900
Koopman et al.
The age-related changes in skeletal muscle mass
sion
[45]
in the elderly. This response has been sug-
gested as a potential mechanism for the attenuated
are attributed to a disturbance in the regulation of
protein synthetic response in the elderly.
muscle protein turnover, which results in a chronic
In addition, the decrease in protein synthesis with
imbalance between muscle protein synthesis and
aging is strongly associated with changes in genome
degradation. Many studies have reported either sim-
integrity and gene expression,
[91]
and in translation
ilar,
[78-81]
or reduced
[1,7,29,30,82-84]
skeletal muscle pro-
and post-translational modification of numerous
tein synthesis rates after an overnight fast in the
proteins.
[92]
It has recently been shown that skeletal
elderly compared with young adults. Some of the
muscle tissue in the elderly contains significantly
discrepancy between earlier reports may have been
lower amounts of total mTOR, S6K11 and eIF2B,
due to the standardisation of prior physical activi-
all key factors in the regulation of translation initia-
ty,
[77]
selection of subjects,
[84]
or selection of differ-
tion.
[93]
Another proposed mechanism involves dif-
ent precursor pools to calculate muscle protein syn-
ferences in protein turnover in the splanchnic region
thesis.
[85]
However, based on the observation that
between the young and elderly.
[80,94]
A percentage of
muscle wasting with aging occurs with about 38%
the ingested amino acids will not be made available
per decade,
[86,87]
it seems unlikely that basal muscle
for muscle protein synthesis due to utilisation in the
protein fractional synthetic rates are diminished by
splanchnic tissues. The latter has been suggested to
2030% as reported previously.
[1,7,29,82]
In addition,
increase with age.
[80,94]
Nevertheless, Volpi et al.
[80]
it is not likely that muscle protein breakdown is
demonstrated that despite a greater first-pass extrac-
elevated by as much as the 50% sometimes reported
tion by the gut and liver of ingested amino acids, the
in the elderly compared with the young.
[88]
At pre-
amino acid-induced muscle protein anabolism in the
sent, it is now generally believed that defects exist in
elderly did not differ from the young. Interestingly,
the ability of the elderly to make adequate use of
it has been shown that protein digestion rates can
dietary protein, which would be consistent with a
differentially affect the anabolic response in young
more gradual loss of skeletal muscle mass and quali-
and older adults.
[95]
Slowly digested protein (casein)
ty with aging.
simulates protein anabolism at the whole-body level
It has been recently reported that muscle protein
in the young,
[96]
whereas faster digested proteins
synthesis is stimulated to a similar extent in young
(whey protein) seem to favour anabolism in the
and elderly subjects following the intravenous
[76]
or
elderly.
[95]
More research is warranted to assess the
oral administration
[60]
of amino acids.
[9]
A more
preferred dietary protein source to stimulate protein
practical mode of amino acid administration (i.e.
synthesis in the elderly.
ingestion of a 15g amino acid mixture) has been
shown to stimulate net muscle protein synthesis to a
3.2 Interventions to Improve Muscle Protein
similar extent in young and elderly subjects.
[78]
In-
Synthesis in the Elderly
terestingly, more recent studies suggest that the ana-
bolic response to the ingestion of smaller amounts of
There is substantial evidence that muscle protein
EAA (i.e. 67g) is severely reduced in the elderly
synthesis is responsive to exercise in both the young
when compared with the young.
[89]
In accordance, a
and elderly.
[6]
In studies performed in both young
blunted anabolic response to the combined ingestion
and elderly persons, resistance
[82]
and endurance
of glucose and essential amino acids has been re-
exercise
[75]
have been shown to stimulate the syn-
ported previously in the elderly.
[79]
This blunted
thetic rate of mixed muscle protein. Resistance exer-
anabolic response could at least partly be related to a
cise training mainly affects muscle mass and
blunted insulin response following carbohydrate co-
strength and can increase specific muscle proteins,
ingestion in the elderly subjects. This could likely
such as MHC.
[82]
In contrast, endurance exercise
impair the responsiveness of S6K1 activation
[90]
and
improves many metabolic functions, including insu-
blunt the postprandial increase in muscle perfu-
lin-induced glucose disposal and mitochondrial
©
2007 Adis Data Information BV. All rights reserved.
Sports Med 2007; 37 (10)
Study 