literature suggests that exercise and heat stress
results in a shift towards increased carbohydrate
catabolism.
The increase in carbohydrate oxidation indi-
cates that lipid utilization is decreased during
exercise in the heat. Few studies, however, have
examined the effect of exercise and heat stress on
lipid catabolism. Plasma free-fatty acid concen-
tration (Fink et al. 1975; Nielsen et al. 1990) and
uptake (Nielsen et al. 1990) are similar when com-
paring exercise in the heat with that in a cooler
environment. These findings, however, do not
demonstrate unequivocally that lipid utilization
is unaffected by heat stress during exercise, since
Finket al. (1975) also observed a decreased intra-
muscular triglyceride utilization. These data,
along with the consistent observation of an
increased RER during exercise and heat stress,
suggest a substrate shift away from lipid.
Recently, Mittleman et al. (1998) have demon-
strated that branched-chain amino acid (BCAA)
supplementation increased endurance perfor-
mance during exercise in the heat. This finding is
in contrast with studies conducted during exer-
cise in cooler environments (van Hall et al. 1995;
Madsenet al. 1996). This discrepancy could arise
because protein catabolism may be augmented
during exercise in the heat. We have observed an
increase in ammonia (NH 3 ) accumulation during
exercise and heat stress (Snow et al. 1993;
Febbraioet al. 1994b). Although a major pathway
for NH 3 production during exercise is via the
deamination of adenosine 5¢-monophosphate to
form NH 3 and inosine 5¢-monophosphate (IMP),
NH 3 can also be formed in skeletal muscle via the
oxidation of BCAA. Accordingly, BCAA supple-
mentation augments muscle NH 3 production
during exercise (MacLean et al. 1996). During our
study (Febbraio et al. 1994b), the augmented
muscle NH 3 accumulation when comparing
exercise in the heat with that in a cooler environ-
ment was observed in the absence of any differ-
ence in IMP accumulation, suggesting that
enhanced BCAA oxidation may have accounted
for the increase. It should be noted, however, that
others (Dolny & Lemon 1988) have estimated
protein degradation, as measured by urea excre-
tion, to be reduced during exercise in the heat.
Further work examining the effect of exercise
and heat stress on protein catabolism is
warranted.Factors influencing fatigue during exercise in
the heat: substrate depletion vs. hyperthermia
During submaximal exercise in comfortable
ambient temperatures, the rate of energy utiliza-
tion is closely matched by rates of energy pro-
vision. It is well established that in these
circumstances fatigue is often associated with
glycogen depletion and/or hypoglycaemia
(Coyle et al. 1986; Sahlin et al. 1990) and
endurance can be increased by providing exoge-
nous carbohydrate during exercise (Coyle et al.
1986; Coggan & Coyle 1987). At fatigue the
muscle is characterized by low glycogen levels
and a concomitant elevation in IMP accumula-
tion (Sahlin et al. 1990; Spencer et al. 1991), since
glycogen depletion may impair the tricarboxylic
acid cycle and adenosine triphosphate must be
generated from alternative pathways such as the
adenylate kinase reaction. Since carbohydrate
utilization is augmented during exercise in the
heat and fatigue often coincides with depletion
of this substrate, it is somewhat paradoxical
that fatigue during exercise in the heat is often
related to factors other than substrate depletion.
We (Parkin et al. 1999) and others (Nielsen et al.
1990) have demonstrated that intramuscular
glycogen content is approximately 300 mmol ·
kg–1dry weight at fatigue when, during exercise
in cooler environments, this figure is usually less
than 150 mmol · kg–1dry weight (Fig. 38.2). This
may be because hyperthermia may lead to
fatigue prior to carbohydrate stores being com-
promised. This hypothesis is supported by the
observations that, when exercising in the heat to
exhaustion, subjects will fatigue at the same
body core temperature even if interventions
such as acclimatization (Nielsen et al. 1993) or
fluid/carbohydrate ingestion (Febbraio et al.
1996a) alter the duration of exercise. There may
be circumstances, however, where carbohydrate
may be limiting during exercise in the heat. If the