Overtraining Syndrome: What Could Be Causing It

Defining Overtraining Syndrome vs. Functional Overreaching

OTS sits at the far end of a maladaptation spectrum. Functional overreaching (FOR) produces short-term performance decrements that resolve within days to two weeks with planned rest. Nonfunctional overreaching (NFOR) extends that timeline to weeks. OTS, by contrast, means months of unexplained performance decline despite adequate recovery time, accompanied by psychological and neuroendocrine disturbances [1].

The European College of Sport Science and the American College of Sports Medicine published a joint consensus in 2013 distinguishing these stages and emphasizing that OTS can only be diagnosed retrospectively, after organic disease has been excluded and recovery has failed within an expected timeframe [1]. This retrospective nature creates clinical frustration. Athletes and clinicians often want a definitive marker, but none exists. Resting testosterone-to-cortisol ratio, once thought useful, lacks sensitivity and specificity when evaluated in controlled settings [2].

A practical distinction: if two weeks of rest restores performance, the athlete had FOR. If performance remains impaired after a structured recovery block lasting four or more weeks, and labs exclude disease, OTS becomes the working diagnosis.

Primary Causes and Contributing Mechanisms

The root cause is a mismatch between total physiological stress and recovery capacity. Training volume alone does not explain OTS. Research from Lehmann and colleagues demonstrated that monotonous training (low variation in intensity distribution) predicted overtraining symptoms more reliably than total volume [3].

Contributing factors include:

Excessive training load without periodization. Athletes who increase weekly volume by more than 10% without rest weeks face exponentially higher risk. A prospective study of collegiate swimmers found that those developing OTS symptoms had 1.8 times greater monotony scores than matched controls [3].

Chronic energy deficit. Inadequate caloric intake relative to expenditure depletes glycogen stores and suppresses hypothalamic-pituitary function. Loucks and Thuma showed that energy availability below 30 kcal/kg of lean body mass per day disrupted LH pulsatility within five days [4]. This same threshold triggers the cascade now classified as RED-S.

Sleep insufficiency. A 2024 systematic review in the British Journal of Sports Medicine reported that athletes sleeping fewer than seven hours per night had 1.7 times the odds of developing overreaching symptoms compared to those sleeping eight or more hours [5].

Psychosocial stressors. Competition anxiety, travel fatigue, academic or occupational demands, and interpersonal conflict all feed the allostatic load. The hypothalamic-pituitary-adrenal (HPA) axis does not distinguish between training stress and life stress.

Infection and immune suppression. Heavy training transiently suppresses mucosal immunity. Repeated bouts without recovery create an "open window" for upper respiratory tract infections, which compound fatigue and extend recovery timelines [6].

The Neuroendocrine Hypothesis

The most widely cited mechanistic model posits that chronic stress exposure leads to hypothalamic dysfunction. Meeusen and colleagues demonstrated that athletes meeting OTS criteria showed blunted prolactin and ACTH responses to pharmacological challenges (buspirone and dexfenfluramine), suggesting reduced sensitivity in serotonergic and dopaminergic pathways [7].

Dr. Romain Meeusen of Vrije Universiteit Brussel stated: "The neuroendocrine changes in OTS reflect a central adaptation to chronic stress rather than peripheral fatigue. The brain protects itself by downregulating the stress response, but this same protection eliminates the drive needed for high performance" [7].

Cortisol patterns shift. Early overreaching produces elevated resting cortisol. Prolonged OTS often shows paradoxically low cortisol, a pattern also seen in burnout and chronic fatigue syndrome. Nocturnal cortisol sampling reveals flattened diurnal curves in affected athletes compared to healthy training controls [8].

Testosterone typically falls. Free testosterone concentrations below the athlete's own baseline (even if within population reference ranges) correlate with performance impairment. The ratio itself matters less than the trajectory.

Differential Diagnosis: What Else Could Be Causing These Symptoms

OTS is a diagnosis of exclusion. The 2013 consensus statement lists mandatory screening targets before labeling an athlete with OTS [1]. Missing even one condition means the diagnosis is premature.

Iron deficiency (with or without anemia). Ferritin below 30 ng/mL impairs oxygen transport and mitochondrial enzyme function even when hemoglobin remains normal. Distance runners lose iron through foot-strike hemolysis, sweat, and GI microbleeding. A 2020 meta-analysis found that iron supplementation improved VO2max by 3.7% in iron-depleted nonanemic athletes [9].

Hypothyroidism and subclinical thyroid dysfunction. TSH, free T4, and free T3 should be measured. Intense training can suppress T3 conversion (low T3 syndrome), creating a biochemical picture that mimics hypothyroidism without true gland failure.

Relative Energy Deficiency in Sport (RED-S). Formerly the female athlete triad, RED-S encompasses low energy availability with downstream effects on bone, menstrual function, immunity, cardiovascular health, and performance. The 2023 IOC consensus broadened criteria and explicitly noted overlap with OTS [10].

Depression and anxiety disorders. Mood disturbance in OTS can be indistinguishable from major depressive disorder. The PHQ-9 and GAD-7 screening tools help differentiate primary psychiatric illness from training-induced mood changes. If mood fails to improve with rest, psychiatric referral is warranted.

Viral reactivation and post-infectious fatigue. Epstein-Barr virus (EBV) reactivation is documented in overtrained athletes. Elevated EBV viral capsid antigen IgG titers have been associated with unexplained underperformance in multiple case series [6].

Cardiac conditions. Myocarditis, arrhythmias, and structural abnormalities present with exercise intolerance. ECG and echocardiography are indicated when symptoms include chest discomfort, palpitations, or syncope.

Diabetes mellitus (type 1 or type 2). Fasting glucose and HbA1c screen for glycemic disorders that impair recovery.

Asthma and exercise-induced bronchoconstriction. Up to 50% of elite winter sport athletes have airway hyperresponsiveness. Reduced oxygen delivery mimics central fatigue.

How Overtraining Syndrome Is Diagnosed

No single test confirms OTS. The diagnostic pathway is structured exclusion.

Step 1: Document performance decline. Objective metrics (time trials, power output, VO2max testing) confirm that underperformance is real, not perceived. A minimum performance decline of 5 to 10% from an athlete's documented baseline supports the clinical picture.

Step 2: Rule out organic disease. The recommended laboratory panel includes complete blood count, ferritin, CRP, TSH, free T4, free T3, fasting glucose, HbA1c, cortisol (morning), total and free testosterone, LH, FSH, creatine kinase, vitamin D (25-OH), and EBV serology [1].

Step 3: Assess training history. Review training logs for the preceding 8 to 12 weeks. Calculate monotony (mean daily load divided by standard deviation of daily load) and strain (weekly load multiplied by monotony). High monotony with high strain predicts maladaptation.

Step 4: Evaluate recovery behaviors. Document sleep duration and quality (actigraphy or validated questionnaires), nutritional intake (three-day food diary with energy availability calculation), and psychosocial stressors (POMS, RESTQ-Sport).

Step 5: Observe response to rest. Prescribe a minimum two-week structured rest period. If performance normalizes, the diagnosis was NFOR, not OTS. If performance remains impaired after four or more weeks of adequate rest and all labs are normal, OTS is the working diagnosis.

The 2013 joint consensus stated: "OTS can only be diagnosed after exclusion of organic diseases, and should be considered as the end result of a long list of possible diagnoses" [1].

Treatment and Recovery Protocols

There is no pharmacological cure for OTS. Recovery requires addressing the root mismatch between stress and recovery capacity.

Relative rest, not complete inactivity. Total cessation of exercise can worsen mood and deconditioning. Low-intensity activity (walking, easy swimming, yoga) at 50 to 60% of maximum heart rate maintains fitness without triggering further HPA axis stress. Duration: start with two to four weeks, then reassess.

Energy repletion. Caloric intake must exceed expenditure during recovery. For athletes with energy availability below 30 kcal/kg lean mass/day, the target is restoration to 45 kcal/kg lean mass/day or above [4]. Carbohydrate availability warrants specific attention. Glycogen-depleted states amplify cortisol release and impair serotonin synthesis.

Sleep optimization. Target eight to ten hours of sleep opportunity per night. Behavioral strategies include consistent sleep-wake timing, removal of blue-light devices 90 minutes before bed, and a cool sleeping environment (18 to 20°C). Melatonin (0.5 to 3 mg) may assist circadian realignment for athletes with shifted schedules [5].

Psychological support. Cognitive behavioral approaches address the perfectionism and compulsive training patterns that frequently underlie OTS. Acceptance and Commitment Therapy (ACT) shows promise in athletic populations for reducing training rigidity.

Gradual return to training. Once symptoms resolve and performance shows upward trajectory in low-intensity testing, volume increases by no more than 5 to 10% per week. Intensity reintroduction lags volume by two to three weeks. Heart rate variability (HRV) monitoring provides a daily readiness signal to guide progression.

Nutritional support. Beyond total calories, specific micronutrients merit attention: iron (if ferritin <50 ng/mL in athletes), vitamin D (target 40 to 60 ng/mL), omega-3 fatty acids (2 to 3 g EPA+DHA daily for anti-inflammatory effects), and magnesium (supports sleep quality and neuromuscular function) [9].

When to Worry: Red Flags Requiring Immediate Evaluation

Not all fatigue in athletes is OTS. Certain presentations warrant urgent workup rather than a "rest and see" approach.

Sudden performance collapse (rather than gradual decline) may indicate cardiac disease, pulmonary embolism, or acute infection. Unintentional weight loss exceeding 5% of body weight in four weeks raises concern for malignancy, uncontrolled diabetes, or hyperthyroidism. Persistent resting tachycardia (heart rate more than 10 bpm above personal baseline for more than one week) should prompt ECG and possibly Holter monitoring. Suicidal ideation or self-harm thoughts in the context of performance distress require immediate psychiatric referral regardless of training status.

Female athletes with amenorrhea lasting more than three months need bone density assessment and endocrine workup. Stress fractures in the setting of fatigue and menstrual irregularity strongly suggest RED-S rather than isolated OTS [10].

Prevention Strategies Supported by Evidence

Prevention outperforms treatment for OTS because recovery timelines are unpredictable.

Periodized training with built-in recovery weeks (typically every third or fourth week at 50 to 60% of peak volume) is the primary structural defense. Polarized intensity distribution (approximately 80% low intensity, 20% high intensity) reduces monotony compared to threshold-heavy programs [3].

Monitoring tools with predictive value include daily wellness questionnaires (fatigue, sleep quality, muscle soreness, mood rated 1 to 5), morning resting heart rate trends, and HRV (specifically the root mean square of successive differences, RMSSD). A decline in RMSSD of more than 10% from an athlete's rolling 7-day average warrants a reduced training day [11].

Energy periodization (matching carbohydrate intake to training demands on a daily and within-day basis) protects glycogen stores without requiring chronic overfeeding. Pre-sleep protein (30 to 40 g casein) supports overnight muscle protein synthesis and may attenuate cortisol elevation [4].

Team environments should normalize rest. Cultures that reward "grinding through" fatigue produce higher OTS rates than those explicitly scheduling and valuing recovery days as training adaptations.

Hormonal Considerations for Athletes on TRT or HRT

Athletes receiving testosterone replacement therapy (TRT) or hormone replacement therapy (HRT) present a modified clinical picture. Exogenous testosterone masks one of the key OTS biomarkers (declining free testosterone), making diagnosis more challenging.

In men on TRT, clinicians must rely more heavily on cortisol patterns, HRV trends, mood questionnaires, and objective performance data rather than hormonal panels alone. A flattened diurnal cortisol curve despite adequate testosterone levels suggests central maladaptation is occurring [8].

Women on HRT (estradiol plus progesterone) may have partial protection against some OTS mechanisms. Estradiol supports glycogen storage, reduces exercise-induced muscle damage, and modulates serotonin receptor sensitivity. Loss of these benefits during perimenopause may explain why some female athletes develop unexplained performance decline that responds to HRT initiation rather than training modification alone.

For patients on GLP-1 receptor agonists (semaglutide, tirzepatide), the caloric deficit inherent to these medications creates an independent risk factor for RED-S overlap if training volume remains high. Energy availability calculations must account for pharmacologically reduced intake.