Anabolic steroids – growth hormone
Skeletal muscle comprises about 40% of the body mass in
humans. An adequate muscle mass is critical for health as
muscle has several important functions: locomotion, breathing,
thermogenesis, protection of internal organs, glucose
and fat metabolism. The regulation of muscle mass is of
interest to a diverse group of people. There are those, such as
power athletes and body builders, who are primarily
interested in increasing their muscle mass. Others are
concerned with preventing muscle loss. This is critical for
the frail elderly, those with myopathies, cancer, sepsis, HIV/
AIDS and other diseases, those suffering from reduced
mobility as a result of injury, and astronauts. Themechanisms
regulating muscle mass maintenance are widely studied due
to the importance of this tissue for health.
Muscle is a highly plastic tissue able to adapt to changing
functional demands. Increased load on muscle results in an
increase in its mass or hypertrophy, whereas unloading or
disuse leads to a decrease in mass or atrophy. Exercise is a key
regulator of muscle mass, as is nutrition (Rennie et al., 2004).
Hormonal factors are also important. It is evident that
men have a greater muscle mass than women. This is
primarily due to the anabolic effects of testosterone. Indeed,
anabolic steroids have long been used by body builders due
to their dramatic effects on muscle bulk. Growth hormone
(GH) and insulin-like growth factor I (IGF-I) have a key role
in the regulation of body size in growing animals but their
role in adults is less clear. IGF-I clearly has anabolic activity
but its mechanism of action as an endocrine, circulating
hormone may be distinct from its activity as an autocrine/
paracrine growth factor.
This review begins with a basic introduction to the GH/
IGF-I axis and the mechanisms of muscle mass regulation.
The evidence for an effect of these molecules on muscle mass
in human, animal and cell culture models is examined
followed by a discussion of their use as performanceenhancing
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