Endurance Nutrition: Fueling Your Body Without Overcomplicating It
Endurance training asks a lot of your body. Long runs, rides, swims, or back-to-back training days require more than grit alone, they require fuel. And not just once in a while, but consistent fueling.
A lot of endurance athletes are exposed to nutrition advice that sounds disciplined and performance-focused but often leads to the opposite result: chronic fatigue, stalled progress, injuries, or feeling “off” without knowing why. Many of these issues trace back to one core problem: not fueling enough to support the work your body is doing.
Why Fueling Enough Matters More Than Eating “Perfectly”
In sports nutrition, we use the term energy availability to describe how much energy is left over for your body’s basic functions after exercise is accounted for.
When intake doesn’t keep up with training demands, low energy availability (LEA) can occur, sometimes unintentionally. Over time, LEA can lead to Relative Energy Deficiency in Sport (RED-S), a condition recognized by the International Olympic Committee that affects athletes of all genders and body sizes.
RED-S isn’t about weight. It’s about whether your body has enough energy to support:
Training adaptation
Recovery
Bone health
Hormone function
Immune health
Focus and mood
You can be eating “well” and still not be eating enough.
RED-S and Endurance Athletes
Endurance athletes are at higher risk for RED-S because training volume is high and energy needs can be easy to underestimate.
RED-S has been linked to:
Declining endurance performance
Slower recovery between sessions
Increased injury risk (especially bone stress injuries)
Hormonal disruptions (including menstrual cycle changes or reduced testosterone)
Frequent illness
GI symptoms
Low motivation, irritability, or trouble concentrating
These signs are often brushed off as “part of training,” but they’re not something athletes should have to push through.
Carbohydrates: Still the Primary Fuel for Endurance
Carbohydrates are the body’s most efficient fuel source for endurance exercise. This hasn’t changed, despite trends that suggest otherwise.
Research consistently shows that adequate carbohydrate intake:
Improves training quality
Reduces perceived exertion
Supports glycogen storage
Helps regulate stress hormones during exercise
Supports immune function
Needs vary depending on training load, intensity, and individual tolerance. But under fueling carbohydrates is a common contributor to fatigue and poor performance in endurance athletes.
Fueling Timing Makes a Difference
It’s not just what you eat, it’s also when you eat.
For many endurance athletes, fueling gaps show up when:
Training sessions are done fasted
Long workouts aren’t fueled during exercise
Meals are delayed for hours after training
Evidence supports:
Eating before training to support output
Using carbohydrates during longer or harder sessions
Refueling after workouts to restore energy stores and support recovery
These habits help prevent small deficits from adding up over time.
A Note on Fasted Training
Fasted training is often discussed in endurance spaces, but research suggests that fasted sessions may increase the risk of low energy availability, especially for athletes training at high volume or intensity.
For many people, prioritizing fuel before and during training leads to:
Better session quality
Improved recovery
Lower injury risk
More consistent performance
Context matters, but fueling is often protective rather than harmful.
GI Symptoms Can Be a Fueling Issue, Too
GI symptoms are common in endurance sport, but they’re not always just about food choices.
Low energy availability has been associated with:
Slower gastric emptying
Changes in gut motility
Increased GI distress during exercise
Before cutting out foods or entire food groups, it’s often helpful to look at total intake, meal timing, and fueling consistency.
Signs You Might Not Be Fueling Enough
Not all under fueling looks dramatic. Some common signs include:
Persistent fatigue or declining performance
Trouble recovering between workouts
Recurrent illness or injuries
Missed or irregular menstrual cycles
Low libido
Feeling cold often
GI issues that don’t improve
Difficulty focusing or low mood
These are signals worth paying attention to, not things to power through.
Supporting Performance Long-Term
Evidence-based endurance nutrition focuses on:
Adequate total energy intake
Matching carbohydrate intake to training demands
Regular fueling throughout the day
Flexibility across training cycles
Individualization based on the athlete, not rigid rules
The goal isn’t perfection, it’s consistency, adequacy, and sustainability.
When Dietitian's Support Helps
Working with a dietitian experienced in endurance sport and RED-S can help athletes:
Identify fueling gaps without focusing on weight
Reduce injury risk
Improve recovery and performance
Address GI symptoms without unnecessary restriction
Support long-term health and participation in sport
If you’re an endurance athlete struggling with energy, recovery, or GI symptoms, working with a dietitian can help.
References
Mountjoy M, Sundgot-Borgen J, Burke L, et al. IOC consensus statement on Relative Energy Deficiency in Sport (RED-S): 2018 update. British Journal of Sports Medicine. 2018
Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the ACSM: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics. 2016
Areta JL, Hopkins WG. Skeletomuscular adaptation to training in low energy availability. Sports Medicine. 2018
Written by Kaitlyn Schlangen, MS, RD
Kaitlyn specializes in: Eating disorders, disordered eating patterns, gastrointestinal (GI) disorders and diseases. She is dedicated to supporting individuals across all spectrums of these challenges.