Six Theories for What Causes Overtraining Syndrome

Why It Matters

Without a clear biomarker, athletes and coaches risk misdiagnosing overtraining, leading to ineffective interventions; a systems‑based approach better safeguards performance and health.

Key Takeaways

• Overtraining results from chronic load exceeding recovery capacity.
• Glycogen depletion theory fails; carbs can't reverse syndrome.
• Serotonin/branched‑chain amino acid hypothesis lacks consistent biomarker support.
• HPA axis dysregulation shows ACTH blunting in most affected athletes.
• Complex systems view suggests no single cause; diagnosis remains exclusionary.

Summary

The video dissects six prevailing biological theories behind overtraining syndrome, emphasizing that none fully explain the condition. It frames the syndrome as a mismatch between total life stress load and an individual’s recovery capacity, rather than a single pathological pathway.

Each hypothesis—glycogen depletion, serotonin/branched‑chain amino acid imbalance, autonomic bias, chronic cytokine elevation, and HPA‑axis dysregulation—is outlined with its supporting evidence and critical gaps. Glycogen shortage cannot account for persistent fatigue despite adequate carbs; BCAA supplementation fails to prevent the syndrome; autonomic shifts appear downstream rather than causal; cytokine spikes resolve quickly after rest; and HPA‑axis studies show reduced ACTH output but rely on small, hard‑to‑replicate tests.

The most cited data come from the Eros study, where 78.6% of athletes meeting overtraining criteria exhibited blunted ACTH responses in an insulin‑tolerance test, and from Armstrong’s 2022 paper proposing a complex‑systems perspective—no single biomarker, only a pattern of multi‑system breakdown. These examples illustrate the difficulty of isolating a definitive mechanism.

Consequently, overtraining syndrome remains a diagnosis of exclusion, urging practitioners to focus on holistic load‑recovery monitoring rather than chasing a solitary lab test. Recognizing its emergent, multifactorial nature can improve prevention strategies and avoid mislabeling athletes who simply face a temporary stress‑recovery imbalance.

Original Description

Sports scientists have six competing theories for what causes overtraining syndrome. None of them are fully supported. Here’s what each one gets right and where each one falls short.

Jordan and Austin walk through all six: the glycogen depletion hypothesis, the serotonin/BCAA central fatigue theory, the autonomic nervous system model, the cytokine/inflammation hypothesis, HPA axis dysregulation, and Armstrong’s complex systems framework. They give special attention to the EROS study, which found ACTH blunting in 78.6% of athletes meeting OTS criteria using insulin tolerance testing — and why even this finding doesn’t settle the question of cause vs. effect. Austin explains why “adrenal fatigue” is mechanistically backwards (the adrenals are intact; the problem is upstream at the pituitary), and why the best-supported mechanism requires a test nobody runs.

Timestamps:

0:00 Six theories, each with evidence and gaps

0:43 The HPA axis as the central stress-response coordinator

1:29 Theory 1: Glycogen depletion

2:23 Theory 2: Serotonin / BCAA central fatigue

3:09 Theory 3: Autonomic nervous system bias

3:55 Theory 4: Cytokine / inflammation hypothesis

4:48 Theory 5: HPA axis dysregulation — strongest current evidence

5:35 Why “adrenal fatigue” is the wrong framing

5:58 The EROS study: ACTH blunting in 78.6% of OTS athletes

8:08 Austin: is HPA dysregulation a cause or an effect?

10:56 Theory 6: Complex systems framework (Armstrong 2022)

11:59 Has “true” OTS ever been cleanly characterized?

Resources:

• Cadegiani & Kater 2017 — EROS study, HPA axis: pmc.ncbi.nlm.nih.gov/articles/PMC5722782

• EROS study — extended findings: pmc.ncbi.nlm.nih.gov/articles/PMC6590962

• Armstrong & VanHeest 2002 — OTS and depression overlap: pubmed.ncbi.nlm.nih.gov/11839081

• Full episode: https://www.youtube.com/watch?v=ssnapz_SnBs&feature=youtu.be