compared with the placebo. However, there was
no added benefit of BCAA despite increases in
plasma (120%) and muscle (35%) concentrations
of BCAA. Van Hall et al.(1995) used more sub-
jects (n=10) and tested a low (6 g · l–1) and high
(18 g · l–1) dose of BCAAs added to a 6% CHO
solution on cycling time to fatigue at 70–75% of
maximal power output. Despite large changes in
plasma concentrations of BCAA, exercise time to
exhaustion (ª122 min) was again not different
from the control treatment (6% CHO). This study
also included a treatment condition in which
tryptophan (3 g · l–1) was added to the 6% CHO
solution. However, this also failed to affect
fatigue. The authors concluded that these nutri-
tional manipulations either had no additional
effect upon brain 5-HT activity or that the change
in 5-HT did not contribute significantly to mech-
anisms of fatigue. It is also possible that the
effects were lessened by the fact that they were
given in a solution with CHO. The CHO would
have suppressed the normally large increase in
circulating levels of stress hormones that is
known to alter TRP transport kinetics.
This brings up the fundamental question of
whether CHO or BCAA supplementation actu-
ally produces the hypothesized effects on brain
5-HT during exercise. This, of course, cannot be
answered in human subjects during exercise. We
recently completed a preliminary study which
partially addressed this issue (Welsh et al.1997).
Solutions containing BCAA, CHO or pure waternutrition, neurotransmitters and cns fatigue 175
TRPTRPTRPTRP
TRP5-HT
5-HT5-HTTRPAAAFFAFFABCAABCAA BCAABCAABCAA f-TRP
BCAAFFAB r a i n C a p i l l a r yTRPTRPTRP
TRPTRP5-HT
5-HT
5-HT5-HTTRP
TRPAAAFFAFFACHO
CHOCHO
CHOCHOBCAABCAATRPf-TRP
BCAAFFACHOB r a i n C a p i l l a r y(a)(b) CHO
+
BCAAFig. 12.2The proposed nutritional effects
on the central fatigue hypothesis during
prolonged exercise. (a) The proposed
effects of CHO ingestion on the
mechanisms of central fatigue with regard
to the attenuation of FFA and f-TRP during
prolonged exercise. (b) The proposed
effects of CHO and BCAA ingestion on the
mechanisms of central fatigue with regard
to the larger decrease in the plasma
f-TRP/BCAA ratio during prolonged
exercise. A, albumin.