Many endurance athletes are told they have Irritable Bowel Syndrome (IBS), or they tell themselves that’s what it must be. They try cutting foods, adjusting fiber intake, timing meals, and avoiding anything that might upset their stomach before training. And yet… the symptoms persist. Bloating, urgency, and a yo-yo of constipation and loose stools. Their gut feels unpredictable at best and completely unreliable at worst. But what if these symptoms are not IBS in athletes? What if the issue isn’t what they’re eating, but that they’re not eating enough?
For many athletes, especially those training at high volumes, these “mystery gut issues” are actually early signs of Relative Energy Deficiency in Sport (REDs), a state of low energy availability that disrupts multiple systems in the body, including the gut.
Read on to learn why IBS in athletes often gets misdiagnosed and how to treat gut-related symptoms related to underfueling.
The Gut as a REDs Warning Sign
REDs doesn’t always show up with a flashing sign announcing its arrival.
There’s not always a missed period, a stress fracture, or obvious performance decline right away. In fact, some of the earliest signs are subtle, and often dismissed.
The gut is one of them.
Digestive changes are frequently one of the first systems to be affected by underfueling, long before more recognized REDs symptoms appear.
That’s because digestion is energetically expensive. When the body senses that energy is scarce, it begins to prioritize survival over processes like digestion, absorption, and gut motility.
So while an athlete may feel like their gut is “the problem,” it may actually be the messenger.
How REDs Disrupts Digestion
When energy availability is low, the body shifts into conservation mode. This has direct downstream effects on the gastrointestinal system.
1. Motility Changes
Athletes may experience swings between constipation and diarrhea.
Slowed motility can lead to constipation, bloating, and discomfort. On the other hand, stress hormones and altered gut signaling can speed things up, leading to urgency or loose stools, especially during exercise.
This variability often gets labeled as IBS, but in the context of REDs, it’s a reflection of a dysregulated system trying to adapt to low fuel.
2. Reduced Absorption Tolerance
Many athletes begin to notice increased sensitivity to foods they used to tolerate well.
Carbohydrates feel “too heavy.” Fiber causes bloating. Even normal portions lead to gas or reflux.
This isn’t random. Underfueling can impair enzyme production and reduce the gut’s ability to properly digest and absorb nutrients. Over time, the gut becomes less tolerant, not because the foods are inherently problematic, but because the system is under-resourced.
3. Gut-Brain Axis Disruption
The gut and brain are in constant communication.
When the body is under stress, whether from high training loads, inadequate nutrition, or both, this communication becomes dysregulated.
Athletes may experience:
- Reduced appetite or early fullness
- A “shut down” feeling around food
- Increased anxiety around eating before training
- Heightened gut sensitivity under stress
This creates a feedback loop: stress suppresses intake → low intake worsens gut symptoms → symptoms increase stress.
Why Athletes Change Food in Ways That Make It Worse
When symptoms show up, athletes do what they think is logical.
They start eliminating foods.
They cut back on carbs before workouts.
They avoid intra-workout fueling.
They eat less overall to “reduce symptoms.”
And temporarily, it might even seem to help.
But over time, this approach often worsens the underlying issue.
Avoiding Fuel When It’s Needed Most
Skipping pre-run carbs or avoiding fueling during long sessions increases the body’s stress response.
Less fuel = more cortisol = more gut disruption.
What feels like a “gut problem” becomes amplified by the very strategies meant to control it.
The “Use It or Lose It” Effect
The gut adapts to what it regularly processes.
When athletes reduce carbohydrate intake or overall food volume, the gut downregulates its capacity to handle those nutrients.
This can lead to:
- Increased bloating when carbs are reintroduced
- Poor tolerance of race-day fueling
- A shrinking list of “safe” foods
Over time, the gut becomes less resilient, not more.
The IBS Trap: Managing Symptoms Without the Root Cause
At some point, many athletes receive (or assume) an IBS diagnosis.
From there, the focus often shifts to symptom management:
- Eliminate trigger foods
- Follow low-FODMAP protocols
- Eat “cleaner” or more restrictively
While these approaches can reduce symptoms short-term, they often miss the root cause when REDs is present.
When “Avoid More Foods” Becomes the Plan
The list of tolerated foods gets smaller.
The anxiety around eating increases.
Meals become more controlled, more rigid, and often… less adequate.
This creates a situation where:
- Energy availability drops further
- Gut function declines further
- Symptoms persist or worsen
Food Fear + Urgency Anxiety
Many athletes start to fear symptoms, especially during training or competition.
They eat less before sessions to avoid urgency.
They avoid fueling to prevent discomfort.
But this backfires.
Low intake increases physiological stress, making symptoms more likely, not less.
What to Do Instead: A Better Starting Point
If an athlete is experiencing ongoing GI issues, it’s worth asking a different question:
Is the gut the problem, or is it responding to low energy availability?
Here’s where to start.
1. Screen for Underfueling
Look beyond total calories and assess within-day energy balance.
Key questions:
- Are there long gaps without eating?
- Is intake matched to training load?
- Is pre-, during-, and post-exercise fueling adequate?
Even athletes who “eat healthy” or “eat enough” may still be underfueling relative to their needs.
2. Rebuild Fueling Consistency
Stabilizing the system starts with consistency.
- Regular meals and snacks
- Adequate carbohydrate intake
- Strategic pre- and intra-training fueling
This helps reduce stress signaling and supports more stable gut function.
3. Rebuild Tolerance Gradually
Instead of removing foods, focus on progressive exposure.
- Reintroduce carbohydrates in small, consistent amounts
- Practice fueling during training
- Allow the gut to adapt over time
Tolerance is trainable, just like fitness.
REDs Training For Providers & Coaches
Not every athlete with GI symptoms has IBS and not every case of IBS in athletes is truly IBS.
For many, the gut is responding exactly as it should to insufficient energy, high stress, and inconsistent fueling.
When you address those root causes, the gut often cooperates.
If you’re working with athletes who present with “mystery GI issues,” this is a critical lens to consider.
Because the difference between managing symptoms and addressing the root cause can change everything, for both performance and long-term health.
This is exactly what I teach inside my REDs Informed Provider Certification Program® and REDs Performance Mastermind.
You’ll learn how to assess GI complaints through an energy availability lens, and confidently guide athletes back to optimal fueling and function.