902
Koopman et al.
fects on translation initiation.
[66,119]
In addition, leu-
leucine co-ingestion substantially elevated the ana-
bolic response following exercise in both the young
cine has been shown to have the potential to affect
and elderly (figure 3).
[127]
muscle protein metabolism by decreasing the rate of
protein degradation,
[120]
most likely via an increase
To assess whether the additional administration
in circulating insulin, and phosphorylation of key
of free leucine further stimulated protein synthesis,
proteins involved in the regulation of protein synthe-
we performed a follow-up study in which subjects
received carbohydrate and protein or carbohydrate,
sis.
[119,121,122]
protein and leucine following physical activity.
Although most in vitro and in vivo animal studies
When ample protein was provided, no surplus effect
report that leucine administration can inhibit protein
of additional free leucine administration could be
breakdown and stimulate protein synthesis, most in
detected (unpublished observations). The apparent
vivo human studies report that leucine and/or BCAA
discrepancy between our study and that of Katsanos
administration reduces muscle protein breakdown,
et al.
[89]
is explained by the fact that besides the prior
without stimulating muscle protein synthesis.
[123]
exercise, much larger amounts of leucine (present in
The apparent discrepancy between observations in
the protein) were provided in our recent study.
humans and animal studies remains to be elucidated.
Therefore, long-term intervention studies are war-
Maximal rates of protein synthesis rates during post-
ranted to address the efficacy of leucine supplemen-
exercise recovery probably require both amino acid
tation as an interventional strategy to attenuate the
signalling and the anabolic signal of resistance exer-
loss of muscle mass in the elderly and/or cachectic
cise. However, under normal in vivo conditions in
patients.
humans it is impossible to discriminate between the
effects of leucine and insulin and resistance exercise
4. Conclusions
(IGF-1 and MGF) on muscle protein synthesis.
Interestingly, recent studies in rodents have sug-
Resistance exercise is a powerful stimulus to
gested a possible role for oral leucine supplementa-
augment muscle protein anabolism, as it improves
tion to restore normal postprandial anabolism in
the balance between muscle protein synthesis and
aged animals.
[124,125]
When supplemented with leu-
cine, muscle protein synthesis in old rats was stimu-
lated and similar to mature rats. Therefore, it was
concluded that meal supplementation with leucine
was sufficient to restore postprandial stimulation of
muscle protein synthesis
[125]
and inhibition of
proteasome-dependent proteolysis
[126]
in old rats.
Recently, Katsanos and colleagues
[89]
showed that
the ingestion of 6.7g of EAA (26% leucine) effec-
tively increased muscle protein synthesis rates in the
young, but failed to augment protein synthesis rates
in the elderly. Interestingly, ingestion of a 6.7g EAA
mixture with a higher leucine content (i.e. 41% vs
26%) increased muscle protein synthesis to the same
extent in both the young and elderly.
As it was proposed that additional leucine could
further stimulate protein synthesis, we assessed the
anabolic response to protein and leucine ingestion
with carbohydrate following activities of daily liv-
ing in both the young and elderly.
[127]
Protein and
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
Young
Elderly
CHO
CHO+PRO+leu
FSR (%/h)
Fig. 3. Fractional synthetic rate (FSR) of mixed muscle protein
following the ingestion of carbohydrate (CHO) or CHO with protein
and leucine (CHO+PRO+leu) in lean young (n = 8) and elderly (n =
8) men using plasma phenylalanine enrichment as a precursor. Age
effect p < 0.05; treatment effect p < 0.001 (reproduced from Koop-
man et al.,
[127]
with permission).
©
2007 Adis Data Information BV. All rights reserved.
Sports Med 2007; 37 (10)
Study 