Overtraining Syndrome — Pathological Counterpart of the Lactate Shuttle

Table of Contents

1. The Overtraining Continuum & Physiological Signature
2. The Immune & Cytokine Signature of Overtraining
3. Diagnosis & Monitoring
4. The Critical Differential: OTS vs Post-Infectious Syndromes

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1 The Overtraining Continuum & Physiological Signature

1.1 Hormesis and the OT Continuum

Exercise follows a biphasic dose–response (Radak 2008): sedentary and chronic-overload sit at the two damaging ends of the same curve. The hormetic sweet spot coincides with MLSS / IAS (Kindermann).

Term	Recovery
FOR — planned overreaching → supercompensation	Days
NFOR — extreme overreaching, no supercompensation	Weeks – months
OTS — prolonged maladaptation, exclusion diagnosis	Months – years

Prevalence (Meeusen 2013): 7–21% per season; career rate ~60% in elite endurance athletes; 91% relapse after a first OTS episode.

1.2 Lactate Curve in OTS (Urhausen 1998, Meeusen 2013)

The lactate–power curve is NOT classically left-shifted in OTS. Instead:

• Submaximal lactate ↓ slightly (deceptively “improved”)
• Maximal lactate ↓
• Max HR ↓
• Time-to-exhaustion at 110% IAS ↓ ~27% ← most sensitive ergometric marker
• 10/30 s anaerobic alactacid power unchanged

A graded test alone misleads. Always combine with a supra-threshold time-to-exhaustion test plus a mood questionnaire.

1.3 Autonomic Nervous System Markers

Israel’s two-form model:

Form	Profile	ANS signature
Sympathicotonic OT	Strength/sprint, acute, young	↑ resting HR, ↑ BP, ↑ catecholamines, anxiety, insomnia
Parasympathicotonic OT	Endurance, chronic	↓ resting HR, depressed mood, fatigue — harder to diagnose

Sympathetic markers: ↓ max HR; ↑ LF/HF (short-term overload); ↓ nocturnal catecholamines (chronic); blunted lactate response; blunted cortisol/ACTH/GH/PRL to exercise.

Parasympathetic markers: ↑ HF power, ↑ RMSSD (Hedelin); ↓ resting HR in chronic endurance OTS.

Unifying mechanism — β-adrenergic receptor down-regulation (Gleeson, Lehmann): chronic catecholamine surges desensitize peripheral β-AR + central HPA, explaining the convergent blunting of max HR, max lactate, cortisol, and pituitary response. Probed directly by the two-bout test (§9).

Diagnostic caveat: HRV is inconsistent across studies — useful only as longitudinal trend in the same athlete (Meeusen 2013).

Link to lactate shuttle: the blunted submax/max lactate is a downstream consequence of sympathetic exhaustion, not a shuttle failure. MCT1/MCT4 machinery remains intact.

One-Minute-Paper Topics

• Define FOR / NFOR / OTS; what single feature distinguishes them?
• Position OTS on the hormesis curve and name the pathway(s) suppressed.
• Why is OTS in most cases a retrospective diagnosis?
• Does the OTS lactate curve shift left, right, or neither? Contrast with detraining / Long COVID.
• Why is submax lactate sometimes ↓ in OTS, and why can this mislead?
• Describe the 110% IAS time-to-exhaustion test and the threshold for OT suspicion.
• Distinguish sympathicotonic vs. parasympathicotonic OT. Which dominates in endurance athletes?
• Name three findings explained by β-adrenergic receptor down-regulation.
• Why is HRV unreliable as a single OTS marker?

Key References

• Radak et al. (2008). Exercise, oxidative stress and hormesis. Ageing Res Rev, 7, 34–42.
• Meeusen et al. (2013). ECSS/ACSM Joint Consensus on OTS. MSSE, 45(1), 186–205.
• Urhausen et al. (1998). Ergometric & psychological findings during overtraining. IJSM, 19, 114–120.
• Israel S (1976). Zur Problematik des Übertrainings. Medizin und Sport, 16, 1–12.
• Lehmann et al. (1998). Autonomic imbalance hypothesis & OTS. MSSE, 30(7), 1140–1145.
• Hedelin et al. (2000). Cardiac autonomic imbalance in an overtrained athlete. MSSE, 32(9), 1531–1533.
• Bosquet et al. (2008). HR & overreaching: systematic review. BJSM, 42, 709–714.

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2 The Immune & Cytokine Signature of Overtraining

2.1 Gabriel et al. 1998 — Immunophenotyping

Companion to Urhausen 1998 (same Saarbrücken cohort). Flow cytometry of CD3/4/8/16/19/45/45RO/56/HLA-DR.

Key finding: peripheral T, B, NK cell counts are NOT altered in OTS. But receptor density of CD45RO on CD4⁺ T-cells is significantly elevated (P < 0.001).

• Self-learning classifier: specificity 92%, sensitivity 93%
• Count-based classification only: 67% / 84%
• Caveat: CD45RO ↑ also reflects subclinical viral (e.g. EBV) reactivation — always screen first
• Meeusen 2013 endorses CD45RO as the single most discriminating immune marker

2.2 Acute OT + Clinical Immune Signature (Fry 1994, Hackney 2012)

Acute OT (5 SAS soldiers, 10 days × 2 sessions/day): performance ↓, mood ↓, IL-2 + lymphocyte proliferation ↓. With recovery, mood normalizes — but fatigue and immune deficits persist.

OTS immune phenotype: ↓ NK cytotoxicity, ↓ neutrophil oxidative burst, ↓ T-cell proliferation, Th2 shift, ↓ sIgA, ↑ resting IL-6 (paradoxical), recurrent URTI, EBV/HSV reactivation, reduced vaccine response. >50% of pre-Olympic OT-symptomatic athletes present with infection vs. 0% of overreached controls.

2.3 Cytokine Hypothesis (Smith 2000)

Training overload → micro-trauma → monocyte activation
                       ↓
        Systemic IL-1β / IL-6 / TNF-α ↑
                       ↓
    CNS sickness behavior  +  Liver gluconeogenesis ↑
    HPA exhaustion         +  Immune dysfunction

OTS = Selye GAS stage 3 (exhaustion), not adaptation. Chronic inflammation downregulates PGC-1α and mitochondrial biogenesis — the same pathways the lactate shuttle (§4 parent) is designed to activate.

2.4 Overtraining and Myokine Monitoring: Red Flags for Excessive Training Load

Ringleb et al. note that excessively prolonged efforts—ultra-endurance events exceeding 5-6 hours of continuous exercise—may paradoxically shift from the adaptive pattern described to potentially detrimental immune responses characterized by sustained rather than transient elevation of pro-inflammatory signals (Ringleb et al. 2026). This observation translates into a practical monitoring approach for coaches and sports medicine practitioners: myokine biomarker profiling in the recovery period can distinguish normal post-exercise adaptation from concerning overtraining patterns.

Normal adaptation pattern: High IL-6 (g > 0.8), concurrent elevated IL-1ra/IL-10, TNF-α normal to mildly elevated, with rapid normalization within 24-48 hours.

Concerning overtraining pattern: Sustained IL-6 elevation at 48-72 hours post-training, delayed or absent IL-1ra/IL-10 elevation, or paradoxical sustained TNF-α elevation suggesting NF-κB-dependent pro-inflammatory signaling. These patterns warrant training load reduction and recovery protocol intensification to prevent progression to overtraining syndrome with its attendant immunological suppression.

One-Minute-Paper Topics

• What did Gabriel 1998 measure that prior studies missed? Specificity/sensitivity?
• Why screen EBV/CMV before attributing CD45RO↑ to OTS?
• Summarize Smith’s cytokine hypothesis in five steps. Which Selye-GAS stage?
• How does chronic inflammation blunt lactate-as-lactormone signaling?

Key References

• Gabriel et al. (1998). Overtraining and immune system. MSSE, 30(7), 1151–1157.
• Fry et al. (1994). Psychological & immunological correlates of acute overtraining. BJSM, 28(4), 241–246.
• Hackney & Koltun (2012). Immune system & overtraining: clinical implications. Acta Clin Croat, 51, 633–641.
• Smith (2000). Cytokine hypothesis of overtraining. MSSE, 32(2), 317–331.
• Ringleb M, Fabritius F, Godde J, Puta C, Bloch W, Javelle F (2026) Circulating myokine responses to acute endurance exercise and their role in immunoregulation: a systematic review and meta-analysis. The FASEB Journal 40(4):e71536. https://doi.org/10.1096/fj.202504780R.
• Kistner TM, Pedersen BK, Lieberman DE (2022) Interleukin 6 as an energy allocator in muscle tissue. Nat Metab 4:170-179.
• Meeusen et al. (2013). ECSS/ACSM Joint Consensus on OTS. MSSE, 45(1), 186–205.

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3 Diagnosis & Monitoring

3.1 Gleeson’s Marker Framework (2002)

Criteria for an ideal OTS marker: sensitive to load, predictive, distinguishable from acute exercise, easy/cheap, non-invasive, measurable at rest or after a short submaximal bout.

Verdict — what works:

Marker	Status
Blunted cortisol response to standardized exercise	Best hormonal probe (saliva-based)
Blunted lactate response to fixed-intensity exercise	β-adrenergic / HPA dysregulation
CD4⁺CD45RO⁺ expression	High specificity + sensitivity (see §5)
Salivary IgA ↓	Good with caveats

What doesn’t work alone: plasma glutamine, CK, urea, cortisol/T ratio, resting catecholamines, CD4/CD8 ratio.

3.2 Psychological Monitoring (Morgan 1987, Meeusen 2013)

• POMS: dose-response with training load; iceberg profile inverts in staleness; up to 80% of OTS athletes show clinical depression
• TDS (7 items): higher discriminant accuracy than full POMS
• RESTQ-Sport (77 items, 19 factors): captures stress and recovery
• Psychomotor speed / reaction-time tests (Stroop, finger pre-cuing): detect central fatigue earlier than biochemistry

3.3 ECSS/ACSM 2013 Consensus (Meeusen et al.)

3.3.1 Two-Bout Maximal Exercise Test

Two incremental tests to exhaustion, separated by 4 hours. Hormonal pattern on bout 2 vs. bout 1:

State	ACTH / PRL / GH on bout 2
Healthy	Comparable
FOR	Less pronounced
NFOR	Hyperreactive
OTS	Bout 1 extreme + complete suppression in bout 2

The only protocol that prospectively differentiates NFOR from OTS.

3.3.2 Foster Session-RPE Load Model

• Daily load = session RPE × duration (min)
• Monotony = daily mean ÷ SD
• Strain = weekly load × monotony

Spikes in monotony/strain prospectively predict illness and injury.

3.3.3 Prevention (the only “treatment” is rest)

1. ≥ 1 passive rest day/week
2. Sleep hygiene
3. CHO 8.5 g/kg/day (Achten 2004); glutamine/BCAA supplementation does NOT help
4. Avoid monotony; individualize intensity (IAS-based)
5. Suspend training during URTI
6. Multidisciplinary monitoring for elite athletes

3.3.4 Diagnostic Flowchart

Underperformance ≥ 2 weeks
  → Rule out organic causes (CBC, ferritin, TSH, EBV/CMV, CK, allergies, cardiology)
  → Lactate test: submax ↓ + max ↓ + max HR ↓
  → 110% IAS time-to-exhaustion test (Urhausen) → t_exh ↓ ≥ 20%
  → Two-bout exercise test (Meeusen) → bout 2 suppression
  → Flow cytometry: CD45RO on CD4⁺ ↑↑ (rule out EBV)
  → POMS / TDS / RESTQ-Sport → TMD ↑, depression ↑↑
  → 2–4 weeks complete rest
        → recovery in weeks → NFOR
        → no recovery in months → OTS

One-Minute-Paper Topics

• Recite Gleeson’s six criteria for an ideal OTS marker.
• Why is a blunted cortisol response more informative than resting cortisol?
• Describe the two-bout test. What hormonal pattern distinguishes FOR/NFOR/OTS?
• Calculate Foster monotony and strain for: 400, 420, 380, 410, 390, 400, 410 (RPE × min).
• State the most important prevention strategy and the CHO recommendation (g/kg/day).

Key References

• Gleeson (2002). Biochemical & immunological markers of overtraining. JSSM, 1, 31–41.
• Morgan et al. (1987). Psychological monitoring of overtraining and staleness. BJSM, 21(3), 107–114.
• Meeusen et al. (2013). ECSS/ACSM Joint Consensus on OTS. MSSE, 45(1), 186–205.
• Foster (1998). Monitoring training with reference to OTS. MSSE, 30, 1164–1168.
• Achten et al. (2004). Higher CHO during intensified training. J Appl Physiol, 96, 1331–1340.
• Kreher & Schwartz (2012). Overtraining Syndrome: A Practical Guide. Sports Health, 4(2), 128–138.

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4 The Critical Differential: OTS vs Post-Infectious Syndromes

4.1 Lactate Clearance: OTS vs. Post-Infectious Syndromes

Critical differential. OTS and ME/CFS / Long COVID look similar (fatigue, low max lactate) but have opposite clearance signatures.

Feature	OTS	ME/CFS / Long COVID
Lactate–power curve	No shift	Left-shifted
Resting lactate	Normal	↑ (correlates with PEM, Lien 2019)
Submax lactate (low W)	↓ or unchanged	↑
Max lactate	↓	↓
Post-exercise clearance	Largely preserved	Slower
2-day CPET VO₂ at AT (day 2)	Variable	Reproducibly ↓ (Snell 2013)
Peripheral O₂ extraction	Preserved	↓ (Haunhorst 2022)
Primary defect	Reduced production (β-AR / HPA)	Reduced clearance / oxidation (mitochondrial + microvascular)
Recovery	Months with rest	Often years; PEM after minimal effort

Mechanism in post-infectious syndromes (Haunhorst 2022, 2024; Joseph 2023; Singh 2022):

• Mitochondrial dysfunction (complex I/IV, PDH ↓; PCr resynthesis ↓)
• Reduced MCT1 expression in muscle subgroups
• Endothelial / microvascular dysfunction
• Autonomic dysregulation (POTS overlap)
• Failed lactate-mediated ETC activation (Cai 2023)
• Immunometabolic dysregulation (Haunhorst 2024)

Brooks-shuttle lens: OTS breaks the upstream sympathetic drive that fuels glycolytic production — shuttle machinery intact. Post-infectious syndromes break the downstream oxidative machinery that consumes lactate — the drain is plugged.

Therapeutic consequence (clinically critical):

• OTS → rest (Meeusen 2013)
• ME/CFS / Long COVID with PEM → pacing within energy envelope — graded exercise can worsen the condition (Haunhorst 2024)

Same lactate-curve abnormality at face value can require opposite therapy depending on whether the defect is autonomic exhaustion or mitochondrial-microvascular dysfunction.

Mixed phenotypes (overtrained athlete + EBV/SARS-CoV-2 infection) are common and require the Meeusen 2013 flowchart expanded with microvascular/immunometabolic work-up.

4.2 Open Questions

• MCT1/MCT4 expression in OTS muscle? (Brooks 2022)
• Histone lactylation as persistence mechanism?
• Does impaired lactate signaling underlie blunted T-cell response in OTS? (Cai 2023)
• OTS as non-infectious analog of PEM in ME/CFS / Long COVID?
• Sex differences + RED-S interactions under-studied
• Multicenter validation of two-bout test

One-Minute-Paper Topics

• Argue for or against: OTS as a non-infectious PEM analog.
• A fatigued athlete shows elevated resting lactate, left-shifted lactate-power curve, and reproducible day-2 VO₂ drop on 2-day CPET. OTS or post-infectious? What therapy is contraindicated, and why?

Key References

• Haunhorst S, … Puta C (2022). Long COVID: putative pathophysiology & physical activity. Oxford Open Immunology, 3(1), iqac006.
• Haunhorst S, … Puta C et al. (2024). Physical activity-induced PEM: microvascular alterations & immunometabolic interactions in post-COVID & ME/CFS. Infection.
• Lien et al. (2019). Elevated resting lactate correlates with PEM severity in ME/CFS. Sci Rep, 9, 19181.
• Singh et al. (2022). Persistent exertional intolerance after COVID-19: invasive CPET. Chest, 161(1), 54–63.
• Joseph et al. (2023). Exercise pathophysiology in ME/CFS and PASC. Chest, 164(3), 717–726.
• Snell et al. (2013). Discriminative validity of 2-day CPET in CFS. Phys Ther, 93(11), 1484–1492.

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References

• Achten et al. (2004). Higher CHO during intensified training. J Appl Physiol, 96, 1331–1340.
• Bosquet et al. (2008). HR & overreaching: systematic review. BJSM, 42, 709–714.
• Brooks et al. (2022). Tracing the lactate shuttle to the mitochondrial reticulum. Exp Mol Med, 54, 1332–1347.
• Cai et al. (2023). Lactate activates the mitochondrial electron transport chain independently of its metabolism. Mol Cell, 83, 3904–3920.
• Foster (1998). Monitoring training with reference to OTS. MSSE, 30, 1164–1168.
• Fry et al. (1994). Psychological & immunological correlates of acute overtraining. BJSM, 28(4), 241–246.
• Gabriel et al. (1998). Overtraining and immune system. MSSE, 30(7), 1151–1157.
• Gleeson (2002). Biochemical & immunological markers of overtraining. JSSM, 1, 31–41.
• Hackney & Koltun (2012). Immune system & overtraining: clinical implications. Acta Clin Croat, 51, 633–641.
• Haunhorst S, … Puta C (2022). Long COVID: putative pathophysiology & physical activity. Oxford Open Immunology, 3(1), iqac006.
• Haunhorst S, … Puta C et al. (2024). Physical activity-induced PEM: microvascular alterations & immunometabolic interactions in post-COVID & ME/CFS. Infection.
• Hedelin et al. (2000). Cardiac autonomic imbalance in an overtrained athlete. MSSE, 32(9), 1531–1533.
• Israel S (1976). Zur Problematik des Übertrainings. Medizin und Sport, 16, 1–12.
• Joseph et al. (2023). Exercise pathophysiology in ME/CFS and PASC. Chest, 164(3), 717–726.
• Kistner TM, Pedersen BK, Lieberman DE (2022) Interleukin 6 as an energy allocator in muscle tissue. Nat Metab 4:170-179.
• Kreher & Schwartz (2012). Overtraining Syndrome: A Practical Guide. Sports Health, 4(2), 128–138.
• Lehmann et al. (1998). Autonomic imbalance hypothesis & OTS. MSSE, 30(7), 1140–1145.
• Lien et al. (2019). Elevated resting lactate correlates with PEM severity in ME/CFS. Sci Rep, 9, 19181.
• Meeusen et al. (2013). ECSS/ACSM Joint Consensus on OTS. MSSE, 45(1), 186–205.
• Morgan et al. (1987). Psychological monitoring of overtraining and staleness. BJSM, 21(3), 107–114.
• Radak et al. (2008). Exercise, oxidative stress and hormesis. Ageing Res Rev, 7, 34–42.
• Singh et al. (2022). Persistent exertional intolerance after COVID-19: invasive CPET. Chest, 161(1), 54–63.
• Smith (2000). Cytokine hypothesis of overtraining. MSSE, 32(2), 317–331.
• Snell et al. (2013). Discriminative validity of 2-day CPET in CFS. Phys Ther, 93(11), 1484–1492.
• Urhausen et al. (1998). Ergometric & psychological findings during overtraining. IJSM, 19, 114–120.