Rehydration and Recovery after Exercise

Chapter 19
Rehydration and Recovery after Exercise
SUSAN M. SHIRREFFS
Introduction
Exercise can lead to a depletion of the body’s
glycogen stores, particularly those of the liver
and in the exercising muscle, and to the development of a body water deficit. This chapter
concerns itself with recovery after exercise: carbohydrate replacement during exercise is discussed in Chapter 8, and fluid replacement
during exercise is described in detail in Chapter
17.
Glycogen depletion results from the mobilization of the stores to provide energy for the
muscular contraction of the exercise, and is a
major factor contributing to fatigue. This has
been discussed in detail in Chapter 6. Depending
on the intensity, frequency and duration of the
exercise sessions, an almost complete emptying
of the glycogen stores in the exercising muscle is
possible.
Dehydration during exercise results largely
from activation of the body’s temperatureregulating mechanisms, and a state of hypohydration will be incurred if fluid is not ingested to
match the seat loss. In an attempt to dissipate the
heat produced due to the mechanical inefficiency
of exercise, the sweating mechanism may be activated and the subsequent evaporation of the
water secreted onto the skin surface removes
with it latent heat of evaporation. This has been
discussed in more detail in Chapters 15 and 17.
Water loss from the respiratory tract, from the
gastrointestinal tract and from urine production
all will add to the body’s water loss. Each of these
256
routes may result in substantial water losses in
some situations, but for most individuals and in
most exercise situations, sweat production will
be the greatest single factor responsible for creating a situation of hypohydration (Chapter 17).
Sweat production, however, is not a situation of
pure water being secreted onto the skin, but
rather a variety of electrolytes and other substances are included in the sweat that is secreted.
A general description of the composition of
sweat with regard to its electrolyte content is
given in Chapter 17.
Effects of muscle glycogen depletion
and hypohydration
Muscle glycogen depletion
If exercise is undertaken when the muscles are
depleted of their glycogen stores, performance
will be poorer than when the muscle glycogen
stores are optimal. This has been shown to be
true for prolonged exercise of 1–2 h duration
(Costill et al. 1988), for high-intensity exercise
lasting only a few minutes (Maughan & Poole
1981), and will also result in a reduction in the
amount of running done by games players
(Jacobs et al. 1982; Bangsbo 1994). In most of these
situations, performance will be closely related to
the size of the glycogen stores at the beginning of
exercise. This has been discussed in detail in
Chapters 5–8.
rehydration and recovery after exercise
Hypohydration
Exercise undertaken by individuals who begin
exercise in a hypohydrated state has been shown
to be impaired relative to that possible when
fully hydrated at the beginning of exercise (see
Chapter 16). However, in addition to these
adverse effects on performance, hypohydration
increases the likelihood of heat illness, and exercise in this state is only likely to accelerate and
exacerbate these effects (Sutton 1990).
Postexercise carbohydrate
replacement
Many of the issues relating to carbohydrate
replacement during exercise are relevant to
carbohydrate replacement after exercise and a
full discussion of this topic can be found in
Chapter 8.
The primary aim of carbohydrate ingestion
following exercise is to promote glycogen resynthesis and restoration of the muscle and liver
glycogen utilized during exercise. This is of
particular importance when a further bout
of exercise is to be undertaken and therefore is of
significance to all athletes in training and in competition where more than one game or round is
involved. Several factors will influence the rate at
which glycogen resynthesis occurs after exercise.
The most important factor is undoubtedly the
amount of carbohydrate consumed: the type of
carbohydrate and the time of ingestion are less
important, but also have an effect.
Amount of carbohydrate to be ingested
The general pattern for glycogen synthesis after
exercise is one of an increasing rate with increasing amount of carbohydrate consumed up to a
certain rate of resynthesis after which there is no
further increase with increasing quantities of
carbohydrate ingestion. This has been demonstrated in studies where subjects were fed different amounts of glucose or maltodextrins every
2 h after exercise (Blom et al. 1987; Ivy et al. 1988a).
The results showed that muscle glycogen synthe-
257
sis occurred at a rate of 2 mmol · kg–1 · h–1 when
25 g of carbohydrate was ingested every 2 h, and
that the replenishment rate increased to 6 mmol ·
kg–1 · h–1 when 50 g was ingested every 2 h.
However, muscle glycogen synthesis did
increase to more than about 5–6 mmol · kg–1 · h–1
even when very large amounts (up to 225 g) of
carbohydrate were ingested every 2 h.
Further, with intravenous glucose infusion of
100 g every 2 h, a muscle glycogen synthesis
of about 7–8 mmol · kg–1 · h–1 has been reported
(Reed et al. 1989). This is not significantly greater
than the rates achieved with oral intake, and
suggests that the failure to keep increasing glycogen synthesis with increasing carbohydrate
consumption is not caused by a limitation in
substrate availability imposed by the gastrointestinal tract. Also, increasing the amount of
carbohydrate ingested will increase the rate of
delivery to the intestine for absorption (see
Chapter 18).
Therefore, it seems that the maximum rate of
muscle glycogen synthesis after exercise is in the
region of 5–8 mmol · kg–1 · h–1, provided that at
least 50 g of glucose is ingested every 2 h after
exercise.
Carbohydrate type and form of ingestion
Glucose and sucrose ingestion both give rise to
similar glycogen synthesis rates when consumed
after exercise. Fructose alone, however, seems
only to be able to promote glycogen synthesis
after exercise at a much lower rate of approximately 3 mmol · kg–1 · h–1 (Jenkins et al. 1984; Blom
et al. 1987). This is likely to be because of the relatively slow rate with which the liver converts
fructose to blood glucose, and even when fructose is consumed in large amounts, the entry of
glucose into the blood does not reach a rate of
50 g every 2 h. Although the use of fructose as a
carbohydrate source is often promoted for athletes, it is poorly absorbed in the small intestine
relative to many other sugars, and ingestion of
large amounts is likely to result in diarrhoea
(Maughan et al. 1989).
There is some evidence that carbohydrates
258
nutrition and exercise
with a high glycaemic index — those carbohydrates which result in a large and sustained elevation of the blood glucose concentration after
ingestion — are the most effective when rapid
glycogen replacement is desired (Coyle 1991).
However, the nature in which carbohydrates
with a high or moderate glycaemic index are consumed after exercise (i.e. as a solid or liquid)
appears to have no influence on glycogen synthesis rates (Keizer et al. 1986; Reed et al. 1989).
advantage to be taken by allowing the increased
rate to be utilized for as long as possible. As a
guide, it is suggested that approximately 0.7 g
glucose · kg–1 body mass should be consumed
every 2 h for the first 4–6 h after exercise in order
to maximize the rate of glycogen resynthesis
(Keizer et al. 1986; Blom et al. 1987). It does not
make any difference whether this carbohydrate
to be consumed is ingested as a few large meals
or as many small, frequent meals (Burke et al.
1996).
Timing of carbohydrate intake
Glycogen synthesis (µmol.g–1 wet wt)
The muscle appears to have a particularly high
affinity for carbohydrate immediately after exercise, and the greatest rate of muscle glycogen
resynthesis occurs over the first 2 h immediately
after exercise (i.e. 7–8 mmol · kg–1 · h–1 vs. the rate
after this time of 5–6 mmol · kg–1 · h–1: Fig. 19.1)
(Ivy et al. 1998b). This increased synthesis rate
can only take place, however, if sufficient carbohydrate is ingested and is available to the body.
Therefore, to optimize this transient increase
in maximal glycogen resynthesis rate, carbohydrate should be consumed as soon as possible
after exercise as this will allow the maximum
20
15
Liver glycogen resynthesis
Liver glycogen restoration occurs less rapidly
than muscle glycogen restoration and indeed,
the fast repletion of muscle glycogen stores may
be at the expense of liver glycogen levels (Fell
et al. 1980). However, whereas fructose does not
promote as rapid a muscle glycogen restoration
as glucose, fructose infusion has been found to
give a greater liver glycogen resynthesis than
glucose (Nilsson & Hultman 1974). Some replenishment of the liver glycogen stores may be possible by gluconeogenesis, but this will not be
sufficient to maintain carbohydrate homeostasis.
After very high intensity exercise, however, such
as multiple sprints in training, a substantial part
of the muscle glycogen that has been converted
to lactate by anaerobic glycolysis will be available as a substrate for hepatic gluconeogenesis.
Postexercise fluid replacement
10
5
0
0–120
120–240
Time after exercise (min)
Fig. 19.1 Muscle glycogen storage during the first 2 h
and second 2 h of recovery from exercise. The subjects
consumed 2 g glucose polymer · kg–1 body mass (as a
23% solution) either immediately following exercise
( ) or 2 h after exercise (䊏). Adapted from Ivy et al.
(1988b).
It has been pointed out elsewhere in this volume
that the athlete who begins exercise in a state of
hypohydration will be unable to achieve peak
performance and will also be at increased risk of
heat illness when the exercise is to be performed
in a warm environment. Where substantial sweat
losses have been incurred, it is therefore essential
that restoration of fluid and electrolyte balance
should be as rapid and complete as the circumstances allow. The opportunities for replacement
may be limited, as when several rounds of a tournament are scheduled for a single day, or when
the time allowed between the weigh-in and com-
rehydration and recovery after exercise
petition is short in weight category sports where
sweating and fluid restriction have been used to
achieve an artificially low body mass.
The primary factors influencing the postexercise rehydration process are the volume and
composition of the fluid consumed. The volume
consumed will be influenced by many factors,
including the palatability of the drink and its
effects on the thirst mechanism, and many different formulation options are open. The ingestion
of solid food, and the composition of that food,
will also be an important factor, but there are
many situations where solid food is avoided
between exercise sessions or immediately after
exercise.
Beverage composition
It is well established that plain water consumed
after exercise is not the ideal rehydration beverage when rapid and complete restoration of fluid
balance is necessary and where all intake is in
liquid form. Costill and Sparks (1973) demonstrated that ingestion of plain water after
exercise-induced dehydration caused a large fall
in serum osmolality with a subsequent diuresis:
the result of this stimulation of urinary water loss
was a failure to achieve positive fluid balance by
the end of the 4-h study period. However, when
an electrolyte-containing solution (106 g · l–1 carbohydrate, 22 mmol · l–1 Na+, 2.6 mmol · l–1 K+,
17.2 mmol · l–1 Cl–) was ingested after exercise
which caused a loss of 4% of body mass, the urine
output was less and net water balance was closer
to the pre-exercise level. Nielsen et al. (1986)
showed differences in the rate and extent of
changes in the plasma volume with recovery
from exercise-induced dehydration when different carbohydrate-electrolyte solutions were consumed: the plasma volume increase was greater
after drinks with sodium as the only electrolyte
(at concentrations of 43 and 128 mmol · l–1) were
consumed than when drinks containing additional potassium (at a concentration of 51 mmol ·
l–1) or less electrolytes and more carbohydrate
were consumed. González-Alonso et al. (1992)
have also confirmed that a dilute carbohydrate-
259
electrolyte solution (60 g · l–1 carbohydrate,
20 mmol · l–1 Na+, 3 mmol · l–1 K+, 20 mmol · l–1 Cl–)
is more effective in promoting postexercise rehydration than either plain water or a lowelectrolyte diet cola: the difference in rehydration
effectiveness between the drinks was a result of
differences in the volume of urine produced. In
none of these studies, however, could the mechanism of the action be identified, as the drinks
used were different from each other in a number
of respects. They did, however, establish that,
because of the high urine flow that ensued, even
drinking large volumes of electrolyte-free drinks
did not allow subjects to remain in positive
fluid balance for more than a very short time.
They also established that the plasma volume
was better maintained when electrolytes were
present in the fluid ingested, and it seemed likely
that this effect was due primarily to the presence
of sodium in the drinks.
The first studies to investigate the mechanisms
that might be involved showed that the ingestion
of large volumes of plain water after exerciseinduced dehydration results in a rapid fall in
plasma osmolality and in the plasma sodium
concentration (Nose et al. 1988a, 1988b, 1988c),
and both of these effects will stimulate urine
output. In these studies, subjects exercised at low
intensity in the heat for 90–110 min, inducing a
mean level of dehydration equivalent to 2.3% of
the pre-exercise body mass, and then rested for
1 h before beginning to drink. Plasma volume
was not restored until after 60 min when plain
water was ingested together with placebo
(sucrose) capsules. In contrast, when sodium
chloride capsules were ingested with water to
give a saline solution with an effective concentration of 0.45% (77 mmol · l–1), restoration of plasma
volume was complete within 20 min. In the NaCl
trial, voluntary fluid intake was higher and urine
output was less; 29% of the water intake was lost
as urine within 3 h compared with 49% in the
plain water trial. The delayed rehydration in the
water trial was a result of a loss of water as urine
caused by a rapid return to control levels of
plasma renin activity and aldosterone levels.
Therefore, the addition of sodium to rehydra-
260
nutrition and exercise
tion beverages can be justified on two accounts.
Firstly, sodium stimulates glucose absorption in
the small intestine (Olsen & Ingelfinger 1968):
water absorption from the intestinal lumen is a
purely passive process that is determined largely
by local osmotic gradients (Parsons & Wingate
1961). The active cotransport of glucose and
sodium creates an osmotic gradient that acts to
promote net water absorption (Sladen 1972),
and the rate of rehydration is therefore greater
when glucose–sodium chloride solutions are
consumed than when plain water is ingested.
This was discussed in detail in Chapter 18. Secondly, replacement of sweat losses with plain
water will, if the volume ingested is sufficiently
large, lead to haemodilution: the fall in plasma
osmolality and sodium concentration that occurs
in this situation will reduce the drive to drink
and will stimulate urine output (Nose et al.
1988b) and has potentially more serious consequences such as hyponatraemia (Noakes et al.
1985).
It has been proposed that drinks used for postexercise rehydration should have a sodium concentration similar to that of sweat (Maughan
1991), but as the electrolyte content of sweat itself
shows considerable variation between individuals and over time (see Chapter 17), it would seem
impossible to prescribe a single formulation for
every individual or every situation. However, a
study to investigate the relation between wholebody sweat sodium losses and the rehydration
effectiveness of beverages with different sodium
concentrations seems to confirm that optimum
rehydration is achieved with a drink with a
sodium concentration similar to that of sweat
(Shirreffs & Maughan 1997b).
Sodium is the major ion in the extracellular
fluid but potassium is the major ion in the intracellular fluid (see Table 17.1). It has been suggested therefore that potassium may also be to
some degree important in achieving rehydration
by aiding the retention of water in the intracellular space. Yawata (1990) undertook experimental
work on rats subjected to thermal dehydration of
approximately 9% of body mass and then given
free access to either tap water, a 150 mmol · l–1
NaCl solution or a 154 mmol · l–1 KCl solution.
The results indicated that despite ingestion of a
smaller volume of the KCl solution compared
to the NaCl solution, there was a tendency for
a greater restoration of the intracellular fluid
space in the KCl group than in the NaCl group.
Maughan et al. (1994) undertook a study in
which men were dehydrated by approximately 2% of body mass by exercising in the
heat, and then ingested a glucose beverage
(90 mmol · l–1), a sodium-containing beverage
(NaCl 60 mmol · l–1), a potassium-containing
beverage (KCl 25 mmol · l–1) or a beverage consisting of the addition of all three. A smaller
volume of urine was excreted following rehydration when the electrolyte-containing beverages
were ingested than when the electrolyte-free
beverage was consumed (Fig. 19.2). An estimated plasma volume decrease of 4.4% was
observed with dehydration over all trials but the
rate of recovery was slowest when the KCl beverage was consumed. Although there were differences in the total amount of electrolyte replaced
as well as differences in the type of electrolytes
present in the drinks, there was no difference in
the fraction of ingested fluid retained 6 h after
finishing drinking the drinks which contained
electrolytes. This may be because the beverage
volume consumed was equivalent to the volume
of sweat lost and subjects were dehydrated,
because of the ongoing urine losses, throughout
the entire study, even following the drinking
period. The volumes of urine excreted were close
to basal levels and significant further reductions
in output may not have been possible when both
sodium and potassium were ingested, over and
above the reductions already induced when the
sodium and potassium were ingested separately.
The importance of potassium in enhancing rehydration by aiding intracellular rehydration over
and above that with sodium seems therefore to
be realistic but further investigation is required
to provide conclusive evidence.
Drink volume
Obligatory urine losses persist even in the dehy-
rehydration and recovery after exercise
261
Fig. 19.2 Cumulative urine
output over time after
rehydration. After exerciseinduced dehydration by
approximately 2% of body mass,
different rehydration drinks in a
volume equivalent to the sweat
loss were consumed, and all the
urine produced was collected. 䊐,
glucose 90 mmol · l-1; 䉭, KCl
25 mmol · l-1; 䊊, NaCl 60 mmol ·
l-1; 䊉, mixture of three drinks. See
text for full explanation. Adapted
from Maughan et al. (1994).
Cumulative urine volume (ml)
800
700
600
500
400
300
200
100
0
0
drated state, acting as a vehicle for the elimination of metabolic waste products. It is clear
therefore that the total fluid intake after exerciseinduced or thermal sweating must amount to a
volume greater than the volume of sweat that has
been lost if an effective rehydration is to be
achieved. Shirreffs et al. (1996) investigated the
influence of drink volume on rehydration effectiveness following exercise-induced dehydration
equivalent to approximately 2% of body mass.
Drink volumes equivalent to 50%, 100%, 150%
and 200% of the sweat loss were consumed after
exercise. To investigate the possible interaction
between beverage volume and its sodium
content, a relatively low sodium drink (23 mmol ·
l–1) and a moderately high sodium drink (61
mmol · l–1) were compared.
With both beverages, the urine volume produced was, not surprisingly, related to the beverage volume consumed; the smallest volumes
were produced when 50% of the loss was consumed and the greatest when 200% of the loss
was consumed. Subjects did not restore their
hydration status when they consumed a volume
equivalent to, or only half, their sweat loss irrespective of the drink composition. When a drink
volume equal to 150% of the sweat loss was consumed, subjects were slightly hypohydrated 6 h
after drinking when the test drink had a low
sodium concentration, and they were in a similar
1
2
3
4
Time after rehydration (h)
5
6
ng
ni
or
M
condition when they drank the same beverage in
a volume of twice their sweat loss. With the highsodium drink, enough fluid was retained to keep
the subjects in a state of hyperhydration 6 h after
drink ingestion when they consumed either
150% or 200% of their sweat loss. The excess
would eventually be lost by urine production or
by further sweat loss if the individual resumed
exercise or moved to a warm environment. Calculated plasma volume changes indicated a
decrease of approximately 5.3% with dehydration. At the end of the study period, the general
pattern was for the increases in plasma volume to
be a direct function of the volume of fluid consumed: additionally, the increase tended to be
greater for those individuals who ingested the
high sodium drink.
Food and fluid consumption
In some situations, there may be opportunities to
consume solid food between exercise bouts, and
in most situations it should be encouraged unless
it is likely to result in gastrointestinal disturbances. In a study to investigate the role of food
intake in promoting rehydration from a hypohydration of approximately 2% of body mass,
induced by exercising in the heat, a solid meal
plus flavoured water or a commercially available
sports drink were consumed (Maughan et al.
262
nutrition and exercise
Table 19.1 Fluid consumed, quantities of major electrolytes ingested and volume of urine produced. Values in
brackets are mean (SEM) or median (range) as appropriate.
Fluid volume (ml)
Electrolytes ingested (mmol)
Na+
K+
6 h urine volume (ml)
1996). The volume of fluid contained within the
meal plus water was the same as the volume of
sports drink consumed, but the volume of urine
produced following food and water ingestion
was less than that when the sports drink was
consumed (Table 19.1). Although the amount of
water consumed with both rehydration methods
was the same, the meal had a greater sodium and
potassium content and it seems most likely that
the greater efficacy of the meal plus water treatment in restoring whole body water balance was
a consequence of the greater total cation content.
Alcohol consumption
Because of the well-known diuretic properties of
alcohol and caffeine, it is usual to advise against
the consumption of drinks containing these substances when fluid replacement is a priority.
However, many people enjoy consuming these
beverages, and where large volumes of fluid
must be consumed in a relatively short time, a
wide choice of drinks will help to stimulate
consumption. In many sports, particularly team
sports, alcohol intake is a part of the culture of
the sport, and athletes are resistant to suggestions that they should abstain completely (see
Chapter 30). However, it is now apparent that the
diuretic effect expected from alcohol, over and
above an alcohol-free beverage having otherwise
the same composition, is blunted when consumed by individuals who are moderately hypohydrated from exercise in a warm environment
(Shirreffs & Maughan 1997a).
After exercise, subjects consumed beer shandy
Meal + water
Sports drink
2076 (131)
2042 (132)
63 (4)
21 (1)
43 (3)
7 (1)
665 (396–1190)
934 (550–1403)
(a peculiarly British drink produced by mixing
beer with lemonade) containing 0%, 1%, 2% or
4% alcohol. The volume of urine excreted for the
6 h following drink ingestion was related to the
quantity of alcohol consumed, but despite a tendency for the urinary output to increase with
increasing alcohol intake, only with the 4% beverage did the increased value approach significance. The calculated decrease in plasma volume
with dehydration was approximately 7.6%
across all trials. With rehydration, the plasma
volume increased, but the rate of increase
seemed to be related to the quantity of alcohol
consumed; 6 h after finishing drinking, the
increase in plasma volume relative to the dehydrated value was approximately 8% with 0%
alcohol, 7% with 1%, 6% with 2% and 5% with
4%. It may be worth noting that the high sugar
content of lemonade (10%) means that beer
shandy has a carbohydrate content of about 5%,
and this carbohydrate may play an important
role in the restoration of muscle and liver glycogen stores after exercise.
Voluntary fluid intake
The information from the work described above
was obtained from studies in which a fixed
volume of fluid was consumed. In practice,
however, intake will be determined by the
interaction of physiological and psychological
factors. A second consequence of ingestion of
plain water is to remove the drive to drink by
causing plasma osmolality and sodium concentration to fall (Nose et al. 1988b). Where a fixed
rehydration and recovery after exercise
volume of fluid is given, this is not important,
but it will tend to prevent complete rehydration
when fluid intake is on a volitional basis
(Maughan & Leiper 1993).
Conclusion
Complete restoration of fluid balance after exercise is an important part of the recovery process,
and becomes even more important in hot, humid
conditions. If a second bout of exercise has to be
performed after a relatively short interval, the
speed of rehydration accomplishment becomes
of crucial importance. Rehydration after exercise
requires not only replacement of volume losses,
but also replacement of the electrolytes, primarily sodium, lost in the sweat. The electrolyte composition of sweat is highly variable between
individuals and although the optimum drink
may be achieved by matching electrolyte loss
with equal quantities from the drink, this is virtually impossible in a practical situation. Sweat
composition not only varies between individuals, but also varies with time during exercise and
is further influenced by the state of acclimation
(Taylor 1986). Typical values for sodium and
potassium concentrations are about 50 mmol · l–1
and 5 mmol · l–1, respectively. Drinks intended
specifically for rehydration should therefore
probably have higher electrolyte content than
drinks formulated for consumption during exercise, especially where opportunities for ingestion
of solid food are restricted.
Where sweat losses are large, the total sodium
loss will be high: 10 l of sweat at a sodium concentration of 50 mmol · l–1 amounts to about 29 g
of sodium chloride. However, a moderate excess
of salt intake would appear to be beneficial as far
as hydration status is concerned without any
detrimental effects on health provided that fluid
intake is in excess of sweat loss and that renal
function is not impaired.
The Oral Rehydration Solution recommended
by the World Health Organization for the treatment of acute diarrhoea has a sodium content of
60–90 mmol · l–1 (Farthing 1994), reflecting the
high sodium losses which may occur in some
263
types of diarrhoea. In contrast, the sodium
content of most sports drinks is in the range of
10–30 mmol · l–1 (see Table 17.2) and in some cases
is even lower. Most commonly consumed soft
drinks contain virtually no sodium and these
drinks are therefore unsuitable when the need
for rehydration is crucial. The problem with a
high sodium concentration in drinks is that some
people find the taste undesirable, resulting in
reduced consumption. However, drinks with a
low sodium content are ineffective at rehydration, and they will also reduce the stimulus to
drink.
Addition of an energy source is not necessary
for rehydration, although a small amount of carbohydrate may improve the rate of intestinal
uptake of sodium and water, and will improve
palatability. Where sweat losses are high, rehydration with carbohydrate solutions has implications for energy balance: 10 l of soft drinks will
provide approximately 1000 g of carbohydrate,
equivalent to about 16.8 MJ (4000 kcal). The
volume of beverage consumed should be greater
than the volume of sweat lost in order to make a
provision for the ongoing obligatory urine
losses, and palatability of the beverage is a major
issue when large volumes of fluid have to be
consumed.
Although water alone is adequate for rehydration, when food is also consumed this replaces
the electrolytes lost in sweat. However, there are
many situations where intake of solid food is
avoided. This is particularly true in weight category sports where the interval between the
weigh-in and competition is short, but is also the
case in events where only a few hours intervene
between succeeding rounds of the competition. It
is in these situations that electrolytes must be
present in the drinks consumed.
If a body water deficit is incurred during exercise, it is important that this is rectified in the
postexercise period if a decrement in performance during a subsequent exercise bout is to be
avoided. If no further exercise is planned, there
may be no urgency for fluid replacement and the
water will generally be replaced over the following day or so by a combination of eating and
264
nutrition and exercise
drinking. If, however, a second bout of exercise is
to be undertaken and a decrement in performance is to be avoided, the water lost must be
replaced as completely as possible before the
exercise commences and further sweat production occurs.
Prioritizing rehydration and recovery
after exercise: carbohydrate vs.
water replacement
Drinks consumed during or after exercise are
generally intended to replace the water and electrolyte losses incurred as a result of sweat secretion, and also to provide carbohydrate to
supplement or replenish the glycogen stores in
the liver and the working muscles. The relative
importance of providing water or substrate is
influenced by many factors. However, disturbances in body fluid balance and temperature
not only can impair exercise performance but are
potentially life-threatening (Åstrand & Rodahl
1986). In comparison, the depletion of carbohydrate stores in the liver and working muscles will
result in fatigue and a reduction in exercise intensity, but on the whole presents no great risk to
health. Therefore, except in situations where
depletion of body water has not occurred, the
first aim of postexercise recovery should be to
restore any fluid deficit incurred, followed by
repletion of liver and muscle glycogen stores.
It must, of course, be recognized that these
aims need not be mutually exclusive. Selection of
suitable food and drinks should provide both
the carbohydrate necessary for optimization of
muscle and liver glycogen resynthesis and the
water and electrolytes necessary for replacement
of sweat losses and restoration of fluid balance.
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