REF-481 · Meeusen et al., 2006 · Sports Medicine, · Paper · PubMed

Central fatigue: the serotonin hypothesis and beyond

Foundational review of CNS fatigue neurochemistry

B+moderate evidenceMeasured on a mixed groupWritten before October 2026, being re-checked

Core Contribution

A detailed analysis of the "serotonin hypothesis" of central fatigue and its modernization.

Key Findings

1

The original hypothesis: extracellular serotonin (5-HT) rises in several brain regions during prolonged exercise and contributes to fatigue — via serotonin's known effects on sleep, lethargy, drowsiness and loss of motivation.

2

Manipulating serotonin has not delivered robust evidence. Nutritional and pharmacological attempts to change central serotonergic activity during exercise "have yet to provide robust evidence for a significant role of serotonin in the fatigue process". (The tryptophan/BCAA transport competition that underlies the nutritional attempts is the classic mechanism of the hypothesis — full text; not checked against the abstract.)

3

The revised hypothesis is a ratio, not one transmitter: a high central serotonin-to-dopamine ratio accompanies tiredness and lethargy and accelerates fatigue; a low ratio favours performance through maintained motivation and arousal.

4

Dopamine's role rests mainly on amphetamine work. Other strategies to manipulate central catecholamines have not changed exercise capacity in temperate conditions.

5

In the heat the picture changes: recent findings support a significant role of dopamine and noradrenaline in performance during exercise in the heat. Serotonergic and catecholaminergic projections reach the hypothalamic thermoregulatory centre, so their activity may contribute to body-temperature control at rest and during exercise.

6

Fatigue during prolonged exercise is "a complex interaction between peripheral and central factors" — the review does not claim the CNS acts alone.

Practical Application

Meeusen et al. 2006 (narrative review): the serotonin-only story of central fatigue is not robustly supported; the revised model is the brain serotonin/dopamine ratio, and catecholamines matter most in the heat. For an athlete this explains why "can't go" at the end of a long, hot race can be central even with willing legs — and why the heat, not temperate conditions, is where brain-chemistry manipulations have shown effects.
BCAA supplementation against central fatigue is not supported — the serotonergic manipulations the hypothesis motivated have not provided robust evidence.
Not from this paper: caffeine (REF-350) and carbohydrate mouth rinse (REF-483) are the base's evidence-backed central-acting tools; this review does not evaluate them in its abstract.
Present the peripheral/central split as an interaction, not a choice: the review itself frames fatigue that way.

Limitations

2006 — predates the era of modern biomarker methods
BCAA and other serotonergic manipulations: the review reports no robust evidence either way
Specific neurotransmitter cut-offs are poorly measurable in vivo in athletes
How this entry was checked and changed
Why this level

regraded 2026-09-27 from A: narrative review without systematic method, from the leading central-fatigue group (Meeusen); a review's level grades its design, not its influence

Content correction 2026-09-27
Level A -> B+: narrative review without systematic method (rubric); kept at B+ as the leading group's review.
Removed acetylcholine, caffeine/adenosine and "training blunts the serotonin response" as findings: none is in the abstract. Tryptophan/BCAA transport kept only as an unverified full-text mechanism.
"Most RCTs null" replaced with the abstract's wording (no robust evidence); added the abstract's main points that the entry lacked: dopamine evidence rests on amphetamine work, catecholamines matter in the heat, hypothalamic thermoregulation link.
Cross-reference pointed at an entry whose content does not match the label (checked against the cited entry); repointed or relabelled.