Nutritional Interventions to Stimulate Muscle Anabolism
899
sponse to exercise. However, ingestion of small
the progressive decline in skeletal muscle mass and
amounts (6g) of EAA 1 and 2 hours post-exercise
function. This process of age-related loss of muscle
resulted in a positive net protein balance for a period
mass and function, or sarcopenia, has substantial
of 2 hours only, after which net protein balance
health consequences, as it is associated with de-
became negative.
[61]
The latter indicates that such a
creased basal metabolic rate, increased body fat
small amount of amino acid was not sufficient to
content, loss of bone mass, reduced capacity for
maintain an anabolic state.
whole-body blood glucose disposal and reduced
strength and functional capacity. Sarcopenia is facil-
Amino acids can also function as potent nutri-
itated by a combination of factors, which include a
tional signalling molecules, and play an active regu-
more sedentary lifestyle and a suboptimal diet. The
latory role in modulating muscle protein metabo-
age-related changes in skeletal muscle mass are
lism. The precise mechanisms responsible for this
attributed to a disruption in the regulation of muscle
amino acid-induced anabolic response have not yet
protein turnover, which results in a chronic imbal-
been fully elucidated. Amino acids can modulate
ance between muscle protein synthesis and degrada-
mRNA translation and thus protein synthesis in
tion rates.
skeletal muscle.
[65]
Increased amino acid availabili-
ty, particularly the branched-chain amino acid leu-
cine, has been shown to activate several key factors
3.1 Aging, Muscle Function and
(mTOR, S6K1) of the mTOR signal transduction
Protein Turnover
pathway, thereby increasing the rate of mRNA
It has been reported that muscle cross-sectional
translation.
[66]
Most of the information on the ef-
area is reduced with aging, which is attributed to a
fects and interaction of nutrition and exercise on
reduction in muscle fibre size and number. Short
translation initiation has been obtained in rodent
et al.
[75,76]
measured the cross-sectional area of mus-
studies.
[67-73]
However, information on the response
cle groups in mid-thigh using CT imaging in healthy
of intracellular signalling to exercise and nutrition in
human subjects (aged 1888 years) and observed a
humans is relatively scarce. This field of research is
25% age-related decline in muscle cross-sectional
not only of potential interest for the athlete, but also
area starting from the fourth decade of life. In addi-
for elderly humans. Measuring the modulation of
tion, a parallel decrease in muscle strength (knee
protein synthesis and signalling by exercise and
extension) accompanies the decline in muscle mass.
nutrition will provide important information, which
Maximal oxygen uptake capacity is another physical
will be instrumental for the development of specific
performance-related factor that has been shown to
interventions to counteract or reduce the loss in
decline with aging.
[75]
As a result, the muscle of
muscle mass with aging.
older individuals is thought to fatigue faster than
those of young adults. The observed decline in mus-
3. Aging
cle mass and strength with aging, in combination
The progressive aging of our population imposes
with a reduced endurance capacity, is at least partly
a major threat to welfare in our Western society. In
attributed to reduced levels of physical activity.
[77]
Europe, the number of people aged
65 years is
The subsequent lower energy expenditure rates in
projected to rise by 70% over the next 50 years, from
the elderly represent a risk factor for the develop-
61 million in 2000 to up to 103 million by 2050. By
ment of obesity. The concomitant alteration in body
the year 2050, people with an age of
65 years will
composition impairs whole-body insulin sensitivity
comprise >25% of the total EU population.
[74]
This
and contributes to the development of type 2 diabe-
aging of our society is responsible for a major in-
tes, hyperlipidaemia and hypertension. The com-
crease in population morbidity, which will put an
bined effect of these metabolic abnormalities induce
increasing demand on our healthcare system. The
an increased risk of developing cardiovascular dis-
increased morbidity in the elderly is associated with
ease and other co-morbidities.
[77]
©
2007 Adis Data Information BV. All rights reserved.
Sports Med 2007; 37 (10)
Study 