The Connection Between Underfueling and the Gut: A Look at the Gastrointestinal Manifestations of Relative Energy Deficiency in Sport
“I know I need to eat more, but I just feel so full all the time. I don’t know how I am going to eat enough for my performance.”
“When my mileage went up, I had constipation like nothing else.”
“My stomach hurts all the time. Maybe I should try cutting out gluten and dairy, I bet that will help.”
Concerns with gut health are common among the athletes with whom I work, and I would not be surprised to hear quotes similar to those written above in my sessions. Many athletes are no stranger to digestive problems in or outside of sport. However, many athletes are unaware that how much they eat can have an effect on their gut.
An athlete enters an energy deficiency when their intake from food and beverages does not match their output from exercise, metabolism, and daily activities. Relative Energy Deficiency in Sport (REDs) is a syndrome that refers to the health and performance consequences an athlete can experience from operating in a prolonged energy deficiency (Mountjoy et al., 2023). The gastrointestinal (GI) tract, responsible for breaking down food, is an organ system that can be affected by REDs.
How Does REDs Affect the GI Tract?
The GI tract, composed of smooth muscle, is in a state of constant repair. When underfueled, these repairs slow down, and the amino acids in the GI cells can become an energy source. Additionally, gut contractions decrease as the body tries to absorb as much nutrition as possible from by increasing the transit time of food (Hedrick, 2022). These changes result in an impaired and sluggish gut.
During exercise, blood is redirected to muscles, reducing blood flow to the GI tract. Increased inflammation from exercise causes increased gut permeability, allowing more particles to pass through the tissue. This decreases gut function and increases the risk of GI issues, especially in a weakened, underfed gut (Angelidi et al., 2024).
Underfueling can also increase pro-inflammatory hormones, weakening the immune system and increasing infection risk. A study of elite female runners found those without periods (another sign of underfueling) had higher rates of GI infections (Shimizu et al., 2012).
As a result, underfed athletes may experience symptoms like bloating, nausea, constipation, abdominal pain, or regurgitation.
Why Typical “Gut Health” Recommendations Fall Short
Interventions for GI issues, like bloating and constipation, typically involve increasing fiber, fluids, eliminating food groups, and adding movement. However, these recommendations may worsen symptoms for individuals with GI issues caused by underfueling.
While fiber and fluids benefit the average person, they may not help athletes with REDs, who often consume more fiber than adequately fueled individuals (Langa et al., 2025). Underfueling athletes need more calories, particularly from energy-dense carbs and fats. Adding fiber or fluids can take up valuable stomach space and worsen constipation by increasing stool bulk without addressing the core issue: slow gut motility. It’s like adding more cars to a traffic jam!
Athletes with gut issues are often advised to eliminate food groups like gluten, dairy, or nightshades, sometimes based on faulty test results. This can limit their diet and reduce gut bacteria diversity, already compromised in underfueling athletes (Carey & Montag, 2021). While food intolerances can occur, unnecessary restrictions may harm gut health further.
Exercise is beneficial, but for high-level athletes, too much movement can suppress hunger and increase fullness. Reducing exercise may also help athletes meet their energy needs by conserving energy expenditure.
How to Address GI Symptoms from Underfueling
The good news – there are nutritional strategies to try when experiencing GI issues from REDs. Here are a few to consider:
Eat regularly to meet energy needs, aiming for a meal or snack every 3-4 hours. Focus on balanced, nutritionally complete meals and snacks, including multiple food groups, especially before and after training.
Avoid diet products, as many low-sugar, low-calorie, high-protein foods contain additives like artificial sweeteners and fiber, which can worsen bloating, gas, or digestive issues.
Try peppermint oil before meals to ease GI symptoms, but consult a doctor first as it may interact with medications.
Reduce gut stress by calming the vagus nerve, which controls digestion. Deep breaths, humming, gentle yoga, or pressure point exercises can help improve digestion before meals. Dietitians such as Rebecca McConville and Em Palmerton have a wealth of resources on these strategies!
Are you an athlete needing support for GI symptoms? Fill out a contact form on my website to see if my nutrition services are a good fit for you!
References
Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee’s (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs) British Journal of Sports Medicine 2023;57:1073-1098.
Carey RA, Montag D. Exploring the relationship between gut microbiota and exercise: short-chain fatty acids and their role in metabolism. BMJ Open Sport & Exercise Medicine. 2021 ;7(2):e000930. DOI: 10.1136/bmjsem-2020-000930.
Hedrick T. The overlap between eating disorders and gastrointestinal disorders. Practical Gastroenterology. August 2022. Available at: https://med.virginia.edu/ginutrition/wp-content/uploads/sites/199/2022/08/August-2022-Eating-Disorders-and-GI-Disorders.pdf.
Angelidi AM, Stefanakis K, Chou SH, et al. Relative Energy Deficiency in Sport (REDs): Endocrine Manifestations, Pathophysiology and Treatments, Endocrine Reviews, Volume 45, Issue 5, October 2024, Pages 676–708, https://doi.org/10.1210/endrev/bnae011
Langa D, Naczyk M, Szymczak RK, Karbowska J, Kochan Z. A Preliminary Study of Nutrients Related to the Risk of Relative Energy Deficiency in Sport (RED-S) in Top-Performing Female Amateur Triathletes: Results from a Nutritional Assessment. Nutrients. 2025 Jan 7;17(2):208. doi: 10.3390/nu17020208.
Shimizu K, Suzuki N, Nakamura M, et al. Mucosal immune function comparison between amenorrheic and eumenorrheic distance runners. J Strength Cond Res. 2012;26(5):1402-1406.