Metabolic dysfunction-associated steatotic liver disease and omega-6 polyunsaturated fatty acids: Friends or foes

Abstract

BACKGROUND

Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most common chronic liver disease worldwide. Its prevalence is closely linked to the dramatic rise in obesity and non-communicable diseases. MASLD exhibits a progressive trajectory that may culminate in development of hepatic cirrhosis, thereby predisposing affected individuals to an elevated likelihood of hepatocarcinogenesis. Diet, especially dietary fatty acids, serves as a key link between nutrient intake and MASLD pathogenesis.

AIM

To explore the impact of various omega-6 fatty acid subtypes on the pathogenesis and therapeutic strategies of MASLD.

METHODS

A systematic literature search was conducted across Web of Science, PubMed, Cochrane Central, Scopus, and Embase databases from inception through June 2024 to identify all original studies linking different subtypes of omega-6 polyunsaturated fatty acids to the pathogenesis and management of MASLD. The search strategy explored the linkage between omega-6 polyunsaturated fatty acids and their subtypes, including linoleic acid (LA), gamma-linolenic acid (GLA), arachidonic acid, conjugated LA, and docosapentaenoic acid, in relation to MASLD and cardiometabolic risk.

RESULTS

By employing the specified search strategy, a total of 83 articles were identified as potentially eligible. During the title, abstract, and full-text screening phases, 27 duplicate records were removed, leaving 56 records for relevance screening. Of these, 43 records were excluded for reasons such as irrelevance and language restrictions (limited to English), resulting in 13 full-text articles being included for detailed assessment (10 human studies,1 animal study, and 2 review articles). Although certain subtypes, as GLA, dihomo-GLA, omega-6-derived oxylipins, and most arachidonic acid-derived eicosanoids, exhibit pro-inflammatory effects, our findings suggest that other subtypes such as LA, cis-9, trans-11 conjugated LA, and docosapentaenoic acid have beneficial effects on fatty liver, cardiometabolic risk factors, and inflammation, even at high intake levels.

CONCLUSION

The varying health effects of omega-6 fatty acids, ranging from anti-inflammatory to pro-inflammatory impacts on the liver, leave the question of their recommendation for MASLD patients unresolved. This underscores the importance of careful selection when considering omega-6 supplementation.

Key Words: Metabolic dysfunction-associated steatotic liver disease; Omega-6 polyunsaturated fatty acids; Conjugated linoleic acid; Arachidonic acid; Lipid metabolism; Oxidative stress; Steatohepatitis; Eicosanoids

Core Tip: Dietary habits extend beyond influencing metabolic dysfunction-associated steatotic liver disease onset, serving as a fundamental component in its therapeutic management. While fats have a deleterious role in the pathogenesis of liver steatosis and fibrosis, unsaturated fats provide a crucial safeguard in metabolic dysfunction-associated steatotic liver disease management. Omega-6 polyunsaturated fat subtypes exhibit varying pro- and anti-inflammatory effects; most arachidonic acid-derived eicosanoids, exhibit pro-inflammatory effects, while linoleic acid and natural conjugated linoleic acid do not increase inflammation, even at high doses, and may even exert beneficial effects. Therefore, further research is needed to explore dietary sources of healthy omega-6 fatty acids.

INTRODUCTION

Formerly denominated as non-alcoholic fatty liver disease (NAFLD), metabolic dysfunction-associated steatotic liver disease (MASLD) has now attained recognition as the most ubiquitous chronic hepatic pathology globally[1]. In 2022, MASLD exhibited an overall prevalence of 32.4%, with the condition continuing to escalate globally[2]. It accelerates the pathological continuum leading to hepatic cirrhosis, thereby amplifying the likelihood of hepatocarcinogenesis[3]. Its high prevalence, along with the associated hepatic and cardiovascular complications, makes MASLD a serious global health concern.

Diet is acknowledged as a leading risk factor in the development and progression of MASLD. In contrast, maintaining a healthy dietary pattern effectively curtails hepatic lipid accumulation, which in turn provides protection against cardiometabolic morbidity and mortality, irrespective of weight loss or total caloric intake[4]. Furthermore, evidence suggests that an unhealthy dietary pattern may serve as a predisposing factor for MASLD, irrespective of obesity status[5]. Various clinical practice guidelines recommend the Mediterranean diet (MedD) as the preferred dietary approach for the treatment of MASLD[6].

The term “essential elements” describes important dietary components that cannot be endogenously produced, requiring their procurement through nutrition. These substances are crucial for overall human health, and among them, two principal polyunsaturated fatty acids (PUFAs) have emerged as areas of significant research focus: Linoleic acid (LA), an omega-6 fatty acid, and alpha-linolenic acid, an omega-3 fatty acid. Both PUFAs influence metabolism and body weight, making them bioactive lipids that can serve as functional foods. Additionally, PUFAs play a role in the development of MASLD, as approximately 15% of hepatic triglycerides (TGs) are derived from dietary intake[7]. The modulation of human metabolism by PUFAs is orchestrated by oxylipins, which bind to G protein-coupled receptors or peroxisome proliferator-activated receptors to mediate these effects. In contrast to the pro-inflammatory actions of omega-6-derived oxylipins, those derived from omega-3 fatty acids are known to elicit anti-inflammatory responses[8].

Through a series of metabolic reactions, LA is metabolized into various omega-6 PUFAs. The process commences with desaturation to generate gamma-linolenic acid (GLA) (18:3n-6), continues with an elongation step resulting in dihomo-GLA (DGLA) (20:3n-6), and concludes with a final desaturation that forms arachidonic acid (AA). AA can subsequently undergo enzymatic transformations to yield additional omega-6 derivatives[9]. Substantial evidence confirms the relationship between AA-derived eicosanoids and inflammation; however, it is important to emphasize that a subset of these eicosanoids facilitates the resolution of inflammation rather than promoting it[10]. While it is a common assumption that increased dietary consumption of omega-6 PUFAs promotes inflammation, human studies have not consistently demonstrated that higher intakes of LA and AA result in elevated inflammatory markers. In fact, epidemiological evidence suggests that LA and AA may be inversely correlated with inflammation[11]. The intricate interplay between ω-6 and ω-3 PUFAs and their metabolically active lipid mediators in orchestration of inflammatory responses exhibit remarkable complexity and are yet to be fully elucidated[12]. This systematic review endeavors to delineate the contributory roles of distinct omega-6 fatty acid subtypes - a subset of PUFAs - in the etiopathogenesis of MASLD and to evaluate their prospective implications in the clinical management of this condition.

MATERIALS AND METHODS

Information sources and search strategy

A comprehensive systematic review of the literature was executed utilizing PubMed, Scopus, Cochrane Central, Web of Science, and Embase databases from inception through June 2024 to identify all original studies linking different subtypes of omega-6 PUFAs to the pathogenesis and management of MASLD. The systematic review was independently performed by four investigators (Hegazy MA, Ahmed SM, Sultan SM, and Afifi OF) using a search strategy that included the following terms: “Omega-6 and NAFLD”, “liver steatosis and omega-6”, “omega-6 and cardiometabolic risk”, “NAFLD and cardiometabolic risk”, “role of omega-6”, “omega-6 subtypes (GLA) and NAFLD”, “eicosanoids and NAFLD”, “AA and NAFLD”, and “docosapentaenoic acid (DPA) and NAFLD”. All original cohort, cross-sectional, and case-control human studies, as well as animal studies, were included, encompassing both observational and interventional designs, including clinical trials. Meta-analysis studies were excluded. Only articles published in English were considered. Additionally, a hand search was conducted in the reference lists of selected retrieved articles.

This systematic approach ensured a comprehensive evaluation of the existing literature, providing a robust foundation for understanding the impact of omega-6 PUFAs on MASLD. A meticulously designed methodological framework facilitated the identification of high-quality studies, offering valuable insights into both the prospective advantages and inherent risks of omega-6 fatty acids within the MASLD paradigm. This systematic review was rigorously executed in full compliance with the PRISMA framework, upholding stringent methodological rigor and conforming to established reporting standards.

Data extraction

Investigators employed a structured data collection instrument to systematically extract pertinent information from each study. This encompassed the study title, first author’s name, study year, publication year, country of origin, number of participants, participant demographics, and the methodologies employed to evaluate linkage between ω-6 fatty acid subtypes and MASLD with respect to pathogenesis, hepatic inflammation, disease progression, and management. Two independent investigators (Sultan SM and Afifi OF) carried out the data extraction process, with subsequent verification and quality assurance conducted by the senior investigator (Ahmed SM).

RESULTS

Utilizing the described search strategy, a total of 83 potentially eligible articles were identified, distributed across databases as follows: 26 from Scopus, 15 from PubMed, 23 from Web of Science, 8 from Embase, and 11 from Cochrane Central. During the title, abstract, and full-text screening phases, 27 duplicate records were removed, leaving 56 records for relevance screening. Of these, 43 records were excluded for reasons such as irrelevance (i.e., studies that did not specifically address the relation between MASLD and ω-6 fatty acids or those that broadly examined dietary fats without a specific focus on omega-6) and language restrictions (limited to English). This left 13 full-text articles for detailed assessment (10 human studies, 1 animal study, and 2 review articles), as shown in Figure 1.

Figure 1  PRISMA checklist.

DISCUSSION

Role of lipid metabolism in the pathogenesis of MASLD

Liver steatosis and NAFLD (MASLD) manifest when the rate of lipid synthesis surpasses that of degradation, resulting in the accumulation of lipids within hepatocytes. This accumulation, combined with the activation of inflammatory pathways, cellular stress responses, and subsequent cell death, exerts a fundamental influence on NAFLD progression[13]. A diverse array of lipid species, including cholesterol, phospholipids, oxysterols, diacylglycerols, fatty acids, and TGs, accumulate within hepatocytes.

The interrelationship between obesity, hepatic fatty acid accumulation, and insulin resistance (IR) is well established. When insulin is unable to effectively inhibit hormone-sensitive lipase-mediated lipolysis in adipose tissue, there is an augmented efflux of free fatty acids from adipose depots, ultimately leading to their enhanced hepatic uptake[14]. IR accompanied by hyperinsulinemia is linked to the hepatic accumulation of cytotoxic lipids, notably free cholesterol. This lipid deposition subsequently triggers the activation of the c-Jun N-terminal kinase signaling cascade, resulting in hepatocyte injury via lipotoxicity and thereby facilitating the development of NAFLD[15]. The overload of TGs, diglycerides, and ceramides triggers endoplasmic reticulum stress and mitochondrial dysfunction, leading to an accumulation of calcium (Ca²⁺) and reactive oxygen species. These factors exert a direct influence on genomic instability by facilitating DNA mutagenesis, driving oncogene activation, or suppressing tumor suppressor functions, thereby fostering the oncogenic progression of hepatocellular carcinoma[16].

MASLD and cardiometabolic risk: Role of omega-6 essential fatty acids

MASLD (NAFLD) is significantly correlated with a heightened occurrence and frequency of both fatal and nonfatal cardiovascular events. Furthermore, cardiovascular disease constitutes the predominant cause of mortality in individuals diagnosed with MASLD[17]. Despite the common belief that n-6 PUFAs have a pro-inflammatory effect on the cardiovascular system[8,10,12], both omega-3 and omega-6 DPA have been shown to inhibit sphingosylphosphorylcholine-induced Ca²⁺ sensitization of vascular smooth muscle contraction by suppressing the activity and translocation of Rho-kinase. Incorporating n-6 PUFAs into the dietary regimen appears promising for reducing the risk of cardiovascular diseases that involve Ca²⁺ sensitization-mediated vascular smooth muscle contraction, notably in cases of cerebral vasospasm and the coronary artery[18].

As early as 1977, orally ingested DGLA was found to induce potential antithrombotic changes in hemostatic function, thereby reducing cardiometabolic risk[19]. Additional evidence was provided in 1994 by a study that examined the impacts of dietary consumption of omega-6 essential fatty acids, GLA and LA, on parameters such as vascular prostacyclin synthesis, platelet functionality, and blood lipids[20]. The study spanned four months and included 12 hyperlipidemic patients (receiving 3 g/day) as well as 12 male Wistar rats (receiving 3 mg/kg/day). Supplementation with GLA in human subjects produced a significant 48% reduction in plasma TGs (P < 0.001) and a 22% augmentation in high-density lipoprotein cholesterol levels (P = 0.01). Omega-6 essential fatty acids further contributed to statistically significant decreases in total and low-density lipoprotein cholesterol concentrations. Parallel human and animal studies revealed that GLA effectively attenuated platelet aggregation - stimulated by low levels of epinephrine and adenosine diphospahe - while reducing serum thromboxane concentrations by 45% and prolonging bleeding time by 40% (P = 0.01). In rats, GLA intake was shown to boost vascular prostacyclin generation, as assessed through radioimmunoassay of 6-keto-prostaglandin F₁α[20]. Such effects of ω-6 essential fatty acids may underpin their potential implication in conferring cardiovascular protection and preventing atherosclerotic disease[20].

Role of MedD in MASLD management

Esteemed as one of the most health-optimizing and ecologically sustainable dietary paradigms, MedD is distinguished by its predominant reliance on plant-derived nutritional sources, including legumes, vegetables, seeds, whole grains, fruits, and nuts, with olive oil serving as the principal lipid constituent. Moreover, it integrates a low to moderate consumption of dairy products, a moderate intake of fish, and a restrained consumption of sweets and meat, particularly red ones. Moreover, it allows for moderate alcohol consumption, primarily in the form of wine, which is traditionally consumed alongside meals[21]. Research has substantiated the beneficial effects of MedD across a spectrum of conditions, including metabolic syndrome, cardiovascular diseases, and type 2 diabetes mellitus (T2DM). These conditions often coexist and share a pathophysiological relationship with MASLD.

Adherence to this dietary pattern has been associated with reduced MASLD severity and has ultimately been incorporated into MASLD management guidelines[22,23]. MedD is rich in monounsaturated fatty acids, PUFAs, and fiber, all of which have been demonstrated to exert favorable effects on lipid and glucose metabolism, thereby contributing to the amelioration of fatty liver disease[24,25]. PUFAs, particularly ω-3 fatty acids, have been documented to enhance insulin sensitivity while mitigating oxidative stress and inflammatory responses[26]. Dietary fiber has been extensively documented to confer cholesterol-lowering effects while modulating gut microbiota composition in a manner that enhances short-chain fatty acids production. These metabolic byproducts may contribute to protective health benefits against MASLD[27]. The MedD is not only nutritionally balanced but also enriched with various bioactive compounds, particularly polyphenols. Emerging evidence suggests that particularly polyphenols may facilitate the management of NAFLD by influencing mitochondrial function and lipid homeostasis, besides mitigating inflammatory and oxidative stress-related pathways[28,29].

Polyunsaturated fat “PUFAs”

PUFAs are structurally distinguished by the incorporation of multiple double bonds within their hydrocarbon backbone. Their categorization is determined by the position of the initial double bond relative to the omega methyl terminus. In this classification, omega-3 PUFAs feature a double bond at the third carbon from the omega terminus, whereas omega-6 PUFAs possess this structural feature at the sixth carbon position[30].

Dietary intake is the exclusive source of essential fatty acids, such as LA (18:2n-6) and alpha-linolenic acid (18:3n-3), which serve as primary substrates for the synthesis of omega-6 and omega-3 PUFAs, respectively (Figure 2). Upon consumption, PUFAs penetrate and incorporate into cell membranes, where they exert various effects on cellular functions. Beyond their fundamental role in preserving cell membrane fluidity, these fatty acids exert regulatory effects on multiple cellular processes. These include attenuating cytokine secretion by monocytes, modulating cellular translocation and motility, reducing the predisposition to ventricular arrhythmias, and inhibiting platelet aggregation[31].

Figure 2 Metabolism of omega-3 and omega-6 polyunsaturated fatty acids. LA: Linoleic acid; GLA: Gamma-linolenic acid; DGLA: Dihomo-gamma-linolenic acid; AA: Arachidonic acid; ALA: Alpha-linolenic acid; EPA: Eicosapentaenoic acid; DHA: Docosahexaenoic acid.

Functioning as key metabolic regulators, oxylipins facilitate the physiological effects of PUFAs by binding to peroxisome proliferator-activated receptors and G protein-coupled receptors, thereby influencing diverse metabolic pathways. In general, omega-6-derived oxylipins exhibit pro-inflammatory properties, whereas omega-3-derived oxylipins exert anti-inflammatory effects. However, oxylipins derived from AA or LA, produced through enzymatic activities of cyclooxygenase, lipoxygenase (LOX), epoxygenase, and ω/ω-1 hydroxylase, may interact with omega-3-derived oxylipins within an exceedingly complex matrix to generate beneficial physiological effects on human health[8].

LA (18:2 omega-6) and AA (20:4 omega-6) are both classified as omega-6 fatty acids. LA undergoes metabolic conversion into other omega-6 PUFAs via a multi-step enzymatic pathway. This process begins with desaturation, yielding GLA (18:3n-6), followed by an elongation step that produces DGLA (20:3n-6). A subsequent desaturation reaction results in formation of AA, which can then be further metabolized into additional omega-6 PUFA derivatives[9].

DGLA, GLA, and AA are relatively scarce in the human diet. GLA is primarily sourced from specific oils, which are commercially available as dietary supplements, including blackcurrant seed oil, borage oil (commonly referred to as starflower oil), and evening primrose oil. In contrast, AA is predominantly present in animal-derived foods, including both white and red meats (such as fish), eggs, and organ meats (e.g., kidney, liver, and brain)[9].

AA is converted into 2-series prostaglandins (PGD₂, PGE₂, PGF₂, and PGI₂) and thromboxanes (TXA₂ and TXB₂) through cyclooxygenase-2 activity, as well as 4-series leukotrienes (LTA₄, LTB₄, LTC₄, LTD₄, and LTE₄) via 5-LOX activity. The resulting lipid signaling molecules exert various pro-inflammatory effects on target tissues and cells, including bronchoconstriction, fever, pain, increased production of inflammatory cytokines such as tumor necrosis factor-α and interleukin-6, platelet aggregation, vasoconstriction, vascular permeability, leukocyte chemotaxis, and the release of reactive oxygen species by granulocytes. AA is primarily obtained from dietary sources such as duck, cured bacon, cod liver, and other meat products[32-34].

AA serves as the precursor for adrenic acid (AdA; 22:4), a long-chain omega-6 PUFA. Like other PUFAs, AdA is metabolized through cytochrome P450 enzymatic pathways, leading to the formation of epoxy fatty acids, specifically epoxydocosatrienoic acids. Functioning as lipid mediators, epoxy fatty acids contribute to various physiological benefits, such as analgesia and the mitigation of endoplasmic reticulum stress. Nevertheless, their bioavailability is compromised due to their rapid hydrolysis into dihydroxy fatty acids by the enzyme soluble epoxide hydrolase[35].

Conjugated LA (CLA) exists in several subtypes, classified based on the position of carbon double bonds. The trans-10, cis-12 (t10-c12) CLA subtype has been found to induce IR in animal models, whereas the cis-9, trans-11 (c9-t11) CLA subtype exhibits therapeutic potential for conditions such as elevated inflammation, impaired lipid metabolism, and reduced insulin sensitivity. However, excessive intake of these fatty acids may have adverse effects[36]. All omega-6 PUFA subtypes are presented in Table 1 and Figure 3.

Figure 3 Omega-6 polyunsaturated fatty acid subtypes. ELOVL: Elongation of very long chain fatty acids.

Table 1 Biochemical structure of omega-6 subtypes.

Omega family	Common name	Systematic name	Abbreviations
n-6	LA	all-cis-9,12-octadecadienoic acid	18:2n-6 or 18:2
	GLA	all-cis-6,9,12-octadecatrienoic acid	18:3n-6 or 18:3
	DGLA	all-cis-8,11,14-eicosatrienoic acid	20:3n-6 or 20:3
	AA	all-cis-5,8,11,14-eicosatetraenoic acid	20:4n-6 or 20:4
	DTA	all-cis-7,10,13,16-docosatetraenoic acid	22:4n-6 or 22:4
	Tetracosatetraenoic acid (TTA n-6)	all-cis-9,12,13 ,5,18-tetracosatetraenoic acid	24:4n-6 or 24:4
	Tetracosapentaenoic acid (TPA n-6)	all-cis-6,9,12,15,18-tetracosapentaenoic acid	24:5n-6 or 24:6
	DPA n-6	all-cis-4,7,10,13,16-docosapentaenoic acid	22:5n-6 or 22:5

LA: Linoleic acid; GLA: Gamma-linolenic acid; DGLA: Dihomo-gamma-linolenic acid; AA: Arachidonic acid; TTA _n-6: Tetracosatetraenoic acid; TPA _n-6: Tetracosapentaenoic acid; DPA _n-6: Docosapentaenoic acid.

The literature primarily identifies an imbalance between omega-6 and omega-3 fatty acids as the key pathogenic mechanism rather than the specific omega-6 subtypes. An excessive intake of omega-6 fatty acids, coupled with an elevated omega-6/omega-3 ratio - characteristic of Western dietary patterns - has been implicated in pathogenesis of multiple diseases, such as malignancies, cardiovascular disorders, and various autoimmune and inflammatory conditions[36,37]. Ultimately, a comprehensive analysis of 15 randomized controlled trials investigating the impact of dietary omega-6 PUFAs, particularly LA, concluded that none of the studies reported an increase in pro-inflammatory markers in healthy adult populations. Moreover, the current body of evidence does not provide a definitive upper limit for LA consumption, given its suggested anti-inflammatory properties in both healthy individuals and those with chronic inflammatory-related conditions[38].

Omega-6 subtypes and MASLD

Emerging research has placed significant emphasis on the interplay between omega-6 PUFA intake and the pathogenesis of MASLD. Several studies have investigated this connection, offering valuable insights into the metabolic and biochemical mechanisms involved, as shown in Table 2[39-51]. Experimental animal studies have demonstrated that excessive dietary intake of LA in rats leads to diminished hepatic lipid accumulation, concomitant with a suppression of hepatic gene expression related to fatty acid uptake and metabolism. Specifically, the expression of fatty acid transporters fatty acid transporter protein-5, fatty acid transporter protein-2, and CD36, along with lipogenic enzymes such as fatty acid synthase, stearoyl-coenzyme A desaturase-1, and acetyl-coenzyme A carboxylase, was significantly lower compared to rats fed a diet with reduced LA levels. Specifically, dietary consumption of CLA has been found to provide partial protection against liver damage by increasing adiponectin levels and reducing fat accumulation in the liver[39,52].

Table 2 Studies reporting connection between metabolic dysfunction-associated steatotic liver disease (also called non-alcoholic fatty liver disease) and omega-6 polyunsaturated fatty acids.

Ref.	Title	Number of cases (human or animal)	Key findings	Results related to MASLD
Montemayor et al[51], 2023	Dietary patterns, foods, and nutrients to ameliorate non-alcoholic fatty liver disease	Review article	The mediterranean diet, high in PUFAs, improves MASLD; sugar-free coffee may be protective; high-quality diet improves liver steatosis	Mediterranean diet rich in omega-6 helps reduce liver fat and inflammation; sugar-free coffee may also help protect against MASLD
Tian et al[50], 2023	Associations between dietary fatty acid patterns and non-alcoholic fatty liver disease in typical dietary population: A United Kingdom biobank study	93399 cases (human study)	PUFA-enriched vegetarian diet negatively associated with NAFLD; animal-source PUFA diet not significantly associated with NAFLD	Vegetarian diet high in omega-6 associated with lower NAFLD risk; animal based PUFA diet not significantly associated with NAFLD risk
Van Name et al[46], 2020	A low ω-6 to ω-3 PUFA ratio (n-6:n-3 PUFA) diet to treat fatty liver disease in obese youth	20 obese adolescents (human study)	A low n-6:n-3 PUFA ratio diet significantly reduced hepatic fat fraction, ALT, and triglycerides in obese youth with NAFLD	The study demonstrated significant improvement in hepatic steatosis and glucose metabolism in obese youth with NAFLD
Heinzer et al[49], 2022	Dietary omega-6/omega-3 ratio is not associated with gut microbiota composition and disease severity in patients with nonalcoholic fatty liver disease	101 NAFLD (human study)	An increased n-6/n-3 ratio in the diet of NAFLD patients is not associated with gut microbiota composition and disease severity	The associations between the dietary n-6/n-3 ratio, the gut bacterial composition, and the disease severity of NAFLD remained unclear
Banaszczak et al[44], 2020	5-lipoxygenase derivatives as serum biomarkers of a successful dietary intervention in patients with non-alcoholic fatty liver disease	68 cases (human study)	Reduction of oxidized omega-6 metabolites serum levels is associated with successful weight reduction and reduced liver steatosis	Reduction in body mass by more than 7% significantly improved steatosis stage, waist circumference, fatty liver index, triglycerides, and cholesterol
Maciejewska et al[41], 2015	Fatty acid changes help to better understand regression of nonalcoholic fatty liver disease	35 Caucasian individuals with steatosis (human study)	With dietary intervention, liver steatosis reduction is associated with changes in fatty acid profiles	EPA and DHA compete with AA and gamma-linolenic acid for cyclooxygenases and lipoxygenases; ameliorate omega 6 oxidation and hence MASLD
Hua et al[42], 2017	Alternation of plasma fatty acids composition and desaturase activities in children with liver steatosis	111 school children (human study)	Children with liver steatosis were highly associated with obesity and insulin resistance	Children with high-grade liver steatosis exhibited higher proportions of dihomo-gamma-linolenic acid (C20: 3n-6), adrenic acid (C22: 4n-6), and docosapentaenoic acid (C22: 5n-6)
Santoro et al[40], 2013	Oxidized metabolites of linoleic acid as biomarkers of liver injury in nonalcoholic steatohepatitis	Review article	The link between oxidative stress and increased production of reactive oxygen species in the liver to oxidation of n-6 polyunsaturated fatty acids and production of specific lipid oxidation metabolites	PNPLA3 plays a role in remodeling TAG in lipid droplets, as they accumulate in response to food intake, and PNPLA3 gene (rs738409) plays its role in the development of fatty liver possibly by interacting with the dietary intake of n-6:n-3 PUFA ratio, dietary intake of n-6:n-3 PUFA ratio
Kaikkonen et al[47], 2021	Associations of serum fatty acid proportions with obesity, insulin resistance, blood pressure, and fatty liver: The cardiovascular risk in young finns study	3596 cases participated in 1980, and follow-up examination carried out 2883 in 2001 (human study)	GLA was positively associated with obesity	The GLA percentage displayed consistent positive outcome association
Mäkelä et al[48], 2022	Associations of serum n-3 and n-6 polyunsaturated fatty acids with prevalence and incidence of non-alcoholic fatty liver disease	2682 males, 920 females (human study)	FLI is a mathematic formula based on BMI, waist circumference, and serum triglyceride and gamma-glutamyl-transferase concentrations for predicting the presence of liver fat	When the n-6 PUFAs were investigated individually, higher LA and AA concentrations were associated with a lower FLI and lower odds for hepatic steatosis, whereas higher GLA and DGLA concentrations were associated with higher FLI
Pertiwi et al[45], 2020	Associations of linoleic acid with markers of glucose metabolism and liver function in South African adults	633 black South Africans (human study)	Dietary and circulating LA were inversely associated with markers of impaired liver function	There is inverse relation between circulating LA and T2D risk
Nagao et al[39], 2005	Dietary conjugated linoleic acid alleviates nonalcoholic fatty liver disease in Zucker (fa/fa) rats	2 groups of male Zucker rats (n = 6) (animal study)	Dietary CLA enhanced fatty acid B-oxidation not only in the liver but also in other tissues in obese rats	CLA alleviates hepatomegaly and triglyceride accumulation in NAFLD rats
Hegazy et al[43], 2019	Diabetes mellitus, nonalcoholic fatty liver disease, and conjugated linoleic acid (omega 6): What is the link?	50 type 2 Egyptian diabetic patients controlled on oral hypoglycemic drugs together with 20 age- and sex-matched healthy participants (human study)	Serum CLA levels were lower in NAFLD patients and associated with insulin resistance and obesity	Low serum CLA levels correlated with advanced NAFLD grades

PUFA: Polyunsaturated fatty acid; MASLD: Metabolic dysfunction-associated steatotic liver disease; NAFLD: Non-alcoholic fatty liver disease; EPA: Eicosapentaenoic acid; DHA: Docosahexaenoic acid; AA: Arachidonic acid; LA: Linoleic acid; GLA: Gamma-linolenic acid; BMI: Body mass index; CLA: Conjugated linoleic acid; T2D: Type 2 diabetes; FLI: Fatty liver index; TAG: Triacylglycerol.

Findings from a longitudinal analysis indicated that individuals with higher serum concentrations of total omega-6 PUFAs, especially LA, demonstrated significantly lower fatty liver index scores and a reduced risk of hepatic steatosis. However, elevated serum GLA levels were correlated with increased fatty liver index scores and a greater odds of hepatic fat accumulation. The associations with other PUFAs were largely weak and statistically nonsignificant, whereas long-chain omega-3 PUFAs exhibited an inverse association with hepatic steatosis. These findings are in concordance with the known cardiometabolic benefits of LA, highlighting the potential role of LA-rich dietary sources - including nuts, seeds, and vegetable oils - in the prevention of NAFLD. Additionally, LA has been reported to provide metabolic benefits in T2DM and to lower LDL cholesterol, total cholesterol, and biomarkers indicative of hepatic dysfunction[45].

Not only has LA been implicated in exerting protective effects against NAFLD, but past studies, including our own, have further established the beneficial influence of CLA in this context. In an animal study, dietary consumption of CLA provided partial protection against liver damage by increasing adiponectin levels, reducing liver enlargement, and decreasing fat accumulation in the liver[37]. In humans, we found that naturally occurring trans fats, specifically c9-t11 CLA, have been suggested to function as bioactive compounds with regulatory effects on energy metabolism and anti-inflammatory properties. In contrast, t10-c12 CLA has been implicated in fat accumulation in the liver by increasing hepatic fatty acid synthase mRNA levels, thereby contributing to the development and progression of IR. CLA, a naturally occurring trans fat, is predominantly found in ruminant-derived food sources, including dairy and meat from cows, sheep, and goats. CLA content in these products is highly reliant on animal’s diet, with grass-fed cattle exhibiting 300%-500% higher CLA levels - primarily in the cis-9, trans-12 form - compared to grain-fed animals. In contrast, industrially synthesized trans fats, commonly present in chemically treated vegetable oils such as sunflower oil, have been linked to negative health outcomes. Additionally, CLA supplements predominantly feature the t10-c12 isomer. Therefore, the composition of CLA, whether a mixture or highly purified isomers, as well as its source, whether from dietary intake such as or artificial supplementation, appears to be critical[43].

In contrast, pediatric patients with advanced liver steatosis displayed elevated levels of omega-6 PUFAs, including DGLA (C20:3n-6) and AdA (C22:4n-6), accompanied by a diminished proportion of the omega-3 PUFA eicosapentaenoic acid (C20:5n-3)[42]. A dietary regimen characterized by a low n-6:n-3 PUFA ratio has been shown to markedly decrease hepatic fat accumulation, lower alanine aminotransferase concentrations, and reduce TGs levels. Additionally, dietary adjustment of this ratio decreases the levels of oxidized LA metabolites[43].

While erythrocyte GLA has been negatively associated with metabolic syndrome, carotid artery plaque, and coronary heart disease, it has shown a positive association with fatty liver[46]. Analogously, plasma concentrations of DGLA have been linked to multiple inflammatory mediators, notably high-sensitivity C-reactive protein and cytokines. Additionally, DGLA levels correlate significantly with waist circumference, body mass index, TGs, aspartate aminotransferase, and alanine aminotransferase, establishing a strong association with both obesity and fatty liver in patients with T2DM[53].

LOXs, particularly 5-LOX, 12-LOX, and 15- LOX, catalyze the metabolism of AA and LA, leading to the formation of leukotrienes, hydroxy-eicosatetraenoic acids, and hydroxyoctadecadienoic acids. Under oxidative stress conditions, the hepatic accumulation of these oxy-lipids may exacerbate disease pathology and facilitate the progression of NAFLD[44].

This controversy can be attributed to the various types and sources of CLA. Recent studies have offered deeper insights into connection between dietary patterns and MASLD. An analysis of dietary habits and their association with MASLD was conducted using data from the United Kingdom biobank. The study found that, in contrast to eicosanoids derived from omega-6 PUFAs, LA does not induce inflammation, even when consumed in high quantities. Nonetheless, a proportion of LA is enzymatically converted in vivo into AA, a key mediator of pro-inflammatory processes. Consequently, the inflammation-inducing effects of omega-6 PUFAs, particularly AA, may partially mitigate the hepatoprotective benefits of LA in the context of MASLD[54]. In general, the dietary patterns of MASLD participants revealed that a PUFA-enriched vegetarian diet was negatively associated with MASLD progression, while animal-derived PUFAs were not significantly linked to any disease risk.

CONCLUSION

Several pro-inflammatory effects of PUFAs, particularly AA and its metabolites, contribute to liver inflammation and the progression to MASH. Maintaining a balance between PUFAs, especially omega-6 and omega-3 fatty acids, is essential for regulating anti-inflammatory and pro-resolving lipid mediators. Despite the potential risks associated with high PUFA intake, different subtypes of omega-6 PUFAs exert distinct biological effects on health. LA, c9-t11 CLA, and DPA exhibit anti-inflammatory properties, even at high doses. However, the pro-inflammatory effects of other omega-6 subtypes raise uncertainty regarding whether omega-6 should be recommended for MASLD patients. This debate remains unresolved, particularly given that most dietary sources contain a variety of omega-6 subtypes.

Recommendations

Nuts and seeds are the primary plant sources of anti-inflammatory omega-6 fatty acids. Additionally, animal-derived food sources, particularly those from grass-fed ruminants such as cows, sheep, and goats, have been shown to contain 300%-500% higher concentrations of the beneficial anti-inflammatory CLA, predominantly in the cis-9, trans-12 isomer, compared to grain-fed cattle. They should be incorporated into a balanced diet, such as MedD, which remains the most beneficial dietary pattern to follow.

However, industrial trans fats, which are primarily found in chemically processed vegetable oils such as sunflower oil and in certain nutritional supplements, are largely composed of t10-c12 isomer of CLA, a pro-inflammatory variant that warrants dietary restriction. Ongoing research is essential to identify dietary sources that are particularly rich in beneficial omega-6 subtypes. Additionally, awareness campaigns should be conducted to educate both physicians and the general population about the potential risks associated with omega-6 supplementation, particularly regarding formulations that may have adverse effects despite their perceived health benefits.