Effects of Amino Acid Supplementation on Muscle protein metabolism and adaptation: a narrative review of effects on muscle mass, strength, and sex differences

Abstract

[Purpose]

To review the effects of amino acid supplementation including essential, branched-chain, and non-essential amino acids on muscle mass, strength, and adaptation, with a focus on sex-specific differences.

[Methods]

PubMed, ScienceDirect, and Google Scholar databases were searched for clinical and nonclinical studies on the effects of supplementation in adults, published between 1990 and 2025.

[Results]

Essential amino acids and leucine showed the most consistent associations with increased MPS and, in older adults, improvements in strength/lean mass. Branched-chain amino acids supplementation generally attenuated muscle damage/DOMS and helped preserve performance, with benefits appearing context dependent (e.g., energy restriction, high training load). Evidence for non-essential amino acids is preliminary. Signals of greater strength oriented effects in men and greater recovery benefits in women are biologically plausible but largely hypothesis-generating due to single sex cohorts and indirect cross study contrasts.

[Conclusion]

Amino acid supplementation can support muscle anabolism and, in selected contexts, strength and recovery, best viewed as an adjunct to resistance training with individualized dosing/timing. Progress requires adequately powered, pre registered, sex comparative RCTs that standardize protocols and prespecify functional and structural endpoints to define dose response, timing, and durability across ages and training states.

INTRODUCTION

Maintaining optimal health throughout the lifespan is a global public health priority, especially in aging populations. Among various determinants of health, skeletal muscle mass and strength have emerged as critical indicators of functional capacity, metabolic health, and overall mortality risk. Low muscle mass and diminished strength, often associated with aging or inactivity, are linked to an increased prevalence of sarcopenia, frailty, type 2 diabetes, cardiovascular disease, and all-cause mortality [1]. In fact, muscle strength has been shown to be a stronger predictor of long-term survival than muscle mass alone [2]. Consequently, muscle mass and strength are not only markers of physical capability but also essential clinical indicators of health status. The European Working Group on Sarcopenia in Older People (EWGSOP) has emphasized the inclusion of muscle strength in the diagnostic criteria for sarcopenia [1]. Given their significance, identifying effective strategies to maintain or improve these parameters has become increasingly important. Among such strategies, resistance training (RT) and nutritional interventions, particularly the supplementation of amino acids are recognized as the most promising non-pharmacological approaches to preserve or enhance muscle health across populations [3,4].

Dietary supplementation has emerged as a practical and evidence- based strategy for enhancing skeletal muscle health, particularly in populations at risk of muscle mass or strength loss. Supplements containing essential amino acids, high-quality proteins, vitamins, and minerals have been shown to support muscle protein synthesis, reduce muscle degradation, and improve functional performance, especially when combined with RT [5,6]. These effects are particularly important in aging individuals, patients with chronic illness, and physically active populations undergoing intense training [7]. Nutritional interventions can serve to bridge dietary gaps, support metabolic recovery, and optimize anabolic signaling, ultimately contributing to the preservation and enhancement of lean body mass [8]. Furthermore, international health authorities and scientific organizations now recognize the role of targeted supplementation as a complementary approach to exercise for improving musculoskeletal outcomes. Reflecting this recognition, global consumption of dietary supplements has grown steadily, with a marked increase in products aimed at muscle health, strength enhancement, and active aging [9]. This growing trend underscores the need for more rigorous, mechanistic studies that evaluate the synergistic effects of various supplement combinations on muscular outcomes.

Adequate protein intake is essential for the maintenance and development of skeletal muscle mass and strength, especially under conditions of aging, immobilization, or exercise-induced muscle stress. Among the constituents of dietary protein, essential amino acids (EAAs) particularly the branched-chain amino acids (BCAAs) leucine, isoleucine, and valine play a pivotal role in stimulating muscle protein synthesis (MPS) by activating the mammalian target of rapamycin complex 1 (mTORC1) pathway [10,11]. Leucine, in particular, serves as a metabolic signal that triggers MPS independent of insulin and has been shown to be especially effective in older adults with anabolic resistance [12]. Several studies have demonstrated that EAA or BCAA supplementation enhances muscle protein turnover, attenuates muscle breakdown during RT, and improves post-exercise recovery [13,14]. Furthermore, long-term supplementation with EAAs has been associated with the preservation of lean mass and improvements in strength, especially in populations at risk of muscle loss such as the elderly or those with chronic illness [15]. Given these findings, optimizing amino acid intake both through dietary sources and supplementation represents a critical strategy to support muscle health. However, despite these well-documented benefits, the combined effects of EAAs on muscle mass and strength remain underexplored, highlighting the need for further investigation in this area.

Current nutritional strategies, particularly those involving EAAs and branched-chain amino acids (BCAAs), are widely recognized for their role in supporting muscle hypertrophy, metabolism, and functional performance [16]. However, these approaches rarely account for physiological sex differences that may alter anabolic responses. Evidence indicates that hormonal milieu, amino acid kinetics, and oxidative pathways differ between men and women [16,17]. For instance, men demonstrate higher leucine oxidation rates during exercise [16], whereas women display lower oxidation rates and higher basal muscle protein synthesis, especially with aging [17,18]. Moreover, estrogen can enhance satellite cell activation and attenuate exercise-induced damage, while testosterone promotes hypertrophy through androgen receptor signaling [19]. These findings underscore the importance of integrating sex-specific physiological factors into supplementation strategies to avoid generalized conclusions and optimize muscle adaptation [19,20].

Although numerous studies have investigated the effects of amino acid supplementation on muscle mass and strength, the findings remain fragmented and lack integrative synthesis. Considering the physiological importance of both nutrient categories and the increasing emphasis on nutritional strategies for musculoskeletal health, a comprehensive review is both timely and necessary. Therefore, the aim of this review is to systematically evaluate the current evidence on the effects of amino acid supplementation including essential, branched-chain, and non-essential amino acids on skeletal muscle mass, strength, and adaptive responses, with particular emphasis on identifying sex-specific differences to guide future research and personalized nutritional strategies. Therefore, the aim of this review is to synthesize and analyze the current evidence on the effects of amino acid supplementation including essential, branched-chain, and non-essential amino acids on skeletal muscle mass, strength, and adaptive responses, with particular emphasis on identifying sex-specific differences to provide information for future research and the development of personalized nutritional strategies.

METHODS

A literature search was conducted using PubMed, ScienceDirect, and Google Scholar for clinical and nonclinical studies published between 1990 and 2025 that investigated the effects of supplementation in adults. The keywords used were “amino acids and muscle,” “amino acids and RT effects,” “amino acids supplement and sex differences,” and “dietary supplement and sex differences.” Experimental and animal studies were excluded from analyses. Additionally, studies were excluded if they had small sample size, lacked clearly defined inclusion criteria, or involved participants receiving medications for conditions known to affect bone or muscle metabolism.

In total, 3,791 records were identified. After de-duplication, 2,087 records remained for title/abstract screening. Two reviewers independently screened records and assessed full texts, resolving discrepancies by consensus. Full-text articles assessed for eligibility: (1); studies included in the qualitative synthesis: (2); of these, randomized trials that directly compared males and females: (3). Inclusion criteria comprised human participants aged ≥18 years; amino acid supplementation (EAA, BCAA, leucine or NEAA) with or without resistance training; and outcomes including at least one of the following: fractional synthetic rate (FSR)/MPS, muscle strength (e.g., 1RM), lean mass, functional performance, delayed-onset muscle soreness (DOMS), or muscle damage enzymes (CK, LDH). We excluded studies with unclear inclusion criteria, insufficient description of protocols (when applicable), or non-English language. Given the narrative scope, we did not pool effect sizes. Instead, we performed a qualitative appraisal across four domains randomization/concealment, blinding, attrition, and selective reporting to contextualize certainty of evidence as higher, moderate, or lower, without formal scoring.

RESULTS

The Effects of Amino Acids Supplementation on Muscle Protein Metabolism and Adaptive Responses

The Effects of EAA Supplementation on Muscle Protein Metabolism and Adaptive Responses

The Effects of Leucine Supplementation on Muscle Protein Metabolism and Adaptive Responses

Owing to its unique capacity to directly stimulate MPS via the activation of the mTORC1 signaling pathway, leucine is considered the most anabolic amino acid among the essential amino acids. Numerous clinical and mechanistic studies have demonstrated that leucine supplementation promotes both muscle hypertrophy and functional strength gain, particularly in populations that are vulnerable to anabolic resistance, such as older adults. Katsanos et al. (2006) reported that in older adults, a leucine-enriched EAA formulation significantly stimulated MPS, whereas a standard EAA blend failed to do so, thereby restoring MPS responses to levels comparable to those observed in young individuals [21]. Dickinson et al. (2014) demonstrated that the ingestion of leucine-enriched EAAs (10 g EAA with 3.5 g leucine) immediately after RT significantly prolonged the post-exercise (MPS rate in older men. Specifically, the MPS rate was sustained for up to 4 h post-exercise before a return to baseline, compared to the control group, which supports the independent efficacy of leucine-enriched formulations in enhancing and prolonging postprandial MPS responses in aging muscle [22]. Oh et al. (2022) found that a 12-week intervention with a leucine-enriched protein supplement (2 g/day leucine), combined with RT, significantly increased the total lean mass and grip strength in Korean adults aged >50 years [23]. Churchward-Venne et al. (2014) observed that the addition of 5 g of leucine to a low-protein mixed macronutrient beverage was associated with a significant increase in post-exercise (MPS in healthy young men [12].

These findings collectively highlight the robust potential of leucine as a key anabolic trigger for enhancing skeletal muscle mass and strength, particularly in aging populations prone to anabolic resistance. The mechanistic basis for these outcomes lies in leucine’s ability to activate the mTORC1 pathway, which initiates downstream signaling via p70S6K and 4E-BP1, thereby facilitating myofibrillar protein synthesis and muscle accretion [25]. This is especially relevant in sarcopenic or elderly individuals, where basal and postprandial MPS responses are blunted, making leucine-enriched supplementation a physiologically targeted countermeasure. Moreover, prolonged stimulation of MPS for several hours post-exercise, as demonstrated by Dickinson et al. (2014), underscores the temporal advantage of leucine in sustaining muscle remodeling beyond immediate post-exercise periods [26]. Collectively, available evidence is consistent with leucine supporting improvements in MPS and strength particularly in older adults while the magnitude and generalizability of effects may vary by age, training status, and protocol. The consistency of evidence across randomized trials, population studies, and mechanistic data reinforces the role of leucine as a critical component of muscle health interventions (Table 1).

Table 1.

Effects of leucine, BCAA, EAA, and NEAA supplementation in humans on muscle strength, muscle mass, and muscle adaptation

Factor	Note		Significant difference	Case number	Ref.
	Age	Details of protein condition
Leucine	66.5 ± 20.2	High leucine proportion (41%) EAAs 6.7g,	(+)	N/A	[21]
	28.8 ± 2.6	At time 0 min (after overnight fast, ~7:30 AM, prior to experimental phase)	Elderly: MPS (FSR: + 0.038%), Young: MPS (FSR: + 0.051%)
	72.4 ± 2	EAA 10g (Leucine 3.5g), at 1 h after exercise	(+)	15	[22]
			MyoPS: + 90%, Phosphorylation of Mtor: p < 0.05
	50-70	leucine rich protein 20g (leucine 2g)/day for 12 weeks	(+)	41	[23]
			(Skeletal muscle mass: + 0.69 kg, lean body mass: + 1.01 kg, grip strength: 1.82 kg)
	19.5 ± 0.1	High leucine Whey protein 6.25g (leucine: 5g), Immediately after resistance exercise	(+)	8	[12]
			(Myofibrillar protein synthesis: + 220%)
BCAA	23 ± 2	BCAA 20g /twice per day (morning and evening) for 12 days	(-)	12	[26]
			(CK: - 86 IU.L, MVC: - 27%)
	19 <, Young men	BCAA 29.2g/day (1st dose: pre-exercise, 2nd dose: after testing, before breakfast, 3rd dose: between lunch and dinner, 4th dose: at bedtime)	(-)	24	[14]
			(Visual analog scale score: - 33%)
	25 ± 1	0.1g/kg BCAA solution ingested pre-warm-up, immediately pre-exercise, during exercise (15 min), and during recovery (15, 30, 60, 90 min)	(+)	7	[27]
			(p70S6k Thr389 phosphorylation: + 240%)
	21-28	18g/day (pre-exercise and post-exercise) for 8 weeks	(+)	12	[28]
			(1RM squat: + 15.1 kg, 1RM bench press: 7.1 kg)
EAA	69.2 ± 2	40g EAA, given orally in small boluses every 10 min for 3 h	(+)	6	[15]
			(FSR: + 33%)
	68.8 ± 5.08	20g EAA/day for 28 weeks	(+)	25	[29]
			(Appendicular skeletal muscle mass-height index: + 8%)
	34 ± 4	15g EAA, bolus ingestion at 11:00 h after overnight fast	(+)	6	[30]
			(FSR: 0.04%/h)
	67 ± 5.6	22g EAA (11 g, twice daily, capsules, between meals with water)/day of 16 weeks	(+)	12	[31]
			(Lean body mass: +1.14 kg, leg strength: + 22.2 %)
	71 ± 6	45g EAA (3 times per day throughout 10d of bed rest)/day for 10 days	(+)	12	[32]
			(Preservation of muscle mass and functional capacity during bed rest)
NEAA	26.5 ± 6.5	Taurine capsule 0.1 g/kg/day for 3 days before and 4 days after eccentric exercise	(-)	10	[33]
			(DOMS: - 18%, serum creatine kinase: - 27%)
	23 ± 3	Single dose (50 mg/kg taurine or placebo) before exercise	(+)	12	[34]
			(Critical power: + 7%)
	23.9 ± 2.8	Single 6 g dose taurine, ingested 60 min before exercise	(+)	30	[35]
			(Peak power: + 5.44%, vertical jump: + 6.91%)
	58.6 ± 14.2	Glycine7 g, twice daily (30 min before breakfast and dinner) for 4 months	(+)	36	[36]
			(Fat‐free mass index by BIA: + 0.27 kg/m2)
	74 ± 1	GlyNAC (glycine 1.33 mmol/kg/day), oral capsules, daily for 24 weeks (compliance checked every 4 weeks)	(+)	8	[37]
			(Gait speed: + 0.3m/s, grip strength: + 2.7 kg)

+: increase, -: decrease.

FSR: Fractional synthesis rates, CK: creatine kinase, MVC: maximal voluntary contraction, MPS: muscle protein synthesis, DOMS: delayed onset muscle soreness.

The Effects of BCAA Supplementation on Muscle Protein Metabolism and Adaptive Responses

BCAAs, including leucine, isoleucine, and valine, have been extensively studied for their role in the regulation of muscle protein metabolism and exercise-induced adaptations. Among these, leucine is the most anabolic. However, evidence suggests that the synergistic effect of all three BCAAs may provide additional benefits in promoting muscle accretion and recovery, particularly in physically active or aging populations [13]. Howatson et al. (2012) demonstrated that 7 days of BCAA supplementation (20 g/day) before and after RT significantly reduced markers of muscle damage (creatine kinase), attenuated delayed-onset muscle soreness (DOMS), and preserved maximal voluntary contraction in RT men compared with placebo [26]. In clinical trials, BCAA supplementation, when paired with RT, enhanced muscle strength and preserved lean mass. Jackman et al. (2010) reported that 10 g/day BCAA supplementation after exercise during a 3-week eccentric training program led to significantly reduced muscle soreness and improved isometric strength recovery in RT men [14]. Karlsson et al. (2004) found that 8-week BCAA supplementation combined with RT in trained athletes significantly increased both arm circumference and maximal strength as compared to the placebo group [27]. Dudgeon et al. (2016) found that 8 weeks of BCAA supplementation (28 g/day) combined with RT during a caloric-restricted diet significantly increased squat 1 repeated maximum (RM) (+ 15.1kg) and bench press 1RM (+ 7.1 kg) [28].

Overall, BCAA supplementation appears to reduce exercise-induced muscle damage, attenuate delayed-onset muscle soreness, and support strength and lean mass when combined with resistance training. The synergistic action of leucine, isoleucine, and valine may extend beyond leucine’s anabolic effect, particularly under conditions such as caloric restriction or high training stress. Nevertheless, existing trials vary widely in dosage, duration, and participant characteristics, and most are short-term with modest sample sizes. Importantly, few studies have directly compared sex- or age-specific responses, which limits the generalizability of conclusions. Thus, current evidence suggests potential benefits of BCAAs, but larger, long-term, and sex-comparative randomized trials are required to confirm these effects.

The Effects of EAA Supplementation on Muscle Protein Metabolism and Adaptive Responses

EAA supplementation plays a pivotal role in promoting skeletal muscle anabolism by stimulating MPS, particularly in individuals with compromised protein metabolism, such as older adults or patients with sarcopenia. Unlike intact proteins, free-form EAAs do not require digestion and are rapidly absorbed, which makes them especially effective for acutely increasing plasma amino acid levels and stimulating MPS through the activation of the mTORC1 pathway [15]. Recent randomized controlled trials (RCTs) and systematic reviews have highlighted that, in older populations, MPS is markedly stimulated only by EAAs and not by intact proteins or non-essential amino acids. [29]. EAA ingestion of 18 g produced effects comparable to those observed with 40 g of balanced amino acids, resulting in a 33% increase in fractional synthetic rate (FSR) [15]. Collectively, this evidence underscores that targeted EAA supplementation is a pivotal nutritional strategy for promoting muscle maintenance and functional capacity in aging populations.

Several RCTs have demonstrated that EAA supplementation enhances Muscle protein synthesis, particularly when combined with RT. Paddon-Jones et al. (2004) found that daily consumption of 15 g EAAs significantly improved FSR (+ 0.04%/h) in participants undergoing RT [30]. additionaly, Børsheim et al. (2008) found that 16 weeks of EAA supplementation (22 g/day) significantly increased lean body mass (+ 1.14kg), improved lower-body strength (+22.2 %) in elderly individuals with impaired glucose tolerance [31]. Moreover, EAA intake demonstrated anabolic benefits in immobilized and hospitalized individuals. In a study by Ferrando et al. (2010), older adults confined to bed rest who received 15 g/day EAAs showed preserved muscle mass and function compared to the placebo, and this emphasizes the role of EAAs in mitigating muscle loss during inactivity [32].

EAA supplementation consistently stimulates muscle protein synthesis, particularly in older adults and clinical populations where anabolic resistance is common. Compared with intact proteins, free-form EAAs act rapidly and can elicit anabolic responses at relatively lower doses, supporting muscle maintenance and function during resistance training or periods of inactivity. Nevertheless, the current evidence base is limited by small sample sizes, heterogeneous dosing regimens, and short intervention durations. In addition, many trials focused on older or metabolically impaired individuals, making it uncertain whether similar benefits extend to younger or athletic cohorts. Thus, while EAAs represent a promising nutritional strategy for preserving muscle health, larger and longer term randomized trials are required to establish optimal dosing, timing, and generalizability across populations.

The Effects of Non-Essential Amino Acids Supplementation on Muscle Protein Metabolism and Adaptive Responses

Taurine, a sulfur containing non-essential amino acids (NEAA) that is abundantly present in skeletal muscles, has shown promising effects on muscle recovery and performance. In a double-blind randomized trial, McLeay et al. (2017) found that taurine supplementation (0.1 g/kg/day for 3 days before and 4 days after eccentric exercise) significantly reduced muscle soreness (DOMS score by approximately 18%, p < 0.05), improved isometric peak torque at 48 h post-exercise by approximately 10% compared to placebo, and attenuated serum creatine kinase (CK) elevations by approximately 27%, which indicated enhanced muscle recovery [33]. In addition, Waldron et al. (2019) demonstrated that 7 days of oral taurine supplementation (3 g/day) in trained males significantly increased critical power by ~7% (p < 0.05) and prolonged time-to-exhaustion during severe-intensity cycling by ~12%, highlighting taurine’s ergogenic potential for sustaining high-intensity performance [34]. Similarly, Buzdağlı et al. (2023) conducted a double-blind, placebo-controlled, crossover study in elite speed skaters and found that a single 6-g taurine dose increased peak anaerobic power by 5.44% and vertical jump performance by 6.91% (p < 0.01), while reducing post-exercise blood lactate by ~11%, supporting its ergogenic potential in anaerobic sports35. Taken together, current human taurine data indicate benefits for muscle recovery, power endurance, and anaerobic performance; however, these findings largely derive from short-duration or small-cohort trials, warranting cautious interpretation when extrapolating to broader athletic or clinical populations.

The evidence from recent research studies highlights the synergistic benefits of GlyNAC supplementation in older adults. Genton et al. (2021) found that 9 months of oral glycine in a randomized double-blind crossover trial increased fat-free mass index versus BCAA (+ 0.27 kg/m² for BCAA vs glycine) [36]. Kumar et al. (2021) found that GlyNAC supplementation in older adults increased gait speed by 0.3 m/s and improved handgrip strength by +2.7 kg, while also correcting glutathione deficiency, reducing oxidative stress, and ameliorating mitochondrial dysfunction [37]. GlyNAC supplementation restores intracellular glutathione synthesis by providing glycine and cysteine, thereby correcting age-related deficiencies, reducing oxidative stress, and protecting mitochondria and contractile proteins. It enhances mitochondrial bioenergetics by improving oxidative phosphorylation efficiency and ATP production, supporting muscle contractility and endurance. Concurrent reductions in systemic inflammation and insulin resistance promote anabolic signaling and protein turnover, while improved endothelial function augments skeletal muscle perfusion. Collectively, these mechanisms explain the observed gains in gait speed and grip strength, underscoring GlyNAC as a potential therapeutic strategy against age-related muscle decline [36,37]. However, both GlyNAC studies enrolled relatively small samples of older adults and should be considered preliminary pending replication in larger, adequately powered randomized cohorts with muscle-specific primary endpoints.

The enzyme phosphoglycerate dehydrogenase (PHGDH), which is involved in endogenous serine synthesis, was recently reported to be essential for skeletal muscle cell biomass formation and mTORC1 signaling. Furthermore, PHGDH inhibition in vitro reduced protein synthesis by approximately 40% and decreased myotube diameter, which indicates serine's crucial role in muscle maintenance [38]. Notably, these findings are mechanistic and derived from cell/experimental models rather than human supplementation trials, underscoring a translational gap that requires clinical validation. The NEAA evidence base remains preliminary, consisting largely of short-duration, small-sample human trials and mechanistic work; accordingly, we frame NEAA related conclusions as signals awaiting confirmation in larger, longer randomized cohorts with muscle-centric primary endpoints.

Across included randomized trials, adequate random sequence generation and allocation concealment were reported inconsistently, blinding of participants/outcome assessors was uncommon in training interventions, and sample sizes were frequently small with short follow-up. Pre-registration and protocol availability were rare. Consequently, we judge the overall certainty as moderate for leucine/EAA effects on MPS and strength in older adults, low-to-moderate for BCAA effects on strength/DOMS, and low for NEAAs (glycine, taurine, serine), warranting cautious interpretation.

Effects of Supplementation with Amino Acids on Muscle Adaptation: Sex Differences

Effects of Leucine Supplementation on Muscle Adaptation: Sex Differences

Leucine is a well-established stimulator of MPS that functions via mTORC1 activation; however, there seem to be sex differences in the anabolic efficacy of leucine. Sex-based differences in the response to leucine supplementation have been observed in multiple clinical studies, suggesting physiological variations in muscle adaptation between men and women. Rieu et al. (2006) demonstrated that in older men, leucine supplementation (0.1 g/kg) significantly enhanced MPS by 33% over a control amino acid mixture [39]. Atherton et al. (2017) found that a leucine-enriched protein drink increased post-exercise MPS by approximately 30% in both young and older men, independently of total amino acid concentration [40]. In contrast, Funderburk et al. (2020) reported that 10 weeks of 5 g/day leucine supplementation combined with RT in untrained middle-aged women did not significantly alter fat-free mass or strength beyond training alone, which indicated limited anabolic responsiveness [41]. Sawicka et al. (2022) found no additive effect of 4 g/day leucine (with or without L-carnitine) during 24-week training in older women on muscle mass or strength, with only minor increases in thigh muscles (+ 2%) and isokinetic knee extension torque (+ 5.5%) [42]. Fujita et al. (2007) showed that basal intracellular amino acid transport and muscle protein turnover were significantly lower in women than in men, which suggests a mechanistic basis for sex-based anabolic resistance [18]. Collectively, these findings highlight a potential sex difference in leucine metabolism and muscle responsiveness, with men displaying greater MPS and strength outcomes, and women exhibiting attenuated or negligible responses under similar supplementation protocols (Table 2).

Table 2.

Sex differences in the effects of leucine or BCAA supplementation in humans on muscle strength, muscle mass, and muscle adaptation

Factor	Note			Significant difference		Case number	Ref.
	Sex	Age	Details of protein condition	Male	Female
Leucine	M	69.5 ± 0.8	0.052 g/kg leucine, semi-liquid diet, 15 aliquots every 20 min for 5 h (after 240-min fast)	(+)	N/A	20	[39]
				MPS (+ 33%)
	M	47 ± 5.5	Leucine-enriched protein drink: leucine 4.2g/immediately after resistance exercise	(+)	N/A	18	[40]
				MPS (+ 30%)
	F	55.3± 6.4	Leucine 5g with resistance exercise/day for 10 weeks	N/A	NS (Fat free mass, Strength)	36	[41]
	F	67.6 ± 0.7	Leucine 4g, once daily with main meal/day for 24 weeks	N/A	NS (Thigh muscles: + 2%, Isokinetic peak torque: + 5.5%)	13	[42]
BCAA	M&F	20-48	BCAA capsules 5g (5/day; 2.5 g leucine, 1.25 g isoleucine, 1.25 g valine), daily for 6 months.	(+)	(+)	100	[43]
				(1RM squat: + 11.2 kg, 1RM bench press: + 9.5 kg, 1RM dead lift: + 11.3 kg) / (DOMS: - 23.6mm)	(1RM dead lift: +9.4 kg) / (DOMS: - 18.1mm
	M	21-28	18g/day (pre-exercise and post-exercise) for 8 weeks	(+)	N/A	17	[28]
				(1RM squat: + 15.1 kg, 1RM bench press: 7.1 kg)
	F	22.5 ± 3.8	BCAA (100 mg/kg, iso:leu:val = 1:2.3:1.2), ingested in morning before squat exercise	N/A	(-)	12	[45]
					(DOMS: - 31%, muscle-force decrease: to ~80% of the value recorded under the control conditions)

+: increase, -: decrease.

M: male, F: female, NS: not significant, N/A: not applicable, RM: repeated maximum, DOMS: delayed onset muscle soreness.

Effects of BCAA Supplementation on Muscle Adaptation: Sex Differences

Sex-based physiological responses to leucine-focused BCAA supplementation revealed notable differences in muscle adaptation outcomes. In a 6-month RCT, male participants receiving 5 g/day of BCAA showed a 10% increase in bench press one-repetition maximum (1RM) although female participants exhibited a markedly greater reduction in DOMS compared to placebo (−18.1 ± 9.4 mm vs −0.8 ± 1.2 mm) [43]. These findings suggest that BCAA supplementation may enhance strength- and recovery-related parameters in men and women, respectively. Supporting this, a trial by Dudgeon et al. demonstrated that BCAA intake (9 g/day) during RT under caloric restriction helped trained males maintain lean body mass (−0.05 kg vs −1.41 kg in control) and preserve bench press 1RM (+1.8 kg vs −2.2 kg) [28]. In contrast, a double-blind, crossover randomized trial in young, healthy, untrained women (n = 12) reported that a single pre-exercise BCAA dose (100 mg·kg⁻¹; isoleucine: leucine: valine = 1:2.3:1.2) before 7 × 20 squat sets significantly reduced DOMS at 48–72 h versus placebo and attenuated the loss of maximal isometric leg force measured 48 h post-exercise; serum myoglobin rose after exercise in the placebo but not the BCAA condition, indicating less muscle damage [44]. Furthermore, a recent systematic review and meta-analysis found that BCAA supplementation significantly lowered creatine kinase and consistently reduced DOMS across follow-up time points after exercise-induced muscle damage [45]. These findings collectively underscore the sex-specific benefits of BCAA supplementation and highlights the role of leucine in strength preservation in men and enhanced recovery potential in women.

Mechanistic Basis of Sex Differences in Muscle Adaptation to Amino Acid Supplementation

Estrogen promotes satellite cell activation and mitigates exercise-induced damage via receptor-mediated pathways, whereas testosterone enhances hypertrophy through androgen receptor signaling [46]. Men also exhibit higher leucine oxidation rates during exercise, while women display lower oxidation and, in some contexts, higher basal muscle protein synthesis [47,48]. These observations provide a biological rationale for differential adaptations to leucine-enriched or BCAA supplementation between sexes. Amino acid kinetics and basal MPS. Large-scale tracer studies indicate that, when controlling for age and training status, basal mixed-MPS and mTORC1 signaling are broadly similar between men and women [49]. However, other isotope-tracer trials reported higher whole-body protein synthesis and muscle fractional synthesis rates in women than in men, suggesting sex-specific variability depending on aerobic fitness, energy expenditure, and hormonal context [47]. Taken together, resting MPS appears similar or slightly higher in women, but sex differences emerge more clearly in the response to amino acid supplementation or exercise, which remain underexplored in direct comparative trials.

Muscle fiber type distribution. Quantitative reviews demonstrate that men exhibit greater cross-sectional area in all fiber types, particularly type II fibers, while women display a higher proportion and relative area of type I fibers [50]. These structural disparities align with functional observations that women are less fatigable during isometric or low-speed tasks, whereas men demonstrate greater potential for high-intensity power outputs. Such characteristics may modulate supplementation outcomes, favoring strength metrics in men versus recovery-related markers in women.

Substrate metabolism and amino acid oxidation. Women consistently oxidize less leucine and protein than men during endurance exercise, indicating a more conserved amino acid metabolism and greater reliance on lipid utilization, while men demonstrate higher carbohydrate dependence [48,51]. This divergence in substrate preference may intersect with leucine availability, mTORC1 activation, and post-supplementation recovery dynamics.

Satellite cells and regenerative capacity. Estrogen signaling sustains the female satellite cell pool, supporting self-renewal and survival, whereas estrogen deficiency (e.g., menopause) reduces satellite cell number and regenerative capacity [46]. This receptor-mediated regulation may underlie why women often show more pronounced recovery benefits, such as reductions in DOMS, following BCAA supplementation, whereas men more consistently demonstrate anabolic outcomes in terms of MPS and strength.

DISCUSSION

Synthesizing the narrative risk-of-bias appraisal, we consider certainty to be moderate for leucine/EAA improving MPS and strength in older adults; low to moderate for BCAA effects on strength/DOMS; and low for NEAAs. Amino acid supplementation, especially EAAs and BCAAs, has been consistently associated with improvements in MPS, muscle strength, or recovery in specific contexts (e.g., older adults, energy restriction), whereas effects are more variable across younger or highly trained cohorts. Patterns suggesting greater strength oriented benefits in men and more pronounced recovery effects in women are hypothesis-generating and require direct sex-comparative randomized trials controlling for hormonal status, training load, and dosing per body mass or fat-free mass. Crucially, direct, head to head randomized comparisons between males and females are scarce, limiting sex-specific inference. The novelty of this review lies in consolidating and problematizing this evidence gap, outlining where sex-specific hypotheses are plausible yet not definitively established.

An integrative review of studies on leucine supplementation highlights its robust potential as a primary anabolic stimulus for improving skeletal muscle mass and strength. The mechanistic basis for these outcomes lies in leucine’s ability to activate the mTORC1 pathway, which initiates downstream signaling via p70S6K and 4E-BP1, thereby facilitating myofibrillar protein synthesis and muscle accretion [24]. This is especially relevant in sarcopenic or elderly individuals, where basal and postprandial MPS responses are blunted, making leucine-enriched supplementation a physiologically targeted countermeasure. Moreover, prolonged stimulation of MPS for several hours post-exercise, as demonstrated by Dickinson et al. (2014), underscores the temporal advantage of leucine in sustaining muscle remodeling beyond immediate post-exercise periods [25]. Collectively, these findings strongly support the use of leucine as a targeted nutritional strategy to enhance muscle mass and strength across diverse populations, particularly among older adults and individuals undergoing RT. The consistency of evidence across randomized trials, population studies, and mechanistic data reinforces the role of leucine as a critical component of muscle health interventions.

Leucine supplementation appears to enhance muscle protein synthesis and strength more effectively in men than in women, possibly because of sex differences in amino acid transport and muscle metabolism [16,17]. Although older and young men consistently showed significant gains in MPS and performance metrics, similar supplementation in women yielded modest or negligible improvements despite identical protocols [17]. This divergence suggests that women may have a higher threshold for leucine responsiveness or may require different dosing strategies [52]. However, variations in age, training status, and study duration across trials have limited direct comparisons of sex differences. Future studies should explore individualized supplementation protocols that consider physiological sex differences to optimize muscle-adaptation outcomes.

Sex-based disparities in amino acid oxidation and anabolic responsiveness may explain the differential effects of leucine supplementation. Women have been consistently shown to oxidize less leucine and protein than men during exercise, which indicates a lower net amino acid catabolism and a more conserved metabolic response [53]. Furthermore, optimal estrogen signaling in women is associated with an increased utilization of lipids, rather than amino acids, and attenuated leucine oxidation, which could blunt the MPS response to leucine intake [42,53]. These findings provide a mechanistic rationale for why men, compared to women, frequently exhibit stronger anabolic responses (e.g., higher MPS rates) to leucine supplementation. While a consistent anabolic role for leucine is supported by tracer and signaling studies particularly in older men direct, head to head randomized comparisons between males and females remain scarce. As such, the oft cited pattern of larger strength or lean mass benefits in men and greater recovery benefits in women should be considered hypothesis-generating rather than confirmatory, given that many studies are single sex cohorts or rely on indirect subgroup contrasts [44,48,53,26].

The efficacy of BCAA supplementation appears to be influenced by dosing strategy and timing, with studies suggesting that pre- and post-exercise ingestion yields the most favorable outcomes [44,54]. Collectively, these findings support the use of BCAAs as a practical and evidence-based supplementation strategy for improving muscle strength, mitigating exercise-induced muscle damage, and preserving lean mass, particularly in individuals undergoing RT or energy restriction [28,55]. Although leucine remains the principal driver of MPS, the inclusion of isoleucine and valine may contribute to energy metabolism, glucose uptake, and recovery, and this highlights the multifaceted value of complete BCAA supplementation [55].

The observed sex-based differences in response to BCAA supplementation suggest a sexually dimorphic adaptation to leucine-enriched amino acid intake during RT. While men appear to benefit more in terms of strength enhancement, women demonstrate greater improvements in muscle recovery parameters such as DOMS, likely due to differential hormonal and neuromuscular responses. Estrogen, known for its anti-inflammatory properties and membrane-stabilizing effects, may synergize with BCAAs to attenuate muscle damage and accelerate recovery in females [56]. In contrast, testosterone-driven anabolic signaling in males may potentiate leucine-mediated mTOR activation, resulting in enhanced muscle protein synthesis and strength gains [57]. However, the variation in BCAA dosage, duration, and participant characteristics across studies presents limitations in establishing standardized sex-specific guidelines. Moreover, the lack of direct mechanistic studies comparing male and female muscle responses to BCAA intake limits definitive conclusions. Nevertheless, the consistent pattern of divergent adaptations reinforces the need for sex-specific nutritional strategies in exercise programming. Future research should focus on elucidating the molecular underpinnings of these sex differences to optimize BCAA interventions for both performance and recovery outcomes. These findings suggest that BCAA supplementation, particularly of leucine-focused BCAAs, elicits sex-specific muscle-adaptation responses. Men tended to experience greater improvements in strength and lean mass preservation, whereas women showed more pronounced reductions in muscle soreness and damage markers. This divergence may stem from sex differences in hormonal regulation, muscle metabolism, or recovery mechanisms. However, variations in the study duration, BCAA dosage, participant age, and baseline training status limit the generalizability of these results. Future research should explore the optimal dosing strategies and mechanisms underlying sex-specific responses to guide personalized supplementation.

Regarding physiological mechanisms, the availability of EAAs is essential for initiating and sustaining MPS because the absence of any single EAA can limit the anabolic response despite the presence of other nutrients. Collectively, these findings provide robust evidence that EAA supplementation is an effective strategy to enhance muscle mass and strength across different populations, including healthy adults, older individuals, and individuals experiencing muscle disuse or catabolism. Consistent positive outcomes across diverse clinical trials underscore the therapeutic potential of EAA interventions in preventing or reversing aging-related muscle loss [6,15].

Although the dietary inclusion of NEAAs is not required, emerging research has indicated that serine, glycine, and taurine play notable roles in muscle physiology, particularly in aging, recovery, and metabolic regulation [35,38]. The beneficial effects of taurine in attenuating exercise-induced muscle damage, improving recovery, and enhancing high-intensity performance highlight its relevance in sports nutrition and muscle adaptation [33]. Serine acts as a precursor for phospholipid synthesis and is essential for maintaining the skeletal muscle cell biomass via the activation of mTORC1 [38]. Glycine is integral to collagen synthesis, antioxidant defense, and neuromuscular function. Glycine supplementation reduced muscle atrophy through enhanced mTORC1–S6 signaling and improved functional parameters when combined with N-acetylcysteine (GlyNAC) supplementation in older adults [58]. These findings suggest that glycine, particularly as part of GlyNAC, may significantly enhance muscle performance and metabolic health in aging populations by supporting antioxidative defenses, mitochondrial function, and neuromuscular integrity [37]. Nevertheless, the current body of evidence for NEAAs is largely composed of short term, small sample, or pilot studies in specific populations (including one animal study and mechanistic in-vitro work), which limits inference strength and generalizability. Accordingly, NEAA focused conclusions in this review are presented as preliminary signals rather than definitive recommendations, and future trials should verify durability of effects, compare clinically meaningful endpoints, and determine dose response relations across diverse cohorts. Collectively, these findings suggest a potential role for NEAAs serine for structural and signaling support, glycine (particularly within GlyNAC) for antioxidant and mitochondrial function, and taurine for recovery and high intensity performance. At present, NEAAs are best considered adjunctive modulators within comprehensive nutrition and exercise frameworks, pending confirmation from larger, longer term randomized clinical trials that prespecify muscle function and morphology as primary outcomes.

While this review synthesizes a substantial body of literature on amino acid supplementation, its interpretation is constrained by several factors. Most notably, direct randomized comparisons between men and women are scarce, and much of the current evidence relies on single sex cohorts, subgroup analyses, or mechanistic models, which substantially weakens the strength of sex-specific inferences. Moreover, considerable methodological heterogeneity including variation in dosage (absolute vs. per kg vs. per fat free mass), timing (pre vs. post-exercise), intervention duration, training status, and outcome selection (e.g., MPS vs. strength vs. DOMS or enzyme leakage) further limits cross study comparability. Importantly, several of the reported sex-related differences are drawn from short term, small sample, or pilot studies, which underscores that the present conclusions should be regarded as preliminary signals rather than definitive evidence. Accordingly, the novelty of this review lies in highlighting the current evidentiary gap regarding sex-specific responses to amino acid supplementation. Future research should therefore prioritize rigorously designed, adequately powered, sex comparative clinical trials that integrate hormonal and life stage variables, employ standardized supplementation protocols, and adopt clinically meaningful functional and structural endpoints. Such efforts will be essential for establishing whether the observed trends greater anabolic and strength oriented benefits in men versus more pronounced recovery effects in women represent true biological dimorphism or study level artifacts. By clarifying these mechanisms, future investigations can provide the evidence base necessary for sex-specific, mechanistically informed supplementation strategies that preserve and enhance musculoskeletal health across diverse populations.

Amino acid supplementation is associated with improvements in muscle protein synthesis and in selected contexts, strength and recovery with the most consistent signals for free form EAAs and leucine in older adults; BCAA benefits appear context dependent, while NEAA evidence remains preliminary. Hints of greater strength gains in men and greater recovery benefits in women are biologically plausible but hypothesis-generating, reflecting single sex cohorts and indirect cross-study contrasts. Overall certainty from our narrative appraisal is moderate for leucine/EAA effects in older adults, low to moderate for BCAAs, and low for NEAAs. Clinically, supplementation should remain an adjunct to structured resistance training, with individualized dosing and timing. Progress requires adequately powered, pre registered, sex comparative RCTs that standardize protocols and prespecify functional and structural endpoints to define dose response, timing, and durability across ages and training states.