Is It IBS… Or are you underfuelling?
Why RED-S can look a lot like Irritable Bowel Syndrome in athletes.
Endurance athletes will often get told they have Irritable Bowel Syndrome (IBS). Or they self-diagnose when they can no longer tolerate fuel or experience gut upset in their day-to-day.
So naturally, the next step for many is: cut foods, fear fibre, avoid eating before training, and start side-eyeing dairy/gluten like it personally ruined your long run.
But here’s the thing. Sometimes it’s not IBS. Sometimes it’s an athlete who’s been underfuelling for months, and their gut is finally waving a giant red flag. When there’s not enough fuel coming in, the gut is usually one of the first systems to start protesting.
Research shows that low energy availability (LEA), the driving factor behind Relative Energy Deficiency in Sport (RED-S), can impact multiple body systems, including gastrointestinal function, hormones, immunity, bone health and performance.[1] So, in this article, we take a closer look at how underfuelling can be a major cause of gut upset amongst athletes and how catching this early can detect the onset of RED-S.
What Is RED-S?
Relative Energy Deficiency in Sport (RED-S) happens when there isn’t enough energy available to support both training demands and normal body function.
Your body has a limited energy budget. If training is taking up most of the budget and not enough food is coming in, the body starts making cuts elsewhere. And unfortunately, your gut, hormones, recovery, immune system and bone health can end up on the chopping block.
RED-S can affect athletes of any gender, body size or performance level.
And no, you don’t have to be dramatically restricting food for it to happen.
Sometimes it looks like:
accidentally under-eating around a heavy training block
failing to meet carbohydrate requirements
avoiding certain foods due to preferences or tolerance levels
not fuelling long sessions properly
training more without increasing food intake
Some of the common signs of RED-S can include:
recurring injuries or niggles
fatigue or poor recovery
declining performance or feeling flat in sessions
disrupted menstrual cycles (in females)
low libido
frequent illness
poor sleep
mood changes
gut symptoms (this one gets overlooked a lot!)
Research shows RED-S affects multiple body systems, including gastrointestinal function, and symptoms can show up long before more “obvious” signs appear.
What Is IBS?
Irritable Bowel Syndrome (IBS) is a functional gastrointestinal disorder.
IBS is very real. And very annoying (IYKYK).
People with IBS commonly experience:
bloating
abdominal pain
constipation, diarrhoea, or both
urgency
unpredictable bowel habits
The Crossover Between IBS and RED-S
We hear this in clinic all the time:
“I bloat after everything.”
“I can’t trust my gut on runs.”
“I swing between constipation and diarrhoea.”
“I feel like I react to random foods.”
Yes, these are the experiences of someone with medically diagnosed IBS. But in athletes? Gut symptoms are often more complicated than that. Especially when training load is high, and food intake isn’t matching output.
In many cases, what looks like IBS can actually be an early sign of RED-S. This might be because your body doesn’t have enough energy available to support both training and normal body function. And unfortunately, digestion is one of the first things to get deprioritised.
Gut Symptoms That Could Indicate RED-S
Your body is smart (and mildly dramatic). When energy availability drops, the body goes into conservation mode. Basically, it starts asking: “What do we absolutely need to survive right now?”
This “energy conservation” response is well recognised in RED-S literature, where the body downregulates non-essential or energetically expensive processes when fuel availability is too low.[1] Gastrointestinal function is one of the systems often affected.
So while your gut might feel like the problem, it could actually be the messenger and an early indicator for RED-S.
So what are these signs?
1. Your gut motility gets weird
One week: constipated. Next week: sprinting to the bathroom before training. Sound familiar? Underfuelling can disrupt gut motility (aka how food moves through the digestive system). Low energy availability can slow things down, resulting in constipation, bloating, and general gut discomfort.[2,3] Gut motility doesn’t just happen. It requires energy to function. So, not only do we see delayed motility in people who are undershooting their intake, but we also see this issue rise a lot in those who follow a low-carb diet. After all, carbohydrates are our body’s primary energy source used for exercise, cognition and yes, digestion.
Stress hormones and training stress can also speed things up.[2,3] Say “g’day” to urgency, loose stools, and “I know every public toilet on my run route” energy.
Digestive symptoms are increasingly recognised as part of the RED-S picture, particularly in endurance athletes training at high volumes. It’s usually assumed that the athlete has food intolerances or IBS, when in fact, there are other factors at play.
2. Foods suddenly feel “harder” to tolerate
This one catches athletes off guard. Foods you used to tolerate? Suddenly not sitting well. Carbs feel “too heavy.” Fibre feels impossible. Normal portions suddenly leave you bloated or uncomfortable. And because symptoms show up after eating, the food gets blamed. But often, the problem isn’t the food.
It’s that the gut is under-resourced. When we chronically underfuel, digestion and absorption can become less efficient. So the system becomes less tolerant — not because the food is “bad,” but because the body is running on empty. Low carbohydrate availability, in particular, has been linked to many of the physiological disruptions seen in RED-S and overreaching athletes.[1]
3. The gut-brain axis starts to crash out
The gut and brain are constantly chatting. And when training stress + life stress + underfueling pile up? That communication gets messy (relatable).
This can look like:
feeling full quickly
reduced appetite
anxiety around eating before sessions
heightened gut sensitivity on training days
Then the vicious cycle begins: Gut symptoms → eating less → worse fuelling → more gut symptoms.
Cool cool cool.
The Accidental Athlete Self-Sabotage
When gut symptoms hit, athletes usually do what feels logical.
They start cutting foods. Less carbs. Less fibre. No pre-training snack. No intra-session fuel because “my gut can’t handle it.” And temporarily? It might even feel better.
Less food moving through the gut = fewer immediate symptoms.
But long term? This often makes things worse. Skipping carbs before training or avoiding fuel during long sessions increases physiological stress.
Less fuel = more stress hormones. More stress hormones = more gut disruption. It’s math that we actually understand.
Which means the very strategy meant to “fix” the gut often keeps the cycle going and going and going.
Gut Training (But Not Good Kind)
This is a big one. Your gut is trainable. If you stop eating carbs during training, stop eating enough overall, or dramatically narrow your food choices, the gut loses some of its ability to tolerate those foods.
Use it or lose it.
Which can look like:
race-day gels suddenly wreaking havoc on your gut
bloating when carbs are reintroduced
feeling like your gut tolerance keeps shrinking
We see this clinically all the time, athletes accidentally “de-train” their gut by chronically avoiding carbs or under-fuelling key sessions. Endurance nutrition research consistently shows the gut adapts to fuelling practice.
So… is it IBS or REDs?
If you’re training hard and noticing symptoms like bloating, constipation, urgency, reflux, or random food intolerances, it’s worth zooming out before assuming your gut is the enemy.
Sometimes the problem isn’t what you’re eating. It’s that your body needs more food than it’s currently getting.
The uncomfortable truth? You can’t out-supplement, low FODMAP, or gut-health-hack your way around chronic underfuelling. Sometimes the most gut-friendly thing you can do is eat enough.
Especially carbs.
Because a well-fuelled gut is usually a happier one.
References
Angeliki M Angelidi et al., Relative Energy Deficiency in Sport (REDs): Endocrine Manifestations, Pathophysiology and Treatments, Endocrine Reviews, Volume 45, 5, (2024), 676–708, https://doi.org/10.1210/endrev/bnae011
Colangelo, J., Smith, et al., Exploring the presentation of REDs in ultra endurance sport: a review. Journal of Eating Disorders, Volume 13, 210 (2025). https://doi.org/10.1186/s40337-025-01381-0
J. Chantler, et al., Educational interventions for REDs in athletes: A scoping review, Science & Sports, Volume 41, 3, (2026), 218-229, https://doi.org/10.1016/j.scispo.2025.09.003
This article was written by Michelle Keaughran.
Founder of The Digestion Co.
Accredited Practising Dietitian and Sports Dietitian
Michelle’s specialty is working with people who have gastrointestinal conditions and food intolerances. Michelle brings her expertise and own lived experience of having coeliac disease and being a runner, to support athletes with digestive concerns.
