How Underfueling Can Be Detrimental To Endurance Athletes And Lead To RED-S

Updated: Sep 25

By Holley Samuel MEd, RD, LD, CPT

Underfueling has been discussed more and more lately in online nutrition spaces for athletes, and for a good reason. Endurance athletes of all levels may feel the need to hit a certain number on the scale to achieve a level of performance and may associate “lighter” with “faster” or “healthier.” In an effort to achieve this, they may reduce their calories, increase their training load (eat less and move more, right?) to get the job done. Or maybe an athlete doesn’t even have a goal weight on their radar but perhaps gets too busy at work and forgets to eat enough to fuel their nutritional requirements.

While occasionally missing the mark on your nutrition doesn’t necessarily cause long-term harm, consistently underfueling - whether intentional or unintentional - may have detrimental long-term effects on performance, health, and body composition.

While many athletes achieve results when they first implement the “eat less move more” strategy, the long-term effects of chronic underfueling during training cause this method to backfire.

Underfueling consistently can lead to low energy availability (LEA) and, in chronic situations, relative energy deficiency in sport (RED-S). RED-S was formerly referred to as the Female Athlete Triad but has since been renamed since this condition affects all genders, not only females.

Energy availability refers to the amount of energy your body requires to function properly and the amount of energy you consume as food. It can be described mathematically as:

EA (energy availability)= EI (energy intake in calories)- EEE (exercise energy expenditure in calories) / FFM (fat-free mass in kg)

How To Assess If You Are Eating Enough To Support Your Training

Start by calculating if your current intake equates to at least 45 calories/kg of fat-free mass, as this is the starting point of energy intake required to fuel an active individual.

For example, if you weigh 145 lbs and have 25% body fat (75% fat-free mass):

145x0.75=108.75 lbs fat-free mass

108.75/2.2= 49.4 kg fat-free mass

49.4x45= 2,224 calories per day required to prevent low energy availability (more may be necessary to enhance performance, and this will vary from individual to individual)

Once an athlete is chronically experiencing LEA for extended periods, they may develop RED-S, which affects all body systems. RED-S needs to be taken seriously and treated with a team-based approach, including a physician, dietitian, mental health professional, and coach.

Symptoms of LEA (Low Energy Availably):

  • Poor performance

  • Low libido

  • Poor ability to concentrate

  • Fatigue

  • Feeling flat in general

  • Erratic appetite (feeling very hungry or even experiencing low appetite)

Symptoms of RED-S (Relative Energy Deficiency In Sports):

  • Recurring injuries or poor healing injuries

  • Irregular periods or missing periods

  • Fertility concerns

  • Low libido and sexual dysfunction

  • Poor immunity/increase in sickness

  • Nutrient deficiencies

  • Delayed growth and development (especially in young populations)

  • Disordered eating and eating disorders

Whether an athlete is underfueling intentionally or unintentionally, it is essential to identify if this is happening, preferably before these complications occur, especially since RED-S is prominent in athletic communities.

One study conducted on Australian athletes showed that 80% of participants presented with at least one symptom of RED-S, with 37% having 2-3 symptoms3. Despite the prevalence of RED-S and LEA in athletics, there is a discrepancy in confidence and ability of coaches and physicians to identify athletes with symptoms of RED-S and the Female Athlete Triad.

One study shows that less than 50% of healthcare professionals and coaches who work with athletes can identify the Female Athlete Triad components. In another study involving physicians, only 37% were comfortable identifying and treating the triad.2 This discrepancy in prevalence and confidence in determining if an athlete presents with the condition should be addressed by improving education on RED-S and LEA in athletic and medical settings.

In runners, there is pressure to be lean or have a certain appearance to produce perceived optimal performance.

Common characteristics in competitive athletes include perfectionism, compulsive behavior with exercise, the ability to push their bodies to the limit. In runners, there is pressure to be lean or have a certain appearance to produce perceived optimal performance. These factors combined can make runners at risk for obsessive-compulsive disorder, eating disorders, and disordered eating, resulting in RED-S.1 Runners should look to treat their RED-S, but treatment may also include treatment for potential root causes of why RED-S occurred. This is why it can be important to approach treatment with a multi-facet health care team.

From a nutritional standpoint, treating RED-S often includes eating enough (and plus some, in some cases) to meet daily energy needs. Some treatment plans also include a temporary reduction in physical activity to allow the athlete to improve their energy availability. Excess physical activity can sometimes blunt the appetite, so reducing physical activity when a period of increasing energy intake is necessary can allow the appetite to return and make the required increase in food consumption more feasible. Some studies show that eating more frequently throughout the day through incorporating more meals or snacks and focusing on calorie-dense foods are the most potentially successful strategies to implement when treating RED-S. Athletes should also work with a physician and registered dietitian to assess if supplementation of specific nutrients is necessary based on the individual’s unique lab work and history of potential nutrient deficiencies.

Want to learn more about underfueling? Check out these resources!

Podcast episode with Jackson Long on RED-S


  1. Logue DM, Madigan SM, Melin A, et al. Low Energy Availability in Athletes 2020: An Updated Narrative Review of Prevalence, Risk, Within-Day Energy Balance, Knowledge, and Impact on Sports Performance. Nutrients. 2020;12(3):835. Published 2020 Mar 20. doi:10.3390/nu12030835

  2. Mountjoy M, Sundgot-Borgen JK, Burke LM, et alIOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update British Journal of Sports Medicine 2018;52:687-697.

  3. Rogers MA, Appaneal RN, Hughes D, Vlahovich N, Waddington G, Burke LM, Drew M. Prevalence of impaired physiological function consistent with Relative Energy Deficiency in Sport (RED-S): an Australian elite and pre-elite cohort. Br J Sports Med. 2021 Jan;55(1):38-45. doi: 10.1136/bjsports-2019-101517. Epub 2020 Nov 16. PMID: 33199358.