NUTRITION IN SPORT

have measured a 113% increase in the active
muscle urea nitrogen content (268±68 to 570± 89
mg·g–1muscle wet mass) of rodents immediately
following 1 h of running exercise at 25 m · min–1
(unpublished data). Moreover, increased rates
of muscle protein degradation (Kasperek &
Snider 1989) and significant muscle damage
(Armstrong et al. 1983; Newman et al. 1983;
Fridenet al. 1988; Evans & Cannon 1991; Kuipers
1994) with exercise are well documented in
several mammalian species (including humans),
especially when the exercise has a significant
eccentric component. Lysosomal proteases, i.e.
cathepsins, have been implicated in this exercise
catabolic response (Seene & Viru 1982; Tapscott et
al.1982; Salminen et al. 1983; Salminen & Vihko
1984) but some believe (Kasperek & Snider 1989)
these do not play a major role. Recently it has
been suggested (Belcastro et al. 1996) that non-
lysosomal proteases, perhaps a calcium-
activated neutral protease (calpain), stimulated
by an exercise-induced increased intracellular
calcium may be primarily responsible for the
initial damage which occurs immediately after
exercise. Evidence for this comes not only from
the observation that isozymes of calpain increase
22–30% with exercise (Belcastro 1993) but also
because the pattern of exercise-induced myofib-
rillar damage is similar to that induced by
calpain (Goll et al. 1992). Lysosomal protease

activity may play an more important role in the

muscle damage that is seen later (several days)

following exercise (Evans & Cannon 1991;

MacIntyre et al. 1995). Whether increased protein

intake can reduce this damage or speed the

subsequent repair processes are interesting

questions.

Together, the large efflux of the amino acids

alanine (Felig & Wahren 1971) and glutamine

(Ruderman & Berger 1974) from active muscle, as

well as the frequently observed accumulation/

excretion of protein metabolism end products,

urea (Refsum & Stromme 1974; Haralambie &

Berg 1976; Lemon & Mullin 1980; Dohm et al.

1982) and ammonia (Czarnowski & Gorski 1991;

Graham & MacLean 1992; Graham et al. 1995)

provide strong indirect evidence that significant

increases in branched-chain amino acid (BCAA)

metabolism occur with endurance exercise (Fig.

10.4). Further, this has been confirmed using

direct oxidation measures (Fig. 10.5) by a number

of independent investigations (White & Brooks

1981; Hagg et al. 1982; Lemon et al. 1982; Babij

et al. 1983; Meredith et al. 1989; Phillips et al.

1993). This is likely the result of an exercise inten-

sity-dependent activation of the limiting enzyme

(branched-chain oxoacid dehydrogenase) in the

oxidation pathway of the BCAA (Kasperek &

Snider 1987). This response is apparently directly

proportional to BCAA availability (Knapik et al.

effects of exercise on protein metabolism 137

Protein concentration (mg

.100 g

–1)

24

10

22

16

Sedentary

controls

0

Time after exercise (h)

20

18

14

12

2 6 24 48

*

Fig. 10.3Effect of prolonged
endurance exercise (10 h
swimming in rodents) on protein
concentration in the red portion of
the quadriceps muscle. Note the
decrease immediately following
the exercise bout. *, P<0.05.
Adapted from Varrik et al. (1992).