chapter 22
Metabolic Homeostasis
it is superseded by fatty acid and plasma glucose utiliza-
tion. Most of the plasma glucose used is oxidized entirely
but 10-20% is converted to lactate. During the exercise
and depending on the past dietary history, branched-chain
amino acid oxidation contributes some energy, and alanine
is formed to eliminate the a-amino nitrogen. Lactate and
alanine are used in the liver for gluconeogenesis and can
provide 5-15% of the glucose required by skeletal mus-
cle. The rest of the glucose is derived from glycogenoly-
sis. Thus, in endurance exercise, hepatic glycogen serves
as a major fuel for muscle, and muscle glycogen is the
initial fuel. As plasma glucose and fatty acids are used,
glycogenolysis in muscle diminishes; despite its smaller
contribution, muscle glycogen plays a critical role since
exhaustion occurs with its total depletion in spite of
the availability of fatty acids and plasma glucose. Thus,
“glycogen loading” is used to extend endurance. Exercise-
depleted glycogen stores can be increased to above-normal
levels for about 24 hours by high-carbohydrate refeeding.
With elevated levels of tissue glycogen, an individual’s
endurance ability is increased. Several other phenomena
contribute to the pattern of metabolite utilization in high-
intensity endurance exercise.
1. Although fatty acids are avidly utilized by skeletal
muscle during exercise, their plasma concentrations
do not differ significantly from those at rest. Their
utilization is being balanced by production from
adipocyte lipolysis, probably as a result of an elevated
catecholamine level.
2. Glucose uptake by skeletal muscle increases with
intensity of exercise even though levels of circulating
insulin decrease. Either this glucose transport is
insulin-independent or exercise enhances insulin
3. The increase in glucose uptake with increased
intensity of exercise is matched by hepatic glucose
production to the extent that arterial glucose
concentrations are elevated by very intense exercise.
Hepatic glycogenolysis probably is enhanced by a
decrease in the level of insulin and by an increase in
levels of glucagon and epinephrine.
4. Exercise may lower insulin levels even though plasma
glucose concentrations are increased. This effect may
result from a-adrenergic inhibition of /3-cell secretion.
Low-Level Nonfatiguing Exercise
This exercise is characteristic of an individual in normal
occupational tasks that could reasonably be continued for
8-12 hours. The principal substrates used are similar to
those in long endurance exercise but without depletion
of phosphocreatine and with minimal muscle glycogen
utilization. The main source of energy is the aerobic oxi-
dation of fatty acids, glucose, and branched-chain amino
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