feine (Vallerand et al. 1989) or ephedrine, caffeine
and theophylline (Vallerand et al. 1993) results in
a significant increase in heat production in cold-
exposed humans, but the ingestion of caffeine
alone produces no such effect (Graham et al.
1991). Likewise, some researchers (Wang et al.
1987) but not others (Vallerand et al. 1993) have
demonstrated that the ingestion of theophylline
during cold exposure attenuates the fall in body
temperature. It appears, therefore, that inges-
tion of b-adrenergic agonists may provide some
means of enhancing thermoregulatory thermo-
genesis, although further work in this area is
required to confirm this theory. In addition, since
b-adrenergic agonists such as ephedrine and caf-
feine are substances banned by the International
Olympic Committee, they may be impractical as
a mechanism for overcoming cold stress during
athletic competition.
Since carbohydrate is the major substrate uti-
lized in shivering thermogenesis, it has been sug-
gested that low endogenous glycogen stores may
reduce cold tolerance. This is true of very lean
individuals (Martineau & Jacobs 1989) but not of
moderately lean and fatter individuals (Young
et al. 1989). Therefore, adequate carbohydrate
stores are not only important to fuel muscle con-
traction during exercise, they possibly allow for a
better maintenance of body core temperature,
especially in leaner athletes.
In summary, during exercise in a cold environ-
ment, effort should be made to ensure that
pre-exercise carbohydrate stores are adequate in
order to offset the potential increase in carbohy-
drate oxidation associated with shivering and
non-shivering thermogenesis. This is especially
important for those individuals who live and
repeatedly exercise in a cold environment. The
concentration of carbohydrate within a fluid bev-
erage should not be increased to more than 12%,
despite the fact that fluid loss via sweating is
minimized or abolished, because of potential
gastrointestinal distress. Finally, the ingestion of
b-adrenergic agonists such as caffeine and theo-
phylline may provide some benefit against acute
cold exposure, but further work examining this
phenomenon is required.
500 practical issues
Exercise in a hot environment
Substrate utilization during exercise
in the heat
Although there is some conflict in the literature,
it is generally accepted that exercise in a hot
environment results in a substrate shift to-
wards increased carbohydrate utilization.
Muscle glycogenolysis (Fink et al. 1975; Febbraio
et al. 1994a, 1994b), liver glucose production
(Hargreaves et al. 1996a) and respiratory
exchange ratio (Febbraio et al. 1994a, 1994b;
Hargreaves et al. 1996a) are higher during exer-
cise in a hot environment. Furthermore, both
muscle (Young et al. 1985; Febbraio et al. 1994a,
1994b) and plasma (Rowell et al. 1968; Fink et al.
1975; Powers et al. 1985; Young et al. 1985;
Yaspelkis et al. 1993; Febbraio et al. 1994a) lactate
accumulation are increased in humans during
exercise in the heat compared with during
similar exercise in a cool environment. The
increase in plasma lactate accumulation is likely
to reflect an increase in muscle lactate produc-
tion, since hepatic lactate removal, although
decreased during exercise in the heat, does
not account for the increase in plasma lactate
accumulation (Rowell et al. 1968) while muscle
lactate efflux is unaffected during exercise and
heat stress (Nielsen et al. 1990). It must be noted,
however, that not all studies have observed an
increase in intramuscular glycogen utilization
during exercise in the heat (Nielsen et al. 1990;
Yaspelkis et al. 1993; Young et al. 1996). It is likely
that the discrepancy in the literature is related to
methodological differences such as the use of
acclimatized subjects (Yaspelkis et al. 1993) or dif-
ferences in pre-exercise glycogen concentrations
(Nielsenet al. 1990; Young et al. 1996) when com-
paring exercise in the heat with that in a cooler
environment. These factors will influence rates of
glycogen utilization, since heat acclimation
attenuated glycogenolysis during exercise in the
heat (King et al. 1985) while pre-exercise glyco-
gen concentration is directly related to rates of
utilization during submaximal exercise (Chesley
et al. 1995; Hargreaves et al. 1995). In general, the