Hyperammonemia as a potential contributor to fatigue in metabolic dysfunction-associated steatotic liver disease: A mechanistic review

Abstract

Non-alcoholic fatty liver disease, now referred to as metabolic dysfunction-associated steatotic liver disease (MASLD), is a global health problem affecting a substantial proportion of the adult population. Fatigue is among the most common and clinically underrecognized systemic symptoms experienced by patients with MASLD and is associated with reduced physical and cognitive performance, impaired daily activities, decreased work capacity, and poorer adherence to treatment. The pathogenesis of fatigue in MASLD is likely multifactorial and may involve systemic low-grade inflammation, mitochondrial and muscle dysfunction, sleep disorders, psychological factors, changes in the gut microbiota, and hyperammonemia, which has recently received increasing attention in the context of MASLD progression. Although experimental and mechanistic studies suggest that hyperammonemia may contribute to fatigue, direct clinical evidence remains limited. The goal of this narrative review is to systematize current evidence on the potential role of hyperammonemia as a biologically plausible mechanism contributing to fatigue in MASLD, and to critically evaluate current data on the influence of dietary factors, skeletal muscle status, and physical activity on ammonia homeostasis. This review is hypothesis-generating and should not be interpreted as a clinical guideline.

Key Words: Metabolic dysfunction-associated steatotic liver disease; Hyperammonemia; Fatigue; Urea cycle; Sarcopenia; Mediterranean diet; Physical activity

Core Tip: Hyperammonemia may develop in precirrhotic metabolic dysfunction-associated steatotic liver disease (MASLD) as a consequence of impaired ureagenesis associated with reduced expression and activity of urea cycle enzymes, together with alterations in glutamine metabolism. Current evidence suggests that ammonia may contribute to both central and peripheral fatigue through interconnected mechanisms involving neuroinflammation, impaired neurotransmission, mitochondrial dysfunction, oxidative stress, and skeletal muscle impairment. However, most available evidence originates from experimental studies, whereas direct clinical evidence in patients with MASLD remains limited. Although dietary factors, skeletal muscle status, and physical activity may influence ammonia homeostasis, their specific role in hyperammonemia in MASLD remains uncertain and should be regarded as hypothesis-generating. A better understanding of ammonia metabolism may facilitate the development of targeted strategies for fatigue management in MASLD.

INTRODUCTION

Non-alcoholic fatty liver disease (NAFLD), increasingly referred to as metabolic dysfunction-associated steatotic liver disease (MASLD) in current terminology, has become a global epidemic affecting up to 30% of the adult population in developed countries[1]. Although clinical attention was traditionally focused on the risks of fibrosis progression and cardiovascular complications, a quality of life in patients is already compromised in the early stages of the disease. Studies assessing the quality of life of patients with MASLD revealed a progressive deterioration in both physical and mental health as the disease progresses[2,3]. One of the most common, yet underestimated, MASLD symptoms is fatigue, with prevalence ranging from 50% to 85%[4]. At present, fatigue is recognized as an important clinical manifestation that negatively affects daily activities, work capacity[5], and adherence to treatment[6]. In addition this symptom has been associated with a poorer prognosis in patients with MASLD[7,8]. Fatigue in patients with MASLD reflects complex pathogenetic mechanisms, including neuroinflammation, mitochondrial and muscle dysfunction, sleep disorders, psychological factors (such as depression and anxiety), and changes in the composition of the gut microbiota[9]. Among these mechanisms, hyperammonemia has emerged as a potential contributor, although its relative role compared with other established factors remains to be fully elucidated. Disturbances in ammonia metabolism are of particular interest because they may provide a mechanistic link between hepatic dysfunction and both central and peripheral manifestations of fatigue through effects on the liver-muscle-brain axis. Moreover, growing experimental evidence suggests that impaired ammonia detoxification may already occur at precirrhotic stages of MASLD, although direct clinical evidence remains limited.

Previous reviews[5,10-14], including our own[15], have discussed fatigue in MASLD as a multifactorial clinical manifestation. In contrast, the present review specifically examines current evidence regarding disturbances in ammonia metabolism as one potential mechanism contributing to fatigue and summarizes current knowledge regarding lifestyle factors that may influence ammonia homeostasis in MASLD.

METHODS

This narrative review summarizes current evidence on the potential role of hyperammonemia in fatigue associated with MASLD. A literature search was performed using the PubMed/MEDLINE and Scopus electronic databases to identify relevant publications. The search primarily included articles published between January 2015 and May 2026, while earlier landmark studies were included when they were considered essential for understanding ammonia metabolism, urea cycle dysfunction, or the pathophysiology of fatigue.

The search strategy combined medical subject headings and free-text terms, including "metabolic dysfunction-associated steatotic liver disease" OR "MASLD" OR "non-alcoholic fatty liver disease" OR "NAFLD", "metabolic dysfunction-associated steatohepatitis" OR "MASH" OR "non-alcoholic steatohepatitis" OR "NASH", "hyperammonemia" OR "ammonia", "fatigue", "urea cycle", "glutaminolysis", "skeletal muscle", "sarcopenia", "exercise", "physical activity", "diet", and "Mediterranean diet". Additional relevant publications were identified through manual screening of reference lists of eligible articles.

Original research articles, systematic reviews, meta-analyses, and relevant clinical guidelines published in English were considered. Studies were selected based on their relevance to the mechanisms of ammonia metabolism in MASLD, the biological basis of fatigue, and potential dietary and lifestyle interventions affecting ammonia homeostasis. Given the narrative nature of this review, no formal systematic review methodology or quantitative synthesis was performed. The selection of studies was based on their scientific relevance and contribution to the objectives of the review.

Because this is a narrative review, the aim was to provide a balanced overview of current evidence and biologically plausible mechanisms rather than a comprehensive systematic synthesis of all available studies.

CENTRAL AND PERIPHERAL FATIGUE AS A MANIFESTATION OF SYSTEMIC HYPERAMMONEMIA IN MASLD

Central fatigue is a symptom resulting from the central nervous system (CNS) dysfunction. This symptom is manifested by a diminished ability to trigger and maintain both physical activity and brainwork requiring conscious effort and motivation[10]. Subjectively, this type of fatigue is experienced as excessive expenditure of energy in the course of completing routine tasks and the rapid onset of exhaustion. Conversely, peripheral fatigue is primarily associated with dysfunction of peripheral tissues and processes, including the neuromuscular transmission, skeletal muscle, and cardiorespiratory systems. Its clinical manifestations include loss of muscle strength, low endurance, and the inability to perform repeated or prolonged muscle contractions[12-14].

To objectively assess fatigue in clinical practice, the following specialized scales are employed: (1) Fatigue Assessment Scale; (2) Fatigue Impact Scale (FIS); (3) Fatigue Severity Scale; (4) Functional Assessment of Chronic Illness Therapy-Fatigue scale; and (5) Multidimensional Fatigue Inventory.

The results of measurements using these scales confirm the incidence of both physical and cognitive components of fatigue in patients with MASLD. Besides that, functional tests that measure muscle strength and the capability to maintain it during repeated muscle contractions can be used to evaluate peripheral fatigue in MASLD and other chronic liver diseases. The study by Goldblatt et al[16], representing an example of using such tests in clinical practice, showed that patients with primary biliary cholangitis (PBC) and severe fatigue exhibit a more rapid decline in handgrip strength during repeated dynamometer squeezes at 5 seconds intervals for 5 minutes vs patients with PBC but without severe fatigue, as well as vs healthy volunteers. Another study performed on 217 patients with chronic liver diseases [PBC (n = 45), primary sclerosing cholangitis (n = 46), autoimmune hepatitis (n = 49), and chronic hepatitis B (n = 57) or C (n = 73)] revealed that the severity of fatigue assessed by the FIS questionnaire was statistically significantly associated with an impairment of the muscle function. The latter was established based on the calf raise test (P = 0.005) and the Timed Up and Go test (P = 0.001)[17]. It is worth noting that, although more advanced stages of fibrosis and cirrhosis are associated with a higher frequency of symptoms, fatigue is routinely recorded at the precirrhotic stages of the disease[15].

Published evidence indicate a strong association between MASLD and cognitive decline. A study by Giménez-Garzó et al[18] evaluating neuropsychological function in patients with MASLD identified impairments in attention, concentration, information processing speed, cognitive flexibility, and short-term memory. These cognitive changes clinically resemble minimal hepatic encephalopathy (MHE), a form of hepatic encephalopathy (HE) in which obvious clinical manifestations are absent, whereas dysfunction is diagnosed solely via neuropsychological testing. However, it remains unclear whether these cognitive changes are mediated by the same pathophysiological mechanisms of hyperammonemia that underlie classical HE. In a study by Wilkinson et al[19] aimed to determine the contribution of hyperammonemia to MHE symptom development ten healthy males received two 4 hours intravenous infusions of either a 2% ammonium chloride solution or a placebo using a double blind cross-over design. It was demonstrated that hyperammonemia in the absence of other complications induced significant sensations of fatigue but did not cause the typically observed performance impairment in individuals with MHE. Therefore, the development of MHE symptoms appears to require other pathological mechanisms, such as hyponatremia, inflammation, neurotransmitter’s imbalance, increasing “GABA-ergic tone”, mitochondrial dysfunction and blood-brain barrier (BBB) leakage[20,21].

To date, no mechanistic studies have directly compared ammonia-mediated cognitive dysfunction in patients with cirrhotic HE and those with precirrhotic MASLD. Therefore, it remains unclear whether hyperammonemia contributes to cognitive impairment through the same molecular pathways in these conditions or whether its effects are modified by other pathogenic mechanisms characteristic of MASLD. Further mechanistic and longitudinal clinical studies are required to clarify the specific contribution of ammonia to cognitive dysfunction in precirrhotic MASLD.

Experimental and clinical evidence suggests that hyperammonemia, previously considered primarily in the context of HE in cirrhosis, may exert neurotoxic and hepatotoxic effects even at the early stages of MASLD, including simple steatosis[22], and may contribute to both disease progression and the development of symptoms such as fatigue[22,23]. Although the full picture of chronic intoxication is linked with a twofold or threefold increase in blood ammonia level, it has been established that even moderate hyperammonemia (30%-50% above normal) has neurotoxic potential. The negative impact on the CNS is manifested by the following nonspecific symptoms: Headache, drowsiness, and increased fatigue (which patients often confuse with routine or professional fatigue, as well as nervous or physical overexertion)[24]. Moreover, chronic exposure to moderately elevated ammonia levels has been suggested to promote neurodegenerative processes similar to the pathogenesis of Alzheimer’s and Parkinson’s diseases[25].

LABORATORY DIAGNOSTICS AND INTERPRETATION OF BLOOD AMMONIA LEVELS

Determining serum ammonia concentration is a complex methodological task associated with a high risk of preanalytical errors. Results are very sensitive to the procedural conditions of a sample collection, along with temperature conditions and duration of transportation; even slight delays in laboratory analysis can lead to falsely elevated values and high variability of the results[26]. In combination with the need to rely on local laboratory reference values, this substantially obscures clinical interpretation and the comparability of results obtained at different medical centers.

It is important to take into account that serum ammonia concentration is an integral indicator of nitrogen metabolism (including inputs from muscles and kidneys). This parameter only indirectly reflects the state of intrahepatic metabolism. In this regard, direct methods of determining ammonia in liver tissue are of high interest, especially considering that biopsies in patients with MASLD are often performed to identify metabolic dysfunction-associated steatohepatitis (MASH) and to assess the stage of liver fibrosis[27]. Although ammonia concentrations > 45-60 μmol/L are usually considered the upper limit of the reference range in scientific studies, no universal diagnostic threshold has been established in actual clinical practice[24,28,29]. According to the recommendations of the European Association for the Study of the Liver (EASL)[30] and the joint guidance from the American Association for the Study of Liver Diseases (AASLD) and EASL[31], HE remains a clinical diagnosis. Elevated blood ammonia alone does not establish the diagnosis or determine its severity, whereas a normal ammonia level should prompt reconsideration of the diagnosis and evaluation for alternative causes of altered mental status. The low prognostic value of serum markers is explained by the fact that the critical factor in the pathogenesis of HE is not the absolute concentration of ammonia in the blood, but rather its volume transported through the BBB and the extent of its neurotoxic effect on the CNS[32]. However, with respect to MASLD, the procedure of hyperammonemia assessment may acquire a prognostic value. A moderate but persistent increase in blood ammonia levels during dynamic monitoring in the early stages of the disease can serve as a biomarker of latent mitochondrial dysfunction in hepatocytes, explain early symptoms of CNS and muscle disorders, and be a predictor of sarcopenia[24]. The Russian clinical guidelines, “Hyperammonemia in Adults” (2025)[24], propose three grades of hyperammonemia for use in dynamic monitoring of high-risk patients: (1) Mild (grade I): Up to 100 μmol/L; (2) Moderate (grade II): 100-200 μmol/L; and (3) Severe (grade III): > 200 μmol/L.

PREVALENCE OF HYPERAMMONEMIA IN THE PRECIRRHOTIC STAGE OF MASLD

Several studies have suggested that hyperammonemia may occur in some patients with chronic liver disease before the development of cirrhosis; however, the available evidence remains limited and heterogeneous. In their study, Ageeva and Alekseenko[33] observed significantly higher blood ammonia concentrations in patients with MASLD and chronic hepatitis C at the precirrhotic stage (fibrosis stages F1-F2 based on liver elastography) than in healthy volunteers (56.1 ± 6.2 μmol/L vs 24.0 ± 2.5 μmol/L). In this study, venous blood ammonia concentrations were measured using an enzymatic assay, with a reference interval of 11-35 μmol/L.

In a single-center cohort study including 96 patients with MASLD, including 47 patients with MASH and 16 patients with newly diagnosed compensated liver cirrhosis due to MASLD, hyperammonemia was reported in 91.7% of participants, with a mean blood ammonia concentration of 111.1 ± 4.86 μmol/L (95%CI: 101.3-120.7 μmol/L). The prevalence of hyperammonemia was higher in patients with MASH than in those with simple steatosis. However, in this study, ammonia concentrations were measured in capillary blood using a portable photometric analyzer (PocketChem BA PA-4140, Arkray, Japan), with an upper reference limit of 55 μmol/L. In addition, all participants had type 2 diabetes mellitus and arterial hypertension, while more than half (57.3%) had chronic kidney disease stage G3a-G3b[34]. These clinical characteristics may have influenced blood ammonia concentrations, indicating that the findings should be interpreted with caution and should not be extrapolated to the broader population of patients with MASLD.

In another study, patients with inflammatory bowel disease and concomitant MASLD demonstrated higher blood ammonia concentrations that correlated with non-invasive fibrosis indices (fibrosis-4 score and aspartate aminotransferase to platelet ratio index), particularly in the presence of small intestinal involvement, although these differences did not reach statistical significance[35]. Importantly, in this study, capillary blood ammonia concentrations remained within the reference range (upper reference limit: 60 μmol/L).

Thus, the interpretation of the available evidence requires caution. The published studies differ substantially with respect to patient selection, disease severity, and the presence of potential confounding factors, including liver cirrhosis, renal dysfunction, and concomitant comorbidities. Furthermore, blood ammonia measurements were performed using different analytical methods and reference ranges, making direct comparison between studies difficult. In addition, most studies do not provide detailed information regarding preanalytical procedures, including sample collection, transport, processing time, and storage conditions, all of which may substantially affect ammonia measurements. These methodological differences may contribute to the wide variability in the reported prevalence of hyperammonemia and currently preclude reliable conclusions regarding its true frequency in the broader population of patients with precirrhotic MASLD.

Despite the uncertainty regarding the prevalence of hyperammonemia in precirrhotic MASLD, experimental and clinical evidence provides biological plausibility for its potential role in the development of fatigue. A double-blind, placebo-controlled study involving healthy volunteers demonstrated that experimentally induced hyperammonemia, with a peak ammonia concentration of approximately 225 μmol/L, significantly increased subjective general and physical fatigue assessed using the Multidimensional Fatigue Symptom Inventory-Short Form, without causing detectable impairment in neuropsychological performance characteristic of HE[19]. These findings suggest that elevated ammonia concentrations may contribute to the perception of fatigue even in the absence of liver disease, although their clinical relevance to patients with MASLD remains uncertain.

Additional evidence suggests that hyperammonemia alone is unlikely to fully explain the neurological manifestations observed in MASLD. In one study, patients with simple steatosis showed no evidence of cognitive impairment, whereas patients with non-cirrhotic MASH exhibited both hyperammonemia and systemic inflammation together with poorer performance on psychometric testing[36]. These observations support the hypothesis that hyperammonemia may interact with systemic inflammation and other disease-related factors in the development of cognitive dysfunction and fatigue. However, the relative contribution of hyperammonemia and its interaction with systemic inflammation and other pathogenic mechanisms in MASLD remain to be established in well-designed prospective clinical studies.

PHYSIOLOGICAL BASIS OF AMMONIA METABOLISM

Nearly half of all ammonia in the body is formed in the bowel. The main sources are bacterial hydrolysis of urea, the breakdown of dietary proteins, and the enzymatic glutamine deamidation. The resulting ammonia diffuses through the intestinal wall into the portal vein and is transported to the liver. Enterocytes, using glutamine as their primary energy substrate, actively synthesize ammonia as a result of high glutaminase (GLS) activity and low glutamine synthetase (GS) activity, which further increases ammonia entry into the portal circulation[37].

In the liver, ammonia detoxification occurs via two major spatially separated pathways. The first pathway is the urea cycle (ureagenesis) localized in periportal hepatocytes. It is a high-performance but low-affinity system that converts ammonia into urea for subsequent urinary excretion. The second mechanism is glutamine synthesis: Excess ammonia (not utilized in the urea cycle) is captured by GS located primarily in perivenous hepatocytes. GS catalyzes a high-affinity but low-efficiency reaction of the ammonia binding to glutamate to form glutamine (Figure 1).

Figure 1 Ammonia metabolism in the liver. Detoxification of ammonia in the liver occurs via two spatially separated pathways: The highly efficient urea cycle, or ureagenesis, in periportal hepatocytes converts ammonia to urea, whereas residual ammonia is utilized by glutamine synthetase in perivenous hepatocytes by binding ammonia with glutamate to form glutamine. OTC: Ornithine transcarbamylase; CPS1: Carbamoyl phosphate synthetase 1; ASS1: Argininosuccinate synthetase 1; ASL: Argininosuccinate lyase; ARG: Arginase; GS: Glutamine synthetase; GLS: Glutaminase.

Thus, the liver has a two-tier ammonia detoxification system characterized by spatial separation and the use of different mechanisms, which increases the efficiency of ammonia utilization under physiological conditions and provides at least partial protection against hyperammonemia in liver damage of various etiologies (when zone 1 or zone 3 hepatocytes are involved in the process)[29,31]. In addition to the liver, GS plays a critical role in ammonia metabolism in other organs, (including skeletal muscle, kidney, and brain), where it serves as the main agent of ammonia detoxification[38].

PATHOGENESIS OF SYSTEMIC HYPERAMMONEMIA IN MASLD

Hepatic ureagenesis dysfunction and impaired glutamine metabolism

The essential basis of nitrogen metabolism disorder in MASLD is dysfunction of the ornithine cycle in the liver, which indicates the development of mitochondrial dysfunction[39]. It is assumed that fat accumulation in hepatocytes (lipotoxicity) contributes to the disruption of urea synthesis processes at the genetic and epigenetic levels[40]. Experimental studies based on animal models of MASLD revealed a reversible reduction in the expression of genes encoding key enzymes of the urea cycle, such as ornithine transcarbamylase (OTC), carbamoyl phosphate synthetase 1 (CPS1), and argininosuccinate synthetase 1[40,41]. It came to light that in patients with MASH, these changes were significantly more pronounced than in simple steatosis, which correlated with higher levels of ammonia in the blood[22,36]. The molecular basis of these alterations has been linked to epigenetic hypermethylation of the promoter regions of the OTC and CPS1 genes[42]. Clinical data supported the results of experimental studies: In patients with MASLD (without diabetes mellitus), a pronounced suppression of the CPS1 gene was revealed (its expression underwent a 3.5-fold reduction, P < 0.0001) compared with the control groups (obese patients without MASLD and healthy volunteers)[43]. Gene expression analysis revealed a selective reduction in the mitochondrial enzyme content of the urea cycle already in the early stages of MASLD. In simple steatosis (without fibrosis), CPS1 expression decreased by 3.1 times compared with healthy controls (P = 0.0003), while OTC1 expression underwent a 3.6-fold reduction (P < 0.0001). In MASH with fibrosis, OTC1 expression exhibited a statistically significant twofold reduction (P = 0.01), while CPS1 expression in individuals with MASH and fibrosis was seemingly decreased compared with control group patents, but differences were not statistically significant[44].

Thus, mitochondrial component dysfunction of the ureagenesis, which manifests itself as diminished expression of CPS1 and OTC1, develops in the early stages of MASLD and precedes severe fibrosis. This fact explains the development of hyperammonemia in MASLD patients already at the preclinical stage. Disease progression to the stage of significant fibrosis (≥ F3) is accompanied by a reduction in the citrulline/ornithine ratio (which is a key indicator of mitochondrial urea cycle activity) and an increase in blood CPS1 levels due to its release from damaged mitochondria. Concurrently, a decrease in arginine (a precursor of ornithine and urea) and an increase in glutamate concentrations are observed, indicating impaired ureagenesis and energy metabolism. These changes are consistent with an increase in blood CPS1 concentration[45]. Mathematical modeling highlights the critical role of CPS1 in ammonia detoxification: Its deficiency leads to a more than threefold increase in blood ammonia levels and, therefore, has a more important impact on nitrogen balance than dysfunction of any other enzyme included in the model (GLS and GS)[46]. It is important to point out that enzyme expression can be restored after elimination of the etiologic factor (dietary correction), indicating the potential reversibility of the disorders[44].

Furthermore, it was established that a combination of inflammatory and metabolic components, particularly insulin resistance, is required for the suppression of ureagenesis. In a study of liver biopsies from children with MASH, a decrease in the expression of urea cycle enzymes was observed only in the group with a combination of high MASH activity and pronounced insulin resistance (severe fibrosis ≥ F2, inflammatory activity ≥ 2 pts, and Homeostatic Model Assessment for Insulin Resistance ≥ 5.5 pts)[47].

Compensatory mechanisms are activated in the liver against the background of ureagenesis impairment. In particular, a more than 1.5-fold increase in GS expression (P ≤ 0.03) was observed, which was inversely associated with the CPS1 level (P = 0.004)[35]. However, as another study suggested that, as the disease progresses to the MASH stage, the effectiveness of this pathway declines: A decrease in the level of GS and a loss of its pericentral allocation were observed, which indicated the depletion of compensatory reserves[42].

Dysregulation of glutamine metabolism

Some studies established that in MASLD, the balance of reactions involved in glutamine metabolism is disrupted, favoring glutaminolysis, which leads to additional ammonia release into the bloodstream. Despite steady serum glutamine concentrations, glutamate levels and the glutamate/glutamine ratio increase significantly in MASLD, especially with the progression of fibrosis[48]. This is driven by increased expression of the glutaminase 1 (GLS1)[37,49]. Immunohistochemical analysis revealed that GLS1 protein levels increase in conjunction with the severity of fibrosis: e.g., its concentration was 1.6-fold higher in F3-F4 fibrosis (P < 0.001). Moreover, GLS1 was detected predominantly in areas of extracellular matrix deposition[48]. Hence, dysregulation of glutamine metabolism (increased GLS1 levels and reduced GS concentrations) is an additional mechanism supporting hyperammonemia in MASLD.

The role of gut microbiota in the pathogenesis of hyperammonemia in MASLD

Changes in the intestinal microflora profile make an additional contribution to the development of hyperammonemia. Compelling evidence demonstrates a close relationship between the composition of the intestinal microbiome, the integrity of the intestinal barrier, and the development of steatosis. The main sources of ammonia in the intestine are bacterial hydrolysis of urea by urease and the enzymatic breakdown (deamination) of amino acids originating from dietary proteins or the body’s own proteins and peptides. In vitro studies have demonstrated that gram-negative anaerobes, representatives of the genera Clostridium, Enterobacter [in particular, Escherichia coli (E. coli)], and Bacillus spp., demonstrate the most pronounced ammoniagenic activity. It is worth noting that patients with steatosis and steatohepatitis consistently exhibit excessive growth of enterobacteria in the intestine, including E. coli[50,51]. Besides urea hydrolysis, ammonia is actively produced in the colon via protein breakdown involving the following major genera: Bacteroides, Clostridium, Propionibacterium, Fusobacterium, Streptococcus, and Lactobacillus[52]. In the general population of patients with MASLD, a characteristic increase in the abundance of the phyla Proteobacteria and Bacteroidetes, as well as the families Enterobacteriaceae (including the genus Escherichia) and Lachnospiraceae, is observed[51].

Although data on changes in the composition of the intestinal microbiota in MASLD resulting in a predominance of ammonia-producing bacterial phyla and families convincingly emphasize its role in the pathogenesis of hyperammonemia in MASLD, the contribution of the intestinal microbiota to the development of systemic hyperammonemia against the background of this liver disease requires further study. Overall, the disruption of the composition and diversity of the intestinal microbiota in MASLD represents another important link in the complex chain of metabolic disorders that contribute to increased ammonia entry into the portal circulation.

Table 1 summarizes the current evidence on ammonia metabolism, hyperammonemia, and fatigue-related mechanisms in MASLD, which are discussed in detail in the following sections.

Table 1 Summary of current evidence on ammonia metabolism, hyperammonemia, and fatigue-related mechanisms in metabolic dysfunction-associated steatotic liver disease.

Study model/source of evidence	Key findings
Patients with pre-cirrhotic MASLD stages[18,22,33-36,42-45,48]	Impaired ureagenesis; reduced urea cycle enzyme activity (CPS1, OTC); altered glutamine/glutamate metabolism; increased glutaminolysis associated with fibrosis progression; increased biomarkers of mitochondrial dysfunction; cognitive impairment has been described (its association with hyperammonemia remains uncertain)
Patients with MASLD and advanced fibrosis or cirrhosis[57,60,87,88,118,119]	Hyperammonemia is associated with sarcopenia, myosteatosis, minimal/overt HE; skeletal muscle detoxifies ammonia through glutamine synthesis; this mechanism is partly BCAA dependent; induced hyperammonemia worsens sleepiness; BCAAs may improve HE-related outcomes
Animal MASLD/MASH models[39-42,48,49,104,108,110]	Impaired ureagenesis; reduced urea cycle enzyme activity (OTC, CPS1, ASS1); hyperammonemia is linked to fibrosis progression and skeletal muscle alterations; ammonia-lowering strategies attenuate fibrosis and liver-muscle injury; hyperammonemia is associated with dynapenia, sarcopenia, sarcopenic obesity; increased hepatic glutaminolysis and GLS1 activity may trigger MASH progression/fibrosis; BCAAs support muscle ammonia detoxification via glutamine synthesis
Cell cultures and ex vivo mechanistic studies[63,66,67,69,81,102,120]	Ammonia induces mitochondrial dysfunction, oxidative stress; ammonia triggers NF-κB/myostatin activation and autophagy in skeletal muscles; ammonia disturbs protein synthesis in skeletal muscles via GCN2/eIF2α activation and mTORC1 suppression, while BCAAs partially restore protein synthesis ex vivo; ammonia induces astrocyte dysfunction
Exercise physiology and MASLD exercise studies[61,113-117,121]	Acute exercise increases ammonia production through purine nucleotide breakdown and amino-acid metabolism; the magnitude depends on exercise intensity, duration, and training status; in MASLD, exercise improves liver fat and metabolic parameters; direct studies assessing exercise-induced ammonia responses in MASLD are lacking

ASS1: Argininosuccinate synthase 1; BCAA: Branched-chain amino acid; CPS1: Carbamoyl phosphate synthetase 1; eIF2α: Eukaryotic translation initiation factor 2α; GCN2: General control nonderepressible 2; GLS1: Glutaminase 1; HE: Hepatic encephalopathy; MASH: Metabolic dysfunction-associated steatohepatitis; MASLD: Metabolic dysfunction-associated steatotic liver disease; mTORC1: Mechanistic target of rapamycin complex 1; NF-κB: Nuclear factor kappa B; OTC: Ornithine transcarbamylase.

PERIPHERAL EFFECTS OF AMMONIA: THE ROLE OF SKELETAL MUSCLE IN THE PATHOGENESIS OF FATIGUE

When the liver is unable to excrete ammonia, skeletal muscle becomes the primary peripheral organ compensating for its accumulation. Despite the low GS activity per unit of muscle mass, muscle tissue is a key organ for ammonia detoxification outside the liver due to its large mass in the body. This statement is supported by increased GS expression in muscle during hyperammonemia and accumulation of ammonia in skeletal muscle of patients with liver disease[53-55]. Several studies demonstrated increased ammonia uptake by muscle and amino acid formation in chronic and acute liver failure accompanied by hyperammonemia[38,56]. These data support the hypothesis that skeletal muscle plays a critical role in maintaining normal levels of ammonia in the blood. Nonetheless, the structural abnormalities of skeletal muscle and the reduced muscle mass that occur in MASLD restrict the muscle’s ability to absorb and detoxify ammonia, thus contributing to the development and maintenance of hyperammonemia[40,57,58].

Ammonia accumulation in muscle tissue has been shown to predict declined muscle strength and fatigue, regardless of actual muscle mass[59]. Venous ammonia concentrations in patients with liver cirrhosis are significantly higher with decreased muscle mass and strength[57,60]. Research shows that ammonia has a direct toxic effect on myocytes via several interconnected mechanisms, leading to energy deficit and atrophy, as well as contributing to the development of fatigue[54].

Energy imbalance and mitochondrial dysfunction in skeletal muscle in the presence of hyperammonemia

The primary mechanism of fatigue development is the disruption of myocyte bioenergetics. Long-term exposure to ammonia inhibits pyruvate dehydrogenase and disrupts the Krebs cycle, thus reducing ATP synthesis and triggering accumulation of lactate[61]. Ammonia detoxification in mitochondria occurs through the reductive amination of α-ketoglutarate by the glutamate dehydrogenase. During this reaction, α-ketoglutarate binds ammonia converting it to glutamate; this process is accompanied by consumption of the coenzyme NADPH. Depletion of the α-ketoglutarate pool due to hyperammonemia interrupts the Krebs cycle leading to reduced ATP synthesis and the development of energy deficit in cells. A reduction in ATP levels inhibits protein synthesis, because the peptide chain elongation step requires significant energy expenditure, and directly contributes to the impairment of skeletal muscle contractility[54,62]. Ammonia also contributes to oxidative stress in mitochondria, thereby stimulating the formation of reactive oxygen species (ROS). The latter triggers apoptosis[63,64]. Furthermore, ammonia metabolism depletes branched-chain amino acids (BCAAs), which (under conditions of hyperammonemia) are consumed for glutamine synthesis, aggravating muscle protein deficiency[65].

Molecular mechanisms of sarcopenia and protein homeostasis disorders

Hyperammonemia has been shown to activate molecular pathways associated with skeletal muscle atrophy. In vitro and in vivo experiments suggested that this process is mediated by activation of the kinase complex (IκB Kinase, IKK) in myocytes. Through a cascade of reactions, activated IKK leads to the release of the transcription factor [nuclear factor kappa B (NF-κB)] and its transfer to the nucleus. In the nucleus, NF-κB binds to the promoter of the myostatin gene (a potent endogenous inhibitor of muscle growth) and activates its expression. Increased myostatin levels block myogenesis and triggers autophagy, which explains the loss of muscle mass[66,67]. In addition, ammonia activates protein kinase, general control nonderepressible 2 (GCN2), in muscles, which inhibits protein synthesis through phosphorylation of the translation initiation factor eIF2α (eukaryotic translation initiation factor 2α) and suppresses mechanistic target of rapamycin complex 1 (mTORC1), a key signaling pathway stimulating muscle growth and repair[58,68]. Consequently, even with normal nutrition, under conditions of hyperammonemia, muscles lose the ability to effectively synthesize proteins, leading to their atrophy and weakness. Notably, in the course of the conventional stress response to amino acid deficiency, activating transcription factor 4 is stimulated, aimed at maintaining cellular homeostasis through protective mechanisms. Yet, in hyperammonemia, this adaptive pathway is not activated; instead, muscles are likely to compensate for the damage by increasing the uptake of leucine (an amino acid critical for activating protein synthesis). However, this is insufficient, since leucine is actively utilized by mitochondria; hence, its intracellular level is not sufficient for the restoration of impaired protein metabolism[69].

Some studies have demonstrated that elevated blood ammonia levels correlate with the intensity of tyrosine nitration in muscle proteins[59,67,70]. Importantly, protein nitration not only impairs their function but also stimulates autophagy aimed at removing damaged molecules. Although autophagy initially plays a protective role, chronic activation of this pathway under conditions of persistent hyperammonemia can contribute to muscle protein degradation and aggravated sarcopenia[67].

Myosteatosis (i.e., fatty infiltration of skeletal muscle) is an additional aggravating factor in MASLD and hyperammonemia. Nardelli et al[57] hypothesized that the replacement of muscle tissue with adipose tissue may contribute to the partial loss of function of GS, which is expressed in muscle cells. A statistically significant inverse correlation was revealed between muscle mass and the ammonia level in venous blood: The concentration of the latter was significantly higher both in sarcopenia (62.6 ± 17.7 μg/dL) and in myosteatosis (65.2 ± 19.2 μg/dL) compared to that in the case of preserved muscle structure and mass (P < 0.001).

Although these experimental findings support a potential role of hyperammonemia in skeletal muscle dysfunction, the relationship between hyperammonemia, skeletal muscle abnormalities, and MASLD remains complex and incompletely understood. Hyperammonemia may promote muscle dysfunction through mitochondrial alterations, oxidative stress, and impaired protein homeostasis, whereas skeletal muscle abnormalities in MASLD also arise from shared pathogenic mechanisms, including insulin resistance, chronic systemic inflammation, and vitamin D deficiency[71-73]. Once established, sarcopenia and reduced skeletal muscle mass may further impair extrahepatic ammonia detoxification, thereby contributing to ammonia accumulation and potentially reinforcing a vicious cycle.

THE ROLE OF AMMONIA IN THE DEVELOPMENT OF CENTRAL FATIGUE

Hyperammonemia has emerged as a potential metabolic contributor to the development of central fatigue. As early as the 1980s, the ammonia fatigue theory was proposed, linking ammonia accumulation in the blood and tissues with reduced physical performance[74]. However, current evidence indicate that the effect of ammonia on fatigue in humans is realized primarily through cognitive impairment: Attention deficit, slower decision-making, and reduced ability to perform complex tasks[75]. Although most available evidence is derived from experimental models, several complementary mechanisms of ammonia-induced neurotoxicity have been identified, including BBB dysfunction, astrocyte dysfunction, neuroinflammation, and impaired neurotransmission in the basal ganglia, all of which may contribute to central fatigue. Figure 2 summarizes the proposed role of ammonia in the development of central and peripheral fatigue.

Figure 2 The role of ammonia in the development of central and peripheral fatigue. When the detoxifying function of the liver is impaired, ammonia is utilized primarily in skeletal muscle and brain astrocytes. Glutamine synthetase converts glutamate into glutamine in the brain. Excessive intracellular accumulation of the latter can lead to osmotic cerebral edema and astrocyte swelling. In muscles, chronic hyperammonemia activates myostatin (which suppresses muscle growth) and general control nonderepressible 2 kinase. Through phosphorylation of alpha subunit of eukaryotic translation initiation factor 2, the latter inhibits protein synthesis and the mammalian target of rapamycin complex 1 pathway, which is necessary for muscle growth and repair. GS: Glutamine synthetase; α-KG: Alpha-ketoglutarate; eIF2α: Eukaryotic translation initiation factor 2α; GCN2: General control nonderepressible 2; GDG: Glutamate dehydrogenase; mTORC1: Mechanistic target of rapamycin complex 1.

Transport across the BBB and metabolism in astrocytes

At present, there is compelling evidence that ammonia crosses the BBB not only by means of passive diffusion of the NH₃ molecule but also via active transport of the NH₄⁺ cation through ion channels[58,76]. Moreover, ammonia has been shown to modulate BBB permeability through altering the penetration of various molecules via both transcellular and paracellular pathways by means of disrupting the integrity of barrier components[77]. After penetrating the brain parenchyma, ammonia accumulates primarily in astrocytes, the major glial cells responsible for CNS homeostasis. This is due to the circumstance that GS enzyme demonstrates high activity in astrocytes, and this enzyme is responsible for the detoxification of ammonia by binding to glutamate. This results in the formation of glutamine, which is an osmotically active substance. Its excessive intracellular accumulation results in osmotic edema and swelling of astrocytes. Even in the absence of clinically evident encephalopathy, minimal astrocyte swelling can have important functional consequences by disrupting glial-neuronal interactions[78,79].

Mitochondrial dysfunction and oxidative stress

Glutamine accumulation in astrocytes leads to its secondary hydrolysis in mitochondria, a reaction that regenerates ammonia and produces ROS[78,80]. In vitro and ex vivo studies have demonstrated that hyperammonemia is associated with mitochondrial membrane hyperpolarization, excessive ROS production, and lipid peroxidation. The combined effects of ammonia and oxidative stress lead to elevated calcium levels in the cytoplasm, opening of mitochondrial permeability transition pores, membrane potential collapse, and oxidative phosphorylation uncoupling. This cascade results in energy deficit (ATP resource depletion), thereby making neurons and glia extremely vulnerable to toxic effects and, in the long term, may contribute to neurodegeneration[81].

Neuroinflammation and neurotransmitter imbalance

Hyperammonemia-induced cognitive dysfunction is critically exacerbated by systemic inflammation. It has been suggested that ammonia triggers an increase of BBB permeability, sensitizing the brain to the effects of circulating proinflammatory factors[79]. In the rat hippocampus (a brain structure responsible for learning and memory), ammonia induces microglia activation and hyperproduction of proinflammatory cytokines: Interleukin (IL)-6 and IL-1β[82]. Neuroinflammation and the direct action of ammonia yield an imbalance in key neurotransmitter systems: e.g., a disruption of glutamatergic transmission through the glutamate-nitric oxide-cyclic guanosine monophosphate signaling pathway and dysfunction of N-methyl-D-aspartate receptors associated with their decreased membrane expression and extrasynaptic accumulation of glutamate, which underlies disorders of synaptic plasticity and cognitive dysfunction[29,82]. Wilkinson et al[75] proposed that dysfunction of this pathway may contribute not only to cognitive impairment but also to central fatigue in patients with chronic liver disease, manifested by reduced motivation, slowed psychomotor responses, and increased exhaustion. Besides that, ammonia boosts GABAergic inhibition through the stimulation of benzodiazepine receptors by endogenous neurosteroids, the synthesis of which is potentiated by ammonia. The efficacy of neurosteroid antagonists (such as golexanolone) in hyperammonemia supports this mechanism[65,82]. Thus, hyperammonemia disrupts the balance of excitatory and inhibitory signals in synapses.

Basal ganglia dysfunction and reward system disruption

Changes in the basal ganglia, including subcortical nuclei such as the subthalamic nucleus, globus pallidus, substantia nigra, and striatum, which in turn comprises the nucleus accumbens, putamen, and caudate nucleus, play an equally significant role in the pathogenesis of fatigue[12]. These structures control motor functions and play an important role in learning, motivation, and reward-driven behavior. There is a hypothesis that changes in the basal ganglia, similar to those seen in Parkinson’s disease and multiple sclerosis, may lead to the development of central fatigue due to disruption of the neural circuits responsible for motivation and an imbalance in the effort-reward system[11]. Neuroimaging has revealed structural changes in these regions already in the early stages of liver disease in patients with complaints of increased fatigue[83,84].

Experimental studies have shown that hyperammonemia disrupts dopaminergic transmission from the nucleus accumbens to the prefrontal cortex while activating alternative pathways involving the substantia nigra. This is accompanied by a decrease or paradoxical increase in motor activity[85]. Dopaminergic dysfunction of these structures leads to an imbalance in the effort-reward system and a decrease in motivation, which is clinically perceived by the patient as an inability to exert effort for performing an action[86].

Sleep disturbances

In fatigue, sleep disturbances are considered an independent pathophysiological mechanism; however, several studies have demonstrated that hyperammonemia may impair sleep quality and reduce sleep duration. For example, Bersagliere et al[87] showed that oral administration of an amino acid mixture (induced hyperammonemia) resulted in a significant increase in subjective daytime sleepiness and alterations in the electroencephalographic architecture of the subsequent sleep episode in patients with liver cirrhosis, suggesting a reduced capacity of the body to achieve restorative sleep under hyperammonemic conditions. In turn, Kann et al[88] demonstrated that induced hyperammonemia led to increased sleepiness in patients with liver cirrhosis. At the same time, the patients themselves reported poor sleep quality. As a possible mechanism underlying the adverse effects of ammonia on sleep, Granados-Fuentes et al[89] proposed that hyperammonemia may affect the circadian regulation of rest-activity behavior by increasing extracellular glutamate levels in the central pacemaker, namely the suprachiasmatic nuclei of the hypothalamus.

Table 2 summarizes the current clinical and experimental evidence regarding the potential role of hyperammonemia in the pathogenesis of fatigue in MASLD.

Table 2 Current evidence supporting the role of hyperammonemia in fatigue-related mechanisms in metabolic dysfunction-associated steatotic liver disease.

Mechanism/finding	Evidence in MASLD patients	Experimental evidence (animal models and non-MASLD human studies)	Strength of evidence
Elevated blood ammonia[15,19,22,33-37,42,79]	Small cohorts, inconsistent	Moderate, human studies	Low to moderate
Urea cycle dysfunction[39,42-45]	Yes	Strong	Moderate to strong
Altered Gln/Glu metabolism, increased glutaminolysis[48,49,53,122]	Yes	Strong	Moderate
Muscle ammonia detoxification[57,60,69,123]	Indirect, stronger in advanced CLD	Extensive	Moderate to strong
Ammonia-induced muscle dysfunction/sarcopenia[54,55,57,111,112]	Limited/indirect	Strong	Moderate
Mitochondrial dysfunction/oxidative stress[39-41,43-45,81]	Indirect	Strong	Moderate
Neuroinflammation/astrocyte dysfunction[81,82]	Indirect	Moderate	Low to moderate
Ammonia-related cognitive impairment[18,36]	Limited	Moderate to strong	Moderate
BCAA involvement in ammonia handling[102-104,118-120]	No MASLD-specific data	Moderate	Low to moderate

BCAA: Branched-chain amino acid; CLD: Chronic liver disease; Gln: Glutamine; Glu: Glutamate; MASLD: Metabolic dysfunction-associated steatotic liver disease.

NONPHARMACOLOGICAL APPROACHES TO REDUCING HYPERAMMONEMIA AS A WAY TO COMBAT FATIGUE IN MASLD

In this section, we review the available evidence regarding the potential influence of dietary factors and physical activity on ammonia homeostasis in MASLD. We also discuss current knowledge gaps concerning the relationship between lifestyle interventions and ammonia metabolism.

Nutrition in MASLD as a way to influence the risk of developing hyperammonemia

Current EASL[90] and AASLD[91] guidelines recommend the Mediterranean diet for patients with MASLD, with an approximate macronutrient distribution of 20% protein, 30% fat, and 50%-60% carbohydrates. Among these macronutrients, dietary protein is of particular interest in the context of ammonia metabolism because amino acids represent the major source of nitrogen entering the urea cycle. However, despite the biological plausibility of a relationship between dietary protein, BCAAs, and ammonia metabolism, direct evidence linking dietary protein composition or BCAA intake with hyperammonemia in patients with MASLD is currently lacking. Therefore, the available evidence should be interpreted with caution, as it is derived primarily from studies evaluating metabolic outcomes rather than ammonia metabolism, and from experimental models.

Several clinical studies have demonstrated that dietary protein composition influences metabolic outcomes in MASLD. In patients with type 2 diabetes and MASLD, both animal- and plant-based high-protein diets significantly reduced liver fat content and improved insulin resistance and markers of hepatic inflammation[92]. Similarly, a randomized study in patients with obesity and MASLD demonstrated greater reductions in intrahepatic lipid content with a high-protein diet than with a low-protein diet despite comparable weight loss[93]. In pediatric and adolescent populations, higher consumption of animal protein has been associated with an increased likelihood of MASLD, whereas greater intake of plant-derived protein has been associated with a lower risk of the disease[94]. Moreover, elevated circulating BCAA concentrations have consistently been associated with insulin resistance and metabolic dysfunction in MASLD[95-97]. Collectively, these studies suggest that dietary protein quality influences metabolic health in MASLD; however, none of them evaluated ammonia production or nitrogen metabolism directly.

Preservation of skeletal muscle mass represents another important aspect of MASLD management because skeletal muscle serves as a major extrahepatic site of ammonia detoxification through glutamine synthesis. Current clinical recommendations for sarcopenic obesity emphasize resistance exercise together with adequate dietary protein intake to preserve muscle mass[98]. Clinical studies have shown that higher protein intake combined with resistance exercise improves lean body mass in individuals with sarcopenic obesity[99,100], while adherence to the Mediterranean diet has been associated with better preservation of muscle mass and function in older adults[101]. Whether these interventions improve ammonia detoxification or reduce hyperammonemia specifically in patients with MASLD remains unknown and requires dedicated clinical investigation.

Currently, no studies have directly evaluated the effects of dietary BCAA intake on ammonia metabolism in patients with MASLD. Experimental evidence remains limited and heterogeneous. In cultured human astrocytes exposed to hyperammonemia, BCAA supplementation altered intracellular metabolic pathways and adversely affected cellular metabolism[102]. Likewise, an experimental animal study demonstrated that excessive dietary BCAA intake modified the intestinal microbiota, increased the abundance of ammonia-producing microorganisms, and promoted systemic ammonia accumulation[103]. Although these findings suggest that BCAAs may influence ammonia metabolism under specific experimental conditions, their relevance to human MASLD has not been established.

BCAA supplementation has demonstrated clinical benefits in patients with liver cirrhosis, particularly in improving manifestations of HE. However, these findings cannot be directly extrapolated to MASLD because the mechanisms underlying hyperammonemia differ substantially between cirrhotic and precirrhotic liver disease. The beneficial effects of BCAAs in cirrhosis are thought to be related primarily to improvements in nutritional status, preservation of skeletal muscle mass, and enhanced extrahepatic ammonia detoxification rather than to a direct ammonia-lowering effect[104-106].

Overall, the available evidence does not permit conclusions regarding the effects of dietary BCAA intake on ammonia metabolism in MASLD. Although experimental studies suggest that BCAAs may influence ammonia metabolism under specific conditions, these findings cannot be directly extrapolated to patients with MASLD. At present, there is insufficient clinical evidence to support dietary recommendations regarding BCAA intake for the prevention or treatment of hyperammonemia in MASLD. Dedicated mechanistic and prospective clinical studies are required to clarify this relationship.

Physical activity and its effect on hyperammonemia in MASLD

Regular physical activity is a cornerstone of MASLD management and is consistently recommended by current international (AASLD)[91] and European (EASL)[90] clinical practice guidelines because of its beneficial effects on hepatic steatosis, insulin resistance, body composition, cardiometabolic risk, and quality of life. Current recommendations encourage patients with MASLD to engage in at least 150 minutes of moderate-intensity or 75 minutes of vigorous-intensity physical activity per week, together with regular resistance exercise. These recommendations are supported by substantial clinical evidence and should not be modified based on the currently limited evidence regarding ammonia metabolism.

During exercise, ammonia is transiently produced in contracting skeletal muscle through purine nucleotide degradation and amino acid transamination, thereby contributing to energy metabolism. Under physiological conditions, this transient increase in ammonia is efficiently detoxified by the liver through the urea cycle and, to a lesser extent, by skeletal muscle through glutamine synthesis. However, impaired hepatic ureagenesis has been demonstrated in several experimental models of MASLD, including rats with diet-induced steatosis[39,41], mice fed methionine- and choline-deficient diets[107], and genetically modified foz/foz mice receiving a high-fat diet[40]. These studies consistently demonstrated reduced expression and activity of urea cycle enzymes, particularly OTC, resulting in impaired urea synthesis and increased circulating ammonia concentrations. Importantly, these alterations were reversible following resolution of steatosis[42].

Clinical studies have similarly demonstrated reduced expression and activity of urea cycle enzymes together with impaired ureagenesis in patients with both simple steatosis and MASH[42,43]. Although the mechanisms underlying these alterations remain incompletely understood, epigenetic regulation of urea cycle genes and premature hepatocyte senescence have been proposed as potential contributors[22].

Collectively, these findings raise the possibility that impaired ammonia detoxification in MASLD could modify the physiological ammonia response to exercise. However, this hypothesis has not been directly evaluated in clinical studies. At present, no studies have investigated whether patients with MASLD experience greater exercise-induced increases in blood ammonia concentrations than healthy individuals or whether such changes have any clinical consequences.

Another important consideration is skeletal muscle dysfunction, which is increasingly recognized across the spectrum of MASLD. Myosteatosis, dynapenia, reduced muscle mass, and sarcopenia have all been reported in patients with MASLD[72,108,109]. Because skeletal muscle represents the principal extrahepatic site of ammonia detoxification, impaired muscle quantity or function may theoretically reduce ammonia disposal capacity. In an experimental model of MASH, Wang et al[110] demonstrated an inverse association between blood ammonia concentrations and muscle strength, accompanied by progressive development of sarcopenic obesity. Similar associations between skeletal muscle abnormalities and MASLD severity have subsequently been reported in clinical studies[111,112]. Nevertheless, whether preservation or improvement of skeletal muscle function translates into enhanced ammonia detoxification in patients with MASLD remains to be determined.

Physical activity itself has consistently been shown to improve the clinical course of MASLD. In a large population-based study involving more than 21000 participants, Park et al[113] demonstrated that regular recreational physical activity was associated with a lower prevalence of MASLD, while resistance exercise performed at least twice weekly was independently associated with a reduced likelihood of MASLD regardless of overall activity level. Likewise, recent systematic reviews and network meta-analyses have concluded that combining aerobic and resistance exercise provides the greatest improvements in hepatic steatosis, metabolic parameters, and liver function[114]. Resistance exercise may be particularly suitable for patients with reduced cardiorespiratory fitness because it confers similar hepatic benefits with lower overall energy expenditure[115].

Experimental studies indicate that blood ammonia concentrations increase with increasing exercise intensity and duration[61,116]. However, physiological responses vary considerably according to age, cardiorespiratory fitness, medications, and underlying cardiometabolic disease, precluding recommendations based on fixed heart-rate thresholds. Importantly, the clinical significance of transient exercise-induced ammonia elevations in MASLD remains unknown. Additionally, whether these transient increases differ in magnitude or duration from those observed in healthy individuals has not yet been investigated.

To date, no studies have directly examined exercise-induced ammonia responses in patients with MASLD. The only available clinical evidence comes from patients with advanced liver cirrhosis. Lin et al[117] demonstrated that venous ammonia concentrations increased transiently during cardiopulmonary exercise testing but returned to baseline during recovery, without precipitating HE during the subsequent follow-up period. Although these findings suggest that transient exercise-induced hyperammonemia may be well tolerated in compensated cirrhosis, they should not be extrapolated to patients with MASLD because ammonia metabolism differs substantially between cirrhotic and precirrhotic liver disease.

Overall, current evidence does not support modifying existing physical activity recommendations for patients with MASLD because of concerns regarding exercise-induced hyperammonemia. Regular physical activity remains one of the most effective non-pharmacological interventions for MASLD and should continue to be recommended in accordance with current clinical guidelines. Although impaired ureagenesis in MASLD raises the hypothesis that exercise-induced ammonia responses may differ from those of healthy individuals, this concept remains speculative. Dedicated mechanistic and prospective clinical studies are needed to determine whether exercise modality, intensity, duration, and skeletal muscle status influence ammonia metabolism in MASLD and whether such changes have clinical relevance.

CONCLUSION

Unlike previous reviews addressing fatigue in MASLD from a broad perspective, the present review specifically highlights hyperammonemia as a potential mechanistic link between liver dysfunction and the development of both central and peripheral fatigue. Current evidence suggests that ammonia may contribute to fatigue through multiple interconnected mechanisms involving impaired ureagenesis, skeletal muscle dysfunction, mitochondrial alterations, oxidative stress, neuroinflammation, and disturbances in neurotransmission. However, most available evidence originates from experimental studies, whereas direct clinical evidence in patients with MASLD remains limited and a causal relationship has not yet been established. Therefore, the conclusions of this review should be regarded as hypothesis-generating and should not be interpreted as clinical guidance. Modifiable lifestyle factors may influence ammonia homeostasis in MASLD; however, the available evidence remains limited. Current clinical guidelines continue to recommend the Mediterranean diet and regular physical activity as the cornerstone of MASLD management because of their well-established benefits for liver and cardiometabolic health. Although dietary protein composition, skeletal muscle status, and physical exercise may theoretically affect ammonia production and detoxification, direct evidence linking these factors to hyperammonemia in patients with MASLD is currently lacking. Consequently, no modifications to existing dietary or physical activity recommendations can be justified on the basis of concerns regarding ammonia metabolism. Further studies are required to determine how lifestyle interventions influence ammonia homeostasis in MASLD. Future longitudinal and mechanistic studies should clarify the role of hyperammonemia in the early manifestations of MASLD, identify reliable approaches for assessing ammonia metabolism in clinical practice, and determine whether targeting ammonia metabolism improves fatigue and other clinically relevant outcomes. Such evidence may facilitate the development of approaches to fatigue management in MASLD.