Potential Application of Essential Amino Acid Supplementation to Treat Sarcopenia in Elderly People

Aging is universally associated with a progressive decline in both physical and metabolic function across a diverse rangeofspeciesfromnematodestohumans.Moreover,progressive reductions in skeletal muscle mass, strength, and quality (referred to as sarcopenia) are often observed with aging. An associated problem is decline in maximal aerobic capacity, and all of these factors together contribute to the physical frailty of older people. In light of the rapidly expanding aging population, sarcopenia and related disorders are emerging as a major public health problem of the 21st century. This is particularly pertinent considering the loss of independence that is associated with frailty and falls that increase the risk of fractures. Another consequence of the sarcopenia of aging is a deleterious change in body composition resulting in many metabolic alterations leading on to metabolic syndrome and increased cardiovascular disease risk. Indeed, sarcopenia has received considerable attention by both researchers and clinicians over the last two decades. Muscle changes with age include reduction in fiber numbers, motor units, and a relative loss of type II fibers with a preservation of type I fibers (1) that may result from reductions in transcript levels of myosin heavy chain isoforms IIa and IIx without any decline of isoform I as reported in older people (2). Thus, a relative increase of type I fibers occurs with age, but these fibers are lower in abundance of mitochondria (3) than those in longdistance runners who also have a relatively high proportion of type I fibers but with high abundance of mitochondria. Reductionsinskeletalmuscleproteinabundances(thebalancebetween protein synthesis and degradation) are a key contributing factor to the development of sarcopenia. Mechanistically, it has been extensively reported that skeletal muscle protein synthesis is reduced in aging adults (2, 4, 5). The reduction in skeletal muscle mitochondrial protein synthesis (6) occurs in conjunction with reducedmitochondrialcapacityforATPproduction(3).Thank