NUTRITION IN SPORT

In the latter hypothesis, accumulation of intersti-
tial potassium in muscle may play a major role
(Sjogaard 1991; Bangsbo 1997).
When repeated bouts of maximal exercise are
performed, the rates of muscle PCr hydrolysis
and lactate accumulation decline. In the case of
PCr, this response is thought to occur because of
incomplete PCr resynthesis occurring during
recovery between successive exercise bouts.
However, the mechanism(s) responsible for the
fall in the rate of lactate accumulation is unclear.
It is commonly accepted that nutrition is not of
great importance to individuals involved in
high-intensity exercise. Muscle glycogen avail-
abilityper seis not usually considered to be
responsible for fatigue during high-intensity
exercise, providing the pre-exercise glycogen
store is not depleted to below 100 mmol · kg–1dm.
It is even unlikely that glycogen availability will
limit performance during repeated bouts of exer-
cise, due to the decline in glycogenolysis and
lactate production that occurs under these condi-
tions. However, there is a growing body of
evidence to indicate that dietary creatine intake
may be a necessary requirement for individuals
wishing to optimize performance during high-
intensity exercise.

Prolonged exercise

The term prolonged exerciseis usually used to
describe exercise intensities that can be sustained
for between 30 and 180 min. Since the rate of ATP
demand is relatively low compared with high-
intensity exercise, PCr, CHO and fat can all con-
tribute to energy production. The rates of PCr
degradation and lactate production during the
first minutes of prolonged exercise are closely
related to the intensity of exercise performed,
and it is likely that energy production during this
period would be compromised without this con-
tribution from anaerobic metabolism. However,
once a steady state has been reached, CHO and
fat oxidation become the principal means of
resynthesizing ATP. Muscle glycogen is the prin-
cipal fuel during the first 30 min of exercise at
60–80%V

.

o2max.. During the early stages of exer-

cise, fat oxidation is limited by the delay in the

mobilization of fatty acids from adipose tissue.

At rest following an overnight fast, the plasma

FFA concentration is about 0.4 mmol · l–1. This is

commonly observed to fall during the first hour

of moderate intensity exercise (Fig. 2.8), followed

by a progressive increase as lipolysis is stimu-

lated by the actions of catecholamines, glucagon

and cortisol. During very prolonged exercise,

the plasma FFA concentration can reach 1.5–

2.0 mmol · l–1and muscle uptake of blood-borne

FFA is proportional to the plasma FFA concentra-

tion. The glycerol released from adipose tissue

cannot be used directly by muscle that lacks the

enzyme glycerol kinase. However, glycerol

(together with alanine and lactate) is taken up by

the liver and used as a gluconeogenic precursor

to help maintain liver glucose output as liver

biochemistry of exercise 35

6 4 2 0 2 1 0

400

300

200

100

0

30 60 90 120

Exercise duration (min)

Plasma glucose

(mmol

–1.l

)

Plasma FFA(mmol

.–1l

)

Muscle glycogen(mmol

.kg

–1

dm)

(a)

(b)

(c)

30 60 90 120

30 60 90 120

Fig. 2.8Changes in the concentrations of (a) plasma

glucose, (b) plasma free fatty acids (FFA), and (c)

muscle glycogen during continuous exercise at an

intensity equivalent to about 70% V

.

o2max..