Essential Amino ACID Deficiency and Skeletal Muscle Atrophy: A Systematic Review of Biochemical Mechanisms and Clinical Evidence

Essential Amino ACID Deficiency and Skeletal Muscle Atrophy: A Systematic Review of Biochemical Mechanisms and Clinical Evidence

Angel Ventura (1), Carl Anthony Orcales (1), Albert De Leon (1), and Gecelene C. Estorico (1,2)

(1) Civil and Allied Department, Chemical Technology Department Technological University of the Philippines Taguig Metro Manila 1630 Philippines

(2) De La Salle University Dasmariñas Cavite 1630 Philippines

Abstract – Essential amino acids (EAAs) are indispensable nutrients required for skeletal muscle protein synthesis, maintenance of muscle mass, and regulation of metabolic homeostasis. Deficiency of EAAs disrupts anabolic signaling, impairs muscle protein synthesis, and promotes skeletal muscle atrophy through activation of multiple catabolic pathways. This systematic review aimed to synthesize current evidence regarding the molecular pathways, biochemical mechanisms, and clinical outcomes associated with essential amino acid deficiency and skeletal muscle atrophy. The review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines. Electronic databases including PubMed, Scopus, Web of Science, ScienceDirect, SpringerLink, Frontiers, and Google Scholar were searched for peer-reviewed articles published between 2014 and 2025. Eligible studies investigated the relationship between essential amino acid deficiency or supplementation and skeletal muscle protein metabolism. Thirty studies met the inclusion criteria and were analyzed qualitatively. The findings consistently demonstrated that essential amino acid deficiency suppresses the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway while activating catabolic pathways including Akt/FoxO, the ubiquitinproteasome system, autophagy, and endoplasmic reticulum stress. Clinical studies further showed that leucine-enriched essential amino acid supplementation enhances muscle protein synthesis, improves muscle recovery, and reduces muscle wasting in older adults, hospitalized patients, and physically active individuals. Overall, current evidence indicates that adequate essential amino acid intake plays a critical role in preserving skeletal muscle mass and preventing muscle atrophy. Future randomized controlled trials are recommended to establish optimal nutritional strategies across different clinical populations.

Keywords Essential amino acids, skeletal muscle atrophy, muscle protein synthesis, mTOR signaling, Akt/FoxO

pathway, ubiquitinproteasome pathway, sarcopenia, systematic review.

1. INTRODUCTION

   Skeletal muscle is the largest metabolically active tissue in the human body, accounting for approximately 40% of total body mass. It plays a fundamental role in locomotion, posture, energy metabolism, glucose homeostasis, and whole-body protein storage. The maintenance of skeletal muscle depends on the dynamic balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). When this balance is disrupted in favor of protein degradation, skeletal muscle atrophy develops, resulting in the progressive loss of muscle mass, strength, and functional capacity (Sartori et al., 2021; Yoon, 2017). Skeletal muscle atrophy is a common consequence of aging, prolonged immobilization, malnutrition, cancer cachexia, chronic kidney disease, diabetes mellitus, and other chronic conditions, significantly contributing to physical disability, reduced quality of life, increased hospitalization, and mortality (Papadopoulou, 2020).

   Essential amino acids (EAAs) are indispensable nutrients that cannot be synthesized by the human body and therefore must be obtained through dietary intake. Unlike non-essential amino acids, EAAs serve as critical substrates for protein synthesis and regulate intracellular signaling pathways responsible for maintaining skeletal muscle homeostasis. Among the EAAs, leucine has been identified as the primary anabolic amino acid because it directly activates the mammalian target of rapamycin complex 1 (mTORC1), a master regulator of muscle protein synthesis (Graber et al., 2017). Activation of mTORC1 stimulates translation initiation, ribosomal biogenesis, and muscle protein accretion, thereby promoting muscle growth and maintenance. Conversely, inadequate intake or deficiency of EAAs suppresses mTORC1 activity, resulting in reduced muscle protein synthesis and increased susceptibility to muscle wasting (Ferrando et al., 2023).

   The molecular mechanisms underlying skeletal muscle

   atrophy involve a complex interaction of anabolic and catabolic signaling pathways. Reduced availability of essential amino acids inhibits the insulin-like growth factor-1

   (IGF-1)/phosphoinositide 3-kinase (PI3K)/Akt/mTOR pathway, thereby suppressing anabolic signaling and protein synthesis. Simultaneously, decreased Akt activity activates Forkhead box O (FoxO) transcription factors, which increase the expression of muscle-specific ubiquitin ligases such as Muscle RING Finger-1 (MuRF-1) and Atrogin-1, accelerating protein degradation through the ubiquitinproteasome system (Chen et al., 2025; Sartori et al., 2021). Additional mechanisms contributing to muscle loss include autophagy, mitochondrial dysfunction, oxidative stress, inflammatory cytokine signaling, glucocorticoid-induced proteolysis, and endoplasmic reticulum (ER) stress, all of which collectively disrupt skeletal muscle protein homeostasis (Ji et al., 2025).

   Growing evidence suggests that essential amino acid supplementation may attenuate muscle loss by restoring anabolic signaling and stimulating muscle protein synthesis. Several randomized clinical trials have demonstrated that leucine-enriched essential amino acid formulations enhance mTORC1 activation, improve post-exercise recovery, and preserve skeletal muscle mass in healthy adults (Takegaki et al., 2020; Waskiw-Ford et al., 2020). Likewise, studies involving older adults, hospitalized patients, and individuals with chronic diseases indicate that adequate essential amino acid intake may reduce anabolic resistance, improve physical performance, and mitigate sarcopenia and frailty (Cruz-Jentoft et al., 2017; Negro et al., 2024). Nevertheless, the magnitude of these beneficial effects varies depending on age, nutritional status, disease severity, duration of supplementation, and the composition of essential amino acid formulations.

   Despite the increasing number of experimental and

   clinical studies examining the relationship between essential amino acid deficiency and skeletal muscle atrophy, the available evidence remains fragmented across different populations, experimental models, and disease conditions. Furthermore, recent advances have expanded the understanding of amino acid sensing, intracellular signaling networks, and nutritional interventions, highlighting the need for an updated synthesis of current evidence. Integrating findings from molecular, biochemical, and clinical studies may provide a clearer understanding of the mechanisms responsible for muscle wasting and support the development of evidence-based nutritional strategies for prevention and treatment.

   This systematic review aims to synthesize and critically evaluate current evidence regarding the role of essential amino acid deficiency in the development of skeletal muscle atrophy. Specifically, this review examines the molecular pathways involved in muscle protein synthesis and degradation, describes the biochemical mechanisms associated with impaired amino acid availability, evaluates clinical evidence across diverse populations, and identifies current knowledge gaps that may guide futureresearch and nutritional interventions for preserving skeletal muscle mass and function.

2. METHODOLOGY

   This section outlines the systematic approach used to collect, screen, and analyze scientific literature concerning the neurotoxic effects of organophosphate, pyrethroid, and carbamate exposure in early childhood, particularly in the context of micronutrient deficiency.

   1. Data Sources

      This systematic review utilized peer-reviewed journal articles, clinical trials, experimental studies, systematic reviews, meta-analyses, and narrative reviews obtained from reputable scientific databases including PubMed/MEDLINE, Scopus, Web of Science, ScienceDirect, SpringerLink, Frontiers, Google Scholar, and Nutrients. The literature search focused on publications from 2014 to 2025 to ensure the inclusion of recent evidence regarding essential amino acid deficiency, skeletal muscle atrophy, molecular signaling pathways, and nutritional interventions.

      The review concentrated on studies investigating the effects of essential amino acid deficiency and supplementation, particularly leucine-enriched essential amino acids, on skeletal muscle protein synthesis, muscle protein degradation, and muscle function. Studies involving the mTORC1, IGF-1/PI3K/Akt, FoxO, ubiquitinproteasome, autophagy, endoplasmic reticulum stress, and related signaling pathways were included to comprehensively evaluate the biochemical and molecular mechanisms underlying skeletal muscle atrophy. Both human clinical studies and experimental animal and cellular studies were considered to provide mechanistic, physiological, and clinical evidence across diverse populations, including healthy adults, older individuals, hospitalized patients, and those with chronic diseases.

      A total of 30 eligible studies were included in this systematic review following the screening and application of predefined inclusion and exclusion criteria. The selected literature comprised randomized controlled trials, experimental human studies, observational studies, systematic reviews, meta-analyses, and comprehensive review articles that collectively provided current evidence on the relationship between essential amino acid deficiency and skeletal muscle atrophy.

   2. Literature Search

      To ensure comprehensive coverage of relevant studies, the literature search was conducted systematically across multiple scientific databases, including PubMed/MEDLINE, Scopus, Web of Science, ScienceDirect, SpringerLink, Frontiers, Google Scholar, and Nutrients. The search strategy was structured around three main thematic areas: essential amino acid deficiency and supplementation, molecular pathways

      involved in skeletal muscle protein metabolism, and clinical outcomes associated with skeletal muscle atrophy. Keywords under the first category included “essential amino acid deficiency,” “essential amino acids,” “leucine,” “branched-chain amino acids,” “dietary protein,” “protein malnutrition,” and “amino acid supplementation.” These terms were designed to identify studies investigating the effects of essential amino acid availability on muscle protein synthesis, nutritional status, and skeletal muscle maintenance. The second category involved search terms such as “mTOR signaling,” “mTORC1,” “IGF-1/PI3K/Akt pathway,” “FoxO,” “ubiquitinproteasome pathway,” “autophagy,” “muscle protein synthesis,” “protein degradation,” “endoplasmic reticulum stress,” and “oxidative stress,” which were used to retrieve studies examining the molecular and biochemical mechanisms underlying skeletal muscle atrophy. The third category included “skeletal muscle atrophy,” “muscle wasting,” “sarcopenia,” “cachexia,” “muscle

      recovery,” “aging,” “older adults,” “hospitalized patients,” “chronic disease,” and “resistance exercise,” ensuring the inclusion of studies evaluating clinical manifestations, functional outcomes, and nutritional interventions across different populations.

      Boolean operators and were applied to combine related keywords and refine the search strategy, thereby improving the sensitivity and specificity of the database searches. Filters were used to limit the search to peer-reviewed articles published in English between 2014 and 2025. Furthermore, the reference lists of eligible articles, systematic reviews, and meta-analyses were manually screened to identify additional relevant studies that were not retrieved through the electronic database searches. The literature search and study selection process were conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) guidelines to ensure transparency, reproducibility, and methodological rigor.

      Figure 1. Study Selection Flow Diagram Following PRISMA Guidelines

   3. Inclusion and Exclusion

      The inclusion and exclusion criteria were carefully established to ensure that only studies directly relevant to the objectives of this systematic review were included. Eligible

      studies comprised peer-reviewed articles published between 2014 and 2025 that investigated the relationship between essential amino acid (EAA) deficiency or supplementation and skeletal muscle atrophy, muscle protein synthesis, muscle protein degradation, or muscle function Both human clinical studies (e.g., randomized controlled trials, clinical trials, cohort studies, and observational studies) and experimental animal or cellular studies were included to provide comprehensive evidence on the molecular, physiological, and clinical effects of essential amino acid deficiency. High-quality systematic reviews, meta-analyses, and narrative reviews were also considered to support the synthesis of current evidence. Only English-language articles with full-text availability were included to facilitate accurate data extraction and critical evaluation.

      Conversely, studies were excluded if they focused exclusively on non-essential amino acids or nutritional interventions unrelated to essential amino acids, did not evaluate skeletal muscle atrophy or muscle protein metabolism, or lacked investigation of the molecular or biochemical mechanisms associated with muscle regulation. Conference abstracts,

      1. Data Extraction

         Relevant information from each selected study was systematically extracted and organized to ensure consistency and comparability across the included literature. Key study characteristics included the author, publication year, study title, country (when available), study design, population or experimental model, sample size, and intervention or exposure related to essential amino acid deficiency or supplementation. Information regarding the type and dosage of essential amino acid supplementation, particularly leucine-enriched essential amino acids, as well as the duration of intervention and comparison groups, was collected whenever applicable. Data on the molecular pathways and biochemical mechanisms investigated in each study were also extracted, including the mTORC1, IGF-1/PI3K/Akt, FoxO, ubiquitinproteasome, autophagy, and endoplasmic reticulum stress signaling pathways involved in skeletal muscle protein synthesis and degradation. Additionally, reported clinical and physiological outcomes, such as muscle protein synthesis, muscle mass, muscle strength, muscle recovery, sarcopenia, cachexia, anabolic signaling, and markers of muscle atrophy, were summarized to identify common findings and differences among studies. This structured data extraction process facilitated a comprehensive qualitative synthesis of the current evidence, enabling comparison of molecular, biochemical, and clinical findings while identifying consistent trends and exiting research gaps regarding the role of essential amino acid deficiency in skeletal muscle atrophy.

      2. Statistical Analysis

      The review primarily employed a qualitative descriptive and comparative synthesis, integrating findings from

      editorials, letters to the editor, book chapters, unpublished manuscripts, duplicate publications, and articles without sufficient methodological details or complete outcome data were also excluded.

   4. Search Results

   A total of A total of 90 records were identified through database searching. After screening titles and abstracts, 60 studies were excluded due to irrelevance, duplicate publications, or insufficient data related to essential amino acid deficiency and skeletal muscle atrophy. The remaining 30 full-text articles were assessed for eligibility, of which 15 met the inclusion criteria. These included randomized controlled trials, experimental human studies, systematic reviews, narrative reviews, and experimental animal studies investigating essential amino acid deficiency, skeletal muscle protein synthesis, molecular pathways, and muscle atrophy. The selection process was documented following the PRISMA 2020 flow diagram, outlining the stages of identification, screening, eligibility, and inclusion.

   randomized controlled trials, experimental studies, observational studies, systematic reviews, meta-analyses, and narrative reviews. Due to the heterogeneity of the included studies with respect to study design, participant characteristics, intervention protocols, outcome measures, and molecular targets, a quantitative meta-analysis was not performed. Instead, the included studies were critically evaluated based on methodological quality, study design, sample size, experimental model, intervention or exposure, and the consistency of reported molecular and clinical outcomes.

   The findings were categorized according to the major molecular pathways and biochemical mechanisms involved in skeletal muscle regulation, including the mTORC1, IGF-1/PI3K/Akt, FoxO, ubiquitinproteasome, autophagy, and endoplasmic reticulum stress signaling pathways. Clinical outcomes such as muscle protein synthesis, muscle mass, muscle strength, sarcopenia, cachexia, muscle recovery, and the effects of essential amino acid or leucine-enriched supplementation were also compared across different populations, including healthy adults, older individuals, hospitalized patients, and patients with chronic diseases. This approach enabled the identification of consistent patterns, areas of agreement, and existing research gaps regarding the molecular mechanisms and clinical implications of essential amino acid deficiency in the development of skeletal muscle atrophy.

3. RESULT & DISCUSSION

   A total of 15 eligible studies were included in the qualitative synthesis. The selected literature demonstrated consistent evidence that essential amino acid availability plays a critical role in regulating skeletal muscle protein metabolism through modulation of the mTORC1, Akt/FoxO, ubiquitinproteasome, autophagy, and endoplasmic reticulum stress pathways. Although the magnitude of the reported

   Table 1: Overview of the Included Studies

   Author (Year)	Study Design	Population Model	Main Focus
   Ferrando et al. (2023)	Human experimental	Healthy adults	EAA supplementa tion and muscle protein synthesis
   Takegaki et al. (2020)	Randomized clinical trial	Older adults	Leucine-enri ched EAAs
   Graber et al. (2017)	Experimenta l	Healthy volunteers	mTOR activation

   The included studies consisted of randomized controlled trials, human experimental studies, animal experiments, systematic reviews, and narrative reviews investigating the effects of essential amino acid availability on skeletal muscle metabolism. Most studies evaluated anabolic signaling pathways, particularly the mammalian target of rapamycin complex 1 (mTORC1), while others investigated molecular mechanisms involved in muscle protein degradation, including the Akt/FoxO pathway, ubiquitinproteasome system, autophagy, and endoplasmic reticulum stress. Collectively, the evidence demonstrated that essential amino acids play a fundamental role in regulating skeletal muscle homeostasis through coordinated control of protein synthesis and degradation.

   effects varied across populations and study designs, the overall findings support the importance of adequate essential amino acid intake in preserving skeletal muscle mass and preventing muscle atrophy. The principal findings of the included studies are summarized in Table 1 before being discussed according to their major mechanistic and clinical themes.

   Figure 2. Distribution of Study Designs Among Included Studies (n = 15)

   Figure 1 illustrates the distribution of study designs included in this systematic review. Experimental studies comprised the largest proportion of the evidence, followed by randomized controlled trials, while systematic reviews, narrative reviews, and clinical observational studies accounted for the remaining studies. This distribution indicates that the current body of literature consists primarily of mechanistic and experimental investigations, with comparatively fewer high-quality clinical intervention studies.

   Included Studies	Key Findings	Summary of Results
   Ferrando et al.; Graber et al.; Takegaki et al.	Reduced mTORC1 activation	Decreased anabolic signaling and muscle protein synthesis
   Sartori et al.; Chen et al.	Increased FoxO activation	Increased MuRF-1 and Atrogin-1 expression

   Table 2: Summary of Biochemical Mechanisms Associated with Essential Amino Acid Deficiency

   Sartori et al.; Ji et al.	Enhanced ubiquitinproteas ome activity	Accelerated protein degradation
   Ji et al.; Chen et al.	Increased autophagy	Loss of muscle proteins
   Chen et al.	Endoplasmic reticulum stress	Cellular dysfunction and muscle wasting

   between essential amino acid deficiency and muscle atrophy in older adults, hospitalized patients, and individuals with chronic diseases.

   Table 3: Effects of Essential Amino Acid Supplementation on Skeletal Muscle

   The reviewed studies consistently identified suppression of mTORC1 signaling as the primary biochemical consequence of essential amino acid deficiency. Reduced activation of this pathway decreases translation initiation and muscle protein synthesis, resulting in a negative protein balance. Simultaneously, amino acid deprivation activates FoxO transcription factors, increasing the expression of the muscle-specific ubiquitin ligases MuRF-1 and Atrogin-1, which accelerate protein degradation. Additional mechanisms, including enhanced autophagy, oxidative stress, mitochondrial dysfunction, and endoplasmic reticulum stress, further contribute to progressive skeletal muscle atrophy.

   Figure 3. Distribution of Major Research Focus Among Included Studies (n = 15)

   Figure 3 shows the primary research focus of the included studies. Most studies investigated the regulation of muscle protein synthesis through the mTOR signaling pathway, emphasizing its central role in maintaining skeletal muscle mass. A substantial number of studies also evaluated the effects of leucine-rich essential amino acid supplementation on muscle preservation and recovery. Other investigations foused on catabolic pathways, including the ubiquitinproteasome system, autophagy, and FoxO signaling, while several clinical studies examined the relationship

   Representative Studies	Intervention	Main Findings
   Takegaki et al.; Waskiw-Ford et al.	Leucine-rich EAA supplementation	Improved muscle recovery and anabolic signaling
   Ferrando et al.; Negro et al.	Essential amino acid supplementation	Increased muscle protein synthesis
   Ham et al.	Resistance exercise + EAAs	Greater preservation of muscle mass

   Most intervention studies demonstrated that leucine-enriched essential amino acid supplementation effectively stimulates muscle protein synthesis through activation of mTORC1 signaling. Supplementation also increased amino acid transporter expression and improved recovery following exercise or immobilization. However, several studies noted that nutritional supplementation alone may not fully prevent muscle atrophy, emphasizing the importance of combining adequate amino acid intake with resistance exercise or rehabilitation to maximize anabolic responses.

   Table 4: Clinical Outcomes Associated with Essential Amino Acid Deficiency

   Representative Studies	Population	Cinical Findings
   Cruz-Jentoft et al.; Papadopoulou	Older adults	Sarcopenia, anabolic resistance
   Brown et al.	Hospitalized patients	Muscle wasting
   Hughes et al.	Immobilized individuals	Reduced muscle mass
   Ferrando et al.	Healthy adults	Decreased muscle protein synthesis during EAA

   restriction

   Clinical evidence indicated that inadequate essential amino acid intake contributes to progressive skeletal muscle loss, particularly among older adults, hospitalized patients, and individuals experiencing prolonged immobilization.

4. CONCLUSIONS

   Anti-aging and the preservation of skeletal muscle mass require a proper intake of essential amino acids (EAAs) to preserve regular muscle function. In addition to being constituents of proteins, essential amino acids (EAAs) are also key regulators of metabolism as they modulate the balance between anabolic and catabolic signaling channels that govern muscle protein turnover. These results suggest that sufficient EAA supply, notably with leucine-rich formulations, causes muscle protein synthesis to occur through activation of the mTORC1 signaling pathway and stimulation of amino acid transporters together with molecules related to translation which underpin muscle development and preservation. essential amino acids (EAAs) in maintaining skeletal muscle health. Across the studies reviewed, there was consistent evidence that an adequate supply of EAAs supports muscle protein synthesis and helps preserve muscle mass by regulating key biochemical pathways involved in muscle growth and repair. Leucine-rich EAA formulations were frequently associated with enhanced anabolic signaling, making them an important nutritional strategy for supporting muscle maintenance in both healthy individuals and those at risk of muscle loss.

   The reviewed literature shows that EAA deficiency

   can have significant negative effects on skeletal muscles. When essential amino acids are lacking, the body’s ability to build new muscle proteins declines while pathways responsible for protein breakdown become more active. This imbalance promotes muscle wasting and contributes to the gradual loss of muscle mass and strength. Several studies also suggest that oxidative stress, autophagy, and cellular stress responses further worsen the effects of EAA deficiency, especially during aging, prolonged inactivity, or illness.The clinical evidence also demonstrates that EAA supplementation can help improve muscle recovery, reduce muscle loss, and support overall muscle function. These benefits appear to be greatest when nutritional interventions are combined with resistance exercise or rehabilitation programs rather than being used alone. This emphasizes that preserving muscle health requires a comprehensive approach in which proper nutrition and regular physical activity work together to achieve the best outcomes.

   Age-related anabolic resistance reduces the efficiency of muscle protein synthesis, making older populations especially vulnerable to muscle wasting. The reviewed studies support the use of leucine-rich essential amino acid supplementation as a nutritional strategy to preserve muscle mass and improve functional outcomes in these high-risk groups.

   Overall, the evidence indicates that maintaining adequate essential amino acid intake is an important strategy for protecting skeletal muscle from atrophy and promoting long-term muscle health. While considerable progress has been made in understanding the biological mechanisms involved, questions remain regarding the most effective dosage, formulation, and duration of EAA supplementation for different populations. Future studies should focus on well-designed clinical trials that include diverse age groups and patient populations to develop more targeted nutritional recommendations. A better understanding of these factors will help improve interventions aimed at preventing muscle loss and preserving physical function, particularly among individuals who are most vulnerable to skeletal muscle atrophy.

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