Amino Acids, Muscle, and the Sex Divide
A narrative review of effects on muscle mass, strength, and sex differences, after amino-acid supplementation.
A new narrative review explored whether essential amino acids, BCAAs, and non-essential amino acids build enough muscle and strength, and whether they work the same way in men and women. The short answer: yes, often, but it depends heavily on which amino acids you're taking, your age, your sex, and what you're trying to achieve. Let’s dig into it.
Aim
Published in Physical Activity and Nutrition in September 2025, this narrative review by Ki-Woong Noh and Sok Park of Kwangwoon University in Seoul sought to synthesise the research on amino acid supplementation and its effects on muscle mass, strength, and adaptation, with a specific focus on whether those effects differ between men and women. The researchers categorised the topic into three groups: essential amino acids (EAAs), including leucine; branched-chain amino acids (BCAAs); and non-essential amino acids (NEAAs), such as taurine and glycine. The timing is deliberate; as global supplement use keeps climbing, the science needed a proper stocktake
Methods
The team searched PubMed, ScienceDirect, and Google Scholar for clinical and non-clinical human studies published between 1990 and 2025. The initial search returned 3,791 records. After removing duplicates, 2,087 records underwent title and abstract screening, with two independent reviewers resolving any disagreements. Studies were included if they involved adults aged 18 or older, used EAA, BCAA, leucine, or NEAA supplementation, and measured at least one of: muscle protein synthesis rate, muscle strength (e.g. 1RM), lean mass, functional performance, DOMS, or muscle damage enzymes like creatine kinase. Studies with unclear inclusion criteria, insufficient protocol descriptions, or non-English language were excluded. Because this was a narrative review, the authors did not pool effect sizes statistically. Instead, they rated certainty of evidence qualitatively as higher, moderate, or lower across four quality domains: randomisation and concealment, blinding, attrition, and selective reporting.
Results
Leucine and EAAs: The Strongest Signal
Leucine earned top billing. It directly activates the mTORC1 signalling pathway, triggering downstream processes that drive muscle protein synthesis (MPS), and it appears to work even when other anabolic signals are blunted, making it especially useful for older adults who struggle to build muscle from regular protein.
Key findings on leucine:
In older adults, a leucine-enriched EAA formulation (with leucine making up 41% of the blend) stimulated MPS, where a standard EAA blend failed to do so
A 10 g EAA dose providing 3.5 g leucine taken one hour after resistance training prolonged MPS in older men for up to four hours post-exercise
A 12-week leucine-enriched protein supplement (2 g/day leucine) combined with resistance training in adults over 50 increased total lean mass and grip strength significantly
Adding just 5 g of leucine to a low-protein mixed macronutrient beverage boosted MPS in healthy young men
For EAAs more broadly, the evidence was equally compelling in older populations. A dose of 18 g EAA produced a 33% increase in fractional synthetic rate (FSR), matching the effect of 40 g of balanced amino acids. A 16-week EAA protocol of 22 g/day in elderly people with impaired glucose tolerance increased lean body mass by 1.14 kg and improved lower-body strength by 22.2%. Even in bedridden older adults, 15 g/day EAA preserved muscle mass and function compared to placebo.
Overall certainty for leucine and EAA effects on MPS and strength in older adults: moderate.
BCAAs: Context Is Everything
BCAAs (leucine, isoleucine, and valine together) showed strong results for reducing muscle damage and soreness, and for preserving strength under tough conditions.
Notable findings:
20 g/day BCAA for 12 days before and after resistance training significantly reduced creatine kinase levels and DOMS while preserving maximal voluntary contraction
Eight weeks of 18 g/day BCAA combined with resistance training increased squat 1RM by 15.1 kg and bench press 1RM by 7.1 kg
During a calorie-restricted diet, 9 g/day BCAA during resistance training helped trained men maintain lean body mass (losing just 0.05 kg vs 1.41 kg in the control group) and preserve bench press 1RM (+1.8 kg vs -2.2 kg in controls)
Overall certainty for BCAA effects on strength and DOMS: low to moderate.
Non-Essential Amino Acids: Promising but Preliminary
Taurine, glycine, and serine are not traditionally viewed as muscle-building tools, but emerging data suggest they play supporting roles in recovery and metabolic health.
Taurine (0.1 g/kg/day) reduced DOMS scores by approximately 18% and attenuated creatine kinase elevations by about 27% following eccentric exercise
A single 6 g dose of taurine increased peak anaerobic power by 5.44% and vertical jump by 6.91% in elite speed skaters
GlyNAC (glycine combined with N-acetylcysteine) 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 and reducing oxidative stress
Overall certainty for NEAAs: low, with findings described as preliminary signals rather than firm recommendations.
Sex Differences
This is where the review breaks new ground, and also where it finds the most obvious gaps in the research. The authors identified a consistent pattern across studies: men tended to show greater MPS and strength gains from leucine and BCAA supplementation, while women tended to benefit more from recovery-related effects such as reduced DOMS.
The biological rationale is plausible. Men oxidise leucine at a higher rate during exercise, meaning they may have a greater need for exogenous leucine to drive MPS. Women, on the other hand, rely more on fat as fuel, preserve amino acids more conservatively, and benefit from estrogen’s anti-inflammatory and membrane-stabilising effects, which may synergise with BCAAs to reduce muscle damage.
Specific sex-difference findings:
Leucine supplementation boosted MPS by 33% in older men and by approximately 30% in young and older men after resistance exercise
The same leucine protocols in middle-aged and older women produced no significant changes in fat-free mass or strength
In a six-month BCAA trial (5 g/day), men gained significantly in squat 1RM (+11.2 kg), bench press 1RM (+9.5 kg), and deadlift 1RM (+11.3 kg), while women's primary benefit was a pronounced reduction in DOMS (-18.1 mm vs -0.8 mm in placebo)
Women had significantly lower basal intracellular amino acid transport rates than men, suggesting a mechanistic basis for their more muted anabolic response
Critical caveat: the authors stress these patterns are hypothesis-generating, not definitive. Almost no studies have directly compared males and females in a head-to-head randomised design. Most of the evidence comes from single-sex cohorts compared indirectly across different studies.
Takeaways
Here is what lifters and fitness-minded people can actually use from this review:
Leucine is your best friend if you’re older or struggling to recover. Aim for a leucine-enriched EAA supplement post-training, particularly if you are over 50. A dose providing at least 2.5 to 3.5 g leucine appears to extend MPS meaningfully beyond what standard protein alone achieves
BCAAs are most valuable under caloric restriction or heavy training loads. If you're cutting or training at high volume, BCAAs (roughly 9 to 20 g/day, split around training) can help preserve lean mass and blunt soreness.
Men and women may need to adjust expectations. Men appear to get stronger from amino acid supplementation; women appear to recover faster. Neither outcome is trivial, but the data suggest dosing strategies may eventually need to be sex-specific.
NEAAs like taurine and GlyNAC are worth watching, not yet prescribing broadly. Taurine shows genuine promise for power output and recovery; GlyNAC is intriguing for older adults. But the trials are small and short. Consider them supportive tools, not cornerstones.
Supplementation is an adjunct, not a replacement. Across every category, the best outcomes came when amino acid supplementation was combined with structured resistance training. Supplements without the training stimulus underperformed consistently.
The Bottom Line
This review confirms that amino acid supplementation, particularly EAAs and leucine, can meaningfully support muscle protein synthesis and, in the right populations, build real lean mass and strength. The emerging sex-difference story is compelling and biologically grounded, but it needs rigorous, properly designed, head-to-head male vs. female randomised controlled trials before it can inform specific recommendations. Until that research exists, individualise your approach, prioritise resistance training, and treat amino acid supplementation as a precision tool rather than a blanket fix.
Reference
Effects of Amino Acid Supplementation on Muscle protein metabolism and adaptation: a narrative review of effects on muscle mass, strength, and sex differences. Phys Act Nutr. 2025;29(3):53-63. Published online September 30, 2025
DOI: https://doi.org/10.20463/pan.2025.0026
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