Carbohydrate Replacement during Exercise

Chapter 8
Carbohydrate Replacement during Exercise
MARK HARGREAVES
Introduction
Muscle glycogen depletion and/or hypoglycaemia are associated with fatigue during prolonged strenuous exercise (Hermansen et al.
1967; Coyle et al. 1986), highlighting the critical
importance of carbohydrate (CHO) availability for intramuscular adenosine triphosphate
supply (Norman et al. 1987; Sahlin et al. 1990;
Spencer et al. 1991). In the early part of this
century, the benefit of CHO ingestion during prolonged exercise was recognized in a classic field
study (Gordon et al. 1925). Over the years since,
numerous controlled, laboratory studies have
demonstrated that ingestion of CHO during prolonged, strenuous exercise results in enhanced
exercise performance. Many studies have used
exercise time to fatigue as their measure of
endurance capacity and this is increased by CHO
ingestion (Coyle et al. 1983, 1986; Björkman et al.
1984; Coggan & Coyle 1987, 1989; Sasaki et al.
1987; Spencer et al. 1991; Davis et al. 1992; Wilber
& Moffatt 1992; Tsintzas et al. 1996a, 1996b).
Recently it has been argued that time to fatigue
is not a reliable test of endurance performance
(Jeukendrup et al. 1996); however, in wellmotivated subjects, who have been familiarized
with the testing procedures, it remains a useful
laboratory test for elucidating mechanisms of
fatigue. Nevertheless, there are no Olympic
events in ‘exercise time to fatigue’, and it is
perhaps more appropriate to assess endurance
performance by tests that measure the time taken
to complete a standard task or the work output in
112
a certain amount of time. Using such tests, CHO
ingestion has been shown to improve performance as measured by enhanced work output or
reduced exercise time (Neufer et al. 1987; Coggan
& Coyle 1988; Mitchell et al. 1989; Williams et
al. 1990; Murray et al. 1991; Tsintzas et al. 1993;
Below et al. 1995; McConell et al. 1996;
Jeukendrup et al. 1997). The increases in exercise
performance with CHO ingestion are believed
to be due to maintenance of a high rate of
CHO oxidation and increased CHO availability
within contracting skeletal muscle (Coyle et al.
1986; Coggan & Coyle 1987; Tsintzas et al. 1996a).
In addition, the prevention of neuroglucopenia
and effects on central nervous system function
may play a role (Davis et al. 1992). Interestingly,
several recent studies have observed improved
high-intensity and intermittent exercise performance with CHO ingestion when, under normal
circumstances, CHO availability is not thought
to be limiting (Anantaraman et al. 1995; Ball et al.
1995; Below et al. 1995; Nicholas et al. 1995; Davis
et al. 1997; Jeukendrup et al. 1997). The mechanisms underlying the ergogenic benefit of CHO
ingestion under these circumstances remain to be
determined, but may involve small increases in
intramuscular CHO availability under conditions of high CHO utilization. Performance in a
20-km cycle time trial, lasting about 30 min, was
not affected by CHO ingestion (Palmer et al.
1998).
carbohydrate replacement during exercise
Metabolic responses to CHO
ingestion during exercise
Ingestion of CHO during prolonged, strenuous
exercise results in higher blood glucose levels
and rates of CHO oxidation late in exercise (Fig.
8.1) (Coyle et al. 1986; Coggan & Coyle 1987).
Liver glucose output is reduced by CHO ingestion (Fig. 8.2) (Bosch et al. 1994; McConell et al.
1994) and while the tracer method used cannot
distinguish between liver glycogenolysis and
gluconeogenesis, it is likely that there is a significant liver glycogen sparing effect of CHO ingestion. A reduction in splanchnic gluconeogenic
precursor and oxygen uptake during prolonged,
low-intensity exercise following CHO ingestion
113
suggests a lower rate of gluconeogenesis
(Ahlborg & Felig 1976) and this has recently been
confirmed in experiments using tracers to estimate rates of gluconeogenesis (Jeukendrup et al.
1999). Muscle glucose uptake during exercise, as
measured by tracer-determined glucose Rd, is
increased by CHO ingestion (McConell et al.
1994). This is consistent with previous observations of increased leg glucose uptake during lowintensity exercise (Ahlborg & Felig 1976) and
elevated rates of glucose disposal and oxidation
during strenuous exercise when blood glucose
availability is increased (Coggan et al. 1991;
Coyle et al. 1991; Bosch et al. 1994; Hawley et al.
1994; Howlett et al. 1998).
Most, if not all, studies utilizing prolonged
6
Plasma glucose (mM)
5
CHO
*
4
*
*
*
Placebo
*
3
*
2
0
1
2
3
4
Fig. 8.1 (a) Plasma glucose and
(b) rates of carbohydrate (CHO)
oxidation during exercise
to
.
fatigue at 70–74% Vo 2peak with
ingestion of either a placebo (䊊)
or CHO (䊉) solution every 20
min. Values are means ± SEM
(n = 7). *, difference from CHO,
P < 0.05. Adapted from Coyle et al.
(1986).
Carbohydrate oxidation (g.min–1)
(a)
2.6
2.2
CHO
1.8
*
Placebo
1.4
0
(b)
1
2
Exercise time (h)
*
*
3
4
nutrition and exercise
120
100
(a)
80
60
40
*
Muscle glucose uptake (g)
Hepatic glucose production
120
100
80
60
40
20
20
0
0
(b)
*
Net muscle glycogen utilization (mmol.kg–1)
114
120
*
100
80
Fig. 8.2 (a) Total hepatic glucose
production, (b) muscle glucose
uptake, and (c) net muscle
glycogen utilization. during 2 h of
exercise at 70–74% Vo 2peak with
( ) and without ( ) ingestion of
CHO. Values are means ± SEM
(n = 6 – 7). *, difference from no
ingestion of CHO, P < 0.05. Data
from Coyle et al. (1986) and
McConell et al. (1994).
60
40
20
0
(c)
strenuous, continuous cycling exercise have
observed no effect of increased blood glucose
availability on net muscle glycogen utilization,
either measured directly from biopsy samples
(Fig. 8.2) (Fielding et al. 1985; Coyle et al. 1986,
1991; Flynn et al. 1987; Hargreaves & Briggs 1988;
Mitchell et al. 1989; Widrick et al. 1993; Bosch et al.
1994) or estimated from total CHO oxidation and
tracer-determined glucose uptake (Jeukendrup
et al. 1998). Decreases in glycogen use during
cycling have been reported (Erikson et al. 1987),
during the latter stages of prolonged exercise
(Bosch et al. 1996), with a large increase in blood
glucose (Bergström & Hultman 1967) and during
intermittent exercise protocols (Hargreaves et al.
1984; Yaspelkis et al. 1993). In two of these studies
(Hargreaves et al. 1984; Erikson et al. 1987), the
results are potentially confounded by higher preexercise muscle glycogen levels in the control
trial which influences the subsequent rate of
degradation (Hargreaves et al. 1995). It is possible
that during intermittent exercise with periods of
rest or low-intensity exercise, CHO ingestion
may result in glycogen synthesis (Kuipers et al.
1987) and a reduction in net muscle glycogen use.
On balance, however, the effects of CHO ingestion on muscle glycogen use during prolonged,
strenuous cycling exercise appear relatively
small. In contrast, recent studies during treadmill
running indicate that CHO ingestion reduces net
muscle glycogen use, specifically in the type I
fibres (Tsintzas et al. 1995, 1996a), and that the
increase in muscle glycogen availability late in
exercise contributed to the enhanced endurance
capacity that was observed (Tsintzas et al. 1996a).
The ingestion of CHO results in lower plasma
free fatty acid levels during prolonged exercise
(Coyle et al. 1983, 1986; Murray et al. 1989a, 1989b,
1991; Davis et al. 1992; Tsintzas et al. 1996a). The
effects of CHO ingestion during exercise on fat
oxidation do not appear to be as great as those
observed with pre-exercise CHO ingestion, most
likely as a consequence of the smaller increases in
plasma insulin levels which, while still blunting
lipolysis and the exercise-induced increase in
plasma free fatty acid levels (De Glisezinski et al.
1998; Horowitz et al. 1998), may result in a
smaller initial increase in muscle glucose uptake
and relatively less inhibition of intramuscular
lipid oxidation (Horowitz et al. 1998).
Practical aspects of CHO ingestion
during exercise
Type of CHO
There appear to be relatively few, if any, differences between glucose, sucrose and maltodextrins in their effects on metabolism and
performance when ingested during exercise
(Massicotte et al. 1989; Murray et al. 1989a;
Hawley et al. 1992; Wagenmakers et al. 1993). In
contrast, fructose alone is not as readily oxidized
as other CHO sources (Massicotte et al. 1989) due
carbohydrate replacement during exercise
to its slower rate of absorption, which may cause
gastrointestinal distress and impaired performance (Murray et al. 1989a). Interestingly, the
combination of fructose and glucose results in
higher rates of exogenous CHO oxidation than
ingestion of either sugar alone (Adopo et al.
1994). This may be a consequence of activation of
two intestinal transport mechanisms, resulting in
greater appearance of ingested CHO from the
gastrointestinal tract. Such a finding is consistent
with observations of greater fluid absorption
from rehydration beverages containing more
than one CHO (Shi et al. 1995). Whether the
increased exogenous CHO oxidation results in
enhanced exercise performance has not been
tested. Galactose is less available for oxidation
when ingested during exercise (Leijssen et al.
1995) and soluble corn starch is oxidized to a
greater extent than insoluble starch during exercise due to its higher amylopectin/amylose ratio
(Saris et al. 1993). The physical form of the
ingested CHO does not exert a major influence
since liquid and solid CHO supplements elicit
similar metabolic responses during exercise
(Lugo et al. 1993; Mason et al. 1993).
Amount of CHO
There is no clear dose–response relationship
between the amount of CHO ingested during
exercise and subsequent exercise performance
(Mitchell et al. 1989; Murray et al. 1989b, 1991).
Ingestion of CHO at a rate of 13 g · h–1 is insufficient to alter the glucoregulatory hormone
response to prolonged exercise or time to fatigue
(Burgess et al. 1991). The addition of a small
amount of CHO to a rehydration beverage was
shown to increase exercise time to fatigue, compared with water alone, and to be more effective
than a larger amount of CHO (Maughan et al.
1996). This may be the result of better rehydration, due to the stimulatory effect of a hypotonic
glucose solution on intestinal fluid absorption,
and subsequent maintenance of a higher plasma
volume, rather than a metabolic effect of the
ingested CHO. Ingestion of CHO at a rate of 26
and 78 g · h–1 increased 4.8-km cycle performance
to a similar extent, following 2 h of exercise at
115
.
65–75% Vo 2peak (Murray et al. 1991). No differences in physiological responses to exercise were
observed between ingestion of 6%, 8% and 10%
sucrose solutions, but performance was only
enhanced with 6% (Murray et al. 1989b). There is
likely to be little benefit in ingesting CHO solutions more concentrated than 6–8% because this
does not result in increased rates of exogenous
glucose oxidation (Wagenmakers et al. 1993) and
increases the risk of impaired gastrointestinal
function and reduced fluid delivery. The absorption of ingested CHO could potentially limit
exogenous CHO oxidation which is observed to
peak at 1–1.3 g · min–1 (Hawley et al. 1992). Such
values are similar to those reported for gastric
emptying and intestinal absorption of glucose
from a 6% glucose–electrolyte solution under
resting conditions (Duchman et al. 1997). Obviously, the important goal of CHO replacement is
to provide sufficient CHO to maintain blood
glucose and CHO oxidation without causing
impaired fluid delivery. Ingesting CHO at a rate
of 30–60 g · h–1 has been repeatedly shown to
improve exercise performance. This CHO intake
can be achieved by ingesting commercially available sports drinks, at a rate of 600–1200 ml · h–1,
with the added benefit of providing fluid and
reducing the negative effects of dehydration
(Coyle & Montain 1992; American College of
Sports Medicine Position Stand 1996).
Timing of CHO ingestion
The beneficial effects of CHO ingestion are likely
to be most evident during the latter stages of prolonged exercise when endogenous CHO reserves
are depleted. Indeed, ingestion of CHO late in
exercise, approximately 30 min prior to the point
of fatigue, produced increases in exercise time to
fatigue similar in magnitude to those seen with
ingestion of CHO early or throughout exercise or
with intravenous infusion of glucose at the point
of fatigue (Coggan & Coyle 1989; Coggan et al.
1991; Tsintzas et al. 1996b). In contrast, ingestion
of CHO at the point of fatigue is not as effective in
enhancing endurance capacity (Coggan & Coyle
1987, 1991); however, delaying CHO intake until
late in exercise, despite increasing blood glucose
116
nutrition and exercise
availability and CHO oxidation, does not always
enhance exercise performance (McConell et al.
1996). From a practical perspective, because athletes are unable to assess the level of their carbohydrate reserves and their likely point of fatigue,
CHO (and fluid) replacement should commence
early and continue throughout exercise.
Conclusion
In view of the importance of CHO for contracting
skeletal muscle during strenuous exercise, CHO
should be ingested to maintain CHO availability
and high rates of CHO oxidation. Such a strategy
results in enhanced endurance performance.
There appear to be few differences between
glucose, sucrose and maltodextrins, as sources of
CHO, in their effects on exercise metabolism and
performance, while fructose alone is not an effective CHO supplement. Enough CHO should be
ingested to supply CHO to contracting muscle at
about 30–60 g · h–1, without negative effects on
fluid bioavailability.
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