tional increase in insulin-stimulated glucose
transport and glycogen synthesis. Ren et al.
(1994) concluded that a rapid increase in GLUT-4
expression is an early adaptation of muscle to
exercise to enhance replenishment of muscle
glycogen stores.
Effect of carbohydrate supplementation
It is evident that the exercise-induced increase in
muscle permeability to glucose, insulin sensitiv-
ity, and glycogen synthase activity are, together,
not sufficient to result in the rapid repletion and
supercompensation of the glycogen stores, since
only a small increase in muscle glycogen occurs
following exercise in the absence of carbohydrate
feeding (Bergström et al. 1967; Bergström &
Hultman 1967b; Maehlum & Hermansen 1978;
Ivyet al. 1988b). Probable factors that prevent the
rapid repletion of muscle glycogen in the fasted
state are a depressed circulating insulin concen-
tration, and an increase in plasma free fatty acids
and fatty acid oxidation by muscle (Ivy et al.
1988a). These conditions are actually advanta-
geous during fasting as they serve to slow
muscle glucose uptake and conserve blood
glucose for use by the nervous system until suffi-
cient carbohydrate is available.
Carbohydrate feeding after exercise probably
stimulates glycogen synthesis by increasing arte-
rial plasma insulin and glucose concentrations.
The increase in circulating insulin not only func-
tions to increase muscle glucose uptake, but also
functions to keep glycogen synthase activity
elevated. It may also play a role in increasing
GLUT-4 expression which would facilitate
insulin-stimulated glucose transport (Ren et al.
1993; Hansen et al. 1995; Brozinick et al. 1996).
With increasing plasma glucose concentration,
glucose transport increases regardless of the
level of muscle permeability to glucose (Nesher
et al. 1985). Therefore, the increase in arterial
plasma glucose concentration functions to
increase the rate of glucose transport, further
increasing substrate availability, and provid-
ing sufficient G6P for activation of glycogen
synthase.
100 nutrition and exercise
It has also been demonstrated that the in-
sulin response to carbohydrate supplementation
increases over subsequent days while glucose
tolerance remains the same or actually improves
(Ivyet al. 1985). This increase in insulin response
to carbohydrate loading is thought to be the
result of an increase in the pancreatic response to
glucose (Szanto & Yudkin 1969). Since insulin is
required for normal glycogen repletion and
supercompensation, it is possible that the hyper-
insulinaemic response following several days of
high carbohydrate consumption is responsible
for the increased sensitivity of glycogen synthase
to G6P. The elevated plasma insulin may also
serve to increase the rate of muscle glucose trans-
port, thus increasing the availability of glucose to
glycogen synthase, as well as possibly increasing
the intracellular concentration of G6P. Hexoki-
nase activity in muscle is also increased during
subsequent days of carbohydrate loading (Ivy
et al. 1983). This too could be of functional signifi-
cance since an increase in hexokinase activity
would prevent the rate-limiting step in glucose
uptake from shifting from transport to glucose
phosphorylation as the G6P concentration
increased.Glycogen supercompensation
regimens
The discovery by Bergström and Hultman
(1967b) that a high carbohydrate diet following
the depletion of muscle glycogen by exercise
would result in an above normal muscle glyco-
gen concentration led to a series of studies to
identify the regimen of exercise and diet that
would best supercompensate the muscle glyco-
gen stores. Bergström et al. (1967) had subjects
exercise to exhaustion to deplete their muscle
glycogen stores. Six of the subjects then received
a high fat–protein diet for 3 days. This was fol-
lowed by another exhaustive exercise bout and
3 days of a high carbohydrate diet. The remain-
ing three subjects followed the same protocol
as above except the order of administration
of the diets was reversed. When the high car-
bohydrate diet followed the high fat–protein