates the sympathetic system. Mental tension reduces blood flow to the intestines. The body reacts differently, even with the same wattage.
The solution? Simulate competition conditions in training. Use B-Races. Tempo units with realistic carbohydrate intake.
As product developers, we can't force that either. We can only point that out.
What product development can (and cannot) do
Let's be honest.
What we can influence as developers:
- Osmotic properties (isotonic vs. hypertonic)
- carbohydrate composition (mono- vs. multiple transportable carbohydrates)
- Consistency and viscosity
- Taste and acceptance
- Stability and durability
What we can't influence:
- How trained is the athlete's intestine
- How high is the sympathetic stress during competition
- Whether the product is dosed and timed correctly
- Whether the athlete drinks enough water
- Individual genetic differences in transporter expression
The product formulation is necessary - but not sufficient.
The elephant in the room: Individual variability
There is no “one-size-fits-all” solution. It's frustrating for athletes and for us as developers.
example: We are developing a carb mix with 2:1 glucose:fructose, isotonic, can be flexibly dosed.
- Athlete A: “Perfect, I mix 90g per bottle and I get along really well with it”
- Athlete B: “Too concentrated in the bottle makes me feel nauseous - I need it more diluted”
- Athlete C: “I need gels rather than liquid - the consistency makes the difference for me”
Product development cannot cover all individual needs. We can provide products for typical Optimize use cases. But there will always be athletes for whom it doesn't suit.
System thinking: Nutrition as a performance tool, not a quick fix
Competition nutrition is not an isolated event. It is a training goal. The way you train VO2 max. Or lactate threshold. Or strength. You also work out absorption capacity.
That means:
- 12 weeks before the competition: systematic “good training”
- 2-3x per week high-carb workouts (zone 2, long duration)
- Progressive increase from 60g/h to 90-100g/h
- Only then: Test competition strategies
This approach is not a quick fix. But it works. Because it affects the intestine as trainable organ understands. Not as a passive recipient of products.
To systematically implement this approach, we have developed a free tool: The Better Gut Trainer under train.get-better.co. In just a few minutes, it shows you what time potential is realistic for your sport due to optimized carbohydrate absorption - and creates a personalized training plan for you.
The limits of what is feasible: When is medical clarification necessary?
Sometimes GI distress isn't a training or product issue. Sometimes it's pathological.
Warning signs:
- blood in the stool
- Persistent pain (>24h after competition)
- Losing weight despite adequate nutrition
- Chronic symptoms even outside of training/competition
In these cases: Please do not self-optimize. Consult a gastroenterologist.
Possible diagnoses:
- irritable bowel syndrome (IBS)
- Lactose intolerance/FODMAP sensitivity
- Ulcerative colitis/ Crohn's disease (rare among athletes, but possible)
- Exercise-induced anaphylaxis (very rare)
What we would like to share with athletes
After ten years in performance nutrition development:
- There is no magic pill: No product will solve GI problems if the intestines are untrained or the dosage is incorrect.
- Training is the biggest lever: Systematic good training over 8-12 weeks means more than any product change.
- Context is everything: Biking ≠ running. Training ≠ competition. Your intestine ≠ the intestines of your training partner.
- Patience pays off: Bowel adaptation takes months, not weeks. Plan accordingly.
- If nothing helps: Doctor, not marketing. Persistent problems require medical clarification.
Conclusion: From isolated products to holistic strategies
At better, we develop products. Good products, based on the best of our knowledge and based on evidence. But we also know that the product is only part of the system.
We see the best results with athletes who:
- Systematically train your intestines
- Use products in context (correct timing, correct dosage)
- Simulate competition reality in training
- Have realistic expectations (not everyone can tolerate 120g/h)
Gastrointestinal problems in competition are complex. Multifactorial. Individually. There is no easy solution. But there is systematic solutions. And this is exactly where - at the interface of product development, training science and individual adaptation - performance nutrition is created that really works. Not because the product is magical. But because the system is right.
You can take the first step now:
Our free Gut trainer under train.get-better.co shows you in just a few minutes:
- How much time potential is realistic in your sport due to optimized carbohydrate absorption
- What absorption capacity do you need for your goals
- What a personalized training plan would look like for you
No marketing. Just science, system and a clear path forward.
About better: better develops evidence-based performance nutrition for ambitious endurance athletes. Our focus is on maximum compatibility, functional formulations and the understanding that products only have their full effect in the context of a well-thought-out nutritional strategy. We explain rather than persuade - because athletes make better decisions when they understand the connections.
note: This post is a guest post. Enduure assumes no responsibility for the accuracy of the content. All content is from the linked author
