The pathophysiology of lean NAFLD can be impressed by diet and nutrients (53, 54). A matched case-control study included 351 Chinese adults, and found the average weekly exercise and daily sleep duration of lean NAFLD people were lower than that of healthy lean NAFLD people, while the average daily intake of total calories, carbohydrate, total cholesterol, fat and protein was higher than that of healthy lean NAFLD people (55). Total starchy foods intake was higher in those with NAFLD compared to the lean healthy group, but no significant difference was found among lean and non-lean NAFLD participants regarding nutrients and food items, indicating that the chance of lean NAFLD would increase by an increase in carbohydrate, potato, and fat intake (56). But in the study of Younossi et al. (57), no difference was found in macronutrients intake (carbohydrate, fat, and protein), vitamins, and minerals between lean NAFLD subjects and lean healthy controls. They suggested that this may indicate that NAFLD in lean patients has occurred due to other metabolic abnormalities. Others have postulated that the cause of lean NAFLD could be due to genetic characteristics, impaired intestinal motility, and some other metabolic disturbances that are not related to weight status (58). Since studies on the association between lean NAFLD and diet are less at present, and is still controversial, it is urgent to assess dietary intakes and pattern, and diet quality with adequate sample size in lean NAFLD people to find if there is any association between them.

The concerted regulation of glucose uptake, utilization, and storage by tissues is critical to maintaining blood glucose homeostasis (59). Insulin increases glucose uptake into peripheral tissues, primarily skeletal muscle and adipose tissues, which express the GLUT4 isoform of the glucose transporter. In unstimulated fat or muscle cells (basal state), 3~10% of GLUT4 is located at the cell surface and >90% is in intracellular compartments (60). GLUT4 participates in a cycle that consists of exocytic movement of the transporter within post-biosynthetic vesicles of endosomal origin towards the plasma membrane, and of endocytic movement from the membrane back to the sorting endosomal system (61). GLUT4 exocytosis is regulated by Insulin-derived signals, including mobilization to the cellular periphery, vesicle tethering, docking and fusion, thereby increasing the cell-surface amount of GLUT4 and the rate of glucose uptake (62).

Skeletal muscle expresses high levels of GLUT4, responsible for approximately 80% of glucose clearance under physiological conditions (32, 46). In mice, muscle-specific knockout of GLUT4 leads to severe glucose intolerance (63). According to the morphology and metabolic features, muscle fibers are mainly divided into type I and type II subtypes. Muscles composed of different types of muscle fibers show great differences in blood glucose metabolism, and the density of glucose transporter 4 (GLUT4) in type I muscle fibers is higher than that in type II muscle fibers (64). Gaster et al. (65) found that GLUT4 expression was reduced in type I fibers in T2DM patients by immunohistochemistry and morphometry. Therefore, it can be assumed that the decrease in insulin-stimulated glucose uptake in the skeletal muscle of T2DM patients might be due to the decrease of GLUT4 in type I fibers. In addition, muscle fibers (especially type IIb muscle fibers) can ameliorate metabolic abnormalities by secreting proteins and muscle factors, while loss of skeletal muscle due to aging or underlying diseases can exacerbate glucose intolerance (66).

Insulin resistance is an important pathogenesis of sarcopenia and NAFLD, regardless of lean type or non-lean type (67–69). Insulin resistance refers to the reduction of glucose uptake and utilization. To sustain stable levels of blood glucose, islet β cells secret excessive insulin to compensate, leading to hyperinsulinemia (70). Increased insulin content results in decreased affinity to insulin receptors thus, further exaggerating the insensitivity of insulin. Simultaneously, increased levels of adipokines and inflammatory cytokines (such as IL-1, IL-6 and TNF-α) activate NF-κB signaling pathway and promote insulin resistance (71). Meanwhile, the weakened insulin signaling transduction also affects the downstream mediator-glycogen synthase kinase-3β (GSK3β) and glycogen synthesis (72). Collectively, excessive insulin secretion, adipokines, and inflammatory factors directly or indirectly interfere with the insulin signaling pathway, and contribute to the development of NAFLD (73).

Insulin resistance leads to increased lipolysis and consequent release of free fatty acids (FFAs) from adipose tissue to the liver, which is the main contributor to the increase of De novo lipogenesis (DNL) (63, 66). The mechanism of insulin-mediated decrease of muscle anabolic metabolism may be related to the activation of p38 mitogen-activated protein kinase (MAPK) and mammalian target of rapamycin (mTOR)/p70S6 kinase (74). Insulin resistance-induced hyperinsulinemia also increases myostatin levels, which further reduces skeletal muscle mass (48). Insulin resistance leads to inhibition of β-oxidation, increased gluconeogenesis, increased expression of sterol regulatory element binding protein-1c (SREBP-1c), as well as increased production of FFAs, leading to accumulation of triglycerides in skeletal muscle and liver, and the long-term level of inflammation might be responsible for the disruption of environmental homeostasis in muscle cells, ultimately leading to systemic metabolic disorders (75, 76). Conversely, a 2-year follow-up cohort included 194 community-dwelling nondiabetic older adults found that loss of lower limb muscle mass is a significant risk factor for development of insulin resistance independent of obesity (77). Sarcopenic obesity patients had higher HOMA-IR index in contrast to subjects without sarcopenia, indicating that muscle mass is also a determinant for insulin sensitivity (67).

In addition, anabolic hormones such as insulin-like growth Factor1 (IGF-1) also contribute to the progression of sarcopenia and lean NAFLD (78), as well as non-lean NAFLD (68). Skeletal muscle mass loss inhibits the growth hormone/insulin-like growth factor-1 (GH/IGF-1) axis, and blocks skeletal muscle protein synthesis (35). The GH/IGF-1 axis is involved in protein metabolism and bone growth and remodeling in skeletal muscle (79). GH/IGF-1 affects carbohydrate and lipid metabolism and have opposite effects: IGF-1 stimulates glucose uptake and facilitates insulin signaling, while GH induces lipolysis, which induces insulin resistance and elevated levels of free fatty acids (FFAs) (80). IGF-1 level decreases when insulin resistance occurs, leading to lipid deposition in intramuscular and intermuscular adipose cells, increased infiltration of adipose between muscle bundles, and aggravated damage to muscle cells (81, 82). Patients with GH deficiency are often comorbidity with NAFLD, which contributes to NASH development and progression (83). Some studies also reported low levels of IGF-1 in NAFLD patients (84, 85). These indicate that GH/IGH-1 axis is another important association between sarcopenia and lean NAFLD.

Sarcopenia and lean NAFLD often present with systemic chronic inflammation, elevated levels of CRP and pro-inflammatory cytokines, and reduced level of anti-inflammatory cytokines (86). This is due to the high proportion of visceral adipose tissue in patients with lean NAFLD, and then more adipose cells secret inflammatory cytokines, and the muscle tissue is in a state of continuous chronic inflammation, leading to an increased risk of muscle atrophy (87). In patients with metabolic disorders, white adipocytes become hypertrophy and proliferated, and these white adipocytes are infiltrated by activated inflammatory macrophages and other immune cells (88). Contractile myoglobin expression is reduced in muscle tubes cocultured with white adipocytes from obese individuals (89). This may indicate that too much adipose tissue disrupts the living environment of muscle cells and promotes their apoptosis. Adipose tissue produces a large number of pro-inflammatory cytokines, such as TNF-α, monocyte chemotactic protein-1 (MCP-1), IL-6, and C-reactive protein, which promote chronic inflammation, decreased muscle protein synthesis, and insulin resistance (90). The increased production of systemic inflammatory molecules and inhibited production of certain adipokines (such as leptin and appetite suppressor hormone) in lean NAFLD patients, further adversely affect the liver, pancreas and skeletal muscle (75). Altered adipokine secretion leads to increased food intake, decreased energy expenditure, and decreased insulin sensitivity in muscles (90). Immune cells and adipocytes release inflammatory cytokines, which aggravate myocyte insulin resistance and further worsen sarcopenia (91). These processes can also occur in the pathogenesis of non-lean NAFLD/T2DM and sarcopenia (68, 92).

Inflammation and oxidative stress appear to be major factors in the etiology of muscle loss (93). Muscle cells are particularly vulnerable to oxidative damage because they are post-mitotic cells and are particularly prone to the accumulation of oxidative damage molecules (94). In addition, skeletal muscle accounts for a prominent portion of total oxygen consumption, increasing the inherent risk of elevated mitochondrial-derived ROS such as H₂O₂ (95). Increased reactive oxygen species (ROS) production and inefficient clearance in muscle cells trigger cellular senescence and eventually muscle loss (96). The imbalance of mitochondrial unfolded protein response and mitochondrial phagocytosis modified integrated stress response systems involved in the development of sarcopenia (97). Oxidative stress also increases the production and circulation of inflammatory cytokines, which promote protein degradation in muscle (98). TNF-α also induces lipid accumulation in the liver by activating DNL (71). It is reported in many rodent studies that inhibition of TNF-α attenuates muscle proteolysis, while an infusion of TNF-α increases Myo-fibrillation lysis (99, 100). This indicates that pro-inflammatory cytokines (CRP, IL-6, and TNF-α) may increase proteolysis and muscle atrophy.

The development of sarcopenia and lean NAFLD is accompanied by a decrease in some protective factors, such as adiponectin (27, 28). Adiponectin is a hormone secreted by adipose tissue that mediates glucose and lipid metabolism in insulin-sensitive tissues such as the liver and muscle (101). In the liver, adiponectin improves insulin resistance by activating AMP-activated protein kinase (AMPK), thus promoting glucose usage and fatty acid oxidation. In addition, adiponectin plays the role of anti-inflammatory by inhibiting TNF-α, improving liver steatosis and inflammation (102, 103). Fat accumulation and aging damage the signaling pathway of the GH/IGF-1 axis, leading to decreased levels of muscle synthesis (104, 105). In an animal model, NAFLD is reported to be associated with decreased muscle mass and strength as well as decreased IGF-1 levels, suggesting that decreased IGF-1 may contribute to the development of NAFLD-related sarcopenia (106).

In addition, Irisin is also involved in the progression of sarcopenia and NAFLD (107, 108). Irisin, an exercise-induced myokine, increases energy expenditure through peroxisome proliferator-activated receptor α-dependent downstream signaling and improves insulin sensitivity and hepatic steatosis by upregulating fibroblast growth factor-21 (108). It increases glucose uptake by enhancing GLUT4 translocation and FFA β-oxidation through activating AMPK in muscle cells (108). Irisin is reported to be inversely associated with the degree of hepatic steatosis and is a potential cause of sarcopenia and NAFLD (107, 109).

NAFLD is highly influenced by genetic and environmental factors, and its susceptibility, progression and risks of related complications vary greatly between individuals, and the genetic variations of NAFLD are confirmed in genome-wide association studies (GWAS) (110). It is reported that the proportion of the G allele in PNPLA3 and the T allele in TM6SF2 in lean NAFLD patients is significantly higher than that in non-lean NAFLD patients, and the mutations of these two alleles are highly related to the disorder of glucose and lipid metabolism (111). The PNPLA3 rs738409 G allele is associated with higher liver-related mortality than the C allele in a nationwide population study in the United States (112). The C allele frequency of rs2279026 and G allele frequency of rs2279028 in the TBC1D1 gene as well as the C allele frequency of rs780093 and rs1260326 in GCKR gene in lean NAFLD patients are lower than those in obese patients (113). A cohort study of 5387 Chinese residents (≥60 years old) revealed that the AA genotype frequency of SOD2 gene rs4880 in patients with lean NAFLD is lower than that in lean healthy people, and rs4880 site in SOD2 determined the susceptibility to lean NAFLD (114). The PEMT gene controls liver triglyceride secretion in the form of VLDL and is associated with NAFLD. Bale et al. (115) sequenced the intact exon region of lean NAFLD patients and found that the PEMT rs7946 variant is associated with a three-times increase in the risk of lean NAFLD.

Variation in skeletal muscle traits among individuals can be attributed to both genetic and environmental factors (116). Though the influence of environmental factors, such as physical activity and diet, have been broadly investigated, only a few studies have identified the specific genetic influences on skeletal muscle traits (117). Sarlo et al. enrolled 120 Italian healthy women and found a positive correlation between sarcopenic normal-weight subjects and G/A-308 TNF-α polymorphism (118). The gene TP53, encoding p53, is reported to be involved in the pathogenesis of NAFLD (119). It is also affects myoblast differentiation in severe and rapid skeletal muscle atrophy, which represents a hallmark of cachexia and sarcopenia during aging (120). The G/G genotype of TP53 codon 72 in exon 4 polymorphism is found to increase the risk of sarcopenia up to 20% (121). The skeletal muscle mass is reported to be associated with FTO (Fat mass and obesity-associated) genotype variation. In a study enrolled 559 non-athlete subjects, TT genotype and T allele carriers of the FTO rs9939609 variant had greater total body (4.8% and 4.1%) and total appendicular lean mass (3.0% and 2.1%) compared to AA genotype (122). The FTO genotype variants also play an important role in lipid-related parameters (123). In a cohort study, AA genotype of the FTO rs9939609 is found to be associated with LDL levels lean NAFLD subjects but not in overweight and obese NAFLD subjects (113). Besides, the ACTN3, ACE, VDR, IGF1/IGFBP3, APOE, CNTF/R and UCP2/3 gene polymorphism are also found to be associated with muscle phenotypes (124–130), but their association with lean NAFLD is still to be determined. Therefore, certain genetic factors might be associated with both sarcopenia and lean NAFLD, but further studies are needed to verify the association and underlying mechanisms.

Circulating cholesterol is the major metabolic risk of NAFLD patients, with the potential to develop cardiovascular diseases (131, 132). Both low-density lipoprotein (LDL) and high-density lipoprotein (HDL) are carriers of cholesterol, with LDL moving cholesterol into the arteries and HDL clearing cholesterol from the arteries. Dyslipidemia is a common complication of NAFLD (133). Cheng et al. (9) found that among 394 NAFLD patients (16. 5% lean), lean NAFLD patients are older, more female and have higher levels of HDL, but lower serum triglyceride and alanine transaminase (ALT) levels in contrast to non-lean patients. Compared with the lean healthy group, lean NAFLD patients showed a higher percentage of hyperlipidemia, age, and an increase in waist circumference, serum triglyceride, LDL-C, and blood glucose levels (9). A study involving 1,305 Chinese residents found that sarcopenic lean NAFLD patients presented a distinct metabolomic profile that is prone to carotid plaque and liver fibrosis, with increased serum valine, small LDL triglyceride and VLDL5 components, and reduced components of HDL4 (134). Park et al. found that higher relative grip strength, one of the indicators of sarcopenia, is associated with lower TG, TC, and LDL-C and higher HDL-C levels (135). These studies suggest that LDL-C, TG and TC as well as HDL-C might be biomarkers of sarcopenic lean NAFLD.

Bile acids (BAs) are amphiphilic steroid molecules synthesized from cholesterol in the liver (136). BAs regulate lipid and glucose metabolism, as well as the synthesis, transport and reabsorption of BAs through bile acid receptors (137). The dysfunctional BA metabolism promotes the accumulation of fat and infiltration of inflammatory cells in the liver (4). Compared with non-lean NAFLD patients, the total BA, primary BA and secondary BA levels are higher in lean NAFLD patients. BA profiling showed that lean NAFLD patients had lower levels of deoxycholic acid (DCA), glycodeoxycholic acid (GCDCA) and goosenodeoxycholic acid (CDCA), and higher levels of glycocholic acid (GCA) (3). Studies reported that serum cholic acid (CA), ursodeoxycholic acid (UDCA) and DCA impair muscle fiber structure and function and induce sarcopenia (138–140). And the level of CA is significantly higher in patients with severe fibrosis compared with those with none/mild fibrosis (3).

Serum uric acid, a metabolite of purines in the liver, is considered to be a predictor of insulin resistance and the severity of liver injury in NAFLD (141). Serum uric acid levels are reported to be higher in lean NAFLD than in healthy controls, but HDL-c levels are in the contrary. Since the ratio of uric acid to HDL-c (UHR) is independently associated with an increased risk of NAFLD, UHR might be a new promising marker for lean NAFLD (142). Some studies have reported a positive association between serum uric acid levels and relative grip strength (143–145). However, a cross-sectional study of 5,247 adults from Korea found a negative association (146). Therefore, the relationship between serum uric acid level and sarcopenia remains controversial and needs further studies to verify the association.

The gut microbiome has a significant contribution to digestion, vitamin synthesis and pathogen resistance (147). The disruption of intestinal homeostasis and changes in microbiota are involved in the pathogenesis of NAFLD (147). Recent studies highlight that lean NAFLD subjects have a distinct gut microbiota profile from obese ones (18).

In a study conducted in a Chinese population, lean NAFLD subjects demonstrated a reduced enrichment of Firmicutes, including Lachnospiraceae, Ruminococcaceae, Lactobacillaceae, and an increase in lipopolysaccharide-producing Gram-negative bacteria (148). Chen et al. (3) reported a distinct microbiome profile in a Caucasian population, and found that lean NAFLD patients have an increased proportion of Ruminococcaceae compared to obese NAFLD patients and an increased population of Dorea and decreased populations of Marvinbryantia and Christensellenaceae compared with healthy controls. According to the analysis of liver biopsy proved NASH patients, intestinal permeability and bacterial overgrowth are associated with the severity of steatosis (149). The gut microbiomes affect FXR-mediated signaling pathways in the liver and gut by interacting with BAs, which is associated with the pathogenesis of lean NAFLD (150, 151). Gut microbiome and BA alterations may predispose NAFLD development at a lower BMI, and further studies of microbiome modulation are required to better understand its role as both a potential biomarker and a therapeutic option in lean NAFLD patients (152).

Nikkhah et al. found that the alteration in age-related sarcopenia and liver cirrhosis-induced sarcopenia was a reduction in short-chain fatty acids (SCFAs) -producing bacteria. Lachnospiraceae family, consisting of Lachnospira, Fusicatenibacter, Roseburia, and Lachnoclostridium, is significantly decreased in age-related sarcopenia, while in liver cirrhosis-induced sarcopenia, the alpha diversity of gut microbiota decreased in comparison to the control group (153). The richness of gut microbiota is considerably reduced in sarcopenic patients, and the Firmicutes/Bacteroidetes ratio, Agathobacter, Dorea and Butyrate are decreased, whereas Shigella and Bacteroides are enriched in the gut (154). A case-control study also reported that Phylum Bacteroides is significantly decreased in old-women with sarcopenia, whereas genus Prevotella is increased (155). However, the relationship between the types and richness of gut microbiota in sarcopenia and lean NAFLD is not clear.