Introduction

Insulin resistance (IR) is a complex pathological condition that has become a focal point in studies related to metabolic diseases such as type 2 diabetes mellitus (T2DM), cardiovascular disease (CVD), non-alcoholic fatty liver disease (NAFLD), and polycystic ovary syndrome (PCOS).¹ IR is characterized by the unresponsiveness of cells to insulin, which has a significant impact on global public health due to increasing obesity and unhealthy lifestyles.^2,3 Lipids, such as diacylglycerol (DAG) and sphingolipids, are key mediators of impaired insulin sensitivity.⁴ DAG, for example, activates protein kinase C (PKC) which in turn inhibits the insulin signaling pathway through serine/threonine phosphorylation of insulin receptor substrate 1 and insulin receptor substrate 2 (IRS-1 and IRS-2).⁵ Sphingolipids such as ceramide cause stress on the endoplasmic reticulum (ER) and mitochondrial dysfunction, both of which contribute to IR. Ceramide inhibits the Akt signaling pathway by activating protein phosphatase 2A (PP2A), ultimately impairing glucose absorption and nutrient storage. In addition, the accumulation of ceramide in insulin-responsive tissues, such as muscle and liver, impairs insulin sensitivity and contributes to the development of obesity.⁶

As dietary patterns change, for example, the consumption of high-fat foods and carbohydrates increases, the necessity for an in-depth study of the mechanisms of lipid-IR interaction in metabolic diseases is crucial. Human studies, in the form of observational research and clinical trials, may provide important insights into the relationship between lipids and IR. Many studies in humans consistently show that increased levels of circulating free fatty acids (FFAs) and triglycerides (TGs) correlate with decreased insulin sensitivity,^7–9 leading to fat accumulation in non-adipose tissues like muscle and liver, thus further inhibiting insulin action.¹⁰ Nevertheless, to understand the molecular mechanisms underlying this relationship, animal models have become a valuable research platform.

Animal models, such as rats and mice, offer deeper insights into the impact of lipids in IR. Dietary manipulations with high-fat diet (HFD) in these models reveal complex interactions between lipids and insulin pathways, proofing that lipids directly influence insulin sensitivity, accelerate hepatic lipid deposition, and eventually lead to IR in a relatively short time.^11–15 Additionally, animal models allow researchers to investigate the long-term effects of lipid exposure on IR, which is difficult to do in human studies.

In this context, this review aims to evaluate the impact of lipids in causing IR, focusing on findings from studies in humans and animal models. The ways that lipids and IR are connected will be discussed, along with what these results mean for creating ways to prevent and treat conditions that are linked to IR. By bringing together findings on both models, it is anticipated that this review may provide deeper insights into the complexity of interactions between lipid and insulin pathways, as well as identify promising future research directions in this area.

Methods

To collect relevant information about the role of lipids in inducing IR in humans and animal models, a literature search was conducted in the PubMed database, limited to articles published from 2014 to 2024 to ensure that this review captures recent developments in understanding the impact of lipids in IR. The search was carried out using two keywords: (1) “Role of Lipids AND Insulin Resistance AND Humans”, which resulted in a total of 145 articles; (2) “Role of Lipids AND Insulin Resistance AND Animal Models”, which resulted in a total of 456 articles. Following the initial search, articles were screened based on title and abstract to exclude articles that were not directly relevant to the topic. The full texts of the remaining articles were retrieved and evaluated according to the inclusion criteria (free full-text articles written in English, studies related to lipid-induced IR, and studies directly related to the topic of the role of lipids in IR in humans and animal models). Finally, 62 articles met the criteria and were selected for review, as depicted in Figure 1. Additional searches were conducted as needed to ensure comprehensive coverage of the mechanisms underlying the relationship between lipids and IR.

Figure 1 Literature review search method.

Results

Of the 62 articles, 12 articles described studies in humans (Table 1), and 50 articles confirmed studies in animal models (Table 2).

Table 1 Relationship Between Lipids and Insulin Resistance in Humans

Table 2 Relationship Between Lipids and Insulin Resistance in Animal Models

The total participants and/or patients included in the 12 articles were 952 males (933 of those were with T2DM) and 799 females (597 of those were with T2DM). Only 6 of the 12 articles delineate ethnicity, with Chinese being the largest group in the studies. Studies that have been conducted regarding IR involve the participation of two main groups of patients, namely diabetic patients (5 studies), PCOS patients (2 studies), and others (5 studies). Of the five studies conducted on T2DM patients, various aspects related to the management of IR and changes in lipid metabolism were reported. However, the variations in ethnicity and gender diversity in human studies may contribute to the limitation of this review. Furthermore, various types of design studies used, such as RCT (randomized controlled trial), double-blind or single-blind, cross-over, and open-label or placebo-controlled, duration of the study, and the interventions used (synthetic drugs or plant-derived drugs), have also contributed to the limitations of this review.

Of the 50 articles on animal studies, only 3 included the use of extracts from plants as part of their research, including the extraction method and solvent used for extraction. In animal models, a high-fat diet is given to create an animal model of IR. The duration of HFD induction in the study ranged from 4 to 30 weeks. Various types of experimental animals were used, with various characteristics, such as varying ages and different genders, with the majority of male mice, and some were mice with special conditions such as diabetes or obesity. Certain demographic data, such as age, body weight, and sample size, are also included in the results of this review, which can provide information about the animal population used in the experiments. A total of 24 articles mentioned the use of C57BL/6J mice, which is the most widely used type of mouse. The C57BL/6J mouse strain is widely used in metabolic research as an animal model for diet-induced obesity (DIO).⁷¹

This study used standard care drugs; for example, 4 articles mentioned metformin (biguanide class, for treating T2DM),^11,12,26,27 2 articles mentioned pioglitazone (thiazolidinedione class, for treating T2DM),^28,29 and 1 article each mentioned amitriptyline (tricyclic antidepressant)³⁰ and orlistat (weight loss medication).³¹ Based on the research carried out, the results show that exposure to lipids in humans and experimental animals can contribute to IR, and IR is the main trigger factor related to the development of T2DM.

Diabetes Mellitus (DM)

DM is prevalent worldwide and increasing annually. The International Diabetes Federation (IDF) ranks Indonesia 5th globally, after China, India, Pakistan, and America, for the highest number of adults (20–79 years) with DM in 2021 and 2045.⁷² According to WHO, DM is marked by increased blood glucose levels, known as hyperglycemia,⁷³ occurring when the body cannot produce or effectively use insulin.⁷⁴ Insulin, synthesized by pancreatic beta-cells, controls blood glucose levels, facilitates cell respiration, and is crucial for protein and fat metabolism.⁷²

IR, the main trigger for T2DM, occurs when insulin cannot effectively stimulate glucose uptake or when target cells (muscle, adipose tissue, liver) respond poorly to insulin.^75,76 This leads to hyperinsulinemia, and over time, pancreatic exhaustion results in elevated blood glucose and T2DM.⁷⁷ IR also contributes to complications, such as heart disease, stroke, high blood pressure, and other metabolic problems.^78–80

Obesity is a major risk factor for IR and T2DM.^81,82 Obesity is the excessive accumulation of fat due to an imbalance in food intake and expenditure and can cause various health problems.⁷⁷ The main cause of obesity is an unhealthy lifestyle and a poor diet, such as one high in fat and sugar, followed by a lack of physical activity.^83,84 HFD can increase the number of small adipocytes, leading to fat accumulation in the liver and decreased insulin sensitivity.⁸⁵ In obese individuals, there is increased production of FFAs, reactive oxygen species (ROS), and proinflammatory cytokines, which may contribute to the development of IR.⁸⁶ An increase in FFAs concentration occurs for several reasons, such as greater FFA release due to increased adipose tissue mass, and FFA clearance may also be impaired.⁷⁷ Consequently, high concentrations of FFA inhibit the antilipolytic activity of insulin and increase the rate of FFA release into the circulation.⁸⁷ Therefore, it is important to adopt a healthy lifestyle, such as partaking in healthy foods, avoiding excessive calorie consumption, exercising regularly, and maintaining an ideal body weight.^88,89 A vegan diet, which excludes animal products, may help in reducing body weight, fat mass, and visceral fat volume, as well as improve insulin sensitivity.²¹

Metformin is a commonly used antihyperglycemic agent in the management of T2DM, working by increasing insulin sensitivity and reducing glucose production by the liver.⁹⁰ Omega-3 fatty acids, found in fatty fish, also play an important role in improving insulin function by increasing insulin sensitivity, reducing ceramide content, and increasing adiponectin expression in animal models,⁹¹ while reducing inflammation and IR.⁹²

With a better understanding of the link between DM and IR, as well as appropriate prevention efforts, it is hoped that we can reduce the burden of this disease on individuals and society as a whole. Prevention and appropriate management can help reduce the risk of serious complications and improve the quality of life for those living with diabetes.

Studies in Humans

Studies in humans reveal that lipid accumulation, particularly saturated fats, triggers the occurrence of IR.^21,22,93 These lipids, including FFAs and TGs, disrupt insulin signal transduction, produce pro-inflammatory cytokines, and oxidative stress, and inhibit intracellular signaling pathways.⁷⁷ Ceramide is also a key mediator of IR.⁹⁴ Understanding these lipids could lead to more effective prevention and treatment strategies for IR and related diseases.

Several studies highlight the impact of lipid infusions on glucose and insulin metabolism in the context of IR. Lipid infusions can significantly affect glucose and insulin metabolism, leading to IR. It increases the levels of TG and FFAs in the blood, worsening glucose tolerance and insulin sensitivity. Lipids affect pancreatic beta-cell function, causing insulin hypersecretion. However, decreased insulin clearance also contributes to hyperinsulinemia.²³ Research has identified proteins that respond to intralipid and insulin infusions in healthy women and women with PCOS. Intralipid infusion reduces the ability of insulin to stimulate glucose utilization, indicating increased IR. In addition, intralipid infusion decreased fibroblast growth factor 19 (FGF19) levels, indicating impaired regulation of glucose and lipid metabolism.¹⁹

Lipid excess can lead to the accumulation of intramyocellular lipid metabolites (IMCL), which coincides with impaired insulin response. The fat composition in IMCL, especially saturated fat, has an important role in mediating IR.²² Palmitic acid, a saturated fat found in muscle TG, is associated with IR through increased concentrations of ceramide, which interferes with insulin activation of cell receptors and downstream signaling molecules such as IRS-1 and Akt/protein kinase B (PKB).^95,96

Female sex hormones also play a role in protecting against lipid-mediated IR. Postmenopausal women, who naturally have increased levels of saturated fat, tend to accumulate more ceramides in response to high lipid loads, thereby increasing the risk of IR. This suggests that premenopausal women’s hormones may have a protective effect against ceramide accumulation and associated IR.^24,32 Understanding the lipid composition in IMCL and the influence of female sex hormones may provide new insights into the management of IR and diabetes risk.

Ceramide, a type of fat in cells, has been linked to lipid-mediated skeletal muscle IR.^97–99 Research has shown that individuals with T2DM have higher blood levels of ceramide,¹⁰⁰ which is associated with IR, insulin sensitivity, and cardiovascular risk.¹⁰¹ This is consistent with a previous study showing obese and diabetic patients have higher levels of ceramide in their muscles.¹⁰² These findings support the role of ceramide in skeletal muscle IR.

Chiglitazar and lobeglitazone show potential for managing T2DM. Chiglitazar, a PPAR agonist (peroxisome proliferator-activated receptor), activates all three PPAR types (-alpha, -gamma, -delta), controlling glucose and fat levels, decreasing TG, increasing HDL-cholesterol, and enhancing insulin sensitivity.^7,8 Lobeglitazone, a PPAR-gamma agonist, improves insulin sensitivity, reducing IR. Both drugs improve glycemic control, lipid regulation, and IR in T2DM patients, reducing disease complications.¹⁶

Natural substances can improve IR in T2DM and PCOS. Berberine, an alkaloid in Chinese herbal medicine, increases glucose consumption and absorption in cells, inhibits alpha-glucosidase, and improves insulin sensitivity.²⁵ Cranberries, rich in polyphenols such as proanthocyanidins, inhibit carbohydrate digestive enzymes, decrease intestinal glucose absorption, and modulate inflammatory biomarkers, all of which contribute to improved insulin sensitivity.¹⁷ Anthocyanins in colored fruits and vegetables improve insulin sensitivity by increasing adiponectin expression and fatty acid oxidation.¹⁸ Overall, these three natural substances have a complementary role in improving IR, demonstrating their potential as therapeutic agents in managing T2DM and related metabolic conditions.

Studies in Animal Models

Various studies show that mice treated with an HFD experience increased glucose intolerance and IR,³⁷ characterized by increased fasting blood glucose (FBG), insulin,⁵¹ total cholesterol, TG,⁶⁸ and FFA levels, as well as decreased adiponectin levels.⁵³ The underlying mechanisms involve complex biological pathways.

One of the main mechanisms is changes in insulin signaling in the liver and skeletal muscle. Signaling cascades such as IRS-1/Akt/GSK-3β (Glycogen Synthase Kinase-3 beta) have a major role in the regulation of glucose transport, glycogen synthesis, and energy metabolism.¹⁰³ Increased Ser307 phosphorylation of IRS-1 characterizes peripheral IR, inhibits Glucose Transporter Type 4 (GLUT4) translocation to the cell surface, and causes hyperglycemia.¹⁰⁴

In addition, HFD elevates the concentration of sphingolipids, such as ceramide, which interfere with the insulin signaling pathway by inhibiting insulin-induced phosphorylation of Akt/PKB in skeletal muscle and liver. This inhibition of Akt/PKB leads to the inhibition of GSK-3 α/β phosphorylation at Ser21/Ser9, resulting in impaired insulin sensitivity.^13,15

HFD also triggers low-grade inflammation in adipose tissue, which in turn leads to the recruitment of immune cells and the production of proinflammatory cytokines, leading to IR.⁵⁵ Studies in mice show that increased plasma FFA could activate muscle NF-kappaB signaling,¹⁰⁵ which eventually leads to higher levels of proinflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), IL-6, and Monocyte Chemoattractant Protein-1 (MCP-1).¹⁰⁶ This increase in plasma MCP-1 can promote the migration of monocytes from the blood to adipose tissue. When macrophages settle in adipose tissue, they differentiate into inflammatory macrophages, which can release a large number of inflammatory factors.¹⁰⁷ Additionally, HFD-induced increases in FFA also increase Toll Like Receptor-4 (TLR-4) activity, which may trigger inflammatory signaling pathways that contribute to the development of IR and atherosclerosis.⁶⁸

Overall, this review shows that HFD significantly escalates the risk of developing IR in mice. The multiple mechanisms involved highlight the complexity of the role of HFDs in the development of IR, which has significant implications for metabolic health and the development of diseases such as T2DM. Therefore, a better understanding of these mechanisms is important for the development of appropriate prevention and treatment strategies.

This review discusses the unique role of proteins such as TNFSF14 (Tumor Necrosis Factor Superfamily Member 14) and NGBR (Nogo‐B Receptor) in influencing glucose metabolism and IR. TNFSF14, specifically Peptide 7, increases insulin signaling and fatty acid oxidation in skeletal muscle cells, improving glucose management and IR in mice fed with HFD.³⁷ Overexpression of NGBR suggests that this protein functions in the improvement of insulin sensitivity and the reduction of endoplasmic reticulum stress in the liver and skeletal muscle.³⁸

Bioactive compounds have also been identified as potential treatments for DM. Polygonatum sibiricum saponin (PSS) has demonstrated its ability to reduce IR, improve pancreatic beta-cell function, and facilitate the normal biological role of insulin. The mechanism of action of PSS involves increasing Akt which triggers GLUT4, thereby helping control blood glucose and improving glucose tolerance.²⁶ Berberine, a bioactive compound from Rhizoma coptidis, shows potential in improving IR by reducing the accumulation of ceramide, which interferes with insulin signaling and activates AMP-activated protein kinase (AMPK) signaling.¹³ Cannabidiol (CBD) from Cannabis sativa lessens the accumulation of sphingolipids and exhibits anti-inflammatory and antioxidant effects that help improve the condition of IR and other diabetic complications.¹⁵ Stevioside, found in Stevia rebaudiana leaves, increases glucose uptake and oxidation in muscles by activating the IR/IRS-1/Akt/GLUT4 pathway and inhibiting oxidative stress.¹² Not only that, resveratrol and cucurbitacin E also indicate the potential to overcome IR. Resveratrol increases glucose uptake via the PI3K/Akt signaling pathway to attenuate IR,¹¹ while cucurbitacin E affects several pathways, including the inhibition of Janus Kinase/Signal Transducers and Activators of Transcription (JAK-STAT5), influences adipokines, decreases lipogenesis, activates AMPK, and regulates insulin signaling.³¹

Glucagon-like Peptide-1 (GLP-1) analogs, synthetic versions of the GLP-1 hormone, regulate glucose levels with a prolonged effect.^108,109 They increase insulin release, reduce glucagon release, and slow gastric emptying.¹¹⁰ GLP-1 analogs maintain healthy blood glucose by modulating calcium signals in hepatocytes and reducing lipid accumulation.⁴⁰ They restore insulin sensitivity by promoting GLUT4 translocation, facilitating glucose uptake, and reducing blood glucose. In HFD-fed mice, GLP-1 restored reduced phosphoinositide 3-kinase (PI3K) expression, improving the insulin signaling pathway.⁵⁸

High levels of pancreastatin can cause IR in T2DM. PST inhibitor peptide-8 (PSTi8) improves insulin sensitivity in mice with IR by increasing glucose clearance, glycogenesis, glycolysis, and gluconeogenesis. PSTi8 targets the IRS1/2-PI3K-Akt signaling pathway, making it a promising therapeutic agent for metabolic diseases, particularly DM.⁸⁰

The intricate interplay of lipid accumulation, inflammatory responses, and disrupted insulin signaling pathways underscores the multifaceted mechanisms through which an HFD-induced IR, highlighting the critical need for targeted prevention and therapeutic strategies to mitigate metabolic diseases such as T2DM.

Lipid-Induced Insulin Resistance

Adipose tissue, composed of adipocytes, can expand and store lipids through hypertrophy (enlargement of the cell size of the adipocyte) or hyperplasia (proliferation). When fat absorption increases, hypertrophic adipocytes stimulate hyperplastic adipocytes, which then attract more macrophages to enter and release inflammatory substances called adipokines. Over time, these hypertrophic adipocytes become dysfunctional and highly lipolytic, producing excessive FFA and contributing to the development of IR. This process is facilitated by the elastic properties of the tissue.¹¹¹ Excess FFA will accumulate ectopically, thereby exceeding the rate of fatty acid oxidation and intracellular storage. Ectopic lipid accumulation is the buildup of fat outside normal adipose tissue, such as in the liver and skeletal muscle, which can cause severe IR.¹¹¹ Ectopic fat produces toxic lipids, for example, ceramide and DAG.¹¹²

Lipid-induced IR may occur due to the aggregation of specific lipid mediators. Lipid mediators are lipid molecules that function as intermediaries in cellular signals.¹¹³ DAG and ceramide have been widely studied as mediators of lipid-induced IR in the liver and skeletal muscle, able to interfere with insulin signaling pathways.^114,115 DAG can accumulate in IR individuals due to an imbalance between its production and the addition of triacylglycerol (TAG) or triglyceride hydrolysis pathways. Ceramide coordinates cellular responses to cytokines or nutritional signals, such as saturated fatty acids.¹¹³

DAG is a signal intermediate that activates members of the PKC family. There are three groups in the PKC family: conventional (α, βI, β II, γ), novel (δ, ε, η, θ), and atypical (ζ and λ), and novel PKCs (nPKCs) have a greater affinity for DAG is known to mediate the role of DAG in IR.¹¹⁶ PKC-ε is mainly involved in tissues such as liver and adipose tissue.¹¹⁷ PKC-δ may also be increased in the liver of obese individuals, causing IR through a significant decrease in insulin-stimulated IRS-1 phosphorylation at Tyr612 and a decrease in Akt phosphorylation at Ser473.¹¹⁸ DAG activates PKC-θ in skeletal muscle, leading to IR through increased IRS-1 serine 1101 phosphorylation and inhibition of insulin-stimulated IRS-1 tyrosine phosphorylation and Akt2 phosphorylation.¹¹⁹ Through PKC activation, sn-1,2-DAG mediates lipid-induced insulin resistance in the liver, skeletal muscle, and white adipose tissue (WAT).^120,121

Figure 2 illustrates the mechanism of IR in skeletal muscle. Cells take up fatty acids (FA) via a specific transport protein in the cell membrane (CD36).¹²² Esterification of fatty acids with coenzyme A forms Long Chain Fatty Acid-CoA (LCFA-CoA). When there are too many FAs and/or mitochondrial problems that prevent β-oxidation from occurring, LCFA-CoA builds up and is broken down into DAG and ceramide. DAG induces sustained activation of serine/threonine kinases such as PKC-θ, which facilitates serine phosphorylation of IRS-1. The serine-phosphorylated form of IRS-1 is unable to associate with and activate PI3K, resulting in decreased PI3K and Akt2 activity. GLUT4 regulates glucose transport through cell membranes, which in turn reduces glucose intake and glycogen synthesis due to this decrease in Akt2 activity. The liver’s IR mechanism bears similarities to that of skeletal muscle. When there is more DAG in liver cells, PKC- ε gets activated. This lowers the activity of insulin receptor kinase, which in turn lowers the tyrosine phosphorylation of IRS-2 by insulin. This, in turn, lowers the activation of PI3K and Akt2. When Akt2 activation is minimal, there is a decrease in hepatic glucose uptake. Furthermore, reduced Akt2 activity leads to decreased phosphorylation of the transcription factor forkhead box O (FOXO), allowing it to enter the nucleus and activate transcription of gluconeogenesis rate-controlling enzymes. The result is an increase in liver glucose production. This allows glucose to escape via GLUT2, contributing to increased plasma glucose levels.^{111,113,123,124}

Figure 2 The mechanism of insulin resistance (IR) in skeletal muscle. Insulin induces activation of the insulin receptor, resulting in autophosphorylation and activation of the insulin receptor intrinsic tyrosine kinases and subsequently associates with phosphoinositide 3-kinase (PI3K). Activated PI3K will induce protein kinase B (Akt) which contributes a main role in glucose metabolism by regulating the translocation of the cytosolic glucose transporter type 4 (GLUT4), to the skeletal muscle membrane cell. Inhibition of PI3K and/or PKB/Akt attenuates the translocation of GLUT4. Insulin also increases LCFA uptake via a cluster of differentiation 36 (CD36), which leads to the increase of DAG and/or ceramide levels, thus activating PKC and subsequently the occurrence of IR. Created with BioRender.com. Abbreviations: LCFA-CoA, long-chain fatty acid-CoA; DAG, diacylglycerol; PKC, protein kinase C; IR, insulin receptor; IRS, insulin receptor substrate; PI3K, phosphatidylinositol 3-kinase; GLUT4, glucose transport protein 4; G3P, glycerol 3-phosphate; GPAT, glycerol-3-phosphate acyltransferase; LPA, lysophosphatidic acid; SPT, serine palmitoyl transferase; PP2A, protein phosphatase 2A.

The role of ceramide in inducing IR is still controversial. However, the accumulation of hepatic ceramides and certain dihydroceramides (eg, C16:0) is increased in insulin-resistant humans with non-alcoholic steatohepatitis (NASH) and is positively correlated with oxidative stress and hepatic inflammation.¹²⁵ Ceramide is a bioactive sphingolipid produced from intracellular fatty acids and sphingosine.^111,113 Ceramide disrupts insulin action by activating PP2A, which dephosphorylates Akt2, thereby reducing insulin signaling, but not due to issues in upstream signaling steps like IR or PI3K activation.¹¹³ Research shows that liver-specific deletion of Cers6 can reduce ceramide levels in the liver, protect against high-fat diet-induced obesity, and improve glucose tolerance.¹²⁶

In summary, lipid accumulation and the resulting metabolic disturbances in adipose tissue, liver, and skeletal muscle play an important role in inducing IR through multiple pathways, including activation of inflammatory responses, impaired insulin signaling, and alteration of mitochondrial function.

The Role of Inflammation in Insulin Resistance

Inflammation is a significant factor in the progression of IR in both humans and animals. Elevated levels of proinflammatory cytokines, such as TNF-α and IL-6, are commonly observed in obese people with IR, suggesting the role of inflammatory mechanisms in this illness.^127–130 A HFD, in particular, intricately interconnects inflammation and IR. Adipose tissue inflammation in mice that developed obesity due to an HFD begins shortly after the start of the HFD and continues for the duration of the HFD. Nevertheless, the inflammation of adipose tissue decreased promptly following the transition from HFD to a regular diet, suggesting that food significantly influences the regulation of adipose tissue inflammation.¹³¹ The mice in the HFD group had a lot more proinflammatory cytokines in their blood when they were at rest. These included TNF-α, IL-6, leptin, MCP-1, plasminogen activator inhibitor-1 (PAI-1), and resistin.¹³² Research conducted on animal models demonstrates that high-fat diets not only result in elevated levels of proinflammatory cytokines in adipose tissue but also trigger inflammation throughout the entire body. This process entails the activation of the NF-kappaB pathway, which ultimately leads to a decrease in insulin sensitivity. Chiang et al found that giving mice an HFD increased the activation of NF-kappaB. This, in turn, caused higher levels of IKKɛ in both hepatocytes and adipocytes. Furthermore, when IKKɛ was removed in mice, they were shielded from obesity and chronic inflammation caused by the HFD.¹³³

Hotamisligil’s research revealed that pro-inflammatory cytokines can cause IR.¹³⁴ When there is excess fat, adipose tissue is infiltrated by macrophages and other immune cells, which then produce pro-inflammatory cytokines such as TNF-α, leptin, IL-6, resistin, MCP-1, PAI-1, angiotensinogen, visfatin, retinol binding protein-4, and serum amyloid A. Adipose tissue also produces other substances, such as adiponectin, which decreases as body fat increases.¹³⁵ Consuming high-fat foods and obesity causes fat to accumulate in fat cells (adipocytes). This buildup triggers stress in cells and activates inflammatory signaling pathways, namely IKKβ/NF-kappaB and JNK. Proinflammatory cytokines like TNF-α and IL-1β turn on the JNK and IKKβ/NF-kappaB pathways through TNFR and IL-R receptor-mediated mechanisms. Cellular stress, such as oxidative stress and endoplasmic reticulum (ER) stress, also activates JNK and NF-kappaB. Lipid accumulation in adipocytes can increase ROS production, which in turn increases TNF-α, IL-6, and MCP-1 production and reduces adiponectin production. Saturated fats trigger an increase in ceramide synthesis, leading to its accumulation in tissues like muscle and a correlation with IR. Acute lipid infusion activates various PKC isoforms that can also activate IKKβ and NF-kappaB.^135–137

JNK and IKKβ/NF-kappaB are involved in inflammation-induced IR through different mechanisms. JNK makes IR worse by phosphorylating serine residues in IRS-1. This stops the insulin receptor/IRS-1 pathway from working normally, affecting serine-302 (pS302) and serine-307 (pS307). On the other hand, IKKβ targets IκBα for proteasomal degradation, which allows NF-κB to enter the nucleus and stimulate gene expression. In contrast to JNK, IKKβ induces IR by activating NF-kappaB via transcription, thereby increasing target gene expression and thereby increasing IR.¹³⁵ Treatments that inhibit NF-kappaB can increase IR, indicating that the NF-κB pathway plays an important role in inflammation-associated IR.¹³⁸ Furthermore, studies have demonstrated that inhibiting JNK decreases the release of proinflammatory cytokines associated with IR, including TNF- and MCP-1.^139,140

The JAK-STAT pathway, especially through IL-6, plays a part in IR by decreasing the expression of GLUT4 and IRS-1 and increasing the expression of SOCS3.^141,142 This also stops glycogen synthesis,¹⁴³ leads to the degradation of IRS-1 by proteasomes, and stops insulin signal transduction.¹³⁷ IL-6 plays an important role in regulating metabolism and immunity. Humans have shown visceral fat to be an important site for IL-6 secretion.¹⁴⁴ A very low-calorie diet and weight loss significantly reduced IL-6 levels in adipose tissue and serum, as well as improved insulin sensitivity compared to controls.¹⁴⁵ Additionally, treatment with anti-IL-6 antibodies increased insulin sensitivity in diet-induced obese mice.¹⁴⁶

IL-1β is a pro-inflammatory mediator that contributes to IR through multiple mechanisms. It binds to the IL-1R1 receptor, activating the JAK protein kinase that stimulates NF-κB translocation to the nucleus. This triggers the expression of inflammatory genes. IL-1β also reduces IRS-1 tyrosine phosphorylation and disrupts PI3K and Akt signaling.^147,148 Experimental studies in animal models indicate that IL-1β is a proinflammatory mediator that can activate various other proinflammatory cytokines and chemokines.¹⁴⁹

Obesity-induced inflammation and lipid metabolites contribute significantly to IR through multiple inflammatory and metabolic signaling pathways. Because of this, reducing systemic inflammation and targeting inflammatory signaling pathways and proinflammatory cytokines like TNF-α, IL-6, and IL-1β, may be a good way to fight IR and stop T2DM from happening.

Treatment and Prevention Strategies

It is important to identify gaps in current research regarding the role of lipids in IR. Many studies have linked increased levels of lipids such as DAG and ceramide to IR, but the specific molecular mechanisms of impaired insulin signaling by these lipids remain incompletely understood. For example, Holland et al showed that ceramide synthase inhibitors can increase insulin sensitivity in obese mice.⁹⁹ Li et al confirmed that a Cordyceps extract with myriocin can lower the buildup of ceramide, boost energy use, and improve glucose homeostasis in obese mice. This suggests that it might be useful as a supplement for treating obesity and related metabolic disorders.¹⁵⁰

Dietary intervention for IR is possible. For example, research by Kahleova et al showed that a low-fat plant-based diet significantly reduced body weight, visceral fat, IR, and intramyocellular and hepatocellular lipid levels in overweight adults in 16 weeks.¹⁵¹ Lifestyle interventions and metformin therapy can effectively prevent or delay the disease’s onset. According to research, lifestyle interventions are more effective than metformin, with one case of diabetes prevented per seven people treated over three years.¹⁵² It is also important to conduct in-depth investigations into how specific lipids, such as ceramide and DAG, interact with insulin signaling components such as PKC and PP2A, as well as their impact on mitochondria and oxidative stress.

Getting rid of systemic inflammation and targeting inflammatory signaling pathways and proinflammatory cytokines like TNF-α, IL-1β, and IL-6 may help beat IR and stop T2DM from happening. Studies have shown that anti-TNF-α antibodies can improve insulin sensitivity in peripheral tissues and increase adiponectin concentrations.¹⁵³ Treatment with shRNA can downregulate TNF-α and increase IRS-1 phosphorylation and insulin response.¹⁵⁴ IL-1β antibody treatment makes IR, blood sugar control, and β-cell function better in obese mice that were fed a high-fat diet.^155,156 Tocilizumab is a humanized monoclonal antibody that selectively blocks IL-6 signaling. This lowers HOMA-IR and IR in RA patients who are not diabetic.¹⁵⁷ These studies suggest that inhibiting TNF-α, IL-1β, and IL6 can improve IR and T2DM. In addition, salicylates, an important class of anti-inflammatory agents, have been shown to improve insulin sensitivity and glucose tolerance through inhibition of NF-kappaB and IKKβ.^158,159

Omega-3 is a type of polyunsaturated fatty acid that is known to have various health benefits, including positive effects on insulin metabolism and insulin sensitivity. In contrast to saturated fatty acids, which can increase inflammation and contribute to IR, omega-3 reveals anti-inflammatory properties that help reduce the risk of IR. Omega-3 can stimulate the GPR120 receptor, which then inhibits pro-inflammatory signaling pathways in macrophage cells.¹⁶⁰ For this process to function, GPR120 must bind to β-arrestin-2. This stops the activation of TAK1, an important kinase in inflammatory pathways.¹⁶¹ By combining findings from human and animal studies, we can gain a more comprehensive understanding of the molecular mechanisms linking inflammation to IR, as well as identify potential interventions to reduce the negative impact of this condition.

Conclusion

IR, a condition associated with various metabolic disorders, is influenced by lipid metabolism, especially induced by HFD. Furthermore, excessive FFA production may lead to decreased insulin sensitivity, progressive pancreatic beta-cell atrophy, and ectopic fat accumulation, which eventually activate inflammatory mediators in the insulin pathway. Human studies show elevated FFAs and TGs may correlate with reduced insulin sensitivity. Metformin and omega-3 fatty acids, or PPAR agonists such as chiglitazar (a novel non-thiazolidinedione structured PPAR pan-agonist) may be potential interventions to overcome these dysfunctions. Animal studies show that HFDs disrupt insulin signaling and increase inflammation, with lipid mediators like ceramides contributing to IR. Understanding lipid-induced IR interactions is critical to developing effective treatments for metabolic diseases. It is suggested to further investigate the long-term impact of dietary changes and pharmacological interventions on lipid metabolism and IR. Future research needs to focus on several specific areas to address these gaps. It will be a challenge to discover new lipid-lowering therapies, specifically those evaluating the efficacy of new compounds that target ceramide and DAG to reduce IR.