Sarcopenia refers to the loss of skeletal muscle due to aging. (and the lack of proper exercise and diet to maintain this muscle) Why should you care? IMO, Sarcopenia is the main reason our bodies quit functioning like they did when we were younger. This muscle loss (about 1/2 of pound per year) starts in our 20's. You won't notice the effects until you reach your 60's or 70's.
The good news is it doesn't have to be debilitating if you work on keeping as much muscle as you can.
You want the fountain of Youth? Do what it takes to keep your muscle mass as you age. This takes work.
The following is an article from Ronenn Rounbenoff, MD, MHS; and Carmen Castaneda, MD, PhD.
Sarcopenia Understanding the Dynamics of
Aging Muscle
 Ronenn Roubenoff, MD, MHS; Carmen
Castaneda, MD, PhD
Sarcopenia is not a disease but
rather refers specifically to the universal, involuntary decline in lean body
mass that occurs with age, primarily due to the loss of skeletal muscle.1
Sarcopenia has important consequences. The loss of lean body mass reduces
function, and loss of approximately 40% of lean body mass is fatal.2-5
Sarcopenia is distinct from wasting involuntary weight loss due to inadequate
intake, which is seen in starvation, advanced cancer, or acquired
immunodeficiency syndrome. Sarcopenia also differs from cachexia, a
cytokine-driven loss of lean body mass that occurs despite maintenance of
weight, which is seen in patients with rheumatoid arthritis, congestive heart
failure, or renal failure.6 However,
sarcopenia is the backdrop against which the drama of disease is played out:
a body already depleted of protein because of aging is less able to withstand
the protein catabolism that comes with acute illness or inadequate protein
intake.7
Protein stores in humans have at
least 2 important functions. First, unlike fat, which is truly stored in the
sense that it is in reserve for times of starvation, body proteins are in use
as contractile proteins in muscle, antibodies, enzymes, etc. Thus, loss of
protein means loss of function. Second, during illness, nitrogen must be
mobilized from muscle to provide amino acids to the immune system, liver, and
other organs. If adequate nitrogen cannot be provided, either exogenously
from diet or endogenously from muscle, the body's capacity to withstand an
acute insult declines, andat about 60% of baseline nitrogen
throughputthe body ceases to function.2-5 Thus,
it is likely that some of the explanation for the poorer outcomes observed
with nearly all diseases in older persons relates to their lower body protein
stores. Muscle is the major source of protein for functions such as antibody
production, wound healing, and white blood cell production during illness. If
the body's protein reserves are already depleted by sarcopenia, there is less
to mobilize for illness.
The determinants of sarcopenia
include genetic and environmental factors,8 with a
complex series of poorly understood interactions. Amino acids and proteins
are the primary substrates for skeletal muscle mass maintenance. Therefore,
knowledge of amino acid kinetics and the balance between protein synthesis
and protein breakdown is pivotal to understand how sarcopenia develops. Until
now, most studies have indicated that muscle protein synthesis declines with
age, suggesting that sarcopenia is due to failure of muscle protein
synthesis.9-11
In this issue of THE JOURNAL, Volpi
and colleagues12 report
findings from the largest reported study to date to examine basal muscle
protein synthesis, as well as the first direct measures of protein breakdown
and net muscle protein balance in a group of healthy older men. The
three-compartment model developed by this group of investigators uses amino
acids labeled with stable isotopes to examine the rates of inward and outward
transport of amino acids in muscle and to determine the rates of muscle
protein synthesis and breakdown and the size of the intracellular free amino
acid pool. Contrary to the notion of reduced protein synthesis with age,
muscle protein synthesis was slightly higher in healthy older men compared
with healthy young men. Basal net protein catabolism was not statistically
different between the groups.
These observations strongly suggest
that sarcopenia is not due to inadequate basal (fasting) protein synthesis.
More likely, aging muscle fails to respond to stimuli that are anabolic to
young muscleeg, diet and exerciseperhaps because of hormonal or
immunological changes that occur with age and no longer favor anabolism.8 For
example, an earlier study by Volpi et al13
suggested that the anabolic response to a mixed glucose-amino acid meal was
reduced in older men. Taken together, these 2 studies implicate insulin
resistance or immune factors, such as catabolic cytokines or other hormonal
or immunological factors, acting primarily in the postprandial state as an
important cause of sarcopenia. These observations also suggest that
interventions aimed at treating or preventing sarcopenia should maximize the
response of muscle to anabolic stimuli, such as diet and exercise, rather
than trying to increase basal protein synthesis. Since the latter may be
impossible to achieve or require potentially harmful doses of anabolic
agents, such as growth hormone, insulin-like growth factor 1, or
testosterone, the results of the study by Volpi et al appear to be good news
indeed.
However, several limitations of this
study should be considered. First, only men were studied, and it is possible
that sarcopenia is regulated differently in men and women. Second, these were
remarkably lean, healthy older men, and the generalizability of these data to
the more typical overweight, sedentary population in many developed countries
remains to be determined. For example, if insulin resistance were partly
responsible for sarcopenia, then studying lean subjects would tend to bias
the results toward the null, as was the case in this study. Third, the major
advance of this investigation over earlier studies is the measurement of
protein breakdown and synthesis. Yet, this technique is still very new, and
it is unknown how sensitive it is to factors such as changes in diet and
physical activity. Additional studies would help to clarify whether the
increase in basal protein synthetic rate via enhanced entry of amino acids
into muscle observed in the older subjects represents the end result rather
than the process by which skeletal muscle compensates for the gradual loss of
muscle mass.
The study by Volpi et al indicates
that sarcopenia cannot be explained by differences in basal rates of muscle
protein turnover between young and older men. The authors acknowledge additional
work is needed in the older population to determine the individual and
combined effects on muscle mass of specific stimuli, such as protein and
energy intakes, physical activity, sex hormones, and growth hormone. For the
clinician, the most important message is that sarcopenia exists in all older
individuals. In the face of acute or chronic illness, maximizing muscle mass
and protein stores through adequate nutritional support, aggressive physical
therapy, and exercise programs becomes all the more important if muscle
function and quality of life are to be preserved in the older population.
Author/Article
Information
Author Affiliation: Nutrition,
Exercise Physiology, and Sarcopenia Laboratory, Jean Mayer USDA Human
Nutrition Research Center on Aging, Tufts University, Boston, Mass.
Corresponding Author and Reprints:
Ronenn Roubenoff, MD, MHS, Nutrition, Exercise Physiology, and Sarcopenia
Laboratory, Jean Mayer USDA Human Nutrition Research Center on Aging, Tufts
University, Boston, MA 02111 (e-mail: Roubenoff@hnrc.tufts.edu).
Editorials represent the opinions of
the authors and THE JOURNAL and not those of the American Medical
Association.
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