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Sarcopenia is the degenerative loss of skeletal muscle mass (0.5–1% loss per year after the age of 50), quality, and strength associated with aging. Sarcopenia is a component of the frailty syndrome. It is often a component of cachexia. It can also exist independently of cachexia; whereas cachexia includes malaise and is secondary to an underlying pathosis (such as cancer), sarcopenia may occur in healthy people and does not necessarily include malaise. The term is from Greek σάρξ sarx, "flesh" and πενία penia, "poverty".
Signs and symptoms
Sarcopenia is characterized first by a muscle atrophy (a decrease in the size of the muscle), along with a reduction in muscle tissue quality, characterized by such factors as replacement of muscle fibres with fat, an increase in fibrosis, changes in muscle metabolism, oxidative stress, and degeneration of the neuromuscular junction and leading to progressive loss of muscle function and frailty. Sarcopenia is determined by two factors: initial amount of muscle mass and rate at which aging decreases muscle mass. Due to the loss of independence associated with loss of muscle strength, the threshold at which muscle wasting becomes a disease is different pathologically from person to person.
Simple circumference measurement does not provide enough data to determine whether or not an individual is suffering from severe sarcopenia. Sarcopenia is also marked by a decrease in the circumference of distinct types of muscle fibers. Skeletal muscle has different fiber-types, which are characterized by expression of distinct myosin variants. During sarcopenia, there is a decrease in "type 2" fiber circumference (Type II), with little to no decrease in "type I" fiber circumference (Type I), and deinervated type 2 fibers are often converted to type 1 fibers by reinnervation by slow type 1 fiber motor nerves.
Satellite cells are small mononuclear cells that abut the muscle fiber. Satellite cells are normally activated upon injury or exercise. These cells then differentiate and fuse into the muscle fiber, helping to maintain its function. One theory is that sarcopenia is in part caused by a failure in satellite cell activation.
Oxidized proteins increase in skeletal muscle with age and leads to a buildup of lipofuscin and cross-linked proteins that are normally removed via the proteolysis system. These proteins compile in the skeletal muscle tissue, but are dysfunctional. This leads to an accumulation of non-contractile material in the skeletal muscle. This helps explain why muscle strength decreases severely, as well as muscle mass, in sarcopenia.
One group has suggested that the evolutionary basis for the failure of the body to maintain muscle mass and function with age is that the genes governing these traits were selected in a Late Paleolithic environment in which there was a very high level of obligatory muscular effort, and that these genetic parameters are therefore ill-matched to a modern lifestyle characterized by high levels of lifelong sedentary behavior.
Epidemiological research into the developmental origins of health and disease has shown that 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. One study has shown that lower birth weight is associated with a significant decrease in muscle fibre score, suggesting that developmental influences on muscle morphology may explain the widely reported associations between lower birth weight and sarcopenia.
A working definition for diagnosis was proposed in 1998 by Baumgartner et al which uses a measure of lean body mass as determined by dual energy X-ray absorptiometry (DEXA) compared to a normal reference population. His working definition uses a cut point of 2 standard deviations below the mean of lean mass for gender specific healthy young adults.
Since Baumgartner's working definition first appeared, some consensus groups have refined the definition, including the European Working Group on Sarcopenia in Older People (EWGSOP). Their consensus definition is:
- Low muscle mass, (e.g. >2 standard deviations below that mean measured in young adults [aged 18–39 years in the 3rd NHANES population] of the same sex and ethnic background).
- Low gait speed (e.g. a walking speed below 0.8 m/s in the 4-m walking test)
- Low muscular strength (e.g. grip strength: <30 kg in males, <20 kg in females)
Severe sarcopenia requires the presence of all three conditions.
The ICD-10 Clinical Modification (ICD-10-CM), which is the United States' national adaptation of ICD-10, classifies sarcopenia to code M62.84. (This is an enhancement over the base ICD-10 classification, which only uses the 5th character position within Chapter XVII to identify the anatomical site of occurrence.)
The European Working Group on Sarcopenia in Older People (EWGSOP) developed a clinical definition and consensus diagnostic criteria for age-related sarcopenia, using the presence of low muscle mass and either low muscular strength or low physical performance. Severe sarcopenia requires the presence of all three conditions.
Lack of exercise is thought to be a significant risk factor for sarcopenia. Even highly trained athletes experience its effects; master-class athletes who continue to train and compete throughout their adult lives exhibit a progressive loss of muscle mass and strength, and records in speed and strength events decline progressively after age 30.
Master-class athletes maintain a high level of fitness throughout their lifespan. Even among master athletes, performance of marathon runners and weight lifters declines after approximately 40 years of age, with peak levels of performance decreased by approximately 50% by 80 years of age. However, a gradual loss of muscle fibers begins to occur at around 50 years of age.
Exercise is of interest in treatment of sarcopenia; evidence indicates increased ability and capacity of skeletal muscle to synthesize proteins in response to short-term resistance exercise. A 2009 Cochrane review also found evidence that in older adults progressive resistance training can improve physical performance (gait speed) and muscular strength, which are two key components of sarcopenia.
As of July 2015[update], there are no approved medications for the treatment of sarcopenia; however, β-hydroxy β-methylbutyrate (HMB), a metabolite of leucine which is sold as a dietary supplement, has demonstrated efficacy in preventing the loss of muscle mass in individuals with sarcopenia. A growing body of evidence supports the efficacy of HMB as a treatment for reducing, or even reversing, the loss of muscle mass, muscle function, and muscle strength in hypercatabolic disease states such as cancer cachexia; consequently, as of June 2016[update] it is recommended that both the prevention and treatment of sarcopenia and muscle wasting in general include supplementation with HMB, regular resistance exercise, and consumption of a high-protein diet. Based upon a meta-analysis in 2015, HMB supplementation appears to be useful as a treatment for preserving lean muscle mass in older adults.[note 1] More research is needed to determine the precise effects of HMB on muscle strength and function in this age group.
DHEA and human growth hormone have been shown to have little to no effect in this setting. Growth hormone increases muscle protein synthesis and increases muscle mass, but does not lead to gains in strength and function in most studies. This, and the similar lack of efficacy of its effector insulin-like growth factor 1 (IGF-1), may be due to local resistance to IGF-1 in aging muscle, resulting from inflammation and other age changes.
Testosterone or other anabolic steroids have also been investigated for treatment of sarcopenia, and seem to have some positive effects on muscle strength and mass, but cause several side effects and raise concerns of prostate cancer in men and virilization in women. Additionally, recent studies suggest testosterone treatments may induce adverse cardiovascular events. Other approved medications under investigation as possible treatments for sarcopenia include ghrelin, vitamin D, angiotensin converting enzyme inhibitors, and eicosapentaenoic acid.
Protein intake and physical activity are important stimuli for muscle protein synthesis. A number of expert groups have proposed an increase in dietary protein recommendations for older age groups to 1.0-1.2 g/kg body weight per day. Key nutrient supplementation in older adults is of interest in the prevention of sarcopenia and frailty, since it is a simple, low-cost treatment approach without major side effects.
A study, in community dwelling older adults with an average age of 67 years, found the UK prevalence of sarcopenia to be 4.6% in men and 7.9% in women using the EWGSOP approach. Another study, conducted in the United States among older adults with an average age of 70.1 years, found the prevalence of sarcopenia to be 36.5%. Sarcopenia affects about half of people over 80 in New Mexico.
Society and culture
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Due to the lessened physical activity and increased longevity of industrialized populations, sarcopenia is emerging as a major health concern. Sarcopenia may progress to the extent that an older person may lose his or her ability to live independently. Furthermore, sarcopenia is an important independent predictor of disability in population-based studies, linked to poor balance, gait speed, falls, and fractures. Sarcopenia can be thought of as a muscular analog of osteoporosis, which is loss of bone, also caused by inactivity and counteracted by exercise. The combination of osteoporosis and sarcopenia results in the significant frailty often seen in the elderly population.
Many opportunities remain for further refinement of reference populations by ethnic groups, and to further correlate the degrees of severity of sarcopenia to overt declines in functional performance (preferably using verified functional tests), as well as incidence of hospitalization admissions, morbidity, and mortality. The body of research points toward severe sarcopenia being predictive of negative outcomes, similar to those already shown to exist with frailty syndrome, as defined by the criteria set forth in 2001 by Fried et al.
Future research should aim to gain a deeper understanding of the molecular and cellular mechanisms of sarcopenia and the application of a lifecourse approach to understanding aetiology as well as to informing the optimal timing of interventions.
Diagnosis can be difficult due to the comprehensive measurements used in research that are not always practical in healthcare settings. Hand grip strength alone has also been advocated as a clinical marker of sarcopenia that is simple and cost effective and has good predictive power, although it does not provide comprehensive information.
Exercise remains the intervention of choice for sarcopenia, but translation of findings into clinical practice is challenging. The type, duration and intensity of exercise are variable between studies, so an off the shelf exercise prescription for sarcopenia remains an aspiration.
The role of nutrition in preventing and treating sarcopenia is less clear. Large, well-designed studies of nutrition particularly in combination with exercise are needed, ideally across healthcare settings. For now, basing nutritional guidance on the evidence available from the wider health context is probably the best approach with little contention in the goals of replacing vitamin D where deficient, and ensuring an adequate intake of calories and protein, although there is debate about whether currently recommended protein intake levels are optimal.
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In conclusion, HMB treatment clearly appears to be a safe potent strategy against sarcopenia, and more generally against muscle wasting, because HMB improves muscle mass, muscle strength, and physical performance. It seems that HMB is able to act on three of the four major mechanisms involved in muscle deconditioning (protein turnover, apoptosis, and the regenerative process), whereas it is hypothesized to strongly affect the fourth (mitochondrial dynamics and functions). Moreover, HMB is cheap (~30– 50 US dollars per month at 3 g per day) and may prevent osteopenia (Bruckbauer and Zemel, 2013; Tatara, 2009; Tatara et al., 2007, 2008, 2012) and decrease cardiovascular risks (Nissen et al., 2000). For all these reasons, HMB should be routinely used in muscle-wasting conditions especially in aged people.
- Wu H, Xia Y, Jiang J, Du H, Guo X, Liu X, Li C, Huang G, Niu K (September 2015). "Effect of beta-hydroxy-beta-methylbutyrate supplementation on muscle loss in older adults: a systematic review and meta-analysis". Arch. Gerontol. Geriatr. 61 (2): 168–175. doi:10.1016/j.archger.2015.06.020. PMID 26169182.
RESULTS: A total of seven randomized controlled trials were included, in which 147 older adults received HMB intervention and 140 were assigned to control groups. The meta-analysis showed greater muscle mass gain in the intervention groups compared with the control groups (standard mean difference=0.352kg; 95% confidence interval: 0.11, 0.594; Z value=2.85; P=0.004). There were no significant fat mass changes between intervention and control groups (standard mean difference=-0.08kg; 95% confidence interval: -0.32, 0.159; Z value=0.66; P=0.511).
CONCLUSION: Beta-hydroxy-beta-methylbutyrate supplementation contributed to preservation of muscle mass in older adults. HMB supplementation may be useful in the prevention of muscle atrophy induced by bed rest or other factors. Further studies are needed to determine the precise effects of HMB on muscle strength and physical function in older adults.
- Brioche T, Pagano AF, Py G, Chopard A (April 2016). "Muscle wasting and aging: Experimental models, fatty infiltrations, and prevention". Mol. Aspects Med. 50: 56–87. doi:10.1016/j.mam.2016.04.006. PMID 27106402.
In conclusion, HMB treatment clearly appears to be a safe potent strategy against sarcopenia, and more generally against muscle wasting, because HMB improves muscle mass, muscle strength, and physical performance. It seems that HMB is able to act on three of the four major mechanisms involved in muscle deconditioning (protein turnover, apoptosis, and the regenerative process), whereas it is hypothesized to strongly affect the fourth (mitochondrial dynamics and functions). Moreover, HMB is cheap (~30– 50 US dollars per month at 3 g per day) and may prevent osteopenia (Bruckbauer and Zemel, 2013; Tatara, 2009; Tatara et al., 2007, 2008, 2012) and decrease cardiovascular risks (Nissen et al., 2000). For all these reasons, HMB should be routinely used in muscle-wasting conditions especially in aged people. ... 3 g of CaHMB taken three times a day (1 g each time) is the optimal posology, which allows for continual bioavailability of HMB in the body (Wilson et al., 2013).
- Argilés JM, Campos N, Lopez-Pedrosa JM, Rueda R, Rodriguez-Mañas L (June 2016). "Skeletal Muscle Regulates Metabolism via Interorgan Crosstalk: Roles in Health and Disease". J. Am. Med. Dir. Assoc. 17 (9): 789–796. doi:10.1016/j.jamda.2016.04.019. PMID 27324808.
Studies suggest dietary protein and leucine or its metabolite b-hydroxy b-methylbutyrate (HMB) can improve muscle function, in turn improving functional performance. ... These have identified the leucine metabolite β-hydroxy β-methylbutyrate (HMB) as a potent stimulator of protein synthesis as well as an inhibitor of protein breakdown in the extreme case of cachexia.65, 72, 76, 77, 78, 79, 80, 81, 82, 83, 84 A growing body of evidence suggests HMB may help slow, or even reverse, the muscle loss experienced in sarcopenia and improve measures of muscle strength.44, 65, 72, 76, 77, 78, 79, 80, 81, 82, 83, 84 However, dietary leucine does not provide a large amount of HMB: only a small portion, as little as 5%, of catabolized leucine is metabolized into HMB.85 Thus, although dietary leucine itself can lead to a modest stimulation of protein synthesis by producing a small amount of HMB, direct ingestion of HMB more potently affects such signaling, resulting in demonstrable muscle mass accretion.71, 80 Indeed, a vast number of studies have found that supplementation of HMB to the diet may reverse some of the muscle loss seen in sarcopenia and in hypercatabolic disease.65, 72, 83, 86, 87 The overall treatment of muscle atrophy should include dietary supplementation with HMB, although the optimal dosage for each condition is still under investigation.68 ...
Figure 4: Treatments for sarcopenia. It is currently recommended that patients at risk of or suffering from sarcopenia consume a diet high in protein, engage in resistance exercise, and take supplements of the leucine metabolite HMB.
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There are a number of nutrition products on the market that are touted to improve sports performance. HMB appears to be the most promising and to have clinical applications to improve muscle mass and function. Continued research using this nutraceutical to prevent and/or improve malnutrition in the setting of muscle wasting is warranted.
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