Exercise at Climatic Extremes

Chapter 38
Exercise at Climatic Extremes
MARK A. FEBBRAIO
Introduction
The relationship between nutrition and exercise
has been a major scientific interest area for over
150 years. With the popularization of the muscle
biopsy technique, arteriovenous (a-v) balance
measurements and, more recently, the use of
isotope tracers as metabolic probes during exercise, it has become possible to clearly investigate
the role of nutrition in exercise physiology and
biochemistry. Accordingly, growth in this area
has increased exponentially. Much of the
research which has examined the interaction
between nutrition and exercise has been conducted in comfortable ambient conditions. It is
clear, however, that environmental temperature
is a major practical issue one must consider when
examining nutrition and sport. In extremely
low ambient temperatures, when the gradient
between the skin and surrounding environment
is high, the rate of endogenous heat production,
even during exercise, may be insufficient to offset
body heat loss. In these circumstances, responses
are invoked to reduce heat loss and increase heat
production. In contrast, when exercise is conducted in very high ambient temperatures, the
gradient for heat dissipation is significantly
reduced, which results in changes to thermoregulatory mechanisms designed to promote body
heat loss. In both climatic extremes, these physiological adaptations ultimately impact upon hormonal and metabolic responses to exercise which
act to alter substrate utilization. Hence, environmental temperature is an important factor to
consider when determining optimal nutritional
strategies for exercise performance.
Exercise in a cold environment
Cold stress or attenuated
exercise-induced hyperthermia?
Unlike heat, which can only serve to augment the
exercise-induced increase in body temperature, a
cold environment may invoke varied physiological responses during exercise. These responses
depend on whether the interaction between the
environment and the exercising organism promotes excessive heat loss or attenuates the
normal rise in body core temperature associated
with exercise. Most studies which have observed
relative hypothermia during exercise have done
so using swimming as the mode of exercise
(Holmer & Bergh 1974; Galbo et al. 1979; Doubt
& Hsieh 1991), since water is a much greater
thermal conductant than air. In contrast, when
exercise has been conducted in cold air environments ranging from 3 to 9°C, an attenuated rise,
rather than a fall in body core temperature, has
been observed (Jacobs et al. 1985; Febbraio et al.
1996a, 1996b). The severity of the ‘cold stress’ is
an important consideration when examining
nutritional requirements since a fall in body temperature will result in shivering thermogenesis
(Webb 1992) and an enhanced sympathoadrenal
response (Galbo et al. 1979), while an attenuated
rise in body temperature blunts the exerciseinduced increase in adrenaline secretion
497
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practical issues
(Febbraio et 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 environment, the rate of glycogen utilization in contracting muscle is reduced (Kozlowski et al. 1985;
Febbraio et al. 1996b; Parkin et al. 1999) and exercise performance is increased (Hessemer et al.
1984; Febbraio et al. 1996a; Parkin et al. 1999),
which is not surprising, since fatigue during prolonged exercise often coincides with glycogen
depletion (Coggan & Coyle 1991). In many circumstances, therefore, the cool environment may
be viewed as an ‘ergogenic aid’, since it results in
a conservation of finite endogenous carbohydrate stores within contracting muscles. It must
be noted, however, that even in some circumstances 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 environment. Brotherhood (1973, 1985) has demonstrated 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 temperature. In these circumstances, thermoregulatory
mechanisms are invoked to increase body heat
production and consequent substrate utilization.
These include shivering and non-shivering thermogenesis. 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,
the carbohydrate oxidation rate increases almost
sixfold, while the rise in lipid oxidation is modest
(Vallerand & Jacobs 1989). The rise in carbohydrate 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 utilization 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 concomitant increase in muscle glycogenolysis
(Jannson et 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 carbohydrate 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, modification from that which is recommended for exercise in comfortable ambient conditions. It is
generally accepted that a glucose/sucrose beverage of 6–10% carbohydrate is appropriate for
exercise at climatic extremes
ingestion during exercise (Costill & Hargreaves
1992), since this would provide necessary
glucose while allowing for optimal gastric emptying and intestinal absorption (Mitchell et al.
1989; Rehrer et al. 1989; Gisolfi et al. 1991). It has
been suggested, however, that increasing the
carbohydrate content of a fluid beverage may be
beneficial during exercise in cooler conditions
since the requirement for optimal fluid delivery
may be less important, due to the reduction in
thermoregulatory stress, while the necessity for
sufficient circulating glucose levels is maintained. We have recently tested this hypothesis
and found that increasing the carbohydrate
content of a fluid beverage which is ingested
during exercise in a cool environment is not
advantageous (Fig. 38.1). While such a practice
does elevate blood glucose levels, it results in
increased gastrointestinal discomfort, a less than
efficient maintenance of plasma volume and a
reduction in exercise performance relative to the
ingestion of a 7% carbohydrate beverage (Febbraio et al. 1996a). Therefore, when the endogenous heat produced by exercise in cool ambient
conditions is sufficient to offset body heat loss,
feeding strategies recommended during exercise
in comfortable ambient temperatures should be
adhered to.
During exercise in extremely cold environments which results in a fall in body core temperature, any dietary modification which results in
an increase in whole-body metabolic rate, which
would generate warmer body temperatures and
improve cold tolerance, would be most beneficial. As a result, recent research has focused on
administration of many ergogenic aids designed
to increase thermotolerance during cold stress.
These ergogenic aids include hormones, pharmacological agents and nutrients. Administration of the pharmacological agent dinitrophenol
(Hall et al. 1948) and hormones such as thyroxin,
catecholamines, cortisol and growth hormone
(Sellers 1972; Le Blanc 1975) in cold exposed
animals results in a delay in the onset of
hypothermia. However, while these studies
provide useful information regarding the mechanisms for the induction of thermogenesis, it is
impractical to suggest that they be taken by
humans as ergogenic aids during exercise and
cold stress because of the obvious health risks.
It is possible, that ingestion of b-adrenergic
agonists such as caffeine, ephedrine or theophylline may improve cold tolerance, although
the literature which has examined such a phenomenon has produced conflicting results.
Ingesting the combination of ephedrine and caf-
10
*
*
*
6
† *†
*†
4
*†
250
†
5
0
–5
–10
*†
200
†
Time (min)
8
Change in PV (%)
Glucose (mmol.l–1)
10
499
†
150
100
50
*†
2
(a)
–15
0
50 100 150 200 250
Time (min)
0
(b)
50
100 150 200 250
Time (min)
0
(c)
CON
LCHO
HCHO
Trial
Fig. 38.1 (a) Plasma glucose, (b) change in plasma volume (PV), and (c) time to exhaustion while consuming a
placebo (CON, 䊏),
. 7% carbohydrate (LCHO, 䊊) or 14% carbohydrate (HCHO, 䊉), beverage during fatiguing
exercise at 70% Vo2max. in 5°C conditions. *, difference (P < 0.05) compared with CON; †, difference (P < 0.05)
compared with HCHO. Data expressed as means ± SE (n = 6). From Febbraio et al. (1996a).
500
practical issues
feine (Vallerand et al. 1989) or ephedrine, caffeine
and theophylline (Vallerand et al. 1993) results in
a significant increase in heat production in coldexposed 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 ingestion of b-adrenergic agonists may provide some
means of enhancing thermoregulatory thermogenesis, although further work in this area is
required to confirm this theory. In addition, since
b-adrenergic agonists such as ephedrine and caffeine 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 utilized in shivering thermogenesis, it has been suggested 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 contraction during exercise, they possibly allow for a
better maintenance of body core temperature,
especially in leaner athletes.
In summary, during exercise in a cold environment, effort should be made to ensure that
pre-exercise carbohydrate stores are adequate in
order to offset the potential increase in carbohydrate 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 beverage 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 theophylline may provide some benefit against acute
cold exposure, but further work examining this
phenomenon is required.
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 towards 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 exercise 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 production, 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 differences in pre-exercise glycogen concentrations
(Nielsen et al. 1990; Young et al. 1996) when comparing 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 glycogen concentration is directly related to rates of
utilization during submaximal exercise (Chesley
et al. 1995; Hargreaves et al. 1995). In general, the
exercise at climatic extremes
literature suggests that exercise and heat stress
results in a shift towards increased carbohydrate
catabolism.
The increase in carbohydrate oxidation indicates 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 concentration (Fink et al. 1975; Nielsen et al. 1990) and
uptake (Nielsen et al. 1990) are similar when comparing 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
Fink et al. (1975) also observed a decreased intramuscular 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 demonstrated that branched-chain amino acid (BCAA)
supplementation increased endurance performance during exercise in the heat. This finding is
in contrast with studies conducted during exercise in cooler environments (van Hall et al. 1995;
Madsen et al. 1996). This discrepancy could arise
because protein catabolism may be augmented
during exercise in the heat. We have observed an
increase in ammonia (NH3) accumulation during
exercise and heat stress (Snow et al. 1993;
Febbraio et al. 1994b). Although a major pathway
for NH3 production during exercise is via the
deamination of adenosine 5¢-monophosphate to
form NH3 and inosine 5¢-monophosphate (IMP),
NH3 can also be formed in skeletal muscle via the
oxidation of BCAA. Accordingly, BCAA supplementation augments muscle NH3 production
during exercise (MacLean et al. 1996). During our
study (Febbraio et al. 1994b), the augmented
muscle NH3 accumulation when comparing
exercise in the heat with that in a cooler environment was observed in the absence of any difference 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-
501
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 utilization is closely matched by rates of energy provision. 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 exogenous 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 accumulation (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–1 dry weight at fatigue when, during exercise
in cooler environments, this figure is usually less
than 150 mmol · kg–1 dry weight (Fig. 38.2). This
may be because hyperthermia may lead to
fatigue prior to carbohydrate stores being compromised. 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
502
practical issues
500
1.25
400
1.0
*
IMP (mmol.kg–1)
Glycogen (mmol.kg–1)
*
300
200
100
0.75
0.5
0.25
0
Rest
0
Fatigue
(a)
Rest
Fatigue
(b)
Fig. 38.2 (a) Glycogen content and (b) inosine 5¢-monophosphate (IMP) concentration before (rest) and after
(fatigue) submaximal exercise to exhaustion in different ambient temperatures: 䊏, 40 °C; , 20 °C; 䊐, 3 °C. Data
expressed as mean ± SE (n = 8). From Parkin et al. (1999), with permission.
intensity of exercise is moderate, resulting in a
relatively low rate of endogenous heat production, or the exercise is intermittent in nature
allowing for effective heat dissipation, carbohydrate may be limiting. Accordingly, carbohydrate ingestion may (Murray et al. 1987; Davis et
al. 1988b; Millard-Stafford et al. 1992) or may not
(Davis et al. 1988a; Millard-Stafford et al. 1990;
Febbraio et al. 1996a) increase exercise performance in the heat. The benefit of carbohydrate
ingestion during and following exercise in the
heat may, however, be related to factors other
than exercise performance. Immune function has
been demonstrated to be depressed by increases
in stress hormones such as catecholamines, corticosteroids and growth hormone (Keast et al.
1988). These hormones are elevated when comparing exercise in the heat with that in a cooler
environment (Febbraio et al. 1994a; Hargreaves
et al. 1996a). There may be, therefore, a possible
relationship between exercise in a hot environment and immune suppression. Indeed, it has
been demonstrated that exercise and heat stress
results in a decrease in lymphocyte production
(Cross et al. 1996). Carbohydrate feeding during
exercise in comfortable ambient conditions
results in a decrease in circulating adrenaline
(McConell et al. 1994), cortisol (Mitchell et al.
1990) and growth hormone (Smith et al. 1996). In
addition, plasma elastase, a marker of in vivo
neutrophil activation, is reduced during exercise
with carbohydrate feedings (Smith et al. 1996). It
is possible, therefore, that carbohydrate ingestion during and following exercise in the heat
may attenuate the rise in the counterregulatory
hormones which depress immune function, and
we are currently undertaking experiments to
examine this hypothesis.
As mentioned previously, glycogen content
within human skeletal muscle at the point of
fatigue during exercise in the heat is often adequate to maintain energy turnover via oxidative
phosphorylation. It is somewhat surprising,
therefore, that a marked increase in IMP accumulation at fatigue during exercise and heat stress is
observed despite glycogen concentration being
adequate to maintain the oxidative potential
of the contracting skeletal muscle (Fig. 38.2)
(Parkin et al. 1999).
These data suggest a disruption to mitochondrial function during exercise and heat stress and
support recent findings by Mills et al. (1996), who
observed an increase in plasma concentrations
of lipid hydroperoxides, a marker of oxidative
exercise at climatic extremes
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 mitochondria, 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 temperature, suggesting that for a given oxygen consumption the increase in ADP rephosphorylation
was lower than the rate of ATP degradation.
Interestingly, in our previous studies in which
we observed increased phosphocreatine degradation and IMP formation (Febbraio et al. 1994b;
Parkin et 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 supplementation may be of benefit during exercise in the
heat and we are currently examining such a
phenomenon.
Candas et al. 1986; Hamilton et al. 1991; Montain
& Coyle 1992), which also improves exercise performance (Maughan et al. 1989; Walsh et al. 1994;
McConell et al. 1997). In addition to the physiological 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 intramuscular temperature and a blunted sympathoadrenal response (Hargreaves et al. 1996b). It
is clear from these data that fluid ingestion not
only attenuates the rise in body core temperature, thereby preventing hyperthermia, it also
reduces the likelihood of carbohydrate depletion. Since sweat rate is exacerbated during exercise 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 demonstrated that fluid ingestion improves exercise
performance in a hot environment.
Since the negative effects of dehydration are
well documented, it would be desirable to hyperhydrate prior to exercise in a hot environment.
Accordingly, glycerol added to a bolus of water
Benefit of fluid ingestion
500
400
∆ GLY (mmol.kg–1 dry wt)
Although a more comprehensive review of fluid
ingestion is covered in previous chapters of this
book (see Chapters 15–17), it is necessary to reiterate the importance of fluid when discussing
nutrition for exercise in climatic extremes. In circumstances where the endogenous heat production 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 temperature. Exercise-induced dehydration is associated with an increase in core temperature
(Hamilton et al. 1991; Montain & Coyle 1992),
reduced cardiovascular function (Hamilton et al.
1991; Montain & Coyle 1992) and impaired exercise performance (Walsh et al. 1994). These deleterious physiological effects are attenuated, if not
prevented, by fluid ingestion (Costill et al. 1970;
503
*
300
200
100
0
No fluid
Fluid ingestion
Fig. 38.3 Net 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.
504
practical issues
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 thermoregulatory capacity, especially during exercise in a
hot environment (Lyons et al. 1990). Although not
clearly understood, it appears that the effectiveness 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 particular sodium, is increased. It has been suggested
that sodium be included in rehydration beverages to replace sweat sodium losses, prevent
hyponatraemia, promote the maintenance of
plasma volume and enhance intestinal absorption 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 exercise (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 recommendations 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
(King et 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
exercise in hot conditions must pay careful attention 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 thermoregulatory responses during exercise in humans
(Schwellnus et al. 1990).
When exercising, one should ingest a carbohydrate/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 ingestion of a carbohydrate/electrolyte/fluid beverage 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 electrolyte 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 performance, provided the carbohydrate is not too concentrated. 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 maintenance 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 carbohydrate within a fluid beverage to approximately
10% during exercise in the heat, even though carbohydrate utilization is augmented in these circumstances. 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.
exercise at climatic extremes
–5
–10
–15
*
–20
0
(a)
120
39
100
38.5
38
37.5
37
36.5
20 40 60 80 100 120
Time (min)
39.5
Exercise time (min)
0
Rectal temperature ( C)
Change in PV (%)
5
0
20 40 60
(b)
80 100 120
Time (min)
505
80
60
40
20
0
(c)
CON 4.2% 7%
CHO CHO
14%
CHO
Fig. 38.4 The change in (a) plasma volume (PV), (b) rectal temperature, and (c) time to exhaustion, while
consuming a placebo (CON, 䊏) or carbohydrate (CHO) beverage
of differing concentrations: 4.2% CHO (䊉), 7%
.
CHO (䉭) or 14% CHO (䉮) during fatiguing exercise at 70% Vo2max. in 33°C conditions. *, difference (P < 0.05) from
other trials. Data expressed as mean ± SE. Data from Febbraio et al. (1996a).
Febbraio, unpublished observations). It is also
recommended that the carbohydrate beverage
be ingested into recovery to replenish intramuscular glycogen stores and promote rehydration,
especially important for those individuals
repeatedly exercising in a hot environment.
As previously discussed, there is some evidence to suggest that protein catabolism is
increased during exercise in the heat. One may be
tempted to recommend that protein intake be
increased prior to and during such exercise.
However, it must be noted that there is a relative
paucity of research examining protein requirements during exercise in the heat and more is
required before definitive recommendations
can be made. Likewise, there is some evidence
to suggest that oxyradical generation may be
increased via the combination of exercise and
heat stress and it may be of some benefit to supplement those undertaking repeated exercise
in a hot environment with antioxidants such
as a-tocopherol (vitamin E) and ascorbic acid
(vitamin C). This recommendation is speculative, however, since the hypothesis that such
supplementation is advantageous during
exercise in the heat is yet to be experimentally
investigated.
The deleterious effects of dehydration during
exercise, especially that which is conducted in a
hot environment, have been well documented.
It would be desirable to hyperhydrate before
exercise and, as has been demonstrated, glycerol
ingestion may provide some benefit in achieving
hyperhydration by attenuating urine output. It
appears that a regime consisting of 1 g glycerol ·
kg–1 body weight in approximately 2 l of fluid
ingested in the 120 min prior to exercise provides
some benefit during subsequent exercise (Lyons
et al. 1990). Competitive athletes should, however, experiment with this regime during training, since not all individuals may respond
favourably to glycerol hyperhydration.
In summary, during exercise in the heat, a
balance between preventing hyperthermia and
maintaining adequate fuel supply to fuel muscle
contraction must be maintained. In order to
achieve this, athletes need to closely monitor hydration levels and carbohydrate intake
leading up to exercise. Daily monitoring of body
weight and ensuring that urine is pallid will
provide a guide to hydration status. During competition, a 4–8% carbohydrate/fluid/electrolyte
solution should be ingested at approximately
400 ml every 15 min and such ingestion should be
maintained during recovery to ensure fluid and
energy replacement. Other dietary modifications
such as increased protein intake, antioxidant
supplementation and glycerol hyperhydration
may provide some benefit but further research
in these areas is required before definitive
506
practical issues
recommendations can be made regarding their
efficacy.
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