NUTRITION IN SPORT

1991; Layman et al. 1994) and inversely propor-
tional to glycogen availability (Lemon & Mullin
1980; Wagenmakers et al. 1991), although other
factors may also be important (Jackman et al.
1997). This suggests that dietary protein, dietary
carbohydrate, prior exercise and time since the
previous meal are probably all important deter-
minants of BCAA oxidation during exercise.
The magnitude of this increased BCAA oxida-
tion could be important relative to daily BCAA
requirements because a single bout of moderate
exercise (2 h at 55% V

.
o2max.) can produce an oxi-
dation rate equivalent to almost 90% of the daily
requirement for at least one of the BCAA (Evans
et al. 1983). In addition, it is possible that this oxi-
dation rate could be even higher in endurance-
trained individuals because at least two studies

138 nutrition and exercise

with rodents have shown that the endurance

training process results in further increases

in BCAA oxidation both at rest and during

endurance exercise (Dohm et al. 1977; Henderson

et al. 1985). With endurance exercise, this increase

is proportional to exercise intensity (Babij et al.

1983) but, despite the extremely intense nature of

strength exercise, BCAA oxidation appears to be

largely unaffected by this exercise stimulus (Fig.

10.6) (Tarnopolsky et al. 1991). This is likely due

to the fact that strength exercise is so intense that

a major portion of the necessary energy must be

derived via anaerobic metabolism, i.e. stored

phosphagens and muscle glycogen, rather than

via oxidative pathways.

Interesting data are also available from several

elegant nitrogen status (balance) experiments

Branched-chain

amino acids

BCAAAT

Branched chain

oxoacids

BCOADH

Oxidation (CO 2 ) (released from muscle)Glutamine Urea formation(liver)

Pyruvate

AAT

Alanine (released

from muscle)

2-oxoglutarate Urea formation (liver)

GDH

Glutamate

GS NH+ 4

100

80

60

40

20

0

10 20 30 40 50 60 70 80 90 100

Exercise intensity (%VO2max)

.

Whole-body leucine flux oxidized (%)

Fig. 10.4Overview of branched-

chain amino acid metabolism

showing the production of alanine

and glutamine in muscle, as well

as the formation of urea in the

liver. AAT, alanine amino

transferase; BCAAAT, branched-

chain amino acid amino

transferase; BCOADH, branched-

chain oxoacid dehydrogenase;

GDH, glutamate dehydrogense;

GS, glutamine synthetase; NH 4 +,

ammonium.

Fig. 10.5Effect of endurance

exercise intensity (V

.

o2max.) on the

oxidation of one of the branched-

chain amino acids (leucine) in

four human subjects. Note the

linear increase in oxidation with

increasing exercise intensity. r=

0.93;y=0.71x+8.44. Adapted

from Babij et al. (1983).