Changes in plasma concentrations of aromatic and branched-chain amino acids during sustained exercise in man and their possible role in fatigue.
Level 4 - case-series / case-control
Uncontrolled observational pre-post physiological study in humans
PubMed 3227900 · doi:10.1111/j.1748-1716.1988.tb08388.x
What was done
Plasma concentrations of branched-chain amino acids (BCAAs) and aromatic amino acids were measured in two cohorts undergoing sustained dynamic exercise: 22 subjects who ran the 1986 Stockholm Marathon and 8 subjects who completed an army training session lasting approximately 1.5 hours. Plasma free tryptophan was specifically measured in the marathon group.
What was found
Both exercise types caused a significant decrease in plasma BCAA concentrations with no change in total (free plus albumin-bound) tryptophan. In marathon runners, plasma free tryptophan increased 2.4-fold, driving a marked increase in the ratio of free tryptophan to BCAAs. Numerical baseline and post-exercise values, variance estimates, and exact p-values were not provided in the abstract.
Why it matters
This study provided foundational human data for the central fatigue hypothesis, suggesting that prolonged exercise alters the plasma free tryptophan-to-BCAA ratio in a way that may facilitate brain tryptophan uptake and central 5-hydroxytryptamine (serotonin) synthesis.
Limits
The study had a small sample size (n = 30 total across two different exercise types) and lacked an unexercised control group. Free tryptophan was evaluated only in the marathon cohort. Brain amino acid transport, cerebral serotonin synthesis, and fatigue outcomes were not directly measured.
Cited by
- supports Exercise causes branched-chain amino acids to be taken up into muscle cells, reducing competition for tryptophan transport across the blood-brain barrier.