Sarcopenia

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton, Saliu Balogun, Kim Jackson, Aminat Abolade and Candace Goh  

Introduction

Sarcopenia is defined as being a decline in muscle function (either walking speed or grip strength) associated with loss of muscle mass. Sarcopenia most commonly affects elderly and sedentary populations and patients who have comorbidities that affect the musculoskeletal system or impair physical activity. Sarcopenia leads to disability, falls, and increased mortality. Loss of muscle strength and aerobic function are 2 of the hallmarks of frailty. Sarcopenia has been linked to an increased prevalence of osteoporosis, thus further increasing its propensity to produce fractures.

Sarcopenia is a growing global health concern.

• Sarcopenia has been reported to affect 5-13% of persons aged 60 to 70 years and up to 50% of people over 80 years of age.
• In 2000, the number of people ≥ 60 years old around the world was estimated to be 600 million.
• This population is expected to rise to 1.2 billion by 2025 and 2 billion by 2050.
• Even with a conservative estimate of prevalence, sarcopenia affects >50 million people today and will affect >200 million in the next 40 years.

Risk Factors

Sarcopenia is considered by most to be an inevitable part of aging. However, the degree of sarcopenia is highly variable and is dependent upon presence of certain risk factors:

1. Physical Inactivity:
   • Lack of exercise is believed to be the foremost risk factor for sarcopenia. A gradual decline in myocyte numbers begins around 50 years of age. The decline in muscle fiber and strength is more pronounced in patients with sedentary lifestyle as compared to patients who are physically more active. Even professional athletes such as marathon runners and weight lifters show a gradual (more slow) decline in their speed and strength with aging.
2. Hormone and Cytokine Imbalance:
   • Age-related decreases in hormone concentrations, including growth hormone, testosterone, thyroid hormone and insulin-like growth factor, lead to loss of muscle mass and strength. Extreme muscle loss often results from a combination of diminishing hormonal anabolic signals and promotion of catabolic signals mediated through pro-inflammatory cytokines eg tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6). Elevated levels of both TNF-α and IL-6 have been shown to be present in skeletal muscles of older individuals.
3. Protein Synthesis and Regeneration:
   • A decrease in the body's ability to synthesize protein, coupled with inadequate intake of calories and/or protein to sustain muscle mass, is common in sarcopenia. Oxidized proteins increase in skeletal muscle with aging and lead to a buildup of lipofuscin ( yellowish brown, pigmented, insoluble granules, thought to be the waste products of oxidative metabolism, stored within each cell of the body and accumulate with time) and cross-linked proteins that are inadequately removed via the proteolysis system. This leads to an accumulation of non-contractile dysfunctional protein in skeletal muscles, and is part of the reason muscle strength decreases severely in sarcopenia.
4. Motor Unit Remodelling:
   • Age-related reduction in motor nerve cells responsible for sending signals from the brain to the muscles to initiate movement also occurs. Satellite cells are small mononuclear cells that abut muscle fibers and are normally activated upon injury or exercise. In response to these signals, satellite cells differentiate and fuse into the muscle fiber, helping to maintain muscle function. One current hypothesis is that sarcopenia is caused, in part, by a failure in satellite cell activation.
5. Evolutionary:
   • Evolutionary theories implicate the failure of the body to maintain muscle mass and function with aging on genes that governing these traits. ie genes suited for high levels of obligatory muscular effort required for survival in the Late Paleolithic epoch are ill-matched to a modern lifestyle characterized by high levels of lifelong sedentary behavior.
6. Early Development Factor:
   • Early environmental influences on growth and development may have long-term consequences for human health. Low birth weight, a marker of a poor early environment, is associated with reduced muscle mass and strength in adult life.

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