Experimental principal component analysis of fatigue in cyclists who have taken an oral L-tryptophan supplement

Abstract

Background

Since the 1980s the role of tryptophan in modulating fatigue has been the subject of debate. According to some models, tryptophan delays the onset of fatigue. Conversely, in other models, this amino acid is implicated in generating fatigue. This study attempts to shed some light on the debate.

Method

Ten elite (non-professional) road racing cyclists took part in a double-blind study employing paired data under standardised dietary and training conditions. Each volunteer did two trials, one with tryptophan supplementation (TRP) and the other with placebo (PLB). Various body fuel, plasma amino acid and renal function parameters were analysed in relation to fatigue data obtained using the Borg test. Blood pressure, pupil size and haematocrit values were also monitored. The parameters obtained were used to examine the principal components of fatigue in said test.

Objective

To confirm that, under test conditions, oral L-tryptophan supplementation has a favourable influence on fatigue perception during exercise.

Results

The principal components of fatigue, as reflected by the Borg test in this study, fit the following model: Perception of fatigue attained = 8.406050586 + (0.126275247 * % load intensity) − (0.031521537 * cholesterol mg/dl) + (0.051002322 * HDL cholesterol mg/dl) − (0.017130681 * tryptophan mmol/ml) − (0.004545865 * glycine mmol/ml) + (0.082894085 * methionine mmol/ml). (Multiple correlation coefficient = 0.90; coefficient of determination R² = 0.82; adjusted R² = 0.81; standard error: 1.82; observations: 75).

Renal function

Urea: (TRP: 45.6 ± 6.5 mg/dl vs. PLB: 44.1 ± 5.7; p < 0.4); creatinine: (TRP: 90.4 ± 12.0 vs. PLB: 88.4 ± 11.8 mg/dl; p < 0.9); resting pupil size: (TRP: 0.38 ± 0.07 AU vs. PLB: 0.35 ± 0.08 AU; p < 0.01); haematocrit: (TRP: 45.3% ± 1.5% vs. PLB: 46.1% ± 2.6%; p < 0.40).

Conclusions

Higher tryptophan levels reduced the increased subjective perception of fatigue (SPF). In the component analysis the impact of tryptophan was estimated as 4% in our model. Tryptophan supplementation did not affect renal function. Tryptophan supplementation altered pupil size. At the dose used, tryptophan supplementation proved effective for the first 2 h following the last intake of the tryptophan supplement.