(Febbraioet al. 1996b). Such responses are likely
to alter substrate utilization during exercise.
Substrate utilization during exercise in
a cold environment
When the rise in body temperature is attenuated
during prolonged exercise in a cold environ-
ment, the rate of glycogen utilization in contract-
ing muscle is reduced (Kozlowski et al. 1985;
Febbraioet al. 1996b; Parkin et al. 1999) and exer-
cise performance is increased (Hessemer et al.
1984; Febbraio et al. 1996a; Parkin et al. 1999),
which is not surprising, since fatigue during pro-
longed exercise often coincides with glycogen
depletion (Coggan & Coyle 1991). In many cir-
cumstances, therefore, the cool environment may
be viewed as an ‘ergogenic aid’, since it results in
a conservation of finite endogenous carbohy-
drate stores within contracting muscles. It must
be noted, however, that even in some circum-
stances where measures have been taken to
ensure that body heat loss is eliminated, more
energy is required to undertake many outdoor
activities in a cold than in a temperate environ-
ment. Brotherhood (1973, 1985) has demon-
strated that walking over ice or snow-covered
terrain increases energy demand compared with
walking at a similar speed over dry ground. In
addition, wearing heavy boots and clothing as
a prevention against hypothermia increases
metabolic demands and substrate utilization
(Campbell 1981, 1982; Romet et al. 1986).
There are many athletic events, such as open
water swimming and mountaineering, where
extreme cold can lead to a fall in body tempera-
ture. In these circumstances, thermoregulatory
mechanisms are invoked to increase body heat
production and consequent substrate utilization.
These include shivering and non-shivering ther-
mogenesis. Shivering, an involuntary rhythmic
contraction of skeletal muscle, is usually invoked
in response to a 3–4°C fall in body temperature
(Webb 1992). This increase in muscle contraction
results in an approximate 2.5-fold increase
in total energy expenditure. More importantly,
498 practical issues
the carbohydrate oxidation rate increases almost
sixfold, while the rise in lipid oxidation is modest
(Vallerand & Jacobs 1989). The rise in carbohy-
drate oxidation is accounted for by increases
in plasma glucose turnover, glycolysis and
glycogenolysis (Vallerand et al. 1995). We have
recently observed that when subjects exercised at
3°C, their pulmonary respiratory exchange ratio
(RER) was higher than during exercise at 20°C
despite contracting muscle glycogenolysis and
lactate accumulation being lower (Febbraio et al.
1996b). This suggests that involuntary activity
associated with shivering in otherwise inactive
muscles contributes to an increase in total body
carbohydrate oxidation during exercise in a cold
environment. Hence, carbohydrate availability
is a critical issue during exercise in climatic
conditions where a shivering response may be
invoked.
Apart from the increase in carbohydrate uti-
lization as a result of shivering, cold exposure
may also increase intramuscular carbohydrate
utilization via an augmented sympathoadrenal
response. Plasma catecholamines are elevated
during exercise in response to cold stress (Galbo
et al. 1979; Young et al. 1986) and exogenous
increases in adrenaline often results in a con-
comitant increase in muscle glycogenolysis
(Jannsonet al. 1986; Spriet et al. 1988; Febbraio et
al. 1998) and liver glucose production (Kjær et al.
1993). Shivering thermogenesis is not an absolute
requirement, therefore, for increases in carbohy-
drate utilization during exercise in a cold
environment.Dietary modifications for exercise in
a cold environment
In circumstances where exercise in a cold
environment attenuates the exercise-induced
increase in body temperature, guidelines for
nutritional intake require little, if any, modifica-
tion from that which is recommended for exer-
cise in comfortable ambient conditions. It is
generally accepted that a glucose/sucrose bever-
age of 6–10% carbohydrate is appropriate for