Nutritional Interventions to Stimulate Muscle Anabolism
901
function, mediated by changes in various sets of
Measurements of muscle protein synthesis and in-
tracellular signalling will provide important infor-
proteins.
[97]
The specific adaptive response to exer-
mation on the molecular basis for the defects in
cise depends on the type, frequency, intensity and
muscle protein metabolism that lead to muscle loss
the duration of the exercise regimen. It is likely that
with aging. Such insight will be of great relevance to
the metabolic demand of exercise, leading to adeno-
the development of therapeutic strategies to aug-
sine monophosphate-activated kinase (AMPK)
ment muscle mass and function.
phosphorylation, has a major impact on muscle pro-
tein metabolism during post-exercise recov-
3.3 Leucine
ery.
[69,98,99]
However, the modulating effect of post-
exercise nutrition on AMPK activation in human
The role of leucine as a regulator of protein
skeletal muscle remains be assessed.
metabolism has been studied extensively.
[114]
In the
It has been shown that only modest increases in
1970s, a number of laboratories performed in vitro
strength are achieved in the elderly when exercise
research to determine the key factors controlling
intensity is low.
[100]
Exercise intervention studies in
protein turnover.
[115-117]
High amino acid concentra-
the elderly have shown that the capacity of the
tions were shown to stimulate protein synthesis and
muscle protein synthesis machinery is preserved
inhibit protein degradation. In particular, in isolated
until very old age.
[101,102]
Increases in muscle mass
diaphragm muscle, leucine was shown to effectively
and strength have been reported following resis-
stimulate protein synthesis.
[115]
These studies initiat-
tance exercise training,
[101-109]
while endurance exer-
ed many investigations into the role of leucine in the
cise has been shown to enhance muscle endurance
control of protein metabolism, its mechanism of
capacity,
[75,76]
both enhancing functional capacity.
action, and its possible value for enhancing muscle
Nutritional interventions on top of exercise training
protein balance in healthy subjects and patients in a
have failed to show a greater skeletal muscle adap-
catabolic or cachectic state.
tive response in the elderly. For example, dietary
In general, most animal studies support the find-
interventions using supplementation with carbohy-
ings by Buse and Reid
[115]
and report that leucine
drate, protein and fat,
[110]
protein
[105]
or branched
inhibits muscle protein breakdown and stimulates
chain amino acids (BCAA)
[111]
or by inducing
muscle protein synthesis. For example, Anthony et
changes in meal protein content
[112,113]
have failed to
al.
[67,118]
reported a direct, stimulating effect of leu-
further augment the anabolic response to exercise
cine ingestion on muscle protein synthesis in ro-
training. The latter could be related to the timing of
dents. In line with those data, the same group
the supplementation in those studies. Esmarck et
showed that leucine supplementation enhances pro-
al.
[31]
showed that in the elderly the timing of protein
tein synthesis in skeletal muscle from diabetic rats
ingestion following resistance exercise is crucial, as
through insulin-independent mechanisms.
[68]
These
a significant gain in muscle mass was only observed
studies have increased the understanding of the nu-
when dietary supplementation was performed im-
tritional regulation of protein synthesis at the molec-
mediately post-exercise as opposed to 2 hours after
ular level. Leucine ingestion can stimulate protein
cessation of exercise. More studies are warranted to
synthesis via insulin dependent and independent
investigate the interaction between exercise and nu-
pathways. Leucine has the ability to function as a
trition in the elderly. The assessment of possible
nutritional signalling molecule that modulates mus-
defects in the activation of translation initiation in
cle protein synthesis and/or breakdown following
response to anabolic stimuli such as exercise and
food intake. The stimulatory effect of leucine on
nutrition may play an important role in the aetiology
protein synthesis occurs at the level of transla-
of sarcopenia. However, data on the specific defects
tion initiation and involves signalling through
in translational control in the elderly are scarce and
mTOR,
[66,119]
which is thought to serve as a conver-
the regulation of muscle mass is poorly understood.
gence point for leucine- and insulin-mediated ef-
©
2007 Adis Data Information BV. All rights reserved.
Sports Med 2007; 37 (10)
Study 