Branched-chain amino acids may help or harm in long-term inflammation and injury-related diseases

Leucine is recognized as the most potent natural mTORC1 activator, with its sensing and signal transduction mechanisms involving a complex sensor network. Key steps include leucine binding to the Sestrin2 protein, relieving its inhibition of the GATOR2 complex, subsequently promoting Rag GTPase complex activation, and ultimately recruiting mTORC1 to the lysosomal surface for activation by the Rheb protein (25, 26). Activated mTORC1 promotes protein synthesis and suppresses autophagy through phosphorylation of downstream targets S6K1 and 4E-BP1, thereby coordinating the balance between anabolic and catabolic metabolism (27). It should be noted that mTORC1 represents one major signaling hub within a broader pathological network rather than a singular unifying switch; its activation is therefore described throughout this review as contributing to or being associated with downstream metabolic consequences, rather than as a sole causal determinant. Notably, different amino acid combinations exhibit varying effects on mTORC1 activation. Research demonstrates that compared to leucine alone, BCAA mixtures more potently activate S6K1 following exercise, while mixtures containing all essential amino acids yield optimal effects, suggesting that a complete amino acid profile maximizes mTORC1 activation and protein synthesis responses (28).

Parallel pathological mechanisms beyond mTORC1. While mTORC1/S6K1 hyperactivation is the most extensively characterized node, the pathological consequences of impaired BCAA catabolism involve at least three additional parallel mechanisms that operate independently of mTORC1. First, BCKAs—the immediate transamination products of BCAAs—accumulate when BCKDH flux is impaired and directly inhibit insulin-stimulated Akt phosphorylation, thereby impairing glucose uptake and mitochondrial oxygen consumption through mTORC1-independent routes (20, 29). Second, branched-chain acylcarnitines (BCACs), generated from incomplete BCAA oxidation, accumulate in skeletal muscle of insulin-resistant individuals, competitively inhibit fatty acid oxidation, and are associated with mitochondrial membrane dysfunction and oxidative stress (14, 30). Third, the resulting mitochondrial redox imbalance—characterized by elevated reactive oxygen species (ROS) production and impaired antioxidant capacity—activates NF-κB and NLRP3 inflammasome pathways, amplifying chronic inflammatory responses (31, 32). The integrated pathological network arising from these converging mechanisms is summarized in Figures 2, 3. Where the available evidence is primarily epidemiological or associative, this review deliberately employs qualified language (e.g., “is associated with, “ “may contribute to”) rather than asserting direct causation.

A complex bidirectional regulatory relationship exists between insulin signaling and BCAA metabolism. As illustrated in Figure 4, Under physiological conditions, insulin regulates BCAA metabolism through multiple mechanisms: suppressing whole-body protein breakdown to reduce endogenous BCAA release; promoting peripheral tissue BCAA uptake; and enhancing BCKDH activity via PPM1K activation, thereby accelerating BCAA oxidative catabolism (33).

However, in insulin-resistant states, this sophisticated regulatory network becomes disrupted. Insulin resistance leads to insensitivity to insulin’s anti-proteolytic effects, while peripheral tissue BCAA uptake and oxidative capacity decline, resulting in elevated plasma BCAA and BCKA levels (34). These accumulated metabolites further exacerbate insulin resistance through multiple mechanisms, particularly via excessive activation of the mTORC1/S6K1 pathway, inducing serine phosphorylation of insulin receptor substrate-1 (IRS-1) and thereby inhibiting insulin signal transduction (35). In vitro studies further confirm that BCKAs can directly interfere with insulin signaling, with mechanisms potentially involving mitochondrial dysfunction and reactive oxygen species generation (29).

As the sole site of BCAA oxidative metabolism, mitochondrial functional status directly determines catabolic efficiency. In metabolically healthy individuals, adequate mitochondrial oxidative phosphorylation capacity ensures efficient entry of BCAAs and their derivatives into the tricarboxylic acid cycle for complete oxidation. However, in patients with obesity, insulin resistance, and type 2 diabetes, skeletal muscle mitochondrial oxidative capacity is universally compromised, manifesting as reduced mitochondrial density and decreased respiratory chain complex activity (13). Notably, BCAAs exhibit dual regulatory characteristics on mitochondrial function. At physiological concentrations, BCAAs upregulate mitochondrial biogenesis-related gene expression through AMPK and PGC-1α activation, promoting mitochondrial function; conversely, abnormally elevated BCAA concentrations suppress mitochondrial biogenesis and oxidative gene expression (10). This dose-dependent effect further demonstrates the complexity of BCAA metabolic regulation (see Figure 4).

Substantial epidemiological evidence indicates that patients with obesity and insulin resistance exhibit significantly elevated plasma BCAA levels, closely associated with reduced tissue BCKDH activity (36). In genetic obesity mouse models, systemic suppression of BCAA catabolic gene expression can be observed, with elevated BCKDH phosphorylation (inactivation) levels, and these metabolic abnormalities often precede overt insulin resistance, suggesting that BCAA metabolic dysfunction may represent an early event in obesity-related metabolic disorders. Restoration of BCKDH activity has been proven to effectively ameliorate metabolic dysfunction. Pharmacological activation of BCKDH using small molecule BCKDK inhibitors (such as BT2) significantly reduces plasma BCAA and BCKA levels in obese mouse models and improves insulin sensitivity and glucose tolerance in preclinical settings (37). These studies provide experimental support for a contributory role of BCAA metabolic dysfunction in obesity-associated insulin resistance, though translation to human therapeutic application requires further validation of long-term safety and efficacy. (see Figure 3).

Chronic low-grade inflammation serves as an important link connecting obesity, insulin resistance, and BCAA metabolic dysregulation. In the obese state, adipose tissue undergoes macrophage infiltration and increased secretion of pro-inflammatory cytokines (such as TNF-α and IL-6), which suppress BCAA metabolic enzyme expression and activity through multiple mechanisms (38). For instance, research on diabetic retinopathy revealed that under conditions of hyperglycemia and BCAA accumulation, Müller cells exhibited decreased BCAT and BCKDH protein levels with elevated BCKDK expression, accompanied by increased levels of inflammatory factors such as TNF-α (39). Conversely, overexpression of metabolism-related proteins, such as HMGB1—a protein closely associated with cancer-related muscle wasting (40), cancer-related cardiac dysfunction (41), and skeletal muscle aging (42)—further impairs BCAA metabolism and promotes BCAA accumulation, establishing a vicious cycle of mutually reinforcing metabolic dysfunction and inflammation (43).

A complex reciprocal relationship exists between mitochondrial dysfunction and BCAA accumulation. In age-related sarcopenia, recent mechanistic studies have unveiled an intriguing ‘mTOR paradox’: impaired mitochondrial BCAA catabolism in aged muscle leads to pathological BCAA accumulation, which excessively activates mTORC1. However, this activation not only fails to promote muscle growth but instead causes muscle mass and strength loss through autophagy suppression and mitochondrial dysfunction (44). This finding challenges the traditional notion that mTORC1 activation benefits muscle health, suggesting that under specific pathological conditions, ‘pathological mTORC1 hyperactivation’ fundamentally differs from ‘physiological mTORC1 activation.’ Pharmacological interventions enhancing BCKDH activity to promote BCAA catabolism have demonstrated protective effects in aged mouse models, providing novel strategic approaches for preventing and treating sarcopenia (45). In the cardiovascular disease domain, similar metabolic dysfunction has been observed. Heart failure patients and animal models exhibit elevated cardiac BCAA levels and increased BCKDH phosphorylation. Restoring cardiac BCAA catabolic capacity can ameliorate cardiac tissue remodeling and heart function, indicating that BCAA metabolic dysregulation plays critical roles in pathological processes across different tissues and organs (29).

The metabolism and regulation of BCAAs constitute a complex network system involving absorption, tissue distribution, enzymatic reactions, and signal transduction. From intestinal absorption through tissue-specific metabolism to the sophisticated regulation of the BCKDH rate-limiting step, each component is subject to fine-tuning by nutritional status, hormonal signals, and energy balance. Under physiological conditions, this system effectively maintains BCAA metabolic homeostasis. However, under pathological states such as obesity, chronic inflammation, and mitochondrial dysfunction, impaired BCAA catabolism leads to accumulation, participating in the pathogenesis and progression of insulin resistance, type 2 diabetes, cardiovascular disease, and sarcopenia through multiple mechanisms including excessive mTORC1 activation, insulin signaling interference, and mitochondrial stress induction. This distinctive regulatory relationship manifests as a ‘double-edged sword’ characteristic in disease development and progression. In Figure 3, we summarized the relationship between pathological drivers and multi-system damage.

Leucine-mediated activation of the mTORC1 pathway: BCAAs, particularly leucine, are key regulators of muscle protein synthesis. Leucine activates the mTORC1 signaling pathway through the Sestrin2–GATOR2–Rag GTPase axis described in Section 2.2.1, promoting phosphorylation of S6K1 and 4E-BP1 and thereby stimulating mRNA translation (28, 46, 47). BCAA mixtures activate S6K1 more potently than leucine alone following exercise, and mixtures containing all essential amino acids produce the strongest response (28).

Attenuation of muscle protein degradation: BCAAs also suppress protein degradation. mTORC1 activation inhibits autophagy by phosphorylating ULK1, TFEB, and Beclin-1 (48). Leucine additionally suppresses ubiquitin-proteasome system activity (47).

Improvement of nitrogen balance: BCAA supplementation shifts net protein balance toward positive values during post-exercise recovery (49). Ingestion of 5.6 g BCAAs following resistance exercise increased myofibrillar protein synthesis rate by 22% and reduced phenylalanine release by approximately 6% (49).

The therapeutic benefits of BCAA supplementation in advanced cirrhosis are supported by a robust body of randomized controlled trial evidence and major clinical guidelines, representing one of the most well-established “angel” applications of BCAAs in chronic disease management.

In a multicenter double-blind RCT, long-term oral BCAA supplementation significantly reduced the composite risk of death, liver transplantation, and disease progression in patients with advanced cirrhosis compared with lactalbumin or maltodextrin controls, with event-free survival significantly improved in the BCAA group (9). Among cirrhotic patients with hypoalbuminemia, BCAA granule supplementation (12 g/day for 2 years) significantly improved serum albumin levels, reduced the incidence of ascites and edema, and improved health-related quality of life; benefits were most pronounced in patients with lower baseline albumin and Fischer ratio, identifying the subgroup most likely to derive clinical benefit (30).

At the highest level of evidence, a Cochrane systematic review and meta-analysis incorporating 16 RCTs (n = 827) confirmed that oral BCAA supplementation significantly reduced the risk of hepatic encephalopathy (risk ratio 0.73, 95% CI 0.61–0.88; number needed to treat = 5) and improved overall clinical manifestations in cirrhotic patients, with the beneficial effect remaining robust in trials with low risk of bias (risk ratio 0.71, 95% CI 0.52–0.96) (66). Although a definitive survival benefit has not yet been established, consistent improvements in hepatic encephalopathy, nutritional parameters, and event-free survival across multiple trials provide compelling evidence for clinical application.

These findings are reflected in international clinical nutrition guidelines. Long-term oral BCAA supplementation is formally recommended for decompensated cirrhotic patients presenting with low Fischer ratio, sarcopenia, or protein-energy malnutrition, typically as leucine-enriched formulations at 12–14 g/day for a minimum of six months, targeting Fischer ratio correction, nutritional rehabilitation, hepatic encephalopathy risk reduction, and skeletal muscle ammonia clearance support (71). This constitutes a Grade A recommendation based on consistent RCT evidence.

Taken together, the available evidence positions BCAA supplementation as a disease-modifying nutritional intervention in advanced cirrhosis, with benefits operating through the three converging mechanisms outlined in Section 3.3.2: Fischer ratio correction, skeletal muscle ammonia clearance support, and mTORC1-mediated promotion of hepatocyte proliferation and protein synthesis. Critically, this therapeutic context—decompensated cirrhosis with documented malnutrition and reduced Fischer ratio—is mechanistically and clinically distinct from early MASLD or metabolic syndrome, where elevated circulating BCAAs reflect impaired catabolic disposal rather than insufficient intake, and supplementation is not recommended (see Section 6.3 for disease-stage-stratified clinical recommendations).

The hepatoprotective effects of BCAAs in cirrhosis operate through three converging mechanisms: (1) correction of the Fischer ratio and competitive inhibition of AAA brain entry; (2) support of skeletal muscle ammonia clearance; and (3) mTORC1-mediated promotion of hepatocyte proliferation and protein synthesis. These mechanisms are disease-stage dependent and are discussed in the clinical context in Section 6.3.

Branched-chain amino acids may play important roles in tissue repair and wound healing. Research indicates that BCAAs participate in promoting protein synthesis and cell proliferation, processes crucial for wound healing (60, 69). In cachectic patients, malnutrition and muscle wasting may impair wound healing, and BCAA supplementation as part of nutritional support may help ameliorate this condition (69). However, direct effects of BCAAs on wound healing require further clinical research confirmation.

Branched-chain amino acids are closely related to brain tryptophan and serotonin levels. BCAAs compete with aromatic amino acids for transport across the blood-brain barrier; therefore, changes in BCAA levels may affect neurotransmitter balance in the central nervous system (70). In HE, this mechanism may represent an important pathway through which BCAAs exert therapeutic effects. Additionally, BCAAs may play roles in maintaining mental strength and promoting neuroprotection (60).

Some studies suggest that BCAAs may possess antioxidant properties. In cancer cachexia animal models, although BCAA supplementation did not significantly improve muscle oxidative stress markers, it may exert anti-stress effects in certain situations by supporting mitochondrial function and cellular energy metabolism (43, 57). Furthermore, BCAAs may reduce stress-induced muscle catabolism by improving protein synthesis and reducing protein degradation.

Branched-chain amino acids play important roles in promoting muscle protein synthesis, preventing and treating muscle loss, supporting immune function, and protecting hepatic function. Substantial clinical research confirms that BCAA supplementation improves muscle strength and mass in elderly individuals, ameliorates HE symptoms, and supports immune function in critically ill patients. However, clinical application of BCAAs requires individualized assessment, considering patient-specific disease states, nutritional status, and therapeutic goals. Future research should focus on high-quality RCTs to clarify optimal populations, doses, and durations for BCAA application, and explore synergistic effects of BCAAs with other therapeutic interventions.

Accumulating epidemiological evidence has demonstrated a strong association between elevated plasma BCAAs levels and insulin resistance (IR). In a pioneering metabolomics study published in 2009, Newgard et al. revealed that BCAAs and their related metabolites exhibited the strongest correlation with insulin sensitivity in obese and insulin-resistant individuals, surpassing even many lipid species (10). Subsequently, Newgard further elucidated the interplay between lipids and BCAAs in the development of insulin resistance in 2012 (72). In a comprehensive systematic review published in 2014, Lynch and Adams demonstrated that fasting circulating BCAA concentrations were universally elevated in obese populations—reflecting impaired catabolic disposal rather than excess dietary intake—and positively correlated with the future risk of developing type 2 diabetes (T2D) (14).

Clinical cohort studies have provided robust evidence supporting this association. A follow-up study of women with gestational diabetes mellitus (GDM) revealed that patients who developed T2D after 6 years exhibited significantly elevated plasma BCAA levels, which were closely associated with insulin resistance indices (73). The PREVEND cohort study, encompassing 5, 791 participants with a median follow-up of 7.3 years, demonstrated that higher BCAA levels were independently associated with T2D incidence (hazard ratio [HR] = 1.19), and the association between fatty liver and T2D was partially mediated by elevated BCAAs, with a mediation proportion of 19.6% (74).

Regarding causality, Zhou et al. discovered in 2019 through integrated pathway analysis that systemic inhibition of BCAA catabolic enzymes accompanied by BCAA accumulation in genetically obese mice could be ameliorated by the BCKDK inhibitor BT2, which restored BCAA catabolism and significantly improved insulin resistance (36). Zhao et al., in a series of studies published in 2020 and 2024, further demonstrated that a single BCAA infusion acutely elevated blood glucose and insulin levels, while continuous infusion impaired whole-body insulin sensitivity; notably, BT2 treatment improved glucose tolerance in high-fat diet-fed mice (75, 76).

It is noteworthy that the effects of BCAAs on insulin sensitivity exhibit a concentration-dependent pattern: beneficial at physiological levels but deleterious at pathological concentrations (14, 22). Blair et al.’s 2023 study suggested that elevated BCAAs might serve as biomarkers rather than direct causative factors of insulin resistance (77), whereas Shah et al.’s acute intervention study provided compelling evidence for direct pathogenic effects (76). This controversy warrants further investigation. This metabolic vicious cycle is clearly illustrated in Figure 5.

The mTORC1/S6K1 pathway is one major mechanistic node linking chronically elevated circulating BCAAs—arising from impaired catabolic clearance—to insulin resistance, operating in parallel with BCKA-mediated direct inhibition of Akt signaling and BCAC-driven mitochondrial dysfunction, as described in Section 2.2.1. Under nutrient-excess conditions, sustained leucine elevation drives chronic mTORC1 activation, leading to IRS-1 serine phosphorylation and impaired insulin signaling (14). Dysregulated BCAA catabolism results in branched-chain acylcarnitine (BCAC) accumulation. These intermediates inhibit fatty acid oxidation, induce mitochondrial dysfunction, and promote oxidative stress (14, 72).BCKAs further suppress insulin signaling through mTORC1 activation (36).

Mitochondrial dysfunction and BCAA accumulation form a bidirectional vicious cycle: impaired oxidative capacity reduces BCAA catabolism, while accumulated intermediates exacerbate mitochondrial stress and ROS generation (3, 9). Elevated BCAAs also link to chronic inflammation, with pro-inflammatory cytokines promoting IRS-1 serine phosphorylation (4).

Multiple clinical studies have reported significantly elevated plasma BCAA levels in patients with T2D. The GDM follow-up study demonstrated markedly higher baseline BCAA levels in patients who progressed to T2D (73). The PREVEND cohort study further confirmed the independence of this association and its mediating role (74).

Exercise intervention may represent an effective approach to modulate BCAA metabolism. Research has shown that exercise enhances mitochondrial oxidation of BCAAs, and BCAA supplementation in athletes typically does not induce insulin resistance (78). Zhou et al.’s study demonstrated that dietary BCAA restriction improved insulin sensitivity in obese mice, whereas increased BCAA intake promoted insulin resistance (36), suggesting potential therapeutic value of dietary modification.

Nuclear factor-κB (NF-κB) and NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome represent key pathways mediating chronic inflammation. NF-κB serves as the principal transcription factor for inflammatory responses, regulating the expression of pro-inflammatory cytokines, chemokines, and adhesion molecules (31, 32). NLRP3 inflammasome activation requires two signals: a priming signal (activating the NF-κB pathway) and an activation signal (mediated through mechanisms including potassium efflux, mitochondrial dysfunction, and ROS generation) (79, 80).

Interestingly, NF-κB exhibits a dual role in inflammation regulation, functioning as both an initiator and a negative regulator. Greten et al. first discovered in 2007 that NF-κB could act as a negative regulator of interleukin-1β (IL-1β) secretion (81). Zhong et al. further confirmed in 2016 that NF-κB suppresses inflammasome activation by inducing p62 accumulation and eliminating damaged mitochondria through mitophagy (82).

Although direct evidence of BCAA-mediated activation of these pathways is lacking, mitochondrial dysfunction, ROS production, and metabolic intermediate accumulation resulting from BCAA metabolic dysregulation can all function as damage-associated molecular patterns (DAMPs) to activate inflammatory pathways. Chronic inflammation is characterized by sustained release of pro-inflammatory cytokines including interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), and monocyte chemoattractant protein-1 (MCP-1). These cytokines induce insulin resistance by activating stress kinases that promote IRS-1 serine phosphorylation (14).

In obesity and metabolic syndrome, adipose tissue macrophages predominantly polarize toward the pro-inflammatory M1 phenotype, releasing pro-inflammatory mediators that exacerbate local and systemic inflammation (83). Hotamisligil systematically elaborated in 2017 on the complex interrelationships among inflammation, metaflammation, and immunometabolic disorders (84). Metabolic dysregulation induced by aberrant BCAA metabolism may promote M1 polarization and adipose tissue inflammation through multiple mechanisms.

A vicious cycle exists among obesity, inflammation, and BCAA metabolic dysfunction (14, 73, 74). In the obese state, decreased expression of BCAA catabolic enzymes in adipose tissue leads to BCAA elevation; elevated BCAAs promote lipid accumulation through mTORC1 activation; insulin resistance further increases BCAA appearance rates. Concurrently, inflammation suppresses BCAA catabolic enzyme expression, while BCAA metabolic dysregulation activates inflammatory pathways.

A Chinese population study demonstrated that BCAA levels positively correlated with metabolic syndrome and its components. Subjects in the highest quartile of BCAA levels exhibited a 2.17-fold increased risk of metabolic syndrome and were independently associated with high cardiovascular risk scores (85). Multi-system inflammation in metabolic syndrome patients manifests as immune cell infiltration and pro-inflammatory mediator release in multiple organs, not only exacerbating insulin resistance but also promoting complications including atherosclerosis and MASLD.

Inflammation plays a central role in atherosclerosis development and progression (84). BCAAs promote vascular inflammation and atherosclerosis through multiple mechanisms, including insulin resistance induction, endothelial cell inflammatory response activation, and oxidative stress promotion.

Multiple clinical studies have reported positive correlations between BCAA levels and inflammatory markers. The GDM follow-up study demonstrated that patients progressing to T2D exhibited not only elevated BCAA levels but also significantly increased C-reactive protein (CRP), triglycerides, and free fatty acids (73). In obese adolescents, BCAA levels correlated with inflammatory markers and metabolic abnormalities (86).

Zhou et al.’s study showed that increased BCAA intake promoted insulin resistance in ob/ob mice, an effect associated with inflammatory activation (36). Interventions improving inflammatory status, such as weight reduction and exercise, can restore BCAA catabolic enzyme activity and reduce circulating BCAA levels (78), suggesting bidirectional regulation between inflammation and BCAA metabolism.

Multiple large-scale prospective cohort studies have revealed associations between elevated fasting circulating BCAA concentrations and cardiovascular disease (CVD) risk. In the PREDIMED study published in 2016, Guasch-Ferré et al. found that the highest quartile concentrations of leucine and isoleucine were associated with increased CVD risk, with hazard ratios of 1.70 and 2.09, respectively (87). Tobias et al., in a study of 27, 041 women with a mean follow-up of 18.6 years published in 2018, found that total BCAAs positively correlated with CVD (HR = 1.13), with effect sizes comparable to low-density lipoprotein cholesterol (LDL-C) (88).

A meta-analysis by Wang et al. published in 2022, incorporating 43, 895 participants, demonstrated that elevated circulating isoleucine levels were independently associated with increased CVD risk (89). In the Women’s Health Study, incorporating BCAAs into traditional risk factor models increased the area under the receiver operating characteristic curve (AUC) from 0.718 to 0.721, with predictive value comparable to LDL-C (88).

The association between BCAAs and CVD varies across different populations. Sex, ethnicity, and metabolic status may all influence this relationship. A 2021 genetic study by Hu et al. demonstrated that the protein phosphatase 1K (PPM1K) gene polymorphism rs1440581 CC genotype was associated with elevated serum BCAAs and served as a superior marker for CVD and MASLD risk (90).

Elevated BCAAs promote cardiovascular disease through multiple mechanisms: induction of insulin resistance and metabolic syndrome; promotion of vascular smooth muscle cell proliferation and macrophage foam-cell transformation via mTORC1 activation (see Section 2.2.1); and effects of BCAA metabolites such as 3-HIB on vascular lipid transport (91).

Endothelial dysfunction arises from impaired eNOS activation, oxidative stress-induced endothelial damage, and mTORC1-driven inflammatory responses (91). In heart failure, downregulation of BCAA catabolic enzymes leads to cardiac BCAA accumulation, potentially impairing cardiac metabolism through mTORC1 activation (91).

Branched-chain amino acids may serve as biomarkers for CVD risk stratification. Reducing BCAA levels could potentially become a strategy for CVD prevention, although this must be balanced against the need to maintain muscle mass. Lifestyle interventions, particularly exercise training, may represent an ideal approach to modulate BCAA metabolism (78). While the Mediterranean diet has limited effects on BCAA levels, it may counteract the harmful effects of BCAAs through other mechanisms (87).

BCAA supplementation strategies in heart failure patients remain controversial. Although supplementation may improve nutritional status and exercise capacity, it could also exacerbate cardiac metabolic burden (91). BCAA supplementation in heart failure patients requires individualized assessment, comprehensively considering nutritional status, cardiac function, and metabolic phenotype.

The value of BCAAs as prognostic biomarkers for CVD is being explored. In T2D patients, BCAAs are associated with cardiovascular complications and mortality risk (89). Combining BCAAs with other biomarkers may provide more comprehensive risk assessment.

The pathological mechanisms described in this section pertain specifically to early and non-cirrhotic MASLD/MASH, a stage at which fasting circulating BCAA concentrations are characteristically elevated as a consequence of impaired catabolic disposal. This context is mechanistically and clinically distinct from advanced MASH-related cirrhosis, in which circulating BCAAs decline and supplementation becomes indicated; the latter is addressed in Section 6.3.2. Non-cirrhotic MASLD patients commonly exhibit aberrant BCAA metabolism, characterised by elevated rather than depleted fasting circulating BCAA concentrations. McCormack et al.’s 2017 study of 78 obese adolescents demonstrated that MASLD patients exhibited significantly elevated plasma valine and isoleucine levels, independent of obesity and insulin resistance, with baseline valine levels predicting increased liver fat content during follow-up (86).

The PREVEND cohort study confirmed that fasting circulating BCAA concentrations positively correlated with fatty liver index, independent of the homeostatic model assessment of insulin resistance (HOMA-IR). Mediation analysis indicated that the association between MASLD and T2D was partially mediated by elevated BCAAs, with a mediation proportion of 19.6% (74), suggesting that elevated circulating BCAAs—reflecting impaired catabolic disposal at the non-cirrhotic disease stage—represent a critical metabolic link between MASLD and T2D.

The area and heterogeneity of hepatic lipid droplets correlated with plasma BCAA levels (92). BCAAs may participate in disease progression from simple steatosis to metabolic dysfunction-associated steatohepatitis (MASH, formerly NASH) through multiple mechanisms, though this pro-disease role is specific to the non-cirrhotic stage in which catabolic impairment—rather than deficiency—drives BCAA accumulation, including promotion of lipogenesis, induction of mitochondrial dysfunction, and activation of inflammatory pathways.

In non-cirrhotic MASLD/MASH, chronically elevated circulating BCAAs promote hepatic lipogenesis through several mechanisms. Chronically elevated circulating BCAAs, driven by impaired catabolic disposal, induce selective insulin resistance, increasing adipose lipolysis and hepatic lipogenesis. mTORC1 activation (see Section 2.2.1) upregulates SREBP-1c and fatty acid synthase expression (75, 76). BCAA metabolic intermediates may also provide carbon sources for fatty acid synthesis.

BCAA catabolic intermediate accumulation competitively inhibits fatty acid oxidation and impairs mitochondrial metabolism in MASLD (93). Downregulation of PPARα is associated with decreased BCAA catabolic enzyme expression in hepatocellular carcinoma (94). BCAA infusion reduces insulin-stimulated pAkt in the liver by approximately 30%, indicating hepatic insulin resistance (75). The valine metabolite 3-HIB promotes trans-endothelial fatty acid transport and ectopic lipid accumulation within skeletal muscle, contributing to local lipotoxicity and insulin resistance (23, 91).

In non-cirrhotic MASLD/MASH with elevated fasting circulating BCAAs, dietary BCAA restriction or avoidance of supplementation may improve metabolic outcomes. Animal studies have demonstrated that protein restriction improves metabolic health (36). Galarregui et al.’s 2021 human study suggested that optimizing amino acid composition may benefit liver status in MASLD patients (95). However, nutritional requirements must be balanced to avoid muscle wasting and malnutrition.

Pharmacological therapies targeting BCAA metabolism represent a theoretically appealing but predominantly preclinical avenue. The BCKDK inhibitor BT2 improves insulin sensitivity by promoting BCAA catabolism in animal models (36), though human safety data and optimal dosing parameters remain undefined. Other potential targets under investigation include branched-chain amino acid transaminase (BCAT), BCAA transporters, and related transcription factors; all require prospective clinical evaluation before therapeutic recommendations can be made.

Lifestyle intervention remains the cornerstone of MASLD treatment. Weight reduction of 5-10% can significantly improve MASLD (95, 96). Shi et al.’s 2021 study demonstrated that exercise training enhances BCAA oxidation, reduces circulating BCAA levels, and correlates BCAA changes with liver fat improv MASLD ement (97). The benefits of combined dietary, exercise, and behavioral interventions may exceed those of single interventions.

It should be emphasised that these therapeutic considerations apply exclusively to non-cirrhotic MASLD/MASH. In patients with advanced MASH-related decompensated cirrhosis, the metabolic profile reverses—circulating BCAAs decline, Fischer ratio falls, and oral BCAA supplementation is supported by Grade A guideline recommendations; this distinct clinical scenario is addressed in Section 6.3.2.

Rapidly proliferating tumor cells require substantial BCAA support (98, 99). BCAAs serve as building blocks for protein synthesis, provide nitrogen sources for nucleotide and non-essential amino acid biosynthesis, and contribute to energy production through oxidation. Li and Zhang systematically elaborated in 2016 on how tumor cells reprogram glucose, fatty acid, and amino acid metabolism to support cancer progression (100).

Tumor cells exhibit significant BCAA metabolic reprogramming, with heterogeneity across tumor types (60, 98, 99). Ananieva and Wilkinson’s 2018 review indicated that BCAA catabolic enzyme expression is suppressed in hepatocellular carcinoma and multiple other tumors (99). Ericksen et al.’s 2019 study demonstrated that the degree of enzyme inhibition strongly correlated with tumor aggressiveness and served as an independent prognostic predictor (94). Inhibition mechanisms include genomic deletions and altered transcription factor (e.g., PPARα) function.

Conversely, BCAT1 expression is upregulated in certain tumors, potentially promoting tumor growth through glutamate generation, provision of nitrogen sources for nucleotide synthesis, and mTORC1 signaling activation (60). Vettore et al.’s 2020 review discussed new insights into amino acid metabolism in cancer (98).

The mTOR pathway plays a central role in tumorigenesis and progression (94, 98, 100). BCAAs activate mTORC1 through the Sestrin2-GATOR-Rag GTPase axis. Downregulation of BCAA catabolic enzymes in tumors leads to BCAA accumulation, providing sustained mTORC1 activation signals. Ericksen et al. demonstrated that modulating BCAA accumulation can regulate cancer cell proliferation and tumor burden (94).

In vitro experiments demonstrated decreased BCAA catabolic enzyme expression in tumorigenic hepatocellular carcinoma (HCC) cell lines. Knockout or inhibition of BCAA catabolism reduced cancer cell proliferation rates and mTORC1 activity. BCAA supplementation promoted cancer cell proliferation, which could be blocked by mTOR inhibitors (94).

Animal model studies provided more direct evidence. In diethylnitrosamine (DEN)-induced HCC models, high-BCAA diets promoted tumor development, while BT2 treatment significantly improved tumor burden and overall survival. In xenograft models, BT2 treatment also significantly inhibited tumor growth (94).

Clinical studies demonstrated that BCAA catabolic enzyme expression levels in HCC patient tumor tissues correlated with tumor aggressiveness and prognosis. Human dietary cohort studies showed that replacing carbohydrates or fats with BCAAs was associated with a dose-dependent increase in cancer mortality risk (94). High BCAT1 expression typically correlates with more aggressive tumor growth (60, 99). Xu et al.’s 2023 review discussed the prospects for therapeutic interventions targeting BCAA catabolism (60). Figure 6 synthesizes the aforementioned mechanisms, highlighting the central role of BCAA dysregulation in cancer metabolism and its associated emerging risks.

Cancer patients frequently experience cachexia, characterized by progressive muscle wasting. BCAA supplementation has traditionally been considered potentially beneficial for improving nutritional status. Nishitani et al.’s 2013 study demonstrated that BCAAs might suppress hepatocellular cancer stem cells through mTOR activation (101), suggesting potential anti-tumor effects of BCAAs under specific circumstances. BCAAs play an important role in maintaining muscle mass by promoting protein synthesis and inhibiting degradation through mTORC1 activation.

However, based on evidence that BCAAs promote tumor growth, excessive supplementation may pose risks. High-BCAA diets promoted hepatocellular carcinoma development in mice (94). This contradiction places BCAA supplementation strategies in cancer patients in a dilemma: nutritional support is needed to maintain muscle mass, but excessive supplementation may “feed” tumors.

BCAA management in cancer patients requires individualized precision nutrition strategies, considering factors including: (1) tumor type and stage; (2) nutritional status and degree of muscle wasting; (3) treatment phase; (4) metabolic phenotype and BCAA levels; (5) tumor molecular characteristics (e.g., BCAA catabolic enzyme expression). Future research should integrate these factors to develop individualized nutrition-metabolic intervention strategies (60, 101).

High-protein diets may increase renal metabolic burden. In patients with renal insufficiency, excessive protein or BCAA intake may accelerate renal function deterioration. BCAA metabolism generates ammonia, which may accumulate in hepatic dysfunction leading to hyperammonemia. Additionally, BCAA-induced insulin resistance and metabolic syndrome indirectly increase chronic kidney disease risk.

The relationship between BCAAs and neurodegenerative diseases remains controversial (102). Mechanisms supporting harmful effects include: autophagy inhibition (32), promotion of mitochondrial dysfunction and oxidative stress, and interference with neurotransmitter synthesis (103). However, evidence also suggests BCAAs may possess neuroprotective properties by supporting neuronal energy metabolism and protein synthesis to maintain neuronal function; this area requires further research clarification (104).

Although BCAA supplementation is generally safe in the short term, the risks of long-term high-dose use have not been adequately studied. Potential risks include: (1) sustained mTORC1 overactivation may promote aging; (2) impaired metabolic flexibility; (3) alterations in gut microbiota composition. Excessive BCAAs may interfere with the transport and metabolism of other amino acids (such as aromatic amino acids), leading to amino acid profile imbalance and nutritional dysregulation.

The “dark side” of BCAAs primarily manifests as potential harm to multiple organ systems at pathological levels. From insulin resistance, chronic inflammation, cardiovascular disease, and MASLD to cancer, BCAA metabolic dysregulation participates in the pathogenesis and progression of various metabolic diseases. Core mechanisms underlying these effects include mTORC1 pathway overactivation, mitochondrial dysfunction, inflammatory activation, and metabolic intermediate accumulation.However, it must be emphasized that the harmful effects of BCAAs primarily occur in pathological states with impaired metabolic function, whereas at physiological levels in healthy individuals, BCAAs are essential nutrients. Therefore, the dual nature of BCAAs as both “angel” and “devil” fundamentally reflects differences in metabolic health status rather than inherent problems with BCAAs themselves. The key dimensions of this context-dependent duality—including mTORC1 activation state, circulating BCAA levels, metabolic context, molecular effects, and disease-specific clinical implications—are comprehensively summarized in Table 1. Future research and clinical applications must seek balance between maintaining basic nutritional requirements and avoiding pathological accumulation.

The biological effects of BCAAs demonstrate significant dose-dependent characteristics, with a well-defined “therapeutic window.” Within this range, BCAAs exert beneficial effects, while exceeding or falling below this range may produce adverse outcomes (105). Within the physiological concentration range (plasma BCAA concentrations approximately 200-400 μmol/L), BCAAs primarily perform normal physiological functions, including serving as protein synthesis substrates and participating in energy metabolism and signal transduction. A study in healthy adults demonstrated that a single oral dose of 7g BCAAs significantly elevated plasma BCAA levels within 2–4 hours without inducing metabolic disturbances (106). Therefore, moderate BCAA supplementation (typically 5-20g per day) can provide therapeutic benefits in specific populations.

In patients with liver cirrhosis, daily supplementation of 7.2g BCAAs for 24 weeks significantly improved muscle mass, muscle strength, and liver function, while reducing the incidence of liver-related events (107). In elderly populations, leucine-enriched amino acid supplementation combined with exercise training improved physical function and maintained lean body mass (108).

However, when BCAA concentrations rise to pathological levels (>600 μmol/L), harmful effects may ensue. Studies have shown that high concentrations of BCAAs (20 mM) can inhibit mitochondrial biogenesis and oxidative gene expression in myotube cells, particularly under insulin-resistant conditions (109). High-BCAA diets (1.5 times the standard dose) promoted obesity-related metabolic disorders and tumorigenesis in animal models (94).

The dose-response relationship of BCAAs is non-linear and exhibits significant inter-individual variability, which is modulated by multiple factors. For instance, genetic polymorphisms in BCAA metabolic enzymes are important factors influencing individual BCAA responsiveness. Genome-wide association studies (GWAS) have identified multiple genetic loci associated with plasma BCAA levels, with the rs1440581 polymorphism upstream of the PPM1K gene showing the strongest association (110). Each allele of this variant can alter plasma leucine levels by 0.08 standard deviations and is associated with an 85% increased risk of type 2 diabetes (110).

BCAT2 gene variations also affect individual BCAA metabolic capacity. Mice with adipose tissue-specific Bcat2 knockout showed resistance to high-fat diet-induced obesity, exhibiting enhanced browning of white adipose tissue and increased thermogenic capacity (111). This suggests that individuals carrying different BCAT2 variants may have significantly different metabolic responses to BCAA supplementation.

Furthermore, an individual’s baseline metabolic state significantly influences the direction of BCAA effects. In insulin-sensitive individuals, BCAA supplementation typically does not induce metabolic disturbances; however, in individuals with existing insulin resistance, the same dose of BCAAs may exacerbate metabolic abnormalities (112). Studies have shown that BCAA infusion has limited effects on insulin sensitivity in normal mice but significantly impairs insulin signaling in high-fat diet mice (76).

Mitochondrial functional status is also a critical modulating factor. In individuals with normal mitochondrial function, BCAAs can be efficiently oxidized and utilized; however, under conditions of mitochondrial dysfunction, BCAA metabolic intermediates accumulate, producing toxic effects (113). Research indicates that insulin-resistant patients exhibit decreased mitochondrial oxidative capacity in skeletal muscle and downregulated BCAA catabolic enzyme expression, leading to accumulation of BCAAs and branched-chain acylcarnitines (114).

Finally, different disease types and disease stages exhibit significant differences in BCAAs requirements and tolerance. In patients with liver cirrhosis, the benefits of BCAA supplementation increase with disease severity, with Child-Pugh class C patients showing the most pronounced benefits (107). Conversely, in patients with metabolic syndrome, BCAA levels positively correlate with disease severity, and restricting BCAA intake may be more beneficial (115).

Insulin sensitivity is a core determinant of BCAA effects, with its status directly influencing BCAA metabolic flux and effect direction. Under insulin-sensitive conditions, insulin effectively stimulates skeletal muscle BCAA uptake and oxidation, promotes BCKDH complex activity, and accelerates BCAA catabolism (14). At this time, BCAAs primarily exert beneficial effects by promoting protein synthesis, inhibiting protein degradation, and supporting energy metabolism. Studies have shown that in healthy individuals, moderate BCAA supplementation can enhance exercise-induced muscle protein synthesis and improve exercise performance (49).

On the other hand, insulin also exerts long-term effects through regulation of BCAA metabolic enzyme expression. Following an oral glucose tolerance test (OGTT), skeletal muscle PPM1K expression is significantly upregulated in insulin-sensitive individuals, promoting BCKDH activation and BCAA oxidation (110). This metabolic flexibility ensures that BCAAs do not excessively accumulate.

In contrast, under insulin-resistant conditions, multiple mechanisms lead to BCAA metabolic dysfunction and harmful effects. First, insulin’s stimulatory effect on BCAA catabolism is weakened, while its inhibitory effect on protein degradation is lost, resulting in increased BCAA appearance rate (14). Second, BCAA catabolic enzyme expression is systemically downregulated in adipose tissue and liver, particularly with the BCKDH complex persistently maintained in a phosphorylated (inactivated) state (39). Simultaneously, accumulated BCAAs and their metabolic intermediates exacerbate insulin resistance through multiple pathways. Persistently elevated leucine over-activates the mTORC1/S6K1 pathway, leading to IRS-1 serine phosphorylation and impaired insulin signaling (14, 35). Accumulation of branched-chain keto acids (BCKAs) inhibits insulin-induced Akt phosphorylation, reducing glucose uptake and mitochondrial oxygen consumption (116). Furthermore, the valine metabolite 3-HIB promotes transendothelial fatty acid transport, increasing accumulation of lipotoxic intermediates in skeletal muscle and further impairing insulin signaling (23).

Recent research has revealed mechanisms by which BCAAs acutely induce insulin resistance. A single BCAA infusion can acutely elevate blood glucose and plasma insulin levels. Continuous BCAA infusion during hyperinsulinemic-euglycemic clamp significantly impairs whole-body insulin sensitivity, especially hepatic sensitivity (76). This acute effect provides a mechanistic explanation for how BCAAs disrupt glucose metabolism in the long term.

Mitochondrial functional status is a key determinant of the dual effects of BCAAs, directly affecting BCAA oxidative capacity and metabolic product fate. Under conditions of normal mitochondrial function, BCAAs can be efficiently oxidized, generating ATP and providing carbon skeletons for the tricarboxylic acid cycle. Physiological concentrations of BCAAs can even promote mitochondrial biogenesis and enhance oxidative capacity (13). Athletes, due to their strong mitochondrial oxidative capacity, do not experience metabolic intermediate accumulation and insulin resistance even with relatively high-dose BCAA supplementation (78).

Brown adipose tissue (BAT) plays an important role in BCAA clearance. Cold exposure significantly reduces plasma BCAA levels, which is associated with increased active uptake and oxidation of BCAAs in BAT mitochondria. BCAAs are utilized in BAT mitochondria for UCP1-mediated thermogenesis; defective BCAA uptake and catabolism lead to impaired BCAA clearance and abnormalities in lipid/glucose metabolism (114).

Mitochondrial dysfunction is a core pathological mechanism in obesity and type 2 diabetes, leading to decreased BCAA oxidative capacity and accumulation of incomplete oxidation products (117). In skeletal muscle of insulin-resistant patients, mitochondrial oxidative phosphorylation (OXPHOS) capacity is reduced, BCAA catabolic enzyme expression is downregulated, leading to accumulation of BCAA metabolic intermediates such as C3 and C5 acylcarnitines (114).

High concentrations of BCAAs, in turn, exacerbate mitochondrial dysfunction, forming a vicious cycle. Studies have shown that 20 mM BCAA treatment can inhibit expression of mitochondrial biogenesis markers (Ppargc1α, Cox5a, Atp5) and glycolytic enzymes in myotube cells (109). Myocyte-specific inhibition of OXPHOS accelerates BCAA catabolism, leading to excessive acetyl-CoA production and lipogenesis, which then inhibits mitochondrial respiration through acetylation-mediated mechanisms, increases reactive oxygen species (ROS) production, and promotes inflammatory responses (75).

Studies with the BCKDK inhibitor BT2 provide evidence that restoring BCAA metabolism can improve mitochondrial function. BT2 treatment promotes BCKDH activity, reduces extracellular BCAA levels, while enhancing mitochondrial proton leak and uncoupling, improving metabolic health (36, 118). This suggests that targeting BCAA catabolism may be an effective strategy for improving mitochondrial dysfunction.

Chronic inflammation can inhibit the expression and activity of BCAA catabolic enzymes through multiple mechanisms. Pro-inflammatory cytokines such as TNF-α and IL-6 downregulate transcription of BCAA metabolism-related genes and impair BCAA oxidation by activating BCKDK to increase phosphorylation (inactivation) of BCKDH (14). In obesity-associated inflammatory states, BCAA catabolic enzyme expression is systemically decreased in adipose tissue (39). Inflammation-induced insulin resistance also indirectly affects BCAA metabolism. Inflammation activates stress kinases such as JNK and IKK, leading to IRS-1 serine phosphorylation and impaired insulin signaling, while insulin is an important regulator of BCAA metabolism (14).

On the other hand, accumulated BCAAs and their metabolic products can activate inflammatory pathways, exacerbating inflammatory responses. In db/db diabetic mice, BCAA supplementation led to elevated BCKA levels, promoting excessive macrophage activation. BCKA treatment increased production of pro-inflammatory cytokines (MCP-1, TNF-α) (75). For example, elevated BCAA levels promote secretion of high mobility group box 1 (HMGB1), which is an activator of M1 macrophages (75). This leads to polarization of adipose tissue macrophages toward the pro-inflammatory M1 phenotype, releasing large amounts of pro-inflammatory mediators, forming a vicious cycle of inflammation-metabolic disorder.

BCAA metabolic dysfunction-induced mitochondrial dysfunction and ROS production are also important mechanisms for inflammation exacerbation. ROS can activate NF-κB and NLRP3 inflammasomes, promoting maturation and release of pro-inflammatory cytokines such as IL-1β (79). Interestingly, NF-κB has dual roles in inflammation regulation, both initiating inflammation and negatively regulating NLRP3 inflammasome activation through induction of p62 accumulation and mitophagy (82).

Under acute disease, trauma, or surgical stress conditions, the body is in a hypercatabolic state, with BCAAs being extensively consumed for energy generation and acute phase protein synthesis. At this time, BCAA supplementation can improve nitrogen balance, support immune function, and promote tissue repair (119). For instance, in patients with brain injury, BCAA supplementation has demonstrated neuroprotective and reparative effects. Branched-chain amino acids serve as the major nitrogen donor for glutamate synthesis, playing a buffering role in post-traumatic brain injury glutamate elevation, with leucine contributing nearly 50% of nitrogen in the brain (106). A single oral dose of BCAAs can maintain elevated plasma levels for several hours, providing feasibility for acute treatment (106).

Conversely, in chronic metabolic diseases such as obesity and type 2 diabetes, the body is typically in a state of nutritional overload, with BCAA circulating levels already elevated. At this time, additional BCAA supplementation may exacerbate metabolic burden. For example, BCAA supplementation in high-fat diet mice, while not altering body weight, hepatic lipid content, or glucose-lipid metabolism, did induce dynamic changes in gut microbiota (120). Therefore, impaired BCAA catabolic capacity is a key pathological feature in chronic metabolic diseases. BCAA metabolic enzyme expression is downregulated in adipose tissue and liver, BCKDH activity is reduced, leading to accumulation of BCAAs and their metabolic intermediates (39). In such cases, restricting BCAA intake may be more beneficial than supplementation. A novel dietary intervention study showed that restricting BCAA intake reduced circulating BCAA levels by 50%, improving obesity and diabetes-related metabolic disorders (115).

In patients with malnutrition, cachexia, or sarcopenia, BCAA supplementation can improve nutritional status, maintain muscle mass, and improve prognosis. Patients with liver cirrhosis and sarcopenia receiving 24 weeks of BCAA supplementation (7.2g/day) significantly improved muscle mass, muscle strength, and physical performance, and improved liver function markers and prognosis (107). In elderly individuals, exercise combined with leucine-enriched BCAAs and vitamin D supplementation improved physical test scores and maintained lean body mass, while the non-supplemented group experienced lean body mass decline (108). This suggests that BCAA supplementation has important value in elderly populations at nutritional risk. This benefit is specific to advanced cirrhosis with documented malnutrition and reduced Fischer ratio. In contrast, for patients at earlier non-cirrhotic stages of MASLD/MASH with elevated fasting circulating BCAAs, supplementation is not appropriate; the stage-stratified framework governing this distinction is detailed in Section 6.3.3.

Cancer cachexia patients face special challenges. While BCAAs are beneficial for maintaining muscle mass, they may promote tumor growth. Individualized assessment of nutritional status, tumor type, and metabolic characteristics is crucial for formulating rational BCAA supplementation strategies (99).

In nutritional excess and obesity states, BCAA circulating levels are typically already elevated, reflecting BCAA metabolic dysfunction rather than insufficient intake (72). At this time, restricting BCAA intake may improve metabolic health. Epidemiological studies have shown that dietary BCAA intake in children and adolescents exhibits a dose-dependent positive correlation with high LDL cholesterolemia risk. Each 1g/day increase in isoleucine, leucine, and valine intake increased high LDL cholesterolemia risk by 30%, 11%, and 16%, respectively (121). This suggests that BCAA intake balance should also be monitored in pediatric populations.

However, BCAA restriction requires caution to avoid nutritional deficiency. A study in healthy adults showed that short-term dietary BCAA reduction could decrease postprandial insulin secretion, improve white adipose tissue metabolism, and gut microbiome composition (115). However, the safety and efficacy of long-term restriction require further study.

During disease activity or acute exacerbation periods, metabolic demands increase, and moderate BCAA supplementation may be beneficial. During acute exacerbations of chronic obstructive pulmonary disease (COPD), BCAA supplementation may improve pulmonary rehabilitation outcomes (122). In acute decompensated heart failure, BCAA metabolic dysfunction worsens, and moderate supplementation may support cardiac function (91). However, supplementation during active periods requires weighing benefits and risks. During active periods of inflammatory diseases, pro-inflammatory states may inhibit BCAA metabolism, and supplementation may lead to accumulation and exacerbated inflammation (79).

During disease stable periods, excessive BCAA supplementation may produce adverse effects. In patients with metabolic syndrome, even during disease control periods, BCAA levels remain positively correlated with cardiovascular risk (85). In cancer patients during recovery periods, restricting BCAAs may suppress tumor recurrence (94).

Stable periods should focus on improving BCAA metabolism through lifestyle interventions. Exercise training can enhance BCAA oxidative capacity, reduce circulating BCAA levels, and improve insulin sensitivity without causing nutritional deficiency (78). Healthy dietary patterns such as the Mediterranean diet, while having limited effects on BCAA levels, may counteract the harmful effects of BCAAs through other mechanisms (87).

The BCKDH complex is the rate-limiting enzyme of BCAA catabolism, and genetic variations in this complex significantly affect individual BCAA metabolic capacity. The BCKDH complex consists of multiple subunits: E1α (encoded by BCKDHA), E1β (encoded by BCKDHB), E2 (encoded by DBT), and E3 (encoded by DLD) (123). Pathogenic variants in these genes cause maple syrup urine disease (MSUD), a rare BCAA metabolic disorder (124).

Beyond pathogenic mutations, common genetic polymorphisms also affect BCAA metabolism. Genome-wide association studies have identified multiple loci associated with plasma BCAA levels, with the PPM1K gene region showing the strongest association (110). PPM1K encodes the activating phosphatase of BCKDH, and its variants affect BCKDH activity and BCAA levels (125).

rs1440581, located 21kb upstream of the PPM1K gene, is the single nucleotide polymorphism (SNP) most strongly associated with BCAA levels. Each allele of this polymorphism alters leucine levels by 0.08 standard deviations and increases type 2 diabetes risk by 85% (110). Mendelian randomization analysis supports a causal relationship between BCAA metabolic dysfunction and type 2 diabetes (110).

Recently, a novel PPM1K pathogenic variant was discovered. An 8-year-old boy carrying a novel homozygous missense mutation c.925A>Gp.(Ile309Val) presented with mild MSUD and mildly elevated plasma BCAAs (124). This is the third reported case of PPM1K-related MSUD, further expanding the clinical and genetic spectrum of this gene (124).

Functional studies of PPM1K variants have revealed regulatory mechanisms. PPM1K-deficient fibroblasts exhibited only 14% of normal BCKDH activity, and transfection with wild-type PPM1K restored 35% of normal activity. Additionally, PPM1K-deficient cells showed a 2-fold increase in ROS levels, suggesting increased metabolic stress (125).

DNA methylation and histone modifications regulate transcription of BCAA metabolic genes. In obese and diabetic patients, increased DNA methylation in the promoter regions of BCAA metabolic genes in adipose tissue is associated with downregulated gene expression (39). These epigenetic changes may be induced by nutritional excess and inflammation.

Post-translational protein modifications are important regulatory mechanisms. Acetylation modifications promote BCAT2 degradation, inhibiting BCAA catabolism and exerting pro-oncogenic effects in pancreatic cancer (126). In heart failure, expression of BCAA metabolic enzymes is regulated by transforming growth factor-β-activated kinase 1 (TAK1) and p38 MAPK signaling pathways, with downregulation of the transcription factor KLF15 leading to systemic decreased expression of BCAA metabolic enzymes (91).

Phosphorylation regulation of BCKDH activity is a key post-translational modification. BCKDK phosphorylates the E1α subunit at Ser293 to inactivate it, while PPM1K dephosphorylates to activate BCKDH (125). In obesity states, hepatic BCKDH is excessively phosphorylated, inhibiting its activity (22).

Interestingly, BCKDK and PPM1K, in addition to regulating BCKDH, also regulate other substrates such as ATP-citrate lyase (ACL), integrating BCAA metabolism with lipid metabolism (22). Hepatic overexpression of BCKDK increases ACL phosphorylation, activating lipogenesis. BCKDK and PPM1K transcription levels are regulated by fructose feeding or ChREBP-β expression (22).

BCAT2 gene variations affect the first step of BCAA catabolism. Adipose tissue-specific Bcat2 knockout mice showed resistance to high-fat diet-induced obesity, achieved through enhanced browning and thermogenesis of inguinal white adipose tissue (111). This suggests that individual differences in BCAT2 activity may influence obesity susceptibility and BCAA supplementation effects.AARS (encoding alanyl-tRNA synthetase) gene variations are also associated with BCAA levels. AARS mutations are associated with multiple diseases, including neurological disorders, metabolic disorders, and cancer (111). rs12149660, located at the AARS locus, is positively correlated with levels of all three BCAAs (111).

Studies of polycystic ovary syndrome (PCOS) have revealed the role of PPM1K in complex diseases: PPM1K knockout mice exhibit PCOS-like phenotypes, including elevated BCAA levels, increased androgen levels, and ovarian dysfunction (111). Dietary BCAA restriction can improve endocrine and ovarian dysfunction in Ppm1k-deficient mice (111). Phenome-wide association studies (PheWAS) have shown that PPM1K gene variants are associated with multiple complex traits (111).

These findings suggest that precision nutrition strategies based on individual genetic backgrounds may optimize BCAA management. Individuals carrying high-risk gene variants may require stricter BCAA intake restrictions, while those carrying protective variants may have better tolerance to BCAA supplementation.

The gut microbiota possesses complete BCAA biosynthetic pathways and can synthesize BCAAs from simple precursors. Metagenomic analysis has identified 14/17 gene families encoding leucine, valine, and isoleucine biosynthetic pathways (127). However, the correlation between bacterial BCAA synthesis gene abundance and serum BCAA levels is not significant after adjusting for age, sex, and body fat (127).

Furthermore, Prevotella copri has been identified as the microbe most positively correlated with elevated serum BCAAs and is more abundant in the gut of vegetarians and lacto-ovo-vegetarians (128). P. copri colonization can increase circulating BCAA levels in mice, exacerbating insulin resistance and glucose intolerance (129). Germ-free mice colonized with fecal microbiota from obese individuals recapitulate the obesity phenotype and elevated BCAA levels. PICRUSt analysis revealed enrichment of amino acid-related pathways in the gut microbiome, including valine, leucine, and isoleucine biosynthesis (128).

In addition to synthesis, gut microbiota also take up and degrade BCAAs. Bacterial BCAA inward transporter gene abundance is negatively correlated with circulating BCAAs and HOMA-IR (42). Higher bacterial BCAA inward transport driven by Faecalibacterium prausnitzii is associated with lower serum BCAA levels in early adolescents (127).

Metatranscriptomic analysis showed that BCAA degradation pathways were significantly enriched in vegetarians and lacto-ovo-vegetarians compared to omnivores (128). This suggests that although vegetarians have higher P. copri abundance (promoting BCAA synthesis), enrichment of BCAA degradation pathways may partially offset the increase in synthesis.

Gut microbiota BCAA metabolism is regulated by nutritional status. BCAA content increases significantly during fasting periods, returns to baseline after refeeding, and decreases significantly after 3 months (128). This dynamic change reflects the influence of nutritional status on microbiota-host BCAA metabolic interactions.

Gut microbiota influence circulating BCAA levels through both direct metabolic activity and indirect regulation of host enzyme expression.

In early MASLD and non-cirrhotic MASH, fasting circulating BCAA concentrations are characteristically elevated, reflecting impaired peripheral catabolic capacity—reduced BCKDH activity in skeletal muscle and adipose tissue combined with insulin resistance-driven proteolysis—rather than excessive dietary intake (84, 140). Plasma valine and isoleucine concentrations are significantly elevated in MASLD independently of obesity and insulin resistance, with baseline valine levels predicting increased liver fat content during follow-up (84). The PREVEND cohort further confirmed that fasting circulating BCAAs positively correlated with fatty liver index independently of HOMA-IR, with the MASLD–T2DM association partially mediated by elevated BCAAs (mediation proportion 19.6%) (72). In this context, BCAA supplementation is not recommended: chronically elevated BCAAs sustain mTORC1/S6K1 activation, worsen hepatic insulin resistance via IRS-1 serine phosphorylation, and accelerate de novo lipogenesis through SREBP-1c upregulation (73, 74). Sex-dependent differences in circulating BCAA trajectories during MASLD progression should also be considered when interpreting intervention data (143).

The priority at this stage is lifestyle modification: 5–10% body weight reduction significantly improves MASLD (93, 94), and exercise training enhances skeletal muscle BCAA oxidation with BCAA reduction correlating with hepatic fat improvement (95). BCAA supplementation should be avoided in patients with elevated fasting circulating BCAAs and documented insulin resistance.

As MASH progresses to decompensated cirrhosis, the BCAA metabolic profile undergoes a fundamental reversal: hepatic synthetic capacity declines, protein-energy malnutrition becomes prevalent, and the Fischer ratio falls as circulating BCAAs decline while aromatic amino acid levels rise due to impaired hepatic clearance (66–68, 144). This biochemical signature—low Fischer ratio, sarcopenia, and protein-energy malnutrition—defines the population in whom supplementation transitions from potentially harmful to clearly indicated.

The clinical evidence is robust. A multicenter double-blind RCT demonstrated that long-term oral BCAA supplementation significantly reduced the composite risk of death, liver transplantation, and disease progression, with event-free survival significantly improved in the BCAA group (9). BCAA granule supplementation (12 g/day for 2 years) in cirrhotic patients with hypoalbuminemia significantly improved serum albumin, reduced ascites and edema, and improved quality of life, with the greatest benefits in patients with lower baseline albumin and Fischer ratio (2). A Cochrane meta-analysis of 16 RCTs (n = 827) confirmed a significant reduction in hepatic encephalopathy risk (risk ratio 0.73, 95% CI 0.61–0.88; NNT = 5), robust in low-bias-risk trials (risk ratio 0.71, 95% CI 0.52–0.96) (4). ESPEN guidelines recommend leucine-enriched oral BCAA formulations at 12–14 g/day for a minimum of six months in decompensated cirrhotic patients with low Fischer ratio, sarcopenia, or protein-energy malnutrition, constituting a Grade A recommendation (4).

In early and non-cirrhotic MASLD/MASH, fasting circulating BCAAs are typically elevated (impaired disposal), insulin resistance is dominant, and BCAA supplementation is contraindicated—lifestyle intervention and exercise are priorities. In compensated cirrhosis with preserved synthetic function, BCAA status is variable and individualised assessment is required. In decompensated cirrhosis with malnutrition, low Fischer ratio, or sarcopenia, circulating BCAAs are characteristically reduced, protein-energy malnutrition dominates, and long-term oral BCAA supplementation carries a Grade A recommendation. Assessment of fasting plasma BCAAs, Fischer ratio, albumin, and body composition is essential before any supplementation decision in chronic liver disease.