and ingested has been demonstrated by some
(Lyons et al. 1990; Koenigsberg et al. 1991; Freund
et al. 1995) but not others (Murray et al. 1991) to
increase fluid retention, reduce sweat rate and
consequently result in an enhanced thermoregu-
latory capacity, especially during exercise in a
hot environment (Lyons et al. 1990). Although not
clearly understood, it appears that the effective-
ness of glycerol may be related to an attenuated
rate of free water clearance, and/or an increase in
the kidney’s medullary concentration gradient
resulting in increased glomerular reabsorption
(Freund et al. 1995). On balance, the literature
suggests that glycerol hyperhydration may be
effective prior to exercise in a hot environment.
As sweat rate increases during exercise in the
heat, the potential for electrolyte loss, in particu-
lar sodium, is increased. It has been suggested
that sodium be included in rehydration bever-
ages to replace sweat sodium losses, prevent
hyponatraemia, promote the maintenance of
plasma volume and enhance intestinal absorp-
tion of glucose and fluid (for detailed review, see
Chapter 17). Although the addition of sodium to
a fluid beverage will maintain the drive for
drinking and minimize urinary fluid loss
in recovery from exercise (Nose et al. 1988;
Maughan & Leiper 1995), we have observed little
effect of alterations in beverage sodium content
on glucose or fluid bioavailability during exer-
cise (Hargreaves et al. 1994).
Guidelines for dietary intake when exercising
in the heat
In examining the literature, it is clear that both
carbohydrate and fluid availability are very
important when making dietary recommenda-
tions for those exercising in the heat. The intake
of carbohydrate should be increased with
repeated exercise bouts in the heat because even
though acclimation reduces glycogenolytic rate
(Kinget al. 1985), glycogen use is still higher in
an acclimated individual exercising in the heat
than in an unacclimated individual exercising
in cooler conditions (Febbraio et al. 1994a). In
addition, those individuals who undergo daily
504 practical issues
exercise in hot conditions must pay careful atten-
tion to fluid intake, since heat acclimatization
increases sweat rate (Armstrong & Maresh 1991)
and, hence, body fluid loss. It is important to
note that while a high carbohydrate diet may
exaggerate the core temperature response in rats
(Francesconi & Hubbard 1986), such a diet
does not cause any deleterious thermoregula-
tory responses during exercise in humans
(Schwellnuset al. 1990).
When exercising, one should ingest a carbo-
hydrate/fluid/electrolyte beverage frequently.
Since the relative importance of fluid delivery is
increased during exercise in the heat, one may be
tempted to ingest water in these circumstances.
This practice should be avoided, since the inges-
tion of a carbohydrate/electrolyte/fluid bever-
age empties from the gut at the same rate as
water (Francis 1979; Owen et al. 1986; Ryan et al.
1989), while it can spare muscle glycogen
(Yaspelkis & Ivy 1991), during exercise in the
heat. In addition, the relative importance of elec-
trolyte intake may be increased during exercise
in the heat and thus rehydration beverages
should include electrolytes. The amount of the
carbohydrate within a fluid beverage ingested
during exercise in the heat appears to have little
effect on fluid availability or exercise perfor-
mance, provided the carbohydrate is not too con-
centrated. The change in plasma volume and
exercise performance in the heat is not different
when ingesting beverages containing 0%, 4.2%
and 7% carbohydrate, respectively. Of note,
however, when a 14% carbohydrate solution is
ingested during exercise in the heat, the mainte-
nance of plasma volume is reduced while the rise
in rectal temperature tends to be augmented.
Accordingly, exercise performance tends to fall
(Fig. 38.4) (Febbraio et al. 1996a). It is important,
therefore, to keep the concentration of carbohy-
drate within a fluid beverage to approximately
10% during exercise in the heat, even though car-
bohydrate utilization is augmented in these cir-
cumstances. In terms of volume and frequency, a
practical recommendation might be 400 ml every
15 min since the rate of fluid loss during exercise
in the heat is approximately 1.6 l · h–1 (M.