Sarcopenia is a syndrome characterised by progressive and generalised loss of skeletal muscle mass and strength with a risk of adverse outcomes such as physical disability, poor quality of life and death.1 True prevalence in certain population groups is unknown, but a systematic review on the prevalence of sarcopenia by the European Working Group on Sarcopenia2 (EWGSOP) found the prevalence of sarcopenia to be 1–29% in community dwelling populations, 14–33% in long-term care populations and 10% in one acute care population. 

The EWGSOP has recommended two criteria for identifying sarcopenia: low muscle mass and low muscle function, the latter of which may be measured either by testing muscle strength or performance.1

A current consensus is emerging that sarcopenia is worth diagnosing as there are now interventions that can treat the condition and may potentially reverse the adverse outcomes described above. EWGSOP has set out a screening method with an algorithm that can be used to screen patients over 65 years of age for sarcopenia.1

This algorithm has been criticised by the authors of a study looking at the performance of the algorithm in screening older adults for muscle mass assessment.3 They state that the cut off points used in the EWGSOP algorithm select a high proportion of participants to undergo further muscle mass measurement and modified cut off points tailored to specific characteristics of the population being studied would improve the performance of the algorithm. 

Part of diagnosis is muscle mass measurement and there are a few possible methods of measuring this. Gold standard muscle mass measurement is by Magnetic Resonance Imaging or Computed Tomography (CT).4 Dual X-Ray Absorptiometry (DEXA) scan measurements of muscle mass are highly related to CT measurements5 so this has become an acceptable alternative. Bio impedance analysis can also provide a portable and quick measure of muscle mass, but reliability data on this method are lacking.5

Sarcopenia and falls
Characteristics of sarcopenia such as muscle weakness, reduced walking speed and grip strength are also three major risk factors for falls.6 There have been a few studies that look at the link between sarcopenia and falls. 

Results from the Aging and Longevity in the Sirente geographic area (ilSIRENTE) study7 showed that 27.3% of participants diagnosed with sarcopenia during the study reported falls in the two-year follow up of the study compared to only 9.8% of those without sarcopenia. 

Another study8 investigating elderly community dwelling participants in Japan also found that the prevalence of sarcopenia was higher in those who had fallen. This demonstrates that a strong link exists between sarcopenia and falls, although a review on frailty, sarcopenia and falls9 suggests that a greater understanding of the physiological and metabolic deficits that lead to falls is needed.

There is much interest at present in developing interventions to prevent and treat sarcopenia. Current methods of treatment for sarcopenia that have shown some positive results are resistance training and nutrition including essential amino acid supplementation.2

Amino acids and muscle mass
Skeletal muscle stores a huge amount of amino acids, which when recruited during illness or injury are essential to improve recovery outcomes.10 Amino acid availability is very important in the regulation of muscle protein synthesis and degradation.11 This is especially important in the older population who tend to be less active as it has been shown that an enhanced daily protein intake is important to maintain protein turnover during periods of inactivity.10 This change in protein intake is important because changes in physical activity and amount of protein intake cause adaptations in the efficiency of protein use so that a consistently high protein intake does not mean that skeletal muscle mass will necessarily increase.11

In the older population a phenomenon termed anabolic resistance has been documented, which means that older individuals need to consume a greater amount of protein to stimulate muscle protein synthesis.12 It is unknown why this phenomenon occurs, but there is a theory that impaired blood flow for delivery of nutrients and essential amino acids to muscles is a contributing factor.13 Other factors could be that ribosomes have a reduced capacity for translation with ageing as shown in studies of rats.14 Ageing also reduces skeletal muscle capillaries and increases basement membrane width,15 which may also contribute to this phenomenon. Muscle protein synthesis is primarily stimulated by essential amino acids.16 

Leucine and beta-hydroxy-beta-methylbutyrate (HMB)
Leucine is an essential amino acid, which means that it cannot be made by the body and must be obtained from the diet. It has been shown that giving a leucine supplement with exercise may improve muscle mass.17 One mechanism may be that it acts as a strong activator of the mammalian target of rapamycin (mTOR) signaling pathway to stimulate muscle protein synthesis.18 The mTOR signaling pathway regulates protein synthesis19 and impairs breakdown of proteins.20 This is important for muscle protein synthesis as leucine is able to stimulate this at much lower levels than other amino acids.21 It has been found that approximately 5% of leucine is metabolised into HMB in the body.22 HMB works to build muscle protein and prevent muscle protein degradation.23 Due to these positive effects it has been argued that HMB may be partly responsible for the benefits of leucine supplementation.24

Leucine and sarcopenia
Leucine enriched essential amino acid supplements can stimulate muscle protein synthesis similarly in both young and older people following exercise.25 In the older population essential amino acids need to be provided at the optimal dose required to stimulate muscle protein synthesis: too small a dose diminishing and too large a dose saturating protein synthesis.26 

One study found that twice daily between meals, essential amino acid supplements increased the rate of muscle protein synthesis in healthy older women.26 Another study concluded that the results from their study on the effects of exercise and amino acid supplementation in Japanese sarcopenic women suggest that exercise and amino acids together might be effective in improving muscle strength as well as combinations of muscle mass, gait speed and strength.17 In a systematic review by the International Sarcopenia Initiative2 it was felt that although there are a couple of promising results, further research is needed to show that leucine can improve functional outcomes. 

HMB and sarcopenia
A systematic review of seven randomised controlled trials, including a total of 147 older adults taking HMB as a supplement and 140 as controls, showed greater muscle mass gain in the intervention groups.27 One study28 found that leg strength improved with daily supplementation with a combination of HMB, arginine and lysine. But this was a small study with only 50 female participants (average age 76.7 years). Another small study29 also showed improved muscle strength with HMB only. 

These initial small studies suggest that there is a beneficial effect on muscle mass in older adults from taking HMB, but larger better controlled studies are required to look at relevant outcome measures such as muscle strength and physical performance in sarcopenic populations.30

Resistance exercise and sarcopenia
As discussed above, a combination of exercise and essential amino acids has been shown to stimulate muscle protein synthesis in older people.25  Resistance exercise alone is another intervention to treat sarcopenia that has shown some promising results. A systematic review2 showed that muscle mass improved in two out of four studies included and muscle strength improved in three out of four studies included, as did physical performance with 3–18 months of resistance training. 

In one study31 participants were split into groups that were given either supplements, resistance training, both interventions or neither. The authors concluded that trained participants had increased upper and lower limb strength and walking capacity. Participants were only 48% compliant with taking the supplements, however, and attended only 56% of training sessions. Compliance with taking supplements and doing exercise could be another problem with treating sarcopenia in the older population. 

Another study32 demonstrated that three months of progressive resistance training induced improvements in maximal voluntary thigh muscle strength and whole body fat free mass in participants with physical frailty. Suetta et al33 found that 12 weeks of resistance training led to increases in maximal contractile muscle strength and improvements in a functional outcome measure, which was stair walking power. Landi et al34 state that regular exercise is “the only strategy found to consistently prevent frailty and improve sarcopenia and physical function in older adults”.

Conclusion
Sarcopenia is emerging as an important phenomenon in the older population and in future regular screening may allow us to deliver interventions, which could improve important functional outcomes associated with the condition. Essential amino acids and resistance training exercise show the greatest potential in this regard. Comprehensive geriatric assessment in the future may include the assessment of gait speed and grip strength as screening methods for confirming sarcopenia through the measurement of muscle mass.

Conflict of interest: none declared

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