Insulin resistance and target organs

Insulin resistance is a condition in which an insulin concentration that would be sufficient to produce a given biological response in a healthy organism is no longer capable of achieving the same effect in one or more target tissues. It is therefore not a single, uniform defect, but a heterogeneous abnormality that may involve the liver, skeletal muscle, adipose tissue, endothelium, kidney, central nervous system and, secondarily, the pancreatic beta cell to different degrees. Its clinical relevance lies in the fact that insulin regulates not only blood glucose, but the entire postprandial energy state by suppressing hepatic glucose production, promoting muscular glucose uptake, limiting lipolysis, modulating lipid synthesis, influencing vascular tone and contributing to the control of satiety and energy balance.

In the pathophysiology of diabetes, particularly type 2 diabetes mellitus, insulin resistance is one of the fundamental driving forces in the transition from normoglycemia to dysglycemia. Its significance extends far beyond overt diabetes, however, because it is the biological core linking visceral obesity, hepatic steatosis, hypertriglyceridemia, reduced high-density lipoprotein cholesterol, arterial hypertension, chronic low-grade inflammation and increased cardiovascular risk. Understanding insulin resistance therefore means understanding how different metabolic organs progressively cease to respond coordinately to the insulin signal, generating a systemic disorder in which hyperglycemia is only one of the final manifestations.

Insulin resistance can be described as reduced sensitivity, potency or effectiveness of the cellular response to insulin. Physiologically, this means that higher insulin concentrations are required to achieve the same suppression of hepatic gluconeogenesis, the same glucose uptake in muscle or the same restraint of adipocyte lipolysis. At an early stage, the body compensates through hyperinsulinemia, increasing pancreatic secretion. As long as the beta cell can sustain this demand, blood glucose may remain nearly normal. Once compensation becomes insufficient, postprandial abnormalities emerge first, followed by persistent hyperglycemia.

The defect may occur at several levels of the insulin-signaling cascade. The first is transport of the hormone to tissues and its access to the interstitial compartment. The second is binding to the insulin receptor and appropriate autophosphorylation of the receptor itself. The third concerns intracellular signal transduction, involving insulin-receptor substrates, phosphatidylinositol 3-kinase, AKT (protein kinase B) and numerous metabolic effectors. The fourth involves terminal steps such as trafficking of glucose transporter type 4, modulation of gene transcription, activity of lipolytic or gluconeogenic enzymes, glycogen synthesis and substrate oxidation.

A decisive feature is that insulin resistance is not always global. In some tissues or signaling pathways, insulin action may be markedly attenuated, while other pathways remain relatively preserved. This concept of selective resistance is particularly important in the liver, where insulin may lose part of its ability to suppress glucose production while retaining or even promoting the drive toward de novo lipogenesis. The apparent result is paradoxical but pathophysiologically coherent: simultaneous hyperglycemia and hepatic fat accumulation.

To understand modern metabolic disease, the idea of a single homogeneous defect must therefore be abandoned. Insulin resistance is a set of tissue-specific, temporally progressive and biologically interconnected abnormalities involving nutrient excess, ectopic lipids, inflammation, mitochondrial dysfunction, oxidative stress, endoplasmic reticulum stress, microcirculatory abnormalities and genetic predisposition. This multiplicity of mechanisms explains why two people with the same body mass index may have very different degrees of metabolic risk.

Normal insulin signaling begins when insulin binds to its membrane receptor, a tyrosine kinase that autophosphorylates and recruits intracellular adaptor proteins. Among these, insulin-receptor substrates are central because they transmit the signal to pathways regulating glucose transport, glycogen synthesis, lipid metabolism, cell survival and gene transcription. When this sequence is intact, the body efficiently transitions from the fasting state to the fed state. Insulin resistance emerges when one or more steps in this signal-transduction process are impeded.

One of the most extensively studied mechanisms is intracellular accumulation of bioactive lipid derivatives, such as diacylglycerols and ceramides, in non-adipose tissues. These molecules are not merely inert energy stores, but true toxic signals that activate serine-threonine kinases capable of altering phosphorylation of insulin-receptor substrates. In practice, the insulin signal is diverted or interrupted before reaching terminal effectors. In muscle, this results in reduced translocation of glucose transporters to the membrane; in the liver, inadequate suppression of gluconeogenesis; and in adipose tissue, loss of insulin’s restraint of lipolysis.

This lipotoxic core is accompanied by endoplasmic-reticulum stress, production of reactive oxygen species, inflammatory activation and abnormalities of mitochondrial turnover. Pro-inflammatory signaling pathways activated by nutrient excess, cytokines and immune-cell infiltration interfere with the insulin cascade and help stabilize the resistant phenotype. This is not the acute inflammation of severe infection, but a chronic, silent and persistent inflammation closely linked to adipose-tissue dysfunction and prolonged energy overload.

Another important element is loss of metabolic flexibility, the ability of tissues to switch appropriately between lipid and carbohydrate oxidation according to nutritional state. In insulin-resistant individuals, muscle and liver tend to maintain less adaptable substrate oxidation, with inefficient glucose use after meals and persistence of metabolic fluxes typical of fasting even in the presence of elevated insulin. Insulin resistance is therefore not only a signaling defect, but also a defect in the coordination of overall cellular metabolism.

Skeletal muscle is the principal organ responsible for insulin-stimulated glucose uptake after a meal, and its insulin resistance therefore has a direct impact on postprandial hyperglycemia. Under physiological conditions, insulin promotes glucose transport through translocation of type 4 membrane transporters, increases glycogen synthesis, promotes glucose oxidation and limits preferential use of fatty acids when the carbohydrate load is high. When muscle becomes resistant, postprandial blood glucose rises more readily because a substantial proportion of circulating glucose is not removed efficiently.

At the molecular level, insulin-resistant muscle often shows accumulation of intramyocellular lipids and their bioactive intermediates, reduced activation of intracellular insulin pathways and abnormalities of mitochondrial metabolism. It is important to clarify that triglyceride itself does not explain the defect; rather, the problem is the inability to manage lipid flux correctly and prevent accumulation of species that disrupt insulin signaling. In parallel, sedentary behavior, loss of muscle mass, reduced capillarization and low cardiorespiratory fitness aggravate the problem by reducing both metabolic demand and tissue oxidative capacity.

In muscle, insulin resistance depends not only on the myocyte but also on the microcirculation. Insulin normally facilitates capillary recruitment and improves delivery of nutrients and hormone to the tissue. If endothelial function is impaired, the insulin signal is hindered even before it fully reaches the myocyte. This helps explain why insulin resistance is closely intertwined with vascular dysfunction and cardiovascular risk.

Clinically, insulin-resistant muscle produces a delayed and inefficient response to a meal. The pancreas is forced to secrete more insulin to achieve only partial glucose uptake, placing the beta cell under a progressively increasing chronic burden. Progressive loss of muscle mass, sarcopenia and intramuscular lipid infiltration further amplify this circuit, particularly in older adults, making muscle not only a victim but also a propagator of systemic metabolic deterioration.

The liver occupies a central position in insulin physiology because it determines how much glucose is produced during fasting and how much is stored or used after a meal. Under normal conditions, insulin suppresses hepatic glucose production, reduces gluconeogenesis, promotes glycogen synthesis and modulates lipid metabolism. When the liver becomes insulin resistant, this suppression is attenuated and hepatic glucose production remains inappropriately elevated, contributing especially to fasting hyperglycemia.

In the liver, the link between fat accumulation and insulin resistance is particularly close. Increased fatty-acid flux from adipose tissue, de novo lipogenesis driven by energy surplus and reduced ability to export or oxidize lipids appropriately promote hepatic steatosis. In this setting, certain intracellular lipid intermediates activate mechanisms that disrupt insulin-signal transduction. The liver thus loses the ability to switch off gluconeogenic transcriptional programs, with persistent expression of enzymes that promote release of glucose into the circulation.

A fundamental characteristic is the previously mentioned selective insulin resistance. Even when the insulin pathway that should suppress glucose production is impaired, pathways that promote fatty-acid and triglyceride synthesis may remain active. This explains why an insulin-resistant liver can simultaneously produce too much glucose and too much fat. The systemic consequence is the combination of hyperglycemia, hypertriglyceridemia, increased triglyceride-rich lipoproteins and worsening atherogenic risk.

The liver is not an isolated organ, however. It receives fatty-acid flux from adipose tissue, is affected by muscle insulin sensitivity and communicates with other organs through lipoproteins, metabolites and hepatokines. When the liver becomes metabolically dysfunctional, the entire system becomes unbalanced. Metabolic dysfunction-associated steatotic liver disease should therefore not be considered a simple comorbidity of diabetes, but one of the principal organ manifestations of systemic insulin resistance.

Adipose tissue is not a passive energy depot, but a highly active endocrine and immunometabolic organ. Under physiological conditions, insulin inhibits lipolysis, limits the release of non-esterified fatty acids into the circulation, promotes triglyceride storage and helps maintain an anti-inflammatory and metabolically efficient adipocyte profile. When the adipocyte becomes insulin resistant, one of the first defects to emerge is loss of this restraint of lipolysis. Fatty acids are released into the blood in excessive amounts and reach the liver, muscle, pancreas and other organs, where they promote lipotoxicity and further insulin resistance.

Adipose-tissue injury is not only quantitative but qualitative. With chronic adipose expansion, especially in the visceral compartment, adipocytes become hypertrophic, less well perfused, more hypoxic and more fragile. At the same time, immune-cell infiltration increases and the adipokine profile changes. Signals that favor insulin sensitivity, such as adiponectin, decrease, while pro-inflammatory mediators that amplify local and systemic metabolic disturbance increase. Adipose tissue therefore ceases to be a relatively safe storage site and becomes a continuous source of lipid flux and inflammatory signals.

Anatomical location is decisive. Although subcutaneous fat can contribute to excess weight, it generally has a more tolerable capacity for expansion. Visceral fat, by contrast, drains largely into the portal system and influences the liver more directly. When adipose tissue’s capacity to store energy surplus appropriately is exceeded, excess energy is redistributed to ectopic sites, and this redistribution is one of the key events in the pathophysiology of insulin resistance.

From this perspective, adipose-tissue insulin resistance is often the systemic trigger that precedes or fuels defects in muscle and liver. It is therefore unsurprising that many of the most effective strategies for improving insulin resistance act precisely by reducing visceral adipose mass, inappropriate lipolysis, adipose inflammation and lipid trafficking to other organs.

Although muscle, liver and adipose tissue are the pillars of glucose homeostasis, insulin resistance also involves other organs with important clinical consequences. In the central nervous system, insulin participates in controlling appetite, satiety, body weight, hepatic glucose production and some aspects of cognitive function. Reduced central insulin sensitivity may promote hyperphagia, disturbances of energy balance and impaired neuroendocrine coordination of the metabolic response. This helps stabilize the vicious cycle between excess weight and worsening peripheral insulin resistance.

In the kidney, insulin modulates complex processes including sodium handling, glomerular hemodynamics and tubular energy metabolism. In the setting of compensatory hyperinsulinemia and insulin resistance, abnormalities may develop that favor sodium retention, higher blood pressure and worsening cardiorenal risk. The kidney also contributes to gluconeogenesis and therefore participates, together with the liver, in maintaining fasting blood glucose. When systemic metabolic regulation is disrupted, this component may also contribute to dysglycemia.

The vascular endothelium is a crucial insulin target. Under physiological conditions, the hormone promotes a nitric-oxide-mediated vasodilatory response, facilitates capillary recruitment and improves distribution of blood flow to metabolically active tissues. When the endothelium becomes resistant, part of this vasoprotective effect is lost and a pro-atherogenic, pro-inflammatory and pro-hypertensive environment is favored. Insulin resistance is therefore not merely a disorder of glucose metabolism, but a systemic syndrome that affects the vascular bed early.

The myocardium is also affected by insulin resistance. A healthy heart flexibly uses fatty acids and glucose according to substrate availability and hormonal state. In the insulin-resistant heart, this flexibility decreases, metabolism tends to shift toward predominant lipid use, and energy inefficiency, lipotoxicity and functional vulnerability may develop. This helps explain why diabetes and metabolic syndrome are strongly associated with heart failure, independently of coronary atherosclerosis alone.

Insulin resistance arises from the interaction between biological predisposition and the metabolic environment. Visceral obesity is the main acquired determinant, but not all individuals with excess weight develop the same degree of insulin resistance. This depends on each person’s capacity to expand adipose tissue relatively safely, body-fat distribution, the extent of ectopic infiltration in different organs and genetic background. There is therefore a personal threshold beyond which energy surplus becomes pathogenic, and this threshold varies greatly between individuals.

The genetic component does not determine a single phenotype, but different susceptibility at individual nodes of the system. Some people tend to develop beta-cell functional defects earlier, others show a greater propensity for hepatic or muscular insulin resistance, and still others have a particularly unfavorable adipose distribution. This heterogeneity explains why type 2 diabetes and metabolic syndrome are not identical diseases in every patient, but result from the convergence of different pathophysiological phenotypes.

Sedentary behavior, sleep deprivation, circadian rhythm disruption, chronic stress, a high-calorie diet rich in ultra-processed foods, smoking, certain endocrine disorders and various medications can also worsen insulin sensitivity. In many cases, these factors do not act in isolation but add together. Reduced physical activity, for example, not only promotes weight gain but also lowers muscle’s autonomous capacity to take up glucose and decreases energy-substrate turnover, amplifying the defect independently of body weight alone.

Chronic low-grade inflammation is the common language connecting many of these influences. It should not, however, be regarded as a single autonomous cause. More often, it is the means by which nutrient excess, cellular stress, adipose injury and immunometabolic dysfunction consolidate the resistant phenotype. Insulin resistance is therefore the result of a network, not a single linear lesion.

One of the most important aspects of diabetes pathophysiology is the bidirectional relationship between insulin resistance and compensatory hyperinsulinemia. When peripheral tissues become less responsive to insulin, the beta cell increases secretion to maintain homeostasis. At an early stage, this adaptation is effective and preserves nearly normal blood glucose. This compensation has a biological cost, however, because it imposes a more intense chronic secretory workload on the beta cell, with increased proinsulin synthesis, endoplasmic-reticulum stress, greater energy demand and functional vulnerability.

Over time, the relationship may become circular. Insulin resistance induces hyperinsulinemia; persistent hyperinsulinemia may in turn contribute to receptor and post-receptor abnormalities in some tissues; meanwhile, the overloaded beta cell begins to lose qualitative and quantitative efficiency. At this point, compensation is no longer sufficient and impaired glucose tolerance appears first, followed by overt diabetes. The crucial transition is therefore not the mere presence of insulin resistance, but the point at which insulin secretion can no longer remain adequate for the degree of resistance.

This relationship also explains why not all insulin-resistant individuals develop diabetes at the same rate. A person with a more robust beta cell or lower susceptibility to beta-cell dysfunction may maintain compensated normoglycemia for a long time. By contrast, in individuals with a more fragile beta-cell reserve, genetic predisposition or concurrent injury, the same degree of insulin resistance may more rapidly translate into clinically evident hyperglycemia.

From an interpretive standpoint, insulin resistance and beta-cell deficiency should therefore not be treated as alternatives. They are two interdependent processes that reinforce one another and carry different relative weight at different disease stages and in different clinical diabetes phenotypes.

Direct measurement of insulin resistance is complex. The reference method in physiological studies is the hyperinsulinemic-euglycemic clamp, which quantifies how much glucose must be infused to maintain stable blood glucose during a controlled insulin infusion. The higher the glucose requirement, the greater the insulin sensitivity. This method is sophisticated and expensive, however, and poorly suited to routine clinical practice.

Clinical practice therefore more often relies on indirect indices derived from fasting blood glucose and insulin, dynamic tests or the overall clinical context. These estimates are useful but must be interpreted cautiously because insulin resistance is not identical across all tissues and a single index does not adequately distinguish hepatic, muscular and adipose components. Peripheral insulin levels also reflect not only pancreatic secretion but hepatic insulin extraction, which may vary among individuals.

Clinically, insulin resistance should be suspected when abdominal obesity, hypertriglyceridemia, reduced high-density lipoprotein cholesterol, elevated fasting blood glucose, hepatic steatosis, hypertension, polycystic ovary syndrome, acanthosis nigricans or a strong family history of type 2 diabetes and cardiometabolic disease coexist. None of these findings is pathognomonic, but their clustering increases the likelihood of an insulin-resistant phenotype.

The purpose of assessment is not merely descriptive. Identifying insulin resistance enables better interpretation of the risk of progression to diabetes, the likelihood of metabolic dysfunction-associated steatotic liver disease, the rationale for specific therapeutic strategies and the need for early intervention on body weight, body composition, physical activity and cardiovascular risk factors. In this sense, insulin resistance is not only a pathophysiological concept, but one of the main tools for interpreting a patient’s overall metabolic risk.

Insulin resistance is a point of convergence among many major chronic noncommunicable diseases. When it simultaneously affects the liver, muscle, adipose tissue, endothelium and heart, the result is not only hyperglycemia but a true cardiometabolic syndrome in which glycemic, lipid, blood-pressure, inflammatory and vascular abnormalities coexist. Its clinical significance therefore cannot be confined to diabetology in the narrow sense.

Its progression often follows an understandable sequence: an initial increase in insulin demand, compensatory hyperinsulinemia, adipose-tissue dysfunction, increased fatty-acid flux, hepatic steatosis, worsening muscular glucose response, beta-cell deterioration and finally overt diabetes. In parallel, atherogenicity, endothelial dysfunction, cardiorenal injury and the risk of major events become established. Disease therefore does not evolve in separate compartments, but as a single pathophysiological continuum.

This explains why effective treatment of insulin resistance has effects extending beyond glucose reduction alone. Improving insulin sensitivity means reducing the burden on the beta cell, decreasing lipid flux to ectopic organs, alleviating steatosis, improving the lipoprotein profile, lowering blood pressure and attenuating overall cardiovascular risk. Biologically, it means restoring, at least in part, appropriate communication among the main metabolic organs.

To understand modern diabetes, insulin resistance must therefore be regarded not as a simple numerical defect in the response to insulin, but as a multi-organ disorder arising from loss of metabolic coordination among tissues. Overt diabetes represents the point at which this system can no longer compensate. The pathological phase, however, begins much earlier.