NUTRITION IN SPORT

concentration of glucose in the blood causes a
release of insulin from the b-cells in the pancreas.
Insulin binds to its receptor and stimulates
glucose transport and metabolism, particularly
in heart, skeletal muscle and adipose tissue. The
signalling pathway for insulin has been studied
extensively and during the last decade the
mechanism of action of insulin has become much
clearer (Kahn 1998).
Glucose is transported into cells by proteins
called glucose transporters. There are different
isoforms of the glucose transporters and their
expression is tissue specific (Holman & Kasuga
1997). GLUT-4 is expressed in tissue where in-
sulin stimulates glucose uptake (skeletal muscle,
heart and adipose tissue) and GLUT-4 is named
the insulin-regulated glucose transporter. Insulin
stimulates glucose uptake by recruitment of
GLUT-4 from intracellular sites to the sarcolem-
mal membrane (Fig. 34.1). GLUT-4 is normally
located in vesicles in the cells, but during insulin
stimulation, GLUT-4 is translocated to the cell
membrane by exocytosis (Holman & Kasuga
1997). When GLUT-4 transporter proteins are in
the sarcolemmal membrane, they will transport
glucose into the cells, and the amount of GLUT-4
in the sarcolemmal membrane is regarded as the
regulatory step for glucose uptake. GLUT-4 will
be internalized when the insulin stimulation is

removed and glucose transport will decrease to

basal level again.

Skeletal muscle makes up 30–40% of the body

weight and the 70–90% of the insulin-stimulated

glucose uptake occurs in this tissue (Shulman

et al.1990). Therefore, it is evident that skeletal

muscles play a central role in regulation of

glucose metabolism. Glucose taken up in skeletal

muscle during insulin stimulation is incorpo-

rated into glycogen (Shulman et al.1990), but

skeletal muscles are unable to release glucose

into the bloodstream to maintain blood glucose

concentration. Skeletal muscle glycogen can,

however, be broken down to lactate and released

from skeletal muscle for conversion to glucose in

the liver via gluconeogenesis. Skeletal muscle

glycogen is therefore indirectly a carbohydrate

source for maintaining blood glucose.

Exercise recruits GLUT-4 to the sarcolemmal

membrane in a manner similar to the effects of

insulin. Although both exercise and insulin stim-

ulate glucose uptake by translocation of GLUT-4

to the sarcolemmal membrane, this process

seems to occur via different signalling pathways

(Richter 1996) and exercise stimulates glucose

uptake even in insulin resistant muscles (Etgen et

al.1996). Another effect of exercise is that insulin

sensitivity increases in skeletal muscle after exer-

cise (Richter 1996). This means that lower insulin

the diabetic athlete 459

Glucose

transport

Fusion

Vesicle

Fission

GLUT-4

+

Ce

llm

em

bran

e

In

su

li

Fig. 34.1Schematic illustration showing regulation of glucose transport in skeletal muscle. When insulin binds to
the insulin receptor, GLUT-4-containing vesicles are translocated to the sarcolemmal membrane. GLUT-4
transports glucose into the cell when they are located in the sarcolemmal membrane. In insulin-resistant muscles,
translocation is reduced in response to insulin.