NUTRITION IN SPORT

stress, in horses exercising in the heat. In
addition, when examining the ratio between
adenosine diphosphate (ADP) production and
mitochondrial oxygen consumption (ADP/O
ratio) in isolated rat skeletal muscle mitochon-
dria, Brooks et al. (1971) observed a constant
ADP/O ratio at temperatures ranging from 25 to
40°C. Above 40°C, however, the ADP/O ratio
declined linearly with an increase in tempera-
ture, suggesting that for a given oxygen con-
sumption the increase in ADP rephosphorylation
was lower than the rate of ATP degradation.
Interestingly, in our previous studies in which
we observed increased phosphocreatine degra-
dation and IMP formation (Febbraio et al. 1994b;
Parkinet al. 1999), intramuscular temperature
was greater than 40°C following exercise in the
hot environment but not the control trial. The
data indicate, therefore, that the combination of
exercise and heat stress may affect mitochondrial
function resulting in oxyradical formation.
Although speculative, antioxidant supplementa-
tion may be of benefit during exercise in the
heat and we are currently examining such a
phenomenon.

Benefit of fluid ingestion

Although a more comprehensive review of fluid
ingestion is covered in previous chapters of this
book (see Chapters 15–17), it is necessary to reit-
erate the importance of fluid when discussing
nutrition for exercise in climatic extremes. In cir-
cumstances where the endogenous heat produc-
tion and high environmental temperature result
in fatigue prior to carbohydrate stores being
compromised, fluid ingestion, irrespective of
whether it contains carbohydrate, is of major
importance in delaying the rise in body core tem-
perature. Exercise-induced dehydration is asso-
ciated with an increase in core temperature
(Hamiltonet al. 1991; Montain & Coyle 1992),
reduced cardiovascular function (Hamilton et al.
1991; Montain & Coyle 1992) and impaired exer-
cise performance (Walsh et al. 1994). These dele-
terious physiological effects are attenuated, if not
prevented, by fluid ingestion (Costill et al. 1970;

Candaset al. 1986; Hamilton et al. 1991; Montain

& Coyle 1992), which also improves exercise per-

formance (Maughan et al. 1989; Walsh et al. 1994;

McConellet al. 1997). In addition to the physio-

logical alterations caused by dehydration, we

have also observed that fluid ingestion reduces

muscle glycogen use during prolonged exercise

(Fig. 38.3), since it also results in a reduced intra-

muscular temperature and a blunted sympa-

thoadrenal response (Hargreaves et al. 1996b). It

is clear from these data that fluid ingestion not

only attenuates the rise in body core tempera-

ture, thereby preventing hyperthermia, it also

reduces the likelihood of carbohydrate deple-

tion. Since sweat rate is exacerbated during exer-

cise in the heat, dehydration progresses more

rapidly and therefore the importance of fluid

ingestion is increased during exercise in extreme

heat. Indeed, Below et al. (1995) have demon-

strated that fluid ingestion improves exercise

performance in a hot environment.

Since the negative effects of dehydration are

well documented, it would be desirable to hyper-

hydrate prior to exercise in a hot environment.

Accordingly, glycerol added to a bolus of water

exercise at climatic extremes 503

∆GLY (mmol

.kg

–1 dry wt)

500

0

300

200

No fluid

100

Fluid ingestion

400

*

Fig. 38.3Net muscle glycogen utilization (GLY;

postexercise minus pre-exercise) during 120 min of

exercise in the absence or presence of fluid ingestion.

*, difference (P<0.05) compared with no fluid. Data

expressed as mean ±SE (n=5). From Hargreaves et al.

(1996b), with permission.