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Ramendu Hom Chaudhuri. The role of amino acids in skeletal muscle health and sarcopenia: A narrative review[J]. The Journal of Biomedical Research. DOI: 10.7555/JBR.38.20240167
Citation: Ramendu Hom Chaudhuri. The role of amino acids in skeletal muscle health and sarcopenia: A narrative review[J]. The Journal of Biomedical Research. DOI: 10.7555/JBR.38.20240167

Unproofed Manuscript: The manuscript has been professionally copyedited and typeset to confirm the JBR’s formatting, but still needs proofreading by the corresponding author to ensure accuracy and correct any potential errors introduced during the editing process. It will be replaced by the online publication version.

The role of amino acids in skeletal muscle health and sarcopenia: A narrative review

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  • Corresponding author:

    Ramendu Hom Chaudhuri, Department of Orthopaedics, Sri Aurobindo Seva Kendra, Jodhpur Park, Kolkata, West Bengal 700068, India. Email: health.research.001@gmail.com

  • Received Date: June 06, 2024
  • Revised Date: September 26, 2024
  • Accepted Date: September 30, 2024
  • The skeletal muscle is the largest organ present inside the body and is responsible for mechanical activities like maintaining posture, movement, respiratory function, and support for the health and functioning of other systems of the body. Skeletal muscle atrophy is a condition associated with a reduction in muscle size, strength, and activity, which leads to an increased dependency on movement, an increased risk of falls, and a reduced quality of life. Various conditions like osteoarthritis, osteoporosis, and fractures are directly associated with an increased muscle atrophy. Additionally, numerous risk factors, like aging, malnutrition, physical inactivity, and certain disease conditions, through distinct pathways negatively affect skeletal muscle health and lead to muscle atrophy. Among the various determinants of the overall muscle health, the rate of muscle protein synthesis and degradation is an important parameter that eventually alters the fate of overall muscle health. In conditions of excessive skeletal muscle atrophy, including sarcopenia, the rate of muscle protein degradation usually exceeds the rate of protein synthesis. The availability of amino acids in the systemic circulation is a crucial step for muscle protein synthesis. The current review aimed to consolidate the existing evidence of amino acids, highlight their mechanisms of action, and assess their roles and effectiveness in enhancing skeletal muscle health.

  • Although the bones are the rigid framework that provides necessary structural stability and posture for the body, they lack the ability to initiate and/or maintain the movement of any body part. Skeletal muscles are the type of muscles that are associated with the skeletal system and are responsible for the movement of the body. The rhythmic contraction and relaxation of skeletal muscles generate adequate kinetic energy that is essential for the initiation of any voluntary and (in some cases) involuntary movements of the body. The skeletal muscle is the largest organ present inside the body and comprises approximately 40%–50% of the total body weight of an adult[12]. Skeletal muscle plays a vital role in providing the mass and strength for maintaining posture, initiating and maintaining bodily movements, and respiratory functioning[2].

    Because muscle protein synthesis (MPS) is a highly complex process that involves various molecular pathways, the availability of amino acids (AAs) in the systemic circulation is also crucial for optimal MPS. It is critical to understand the exact pathways by which the AAs are able to modulate the MPS, with a particular emphasis on muscle degenerative conditions like sarcopenia. The current review aimed to consolidate the available evidence on the role of AAs in skeletal muscle health and highlight the mechanisms of AA actions in promoting MPS and preventing muscle protein degradation.

    AAs are small organic compounds that are the building blocks for all proteins in the body. Structurally, AAs are compounds that have an amine and carboxylic acid functional group with a carbon skeleton. Various compounds exist in nature, which are identified as AAs based on their structure, but currently, there are 20 AAs that have been identified to be involved in the protein synthesis process in humans[3]. Other than protein synthesis, AAs also play an important role in the production of various physiologically important substances like polyamines, glutathione, nitric oxide (NO), and various hormones as well[4]. Additionally, AAs also have key roles in various cellular signaling, immune system activity, mood and sleep, metabolism, and osmoregulation[5].

    Various methods are used for classifying these protein-forming AAs based on their structure, function, and other properties. The most widely used classification method divides these AAs into two types based on their availability, namely essential and non-essential amino acids. Essential amino acids (EAAs) are the AAs that are required to be taken from external or dietary sources, because the body is not able to synthesize them, while non-essential amino acids (NEAAs) are the AAs that are not generally required to be taken from external or dietary sources as the body can synthesize them. Out of the 20 AAs, there are nine EAAs (i.e., lysine, leucine, isoleucine, methionine, valine, threonine, phenylalanine, histidine, and tryptophan) and eleven NEAAs (i.e., arginine, alanine, aspartate, asparagine, cysteine, glutamine, glutamate, serine, proline, glycine, and tyrosine)[67]. In certain disease conditions, the metabolism of AAs in the body is altered, which increases the requirement for certain AAs that can be either essential or non-essential. Such AAs are termed as conditionally essential amino acids (CEAAs)[89]. In such conditions, the availability of CEAAs becomes necessary to cope with an increased demand of the body, which, if not met, would lead to a CEAAs-deficient state that would directly affect various physiological pathways, with the major one being the protein synthesis pathway.

    Because AAs are structural blocks for protein formation and the skeletal muscle is largely composed of proteins, AAs play a crucial role in maintaining normal muscle health and are also important for muscle growth, repair, and overall functioning[10]. Among the various determinants of MPS, the presence of an optimal AA level in the systemic circulation is an important parameter that has a strong effect on the fate of the MPS rate.

    The mechanistic target of rapamycin (mTOR) is a kinase molecule present in the cellular cytoplasm. mTOR is considered a master regulator of protein translation activity and is present in two subunits, namely, mTORC1 and mTORC2 [1112]. AAs, particularly leucine and arginine, play a crucial role in directly stimulating the mTOR signaling, thereby initiating the downstream signaling pathways and ultimately causing a spike in the cellular protein synthesis cascade. The presence of AAs activates Rag GTPase to form the heterodimers GTP-bound RagA/B and GDP-bound RagC/D and inversely, in their absence, they form GDP-bound RagA/B and GTP-bound RagC/D. Acting as an upstream of Rag GTPases, the complexes of GTPase-activating proteins towards Rags (GATOR) control the mTORC1 signaling. GATOR1, a subcomplex of GATOR, negatively regulates the activation of Rag GTPase in the absence of AAs[13].

    Leucine plays a role in the mTORC1 activation through the sestrin 2 protein. Sestrin 2 acts as a sensor for leucine. Under leucine deprivation, sestrin 2 binds to and inhibits the GATOR2 complex, which enables GATOR1 to hydrolyze the GTP-RagA/B complex, which prevents the recruitment of mTORC1 to the lysosome, thereby inhibiting mTORC1 activation. When leucine levels are restored, leucine binds to sestrin 2, causing sestrin 2 to dissociate from GATOR2. This dissociation relieves the inhibition of mTORC1, allowing it to be recruited to the lysosome and activated. Besides, arginine activates mTORC1 through the cytosolic arginine sensor for mTORC1 subunit 1 (CASTOR1), a cytosolic arginine sensor. In the absence of arginine, CASTOR1 inhibits GATOR2 by binding to it, which prevents mTORC1 activation. Upon arginine availability, arginine binds to CASTOR1, causing it to dissociate from GATOR2, which allows mTORC1 to be activated. Another important regulator of the mTORC1 activity is the sensor of S-adenosylmethionine (SAMTOR). In the absence of methionine, SAMTOR activates GATOR1 via the KPTN, ITFG2, C12orf66, and SZT2-containing regulators of mTOR (KICSTOR). In the presence of an adequate methionine level, the activity of SAMTOR is reduced, thereby inhibiting the activity of KICSTOR and GATOR1, and therefore stimulating mTORC1 activity[1416]. The activation of mTORC1 initiates protein synthesis by two major mechanisms by: (a) phosphorylating and inhibiting the eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1), thus relieving the eIF4E to form a complex eukaryotic initiation factor 4F (eIF4F) and thereby initiating ribosome recruitment and the protein translational process; and (b) phosphorylating the ribosomal S6 protein by stimulating the ribosomal protein S6 kinase beta-1 (S6K1), an essential step required for the translation (Fig. 1)[17].

    Figure  1.  Molecular mechanisms of mTOR activation and muscle protein synthesis.
    Abbreviations: mTORC1, mammalian target of rapamycin complex 1; CASTOR1, cytosolic arginine sensor for mTORC1 subunit 1; GATOR1/2, GTPase-activating proteins towards Rag 1/2; 4E-BP1, eukaryotic translation initiation factor 4E-binding protein 1; eIF4F, eukaryotic initiation factor 4F; S6K1, ribosomal protein S6 kinase beta-1; SAMTOR, sensor of S-adenosylmethionine.

    Some evidence has also suggested the role of AAs in activating the pancreatic ß-cells, which subsequently improves the insulin secretion rate. As the skeletal muscles are primarily responsible for generating the force required for any locomotory activity, they are also the site of the body that utilizes the highest amount of glucose to maintain an adequate energy level. Insulin is the major hormone in the body and is responsible for the transfer of glucose from systemic circulation into skeletal muscle, which thereby improves the energy level. Also, insulin is suggested to increase the activation of mTOR within skeletal muscles, both directly and indirectly by reducing the AMPK level. Because of this dual role, insulin is considered an anabolic hormone that has protein synthesis-stimulatory activity in skeletal muscles[1112]. Hence, by directly getting incorporated into the cytoskeleton of proteins and by activating the pathways responsible for initiating cellular protein synthesis, AAs have an important role in skeletal muscle growth, development, and overall health[1819].

    In the context of skeletal muscle health, both EAAs and NEAAs play pivotal roles in MPS. However, the effectiveness of specific AAs, either individually or in combination, varies depending on their metabolic roles and mechanisms of the action. Branched-chain amino acids (BCAAs), particularly leucine, are potent stimulators of MPS, particularly in elderly populations and those suffering from sarcopenia. However, while BCAAs alone can enhance MPS, studies suggest that a complete EAA mixture is more effective in sustaining MPS over time[2021]. For instance, a study found that older adults experienced a more significant increase in MPS when consuming a high proportion of leucine-enriched EAAs, compared with those who consumed leucine alone, suggesting that the proportion of leucine within an EAA mixture is critical for maximizing the anabolic response, especially in populations at risk of muscle atrophy, such as those with sarcopenia[20]. Similarly, in another clinical study, it was observed that the group of participants supplemented with the combination of EAAs with intermediates of the tricarboxylic acid showed better muscle health parameters, compared with the group supplemented with BCAAs alone[21]. Such observations underscore the needs of all EAAs for optimal beneficial effects on improving muscle health.

    Sarcopenia is a skeletal muscle disease that is characterized by progressive and generalized loss of muscle mass, strength, and associated functionality. The European Working Group on Sarcopenia in Older People (EWGSOP) has classified sarcopenia into two types, namely primary and secondary sarcopenia. Primary sarcopenia is characterized by muscle loss because of aging, whereas secondary sarcopenia is characterized by muscle loss because of factors other than aging, including bed rest, a sedentary lifestyle, inadequate nutrition, smoking, chronic disease conditions (e.g., metabolic syndrome, depression, Parkinson’s disease, anorexia, anemia, osteoporosis), and certain medications[2223]. The worldwide prevalence of sarcopenia is estimated to be 10% to 27% in the elderly population[24], and sarcopenia is found to be more prevalent in patients than in healthy populations[23].

    Individuals with sarcopenia are highly vulnerable to adverse personal, social, and economic consequences. The personal health consequences of sarcopenia include an increase in the risk of falls and fractures, an impaired activity of daily living, cardiovascular complications, respiratory distress, mental and cognitive impairment, impaired mobility, increased dependency and loss of independence as well as a significantly negative influence on the overall quality of life. Additionally, sarcopenia increases the healthcare cost of the individual by increasing the hospitalization rate and the care during the hospitalization period (Fig. 2). Data from various community-based studies have shown that the hospitalization costs of individuals with sarcopenia are five times more likely to be high, compared with those without sarcopenia. These data are consistent in numerous studies, irrespective of the community setting and age of the participants[25].

    Figure  2.  Causes and consequences of sarcopenia.

    The research on sarcopenia has increased substantially in the recent decade with some evidence suggesting that sarcopenia is a complex pathological condition involving various simultaneously acting pathways, including satellite cell abnormality, alterations in the MPS pathway, biotransformation of muscle fibers, mitochondrial dysfunction, an increase in reactive oxygen species, an increase in fat deposition, an impaired motor-neuron activity, and chronic systemic inflammation (Fig. 3).

    Figure  3.  Pathophysiology of sarcopenia.

    The muscle repair and regeneration processes primarily rely on the activation of satellite cells. After muscle damage, satellite cells are activated, enter the cell cycle, proliferate into myogenic precursor cells, and further differentiate to fuse with existing muscle fibers or form new ones to completely heal the damaged muscle site[26]. In aging and chronic disease conditions, various factors like a reduction in antioxidant capacity, an increased DNA damage, and altered gene expression levels cause a progressive reduction in satellite cell number and activity. This reduced number and activity of satellite cells is associated with an impaired muscle regenerative potential, leading to sarcopenia[2728].

    Among the various determinants of MPS, the major parameters include activation of the mTOR signaling pathway, adequate functioning of insulin in skeletal muscle, and the presence of adequate energy at the cellular level in skeletal muscle. These parameters have a direct and most potent influence on the overall MPS[29]. However, with aging, the sensitivity of mTOR is observed to be reduced, probably due to prolonged and sustained activation of mTOR, which is associated with muscle atrophy[30]. Additionally, in disease conditions that involve impaired insulin sensitivity and increased insulin resistance, the activity of insulin at the skeletal muscle level is altered, reducing the skeletal muscle glucose intake, glycogenic pathways, and muscle cell energy substrate (ATP) level. The reduction in cellular ATP level is further correlated with the reduced ATP/AMP ratio, which leads to the AMPK activation and subsequently inhibits the mTOR signaling cascade, further suppressing the MPS pathway[11]. Other than the impaired MPS, sarcopenia is correlated with increased muscle protein degradation. Various pathways, including autophagy, the ubiquitin proteasomal system and calpains-related signaling pathways, have been identified to play an important role in skeletal muscle protein degradation[31]. This reduced MPS and increased degradation rate collectively result in a net skeletal muscle catabolic state, ultimately causing a reduction in muscle size and strength.

    Based on their functionality, skeletal muscle fibers are classified into two types: slow-twitch (type 1) muscle fibers and fast-twitch (type 2) muscle fibers[32]. During muscle aging, senescent muscles undergo significant alterations at the cellular and molecular level, including a change in muscle fiber activation rate, a reduction in excitation-contraction coupling activity, an altered actin-myosin cross-linking, and an alteration in energy production rate[33]. Because of these alterations, the muscle filaments undergo a characteristic transformation from fast-to-slow twitch muscle fibers, which is reflected by a reduction in fast myosin heavy chain isoforms (MyHC-2a and MyHC-2x) and an increase in slow myosin heavy chain isoform (MyHC-1)[34]. Such alterations cause the molecular and functional switch of type-1 to type-2 muscle fibers, which is characteristically observed in the sarcopenia condition. This functional switch of muscle fiber reduces the activation speed, response time, and functionality of muscles, resulting in the increased stiffness, fatigue, and the reduced functional capability in individuals with sarcopenia.

    Mitochondria is an important energy-regulating cellular component that produces cellular ATP through the oxidative phosphorylation process[35]. The process of energy production within the mitochondria involves a complex chain of reactions, which is termed the respiratory chain process[36]. The respiratory chain process is a single-electron exchange reaction process that yields numerous by-products during the entire reactive cascade, with the most important by-product being reactive oxygen species (ROS). ROS are small, molecularly charged, reactive molecules that have numerous signaling functions within the cell[37]. ROS are generated by almost all cellular components, of which around 90% of cellular ROS is generated by the mitochondria because of their extensive role in utilizing oxygen species in the formation of ATP[38]. Besides the production of ROS, mitochondria have a ROS-scavenging pathway as well, which helps maintain an intricate ROS level in the cell[36,39].

    Various pieces of evidence have suggested the important roles of ROS for optimal cellular functioning, which includes triggering pathways correlated with cellular protection, the initiation of mitochondrial fission, and autophagy reactions to remove any abnormal organelle[39]. An imbalance in the controlled ROS production cascade may lead to either under-production or over-production of ROS, both of which are correlated with characteristic pathological states. In sarcopenia, data from some evidence have suggested a possible and critical link between high ROS generation and muscle cell death[40]. This is because mitochondrial DNA (mtDNA) is around 10–20 times more susceptible to ROS-mediated mutagenesis and damage, compared with nuclear DNA, because of the proximity of mtDNA to the ROS-producing site[41]. This alteration of and damage to the mtDNA reduces the oxidative capability of mitochondria and also damages the mitochondrial structure, causing reduced energy production and initiation of the chain of ROS-induced ROS production and release[39,41]. All these alterations cause the activation of apoptotic signaling pathways, increase cellular oxidative stress and disrupt cellular functioning, leading to cell apoptosis and death. With aging and chronic muscle damage, the increased ROS level in the skeletal muscle may be directly linked with the described pathway, which is associated with the reduced mass and strength observed in sarcopenia[42].

    Adipose tissues also serve a dual role as an immune-endocrine organ and an energy storage site. Triglycerides stored in adipose tissues are broken down into free fatty acids and glycerol that are transported for energy use. In aging individuals, the impaired adipogenesis reduces the ability of white adipose tissues to buffer free fatty acids. Obesity exacerbates the problem by causing abnormal fat deposits in tissues like the liver, muscle, heart, and pancreas. The excessive ectopic lipids in skeletal muscle led to lipotoxicity, characterized by an increase in the release of cytokines, adipokines, and chemokines, ultimately causing muscle wasting and mitochondrial dysfunction[43].

    A motor unit comprises a single alpha motor neuron (α-MN) and the related muscle fibers. In the event of the loss of an α-MN, the related muscle fibers undergo structural changes to connect with adjacent surviving α-MN. Such an adjustment with the adjacent α-MN results in the formation of larger motor units, which ultimately contributes to a decline in muscle efficiency, potentially causing the tremors and fatigue commonly observed in elderly individuals[44]. Also, it was noted that after 70 years of age, the number of α-MN decreases by 50%, leading to a reduced muscle coordination[29].

    Aging is commonly associated with a chronic systemic low-grade inflammatory state, which is a consequence of various factors including a reduced nutrition intake, hormonal changes, and a decreased physical activity[45]. The surge in the level of pro-inflammatory cytokines (including TNF-α and IL-6) has been correlated with a negative effect on the growth and metabolic state of skeletal muscles as well as involved in promoting muscle protein breakdown and impairing the anabolic processes responsible for muscle maintenance and repair. Additionally, the inflammatory environment attenuates the MPS, which in turn disrupts the delicate balance between muscle protein formation and degradation, favoring the latter and leading to a net loss of muscle mass. Moreover, chronic inflammation induces insulin resistance and reduces the level of anabolic hormones, mainly insulin-like growth factor 1 (IGF-1), which further exacerbates muscle protein breakdown. These degradative mechanisms underscored the pathological role of low-grade inflammation in muscle homeostasis[4647].

    T2DM is a chronic metabolic condition characterized by increased systemic glucose levels and insulin resistance. As previously discussed, the skeletal muscle was one of the targets for the insulin action that subsequently led to MPS via the activation of mTOR, raising the possibility that T2DM may have a potential effect on the metabolic state of muscles[11]. The surge in glucose levels and insulin resistance have been correlated with the increased muscle wasting, thereby negatively affecting overall muscle health. Various molecular pathways have been identified that high glucose levels have a significant role in hyperglycemia-induced muscle degradation, with the key role of the ubiquitin-proteasome pathway (UPP) and WWP1/KLF15 pathway. UPP is an endogenous protein-degrading mechanism, which involves the coordinated activity of various proteins and enzymes, leading to the degradation of target proteins via the activity of three essential enzymes, namely ubiquitin-activating enzyme (E1 enzyme), ubiquitin-conjugating enzyme (E2 enzyme), and ubiquitin ligase (E3 ligase). These enzymes collectively facilitate the tagging of proteins with ubiquitin, marking them for subsequent degradation by proteasomes into small peptides, or AAs[48]. WWP1 is an endogenous E3 ligase that is known to prevent muscle atrophy during hyperglycemia by particularly targeting the Krüppel-like factor 15 (KLF15) protein, leading to the UPP-dependent KLF15 degradation. KLF15 is a transcription factor that regulates the metabolisms of carbohydrates, proteins, and lipids. During conditions of insulin resistance, the activity of WWP1 is reduced, which causes the inhibition of UPP-dependent KLF15 degradation, thereby increasing the levels of KLF15. These increased KLF15 levels in muscle cells further lead to the upregulation of genes that are related to muscle atrophy[49]. Additionally, insulin resistance leads to the activation of other UPP enzymes that particularly target the MPS-related gene and signaling factors, thereby preventing the MPS, which collectively causes an increase in muscle protein degradation and a reduction in muscle protein formation, leading to net muscle catabolism[50]. This particular mechanism was confirmed in an streptozotocin-induced diabetic model in which mice deficient in muscle-specific KLF15 showed protection against hyperglycemia-induced skeletal muscle atrophy, while streptozotocin treatment in wild-type mice showed muscle atrophy[49].

    OA is a chronic joint degenerative disease that causes progressive destruction of the articular cartilage, synovial membrane, ligaments, and subchondral bone[51]. Some clinical evidence has shown a correlation between OA and the reduced lower limb muscle strength[5253]. While the direct correlation between OA and muscle wasting is not well understood, various studies have pointed out that OA is correlated with a chronic inflammatory environment and the increased gene expression of muscle-degrading proteins, which lead to muscle degradation. A preclinical study in rats with OA showed that the levels of IL-1β and myostatin increased while the expression level of myogenin decreased[54]. Myostatin is a protein that functions to control the hypertrophy of myoblasts by preventing their proliferation and differentiation rate. Some evidence suggests that an increase in myostatin level is associated with muscle wasting, because it prevents the growth of muscle cells and also promotes the activation of UPP[55]. Myogenin, on the other hand, is a transcription factor specific to skeletal muscle, which is involved in the induction of myogenesis and the increase of skeletal MPS[55]. With the alteration in the levels of myostatin and myogenin, it may be plausible to say that OA has a direct and negative effect on muscle health and promotes muscle atrophy and sarcopenia. The observations of preclinical studies are further confirmed in real-world clinical settings, in which OA patients showed some elevated levels of muscle inflammatory markers, such as monocyte chemotactic protein-1, p65, NF-κB and IL-6, and an increase in the signal transducer and activator of transcription 3 (STAT3) activity, which identifies the chronic inflammatory state as playing an important role in the muscle atrophy condition[55]. Furthermore, patients with moderate knee OA showed a lower density of satellite cells, indicating some impaired muscle regenerative capacity, and high expression of the profibrotic gene, suggesting an increased fibrosis, which concludes the reduction in muscle quality[53].

    A bone fracture is one of the most common orthopedic injuries, characterized by a break or discontinuity in the bone tissues[56]. Patients with hip fractures showed a negative impact on their muscle strength[57], and patients with vertebral fractures showed significantly reduced hand grip strength, leg extension, arm curl, sit-to-stand test, and step test[58]. In pelvic ring fracture patients, hip muscle strength was significantly affected, compared with the controls[59]. In all cases, the primary reason for muscle loss is a reduction in physical activity after a fracture, which is because of the restriction of movements caused by the increased pain[60].

    Various surgical procedures lead to a rise in inflammatory markers and oxidative stress, a disuse of muscles, and a reduced protein intake, which collectively cause muscle wasting[61]. For instance, the patients who underwent total knee arthroplasty showed slower walking speed, more stair climbing time, and low knee extension, compared with the control subjects[62]. Furthermore, the quadriceps and hamstring muscle thickness were found significantly reduced in six weeks after total knee replacement (TKR)[63]. The possible reasons behind the muscle loss might be the increase in catabolic activity and muscle proteolysis, which accelerated as a result of the surgical procedure[64]. In hospitalized patients, the elevated levels were observed in myostatin mRNA that is responsible for muscle atrophy, and the suppression of IGF-1 that promotes the hypertrophy of muscle mass[65].

    While various risk factors and molecular targets have been identified for sarcopenia, a very small number of treatment options are currently available as potential treatment options. The most promising therapy is the resistance training and/or exercise. Various pieces of clinical evidence have supported the positive role of exercise in improving muscle strength and functional performance[66]. Other than resistance exercise, protein supplements are also used as a treatment option. However, the major complication with protein supplementation is the huge variation in clinical benefits because of different protein supplements. As it is evidenced that different protein supplements have different clinical benefits, the generalization of any clinical study results in all available protein supplements needs to be prevented. Such generalization of any clinical study results to a real-world setting may cause detrimental effects on individuals with sarcopenia by increasing their healthcare expenditure and therapy burden, while providing no additional clinical benefits to overall muscle health[67].

    Supplementation of AAs is also practiced in clinical settings as a treatment option for sarcopenia, with the fundamental understanding that AAs are the structural unit of proteins, and also have a better absorption profile, compared with whole protein supplementation. Results from various clinical studies particularly support the notion that plasma AAs concentration and MPS rate were significantly higher in the AA supplements plus whey protein group than in the whey protein alone supplementation group[68]. Additionally, the AA supplementation has been shown to improve muscle mass and muscle strength as well as reduce inflammatory markers[6970]. The use of antioxidants, like vitamin C and vitamin E, is also justified based on the fact that oxidative stress has an important role in muscle wasting, and antioxidants may prevent such oxidative stress-induced muscle wasting[71]. Additionally, vitamin D deficiency has shown a correlation with muscle loss and muscle quality, indicating that supplementation of vitamin D in sarcopenia may be effective in preventing muscle loss, supported by a clinical trial in which supplementation with vitamin D showed an improvement in muscle strength[7273].

    As AAs play a crucial role in MPS, there are certain AAs that become essential in the sarcopenia condition because of either the increased demand for AAs in MPS, a reduced ability of the body to endogenously produce AAs, or both. Such a condition causes a net negative balance in the level of such AAs, and hence these AAs are then classified as CEAAs, and the exogenous supplementation of these AAs becomes utmost necessary for preventing muscle protein degradation and improving MPS. While all 20 AAs have a specific role in the body, the roles of leucine, lysine, arginine, valine, methionine, isoleucine, phenylalanine, threonine, histidine, and tryptophan are widely studied and accepted for skeletal muscle health.

    Leucine has a primary role in activating the mTOR activity in various tissues, including skeletal muscles[74]. Because of its direct mTOR-stimulatory effect, the intake of leucine is correlated with an increase in the number of satellite cells and their activation in skeletal muscles[75]. These observations were supported by the result of a clinical study in which leucine supplementation in a sarcopenia patient resulted in an improvement in muscle mass, walking speed, and knee extension[76]. Similarly, lysine plays an important role in the activation and proliferation of satellite cells in skeletal muscle, via activating the mTORC1 signaling pathway[77]. Arginine, being a biological precursor of NO, is also important for maintaining optimal muscle health. NO is an endogenous signaling molecule, which is released from the endothelial lining and causes vasodilation, reduces platelet aggregation, and inhibits mast cell-induced inflammation[78]. NO production in skeletal muscle is correlated with some improved metabolic functions like blood flow, glucose uptake, and oxidative phosphorylation, resulting in increased energy levels[78]. Hence, it is hypothesized that arginine supplementation in sarcopenia improves overall blood flow and metabolic functioning in skeletal muscles, thereby improving overall muscle health. Methionine is the major AA, which is involved in the endogenous synthesis of glutathione, a potent antioxidant that helps counter oxidative stress[79]. As the deleterious role of oxidative stress in muscle health is very well defined, the supplementation of methionine in the sarcopenia condition may be associated with an improvement in overall antioxidant capacity, thereby reducing oxidative stress and related muscle wasting.

    Leucine, isoleucine, and valine are widely known as BCAAs based on their distinct structures. Other than leucine, the other BCAAs, namely isoleucine and valine, have a crucial role in activating the malate-aspartate shuttle pathway in the muscles. The malate-aspartate shuttle pathway is an important shuttle pathway responsible for maintaining cellular and mitochondrial redox potential, which is thereby essential for the oxidative phosphorylation-related ATP production in the mitochondria[8081]. An impairment in the malate-aspartate shuttle pathway may drastically alter the redox potential and thereby reduce the muscle energy level[82]. In older patients with sarcopenia, lower levels of histidine and tryptophan were found, which are the key AAs required for the proper functioning of muscle[8385] Moreover, threonine and phenylalanine help with skeletal MPS by activating IGF-1[8687].

    Various pieces of evidence have suggested that AAs are involved in protein synthesis, metabolism, and the regulation of signaling pathways in skeletal muscle. Other than being the structural unit for protein synthesis, AAs supplementation is associated with an increase in the number and activation rate of satellite cells, an improvement in the mTOR activation rate, an increase in the level and activity of anabolic hormones and molecules, an anti-inflammatory effect and an antioxidant effect (Fig. 4).

    Figure  4.  Mechanism of action of amino acid supplementation in muscle health.

    Satellite cells are essential precursor stem cells of skeletal muscles, and when activated, they play a crucial role in the regeneration of injured muscle fibers[1]. The result of a clinical study underscored that during conditions of the increased muscle wasting, the number of satellite cells was significantly reduced, but the supplementation of AAs significantly improved the number and activation rate of satellite cells[88]. The study included patients undergoing total knee replacement surgery and they were supplemented with the combination of histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, and valine. After the surgery, it was observed that the patients supplemented with AAs had a significantly greater number of satellite cells, and their activation rate was also significantly improved, compared with patients who were not treated with AAs, which suggests that the optimal availability of AAs is crucial for the activity of satellite cells[88]. While the study identified the anti-inflammatory effect of the supplemented AAs responsible for their roles in satellite cells, future studies are required to confirm these findings[88].

    As discussed earlier, the mTOR pathway acts as a key regulator of MPS. mTORC1, a complex of mTOR, is recognized as a crucial regulator in the control of skeletal muscle mass. Its role extends to various processes, including contraction-induced hypertrophy, mechanical load-induced hypertrophy, synergistic ablation, myotube hypertrophy, and AA sensing[89]. Results from some pre-clinical evidence suggested that the treatment of human fibroblasts with AAs was correlated with reduced protein degradation through distinct signaling pathways[90]. AAs, such as arginine, leucine, and methionine, have shown a potent activation of mTORC1 that promotes anabolism by phosphorylating translation-related proteins like S6K1 and 4E-BP1, ultimately promoting protein synthesis[91].

    Muscle protein anabolism is very well controlled by upstream triggers, such as IGF-1 and intermediary proteins, which activate the mTOR pathway[92]. As IGF-1 binds to its receptor, the cascade starts and phosphorylates p70, which ultimately leads to protein synthesis[93]. MPS and muscle protein breakdown are simultaneous processes, occurring in harmony in healthy muscle. To increase muscle mass, the anabolism of protein in muscle should exceed the muscle protein breakdown[94]. Any imbalance in this homeostasis can result in muscle atrophy, leading to a reduction in muscle strength[94]. A study showed that post the exercise, the AA supplementation increased MPS as well as reduced muscle protein breakdown, concluding that AA supplementation improves the anabolic response of MPS[95].

    With age, chronic inflammation starts to rise in the body, which is associated with muscle catabolism as the body is not efficiently producing the required energy[45,96]. Along with that, the imbalance in ROS production leads to increased ROS levels and ultimately causes damage to the muscle cells[40]. AAs, such as histidine, lysine, and arginine, are known to have strong antioxidant and anti-inflammatory activities[9798]. Moreover, methionine plays an important role in the immune system. The catabolism of methionine yields an increased production of glutathione, a known antioxidant, along with other metabolites[79].

    A randomized placebo-controlled study included 60 patients with TKR for knee OA. The patients were randomized to receive either a blend containing ten AAs, which included nine EAA (i.e., lysine, leucine, valine, methionine, isoleucine, phenylalanine, threonine, histidine, and tryptophan), along with arginine, or a placebo lactose powder. The supplementation was carried out one week before the surgery and two weeks after the surgery. The changes in the rectus femoris muscle area and the quadriceps muscle diameter were the primary endpoints considered for the study. Supplementation with AAs was correlated with an increase in the rectus femoris muscle area and the quadriceps muscle diameter, compared with the placebo group, but statistical significance between the groups was not reached (P = 0.457 and P = 0.861, respectively). However, knee pain after surgery was reduced significantly in the AAs supplementation group (P = 0.038) than in the placebo group[64].

    A similar study was conducted on 52 patients undergoing unilateral TKR with a two-year follow-up. The patients assigned to the treatment group were given a blend containing almost similar AAs as in the previous study, and the placebo group was given lactose powder. The participants consumed the allotted treatment one week before and two weeks after the surgery. At two-year follow-up, a significant improvement was found in the diameter of rectus femoris (P = 0.009), rectus femoris muscle area (P = 0.01), and quadriceps muscle strength (P = 0.02) in the supplementation group than in the placebo group, concluding that the supplementation of AA has a beneficial role on muscle mass even in the long run[99].

    To evaluate the effect of EAAs along with arginine in glucose-intolerant patients, a trial was conducted involving 12 diabetic patients who were supplemented with EAA along with arginine for 16 weeks, and parameters like lean body mass, lower limb strength, and functionality were evaluated. A significant increase in lean body mass (P < 0.05), lower limb strength (P < 0.001), usual gait speed (P = 0.002), timed 5-step test (P = 0.007), and timed floor-transfer test (P = 0.022) was observed after the AA supplementation. While the results were consistent with previous studies, this study highlighted the safety of EAAs along with arginine supplementation in diabetic patients suffering from muscle wasting[100]. Similarly, another clinical study was conducted to evaluate the effect of AA supplementation on pain levels in elderly patients with hip fractures. Forty participants were randomly assigned to receive either the blend of AAs or the placebo maltodextrin for four weeks. After supplementation, a significant decrease in pain level (P < 0.001) was observed in the treatment group than in the placebo group[101].

    While various studies have confirmed the beneficial role of AAs in muscle health, many studies have also confirmed the benefits of protein supplementation (especially the whole whey protein) in improving overall muscle health[102]. Hence, to understand the beneficial effect of individual AA supplementation and whey protein supplementation as a whole in improving MPS rate, a clinical study was conducted to compare the effect of AAs against intact whey protein on MPS. The treatment group (n = 7) received the AA supplement, while the control group (n = 8) received the whey protein in an isocaloric amount. The MPS was measured by the mixed muscle fractional synthetic rate (FSR). The findings revealed that the AA supplementation increased the FSR significantly, compared with the whey protein supplementation. The authors concluded that the observed improved FSR was because of faster and higher absorption of AAs in the systemic circulation from the AA supplements rather than intact whey protein supplements. The improved absorption rate resulted in a three-fold increase in the net FSR (P < 0.05), as observed by the improvement in the net phenylalanine uptake rate (P < 0.05). From these observations, the authors also concluded that for providing equivalent FSR between the AA supplements and intact whey protein supplements, it is essential to supplement with a higher dose (two times higher) of whey protein supplements, compared with the AA supplements[103].

    In conclusion, this comprehensive review underscores the pivotal role of EAAs in preserving skeletal muscle health. By investigating their effects on protein synthesis, cellular signaling, and muscle function, the review also underscores the significance of AAs in preventing muscle wasting through diverse mechanisms. Additionally, the article explores the roles of factors, such as aging, medical interventions, and metabolism, in muscle health, and discusses potential therapeutic applications of amino acids and nutritional strategies. Both pre-clinical and clinical evidence indicate that AAs operate through multifacet mechanisms, enhancing muscle protein synthesis, increasing muscle mass, and improving muscle function. These highlight the crucial importance of AAs in promoting optimal muscle health.

    None.

    None.

    CLC number: R685, Document code: A

    The authors reported no conflict of interests.

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