Text with master prompt + 2 references: Wenger et al 2019, Stellingwerff et al 2021.
AI Model: Sonnet 4 (Claude) on September 26th, 2025.
Expert: Dr Nathalie Wenger

Clinical case

A 16-year-old female triathlete comes in complaining of ­fatigue and decreased performance/stagnation. His coach thinks she has an iron deficiency… what should I do? What is the differential diagnosis, what investigations are recommended, and how should we manage this?

Introduction

As a sports medicine physician working with endurance athletes, I frequently encounter the challenging scenario of unexplained fatigue and performance decline in young triathletes. The complexity of triathlon—combining swimming, cycling, and running—creates unique physiological demands that, when coupled with adolescent developmental factors, can manifest as performance stagnation or frank deterioration.
The understanding of fatigue in endurance athletes has evolved significantly over the past decade. The 2019 IOC consensus statement on Relative Energy Deficiency in Sport (REDs) has fundamentally shifted our diagnostic framework beyond simple iron deficiency or overtraining syndrome [1]. Recent evidence demonstrates significant symptom overlap between overtraining syndrome (OTS) and REDs, with up to 86% of training-overload studies showing indications of low energy availability or carbohydrate deficiency [2]. Current epidemiological data suggest that fatigue affects nearly 40% of adolescents weekly, with athlete prevalence rates for REDs components ranging from 30-90% depending on sport type and diagnostic criteria [3,4].
In triathlon specifically, the metabolic demands are extraordinary. A typical training week for a competitive adolescent triathlete involves 15-20 hours of structured exercise across three disciplines, creating substantial energy expenditure that must be matched by appropriate nutritional intake and recovery strategies [5]. The challenge lies in distinguishing between physiological fatigue from normal training adaptation and pathological fatigue requiring intervention.

Case Reference

The presented case of a 16-year-old triathlete exemplifies the diagnostic challenges I encounter regularly. While the coach’s suspicion of iron deficiency reflects common knowledge, it represents a reductionist approach to a potentially complex multifactorial condition. This case highlights several critical considerations: the tendency to attribute fatigue to single nutrient deficiencies, the importance of systematic evaluation, and the need to consider REDs, overtraining syndrome, and psychological factors in our differential diagnosis.
The triathlon environment often perpetuates a culture of “more is better”, where young athletes may be reluctant to report fatigue for fear of being perceived as weak or uncommitted. This cultural barrier requires careful navigation during clinical assessment.

Clinical Background

Fatigue in adolescent endurance athletes stems from multiple interconnected pathophysiological mechanisms. The primary etiologies include REDs, functional overreaching progressing to overtraining syndrome, iron deficiency with or without anemia, other micronutrient deficiencies (B12, folate, vitamin D), endocrine dysfunction, sleep disorders, and underlying medical conditions [7,8].
REDs represents a spectrum of physiological dysfunction resulting from low energy availability (LEA), defined as energy intake minus exercise energy expenditure relative to fat-free mass <30 kcal/kg/day [9]. In triathletes, LEA can occur inadvertently due to high training volumes combined with inadequate nutritional planning. Importantly, recent research demonstrates that many training-overload studies previously attributed to overtraining may actually represent misdiagnosed REDs, with 86% of such studies showing evidence of decreased energy availability or carbohydrate deficiency [2].
The differential diagnosis must systematically exclude organic causes including thyroid dysfunction, celiac disease, inflammatory conditions, and cardiac abnormalities. Sleep perturbations are particularly common in adolescents due to natural circadian phase shifts, further compounded by early morning or late evening training sessions typical in triathlon [10]. Psychological factors account for 80-90% of fatigue complaints in adolescents, with competitive pressure being especially relevant in triathlon culture [11].
Clinical presentation typically involves a constellation of symptoms: decreased training capacity, prolonged recovery times, mood disturbances, recurrent infections, and in female athletes, menstrual dysfunction. The challenge lies in distinguishing pathological fatigue from normal training adaptation stress, particularly when fatigue persists beyond 1-2 weeks despite adequate recovery periods.

Management

My approach follows a systematic, multidisciplinary framework that emphasizes ruling out REDs before diagnosing overtraining syndrome. Initial investigation includes comprehensive metabolic panel, complete blood count, iron studies (ferritin, transferrin saturation, soluble transferrin receptor), vitamin B12, folate, 25-hydroxyvitamin D, thyroid function, inflammatory markers (CRP, ESR), and urinalysis. In female athletes, I include reproductive hormones and consider bone density evaluation if menstrual dysfunction is present [12].
Energy availability assessment is crucial given the emerging evidence that most training-overload scenarios may actually represent misdiagnosed REDs [2]. The sports dietitian plays a critical role in conducting detailed energy intake analysis using food diaries and metabolic calculations. Studies demonstrate that small energy deficits (300-400 kcal/day) can produce clinically meaningful REDs symptoms, emphasizing the importance of precise assessment [13].
Iron deficiency management follows Swiss Sports Medicine Society guidelines, with ferritin targets of 15 μg/L (<12 years), 20 μg/L (12-15 years), and 30 μg/L (15-18 years). Recent research supports alternate-day iron supplementation (rather than daily) to optimize absorption by avoiding hepcidin elevation [14]. Vitamin D should target levels >75 nmol/L with supplementation of 600-1000 IU daily.
From a training perspective, I collaborate closely with coaches to implement periodized recovery protocols. This involves structured rest days, modified training intensities, and careful monitoring using heart rate variability and subjective wellness questionnaires [15]. The key insight from recent research is that increased exercise energy expenditure often leads to inadvertent energy deficits when athletes fail to compensate with proportional increases in energy intake.
The sports psychologist becomes essential when addressing perfectionist tendencies, competitive anxiety, or underlying eating disorder behaviors that may contribute to LEA. Family involvement is crucial in adolescent cases, requiring education about appropriate nutrition and recovery strategies.
Training environment modifications include temporary reduction in training volume by 20-30% and emphasis on technique refinement rather than intensity work. Sleep hygiene education targeting the 8.5-9.5 hours recommended for adolescent athletes is essential, though often challenging to achieve with triathlon training schedules.

Return to Play

Return to sport progression follows graduated approaches outlined in recent BJSM guidelines, with important modifications based on emerging REDs research [17]. Phase 1 involves addressing underlying deficiencies while maintaining light aerobic activity at intensities that don’t exacerbate energy deficits. Phase 2 introduces sport-specific skills at reduced intensity once biochemical markers normalize and subjective energy improves.
The Stellingwerff framework suggests that many athletes progress through stages from functional overreaching to non-functional overreaching, and potentially overtraining syndrome, often driven by energy availability mismatches rather than training load per se [2]. This understanding has revolutionized my approach to return-to-sport timelines.
For triathletes specifically, I recommend discipline-specific progression, often starting with swimming due to its lower impact nature and reduced mechanical stress, followed by cycling, and finally running. Each discipline progression occurs over 2-3 weeks, with careful monitoring of subjective wellness scores and objective markers including resting heart rate variability and metabolic markers.
The unique considerations for adolescent athletes include growth velocity assessment, pubertal stage evaluation, and coordination with parents regarding nutritional support. Research demonstrates that prevention is crucial, as duration of illness/dysfunction significantly impacts prognosis, making early recognition and intervention essential [18].
Elite-level triathletes require additional consideration of competition schedules and qualification periods, necessitating individualized timelines that may extend 3-6 months for complete recovery. Regular monitoring throughout RTP includes weekly biochemical markers initially, progressing to monthly assessments. Training load progression should not exceed 10% weekly increases, with built-in recovery weeks every fourth week.
The emerging understanding that up to 84% of training-overload situations may represent energy availability deficits rather than true overtraining emphasizes the im­portance of ongoing nutritional monitoring throughout the ­return-to-sport process [2].

Prognosis and Athlete Counseling

Recovery timelines vary significantly based on underlying etiology and intervention compliance. Straightforward iron deficiency may resolve within 6-12 weeks, while REDs or overtraining syndrome can require 6-12 months for complete recovery [19]. Recent evidence suggests that full-blown overtraining syndrome diagnoses are relatively rare (only 15% of athletes already reporting under-performance and fatigue), highlighting the importance of early intervention before progression to more severe stages [2].
Predictors of prolonged recovery include severe energy deficiency (LEA <20 kcal/kg/day), concurrent eating disorder behaviors, poor social support, and continued high training loads during treatment. The research demonstrates that rushed return attempts often result in symptom recurrence and extended recovery periods, particularly when underlying energy availability issues remain unaddressed.
A critical insight from recent literature is that the prevention of either overtraining or under-fueling should be primarily based on awareness and monitoring. REDs prognosis has been shown to be linked to duration of illness/dysfunction, making early recognition and intervention essential when symptoms are still minor [20]. Unfortunately, awareness remains poor, with less than 50% of physicians, coaches, and sports professionals able to identify key diagnostic components.
Shared decision-making involves transparent discussion of recovery expectations, competitive goal adjustments, and long-term health implications. For adolescent athletes, I emphasize the importance of prioritizing long-term development over short-term competitive goals, often requiring difficult conversations about season modifications or competition withdrawal.
The prevalence rates of REDs components (30-90% depending on sport and athlete level) compared to true overtraining syndrome suggest that most fatigue scenarios in endurance athletes may be preventable through appropriate nutritional support and energy availability management [3]. This perspective fundamentally changes the counseling conversation from accepting fatigue as an inevitable consequence of training to treating it as a largely preventable condition.

Take-Home Messages

• • Fatigue in adolescent triathletes requires systematic evaluation beyond simple iron deficiency, with REDs being increasingly recognized as a primary etiology that may be misdiagnosed as overtraining syndrome in up to 86% of cases requiring multidisciplinary management.
• Energy availability assessment is fundamental, requiring collaboration with sports dietitians to calculate actual energy balance, as recent evidence demonstrates that many training-overload scenarios represent energy deficits rather than true overtraining pathophysiology.
• Iron deficiency management should follow sport-specific guidelines (SEMS standards) with alternate-day supplementation protocols, while vitamin D levels should target >75 nmol/L for optimal bone health and immune function.
• Training load modification is essential during evaluation and treatment phases, typically requiring 20-30% volume reduction with emphasis on technique rather than intensity work, informed by emerging understanding of energy availability-performance relationships.
• Return to sport progression should be discipline-specific and graduated over 16-24 weeks, with careful monitoring of both objective biomarkers and subjective wellness scores, recognizing that rushed returns often result in symptom recurrence.
• Long-term athlete development perspective must supersede short-term competitive goals, with early recognition and intervention being crucial as REDs prognosis is directly linked to duration of dysfunction, requiring enhanced awareness among all stakeholders in triathlon coaching and support.

Image credits: Open AI, Model: Dall-E 3.
https://chatgpt.com/
Prompt: “Educational conceptual medical illustration of an adolescent triathlete sitting tired at the side of a training field. The athlete is wearing triathlon gear (running shorts, jersey, and shoes), looking fatigued but neutral, leaning forward with hands on knees. Around the body are semi-transparent overlay icons: a stylized heart glowing red, a plate with food representing nutrition, a moon and bed symbol representing sleep, and a brain icon representing psychology. The illustration conveys fatigue and performance stagnation in young endurance athletes and the need for a multidisciplinary approach. Clean infographic style, minimal neutral background, no text, high detail, 4k resolution.”

References

1. 1. Mountjoy M, et al. International Olympic Committee (IOC) consensus statement on relative energy deficiency in sport (REDs): 2018 update. Br J Sports Med. 2018;52(11):687-697.
2. Stellingwerff T, et al. Overtraining Syndrome (OTS) and Relative Energy Deficiency in Sport (RED-S): Shared Pathways, Symptoms and Complexities. Sports Med. 2021;51(10):2251-2280.
3. Wenger N, et al. La fatigue chez l’athlète adolescent. Rev Med Suisse. 2019;15:1323-1328.
4. Logue DM, 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.
5. Laursen PB, Jenkins DG. The scientific basis for high-intensity interval training. Sports Med. 2002;32(1):53-73.
6. Meeusen R, et al. Prevention, diagnosis and treatment of the overtraining syndrome. Eur J Sport Sci. 2006;6(1):1-14.
7. Thomas DT, et al. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: nutrition and athletic performance. J Acad Nutr Diet. 2016;116(3):501-528.
8. Loucks AB, Thuma JR. Luteinizing hormone pulsatility is disrupted at a threshold of energy availability in regularly menstruating women. J Clin Endocrinol Metab. 2003;88(1):297-311.
9. Clark A, Mach N. Exercise-induced stress behavior, gut-microbiota-brain axis and diet: a systematic review for athletes. J Int Soc Sports Nutr. 2016;13:43.
10. Bergeron MF, et al. International Olympic Committee consensus statement on youth athletic development. Br J Sports Med. 2015;49(13):843-851.
11. Neinstein LS. Adolescent health care: a practical guide. Philadelphia: Lippincott Williams & Wilkins; 2008.
12. De Souza MJ, et al. 2014 Female Athlete Triad Coalition consensus statement on treatment and return to play of the female athlete triad. Br J Sports Med. 2014;48(4):289.
13. Fahrenholtz IL, et al. Within-day energy deficiency and reproductive function in female endurance athletes. Scand J Med Sci Sports. 2018;28(3):1139-1146.
14. Stoffel NU, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women. Lancet Haematol. 2017;4(11):e524-533.
15. Buchheit M. Monitoring training status with HR measures: do all roads lead to Rome? Front Physiol. 2014;5:73.
16. Bergeron MF, et al. International Olympic Committee consensus statement on youth athletic development. Br J Sports Med. 2015;49(13):843-851.
17. Schwellnus M, et al. How much is too much? (Part 2) International Olympic Committee consensus statement on load in sport and risk of illness. Br J Sports Med. 2016;50(17):1043-1052.
18. Kuikman M, et al. A narrative review of non-pharmacological strategies in the treatment of relative energy deficiency in sport. Int J Sport Nutr Exerc Metab. 2021;31(3):268-279.
19. Elliott-Sale KJ, et al. Methodological considerations for studies in sport and exercise science with women as participants. Sports Med. 2021;51(5):843-861.
20. Mountjoy M, et al. RED-S CAT: relative energy deficiency in sport clinical assessment tool. Br J Sports Med. 2015;49(7):421-423.

Not all references were assigned by AI.

The expert’s review

Dr Nathalie Wenger

OK! Frankly, I am pretty impressed, even though I don’t use AI very much!
Overall, it is pretty good if you are looking for a general overview. But it is still quite (or even too) superficial for a medical professional, especially considering the themes covered in the medical history.

My critique and additions

First, in the management section, AI does not start with a medical history, but directly with additional tests, whereas in medicine we usually take a history before prescribing tests.

1. The medical history

• It remains vague and lacks detail in terms of training, workload, duration, intensity and schedule. For me, this is very often the key—or at least a major part—of the problem with these young, tired athletes. Additionally, AI does not mention the necessary rest days per week (1-2 days off per week) for young athletes, which are the recommendations [21,22].
• Similarly, it does not ask any questions about what this young person does in her life, such as her studies, whom she lives with, etc. (HEADSSS for adolescent medical history) [23].
• It does mention menstrual dysfunction in the clinical background but does not present these issues in the medical history.
• Later, in the psychological section, there is no mention of stress and pressure, nor abuse from those around her. Really few details. However, it does address perfectionism (often found in endurance athletes who have RED-S). Young people put enormous pressure on themselves with social media, comparisons with others, etc. Unfortunately, abuse is common, and there is no mention of this. My advice: always ask the question, as sometimes fatigue is the only symptom of this insidious psychological suffering.
• Finally, it should be noted that the concept of confidentiality should always be mentioned and introduced at the beginning of the consultation with an adolescent athlete, and the young athlete should be seen alone for a moment.

2. Clinical examination

There is no information on what to examine. As a preliminary step, the adolescent should always be weighed/measured to assess their growth/puberty, and if possible, compare with previous values on growth charts.
For fatigue, I always rule out lymph nodes/adenopathy, palpate the thyroid, listen to the lungs (asthma?) and the heart to check for rhythm disorders. Hepatosplenomegaly can also be checked for.

3. Management

• Additional tests: AI does not suggest the need for an ECG, which I believe is essential to check for signs of pericarditis, arrhythmias, extrasystoles, etc. It does provide details for laboratory tests, but I disagree with the urine tests (we no longer do them, and I see no link with fatigue—at least not immediately if blood tests/kidney function tests are normal). Vitamin D is not systematically reimbursed in Switzerland in the last 3 years, except in certain cases including fatigue, which still need to be specified. It goes on to talk about biochemical criteria, but I do not see what it means?! Apart from low T3 syndrome or low female reproductive hormones (LH, FSH, estrogen and progesterone and possibly total and LDL-cholesterol) [24], there are few biological diagnostic criteria. As for bone density testing, which should only be discussed after 6-12 months of amenorrhea, there is no specific information on this either.
• AI points to the need for interdisciplinary care with a psychologist, coach, dietician, medical doctor for iron deficiency: all very good. The iron and ferritin reference ­values presented are acceptable. Vitamin D can be substituted without measuring blood levels. It insists on the need for help from dieticians, that is essential in the management. However, I would like to point out that it is very difficult to accurately assess the energy requirements of adolescents, due to the variation in their growth spurt, training, etc. Also, this can induce eating disorders if young people start to calculate their food intake too precisely. Do not talk too much about exact kcal/day, especially with young girls!
• Communication with coaches: very important, yes, but sometimes not easy… this is not mentioned.
• Heart rate variability: is it really an accurate method for measuring fitness?
• Sport psychologist: yes, really essential. Also, parents need to be more involved if the athletes are young, as do the coaches. Sometimes the parents are the ones pushing. Always make sure that the young person enjoys participating in their sport. This is not discussed. Are they doing it for themselves? For their coach or their parents?

4. Return to play

Follow-up with a training plan is generally good to reduce the training load, but it is not certain that this is exactly the same for every athlete, so it needs to be nuanced. AI gives a plan for triathletes, but for example, swimming in cold water is not ideal for anorexic or athletes with a large energy deficit! It talks about individualized timelines after that, which I find better. Furthermore, monitoring once a week using biochemical markers is not what we do. Hormones are measured every three months, even though these are not accurate markers (see above).

Conclusions

In the end, there is a lot of useful general information, but I think we still need human experts to dig deeper and provide proper care and guidance for returning to sport!
That being said, I appreciated that AI emphasized the importance of prevention, shared decision-making and the long-term health of young athletes. Additionally, it pointed out the unfortunate lack of knowledge in the athletes’ environment.
Finally, a small a remark to close…about the figure generated by AI: would it not be smarter to put the heart on the left side of the chest 😉?

Additional references by expert

References 10 and 15 given by AI are the same.

21. Brenner JS. American Academy of Pediatrics Council on Sports Medicine and Fitness. Overuse injuries, overtraining, and burnout in child and adolescent athletes. Pediatrics. 2007 Jun;119(6):1242-5. doi: 10.1542/peds.2007-0887.
22. Brenner JS, Watson A. Council on sports medicine and fitness. Overuse Injuries, Overtraining, and Burnout in Young Athletes. Pediatrics. 2024 Jan 1;153(2):e2023065129. doi: 10.1542/peds.2023-065129.
23. Adelman WP, Klein DA, Goldenring JM et al. The HEEADSSS interview for adolescents. Contemporary OB/GYN. 2014;59:31.
24. Klein M, Goldenring JM et Adelman WP. HEEADSSS 3.0: The psychosocial interview for adolescents updated for a new century fueled by media. Contemporary Pediatrics. January 2014.

Author profile

Dr Nathalie Wenger
nathalie.wenger@ehc.vd.ch

Affiliation: Chief physician in Centre GO2 , Centre romand de chirurgie et médecine du sport et des arts pour les 10-20 ans +, EHC-Morges, 1110 Morges, go2@ehc.vd.ch
Specializations: Internal Medicine FMH and Sports Medicine SEMS
Year of my medical school graduation: 2007
Sports organizations/federations I work with: Swiss Aquatics, numbers of Regional Clubs (Basketball, Volleyball, Track and Field, Sports-study dance program, Ice Skating, Tennis, …)
Fields/topics I am fond of: Global management of young athletes, Women and sports, REDs, Nutrition and Sport, Fatigue in Young athletes
Outside of Sports & Exercise Medicine, I also love: playing sports for myself, spending time with my family, being outdoors, enjoying our beautiful Swiss mountains (summer and winter), and seeing friends to play sports and/or enjoy good meals !
How I use AI: Very little, I must say, except to look up medical references sometimes or get help with computers and ask for general information/explanations (with my children, for example!)