Insulin Resistance

Insulin resistance is a physiological state in which a given concentration of insulin produces a smaller-than-expected biological response. It principally affects the hormone’s regulation of glucose uptake, glucose production, and fat breakdown in skeletal muscle, the liver, and adipose tissue. Insulin resistance can involve reduced sensitivity, reduced maximal responsiveness, or both, and its extent may differ among tissues and metabolic pathways. (ncbi.nlm.nih.gov)

The pancreas can initially compensate by secreting more insulin, allowing blood glucose to remain within its normal range. When this compensation becomes insufficient, elevated glucose concentrations may develop. Insulin resistance is therefore an important contributor to type 2 diabetes, but it is not synonymous with diabetes or prediabetes. It is also associated with obesity, metabolic syndrome, abnormal blood lipid concentrations, and fatty liver disease. Common forms are multifactorial, whereas rare severe forms can result from inherited defects or acquired disorders of insulin receptors and adipose tissue. (niddk.nih.gov)

Understanding insulin resistance requires consideration of hormone signaling, interactions among organs, pancreatic compensation, and the methods used to measure insulin action. Its study connects molecular physiology with clinical medicine and population health; research measurements of insulin sensitivity must be distinguished from the blood glucose tests used to diagnose prediabetes and diabetes. (ncbi.nlm.nih.gov)

Definition and Physiological Boundaries

Insulin resistance denotes a reduced biological response to a given concentration of insulin. In metabolic physiology, the relevant responses include stimulation of glucose uptake and utilization, suppression of glucose production by the liver, and inhibition of fat breakdown in adipose tissue. Resistance therefore concerns the relationship between hormone exposure and tissue response, not simply the blood glucose concentration. Its dose–response curve has two distinguishable properties: sensitivity, commonly expressed as the insulin concentration producing half the maximal effect, or $EC_{50}$, and maximal responsiveness, the greatest effect achievable. A rightward shift increases $EC_{50}$: more insulin is required for the same response, but the maximum may remain intact. A reduced maximum means that even high insulin concentrations cannot restore the original response. Either alteration, or both together, can constitute insulin resistance; measurement at one insulin concentration cannot fully distinguish them. (ncbi.nlm.nih.gov)

Insulin sensitivity is specific to the tissue and biological endpoint examined. Skeletal-muscle glucose uptake, hepatic glucose production, and adipose fat breakdown need not become resistant to the same degree. Even in healthy subjects, different metabolic actions have different concentration–response relationships. In a 1981 study, Robert A. Rizza, Lawrence J. Mandarino, and John E. Gerich administered graded insulin infusions to 15 healthy participants. The insulin concentration associated with half-maximal suppression of glucose production was approximately $29 \pm 2\ \mu\mathrm{U}/\mathrm{mL}$, compared with approximately $55 \pm 7\ \mu\mathrm{U}/\mathrm{mL}$ for half-maximal glucose utilization. These experimental values illustrate endpoint-dependent sensitivity; they are not diagnostic thresholds. Consequently, a whole-body glucose-disposal measurement cannot establish the sensitivity of every organ or every action of insulin. (experts.arizona.edu)

Resistance can also be pathway-selective within a tissue: impairment of one insulin-regulated process does not necessarily imply loss of all insulin signaling. For example, a 2011 study using cultured adipocytes exposed to chronic high insulin concentrations found impaired movement of the GLUT4 glucose transporter to the cell membrane, while insulin-induced exclusion of the FoxO1 transcription factor from the nucleus remained preserved. This cell-model finding demonstrates that responses sharing parts of a signaling network can become uncoupled; it does not establish an identical pattern in every insulin-resistant person. Describing a tissue as insulin-resistant therefore requires identifying the action measured rather than assuming complete hormone unresponsiveness. (pmc.ncbi.nlm.nih.gov)

Insulin resistance is distinct from insulin deficiency, which concerns inadequate insulin availability rather than reduced target-tissue response. The two can coexist: pancreatic beta cells may produce substantial insulin yet still provide too little for the prevailing degree of resistance. Conversely, increased secretion can maintain normal glucose concentrations despite reduced sensitivity. Hyperinsulinemia describes an elevated circulating insulin concentration, not resistance itself. Insulin concentration depends on both secretion and clearance from the circulation. A human study published in 2016 found greater insulin secretion in insulin-sensitive participants with obesity than in similarly insulin-sensitive overweight participants, illustrating why elevated insulin alone is not an interchangeable label for impaired insulin action. (jci.org)

Prediabetes and diabetes mellitus are classifications of abnormal glucose regulation, not direct measurements of insulin sensitivity. Prediabetes denotes glycemia above the normal range but below diabetes thresholds; diabetes meets defined glycemic diagnostic criteria. Neither classification specifies a single underlying mechanism, and resistance can exist without either. Insulin sensitivity is instead a quantitative physiological characteristic with a broad range across individuals. Dividing that range into “resistant” and “sensitive” categories requires an operational threshold tied to the measurement method and reference population. There is no universally applicable cutoff for fasting insulin-based surrogate indices, partly because insulin assays are not fully standardized. A glycemic diagnosis and an estimate of insulin sensitivity thus answer different questions. (niddk.nih.gov)

Normal Insulin Signaling and Metabolic Actions

Insulin binds to the insulin receptor, a receptor tyrosine kinase composed of two extracellular α subunits and two membrane-spanning β subunits linked by disulfide bonds. Binding changes receptor conformation and activates the intracellular kinase domains of the β subunits, which phosphorylate one another on tyrosine residues. This autophosphorylation increases catalytic activity and enables recruitment and phosphorylation of insulin receptor substrate proteins, particularly IRS1 and IRS2. These substrates are signaling scaffolds rather than enzymes: their phosphorylated tyrosines provide docking sites for proteins that transmit the signal into several downstream pathways. (pmc.ncbi.nlm.nih.gov)

In the IRS–PI3K–AKT pathway, phosphorylated IRS proteins recruit class I phosphoinositide 3-kinase (PI3K). PI3K converts the membrane lipid phosphatidylinositol 4,5-bisphosphate into phosphatidylinositol 3,4,5-trisphosphate, which recruits AKT, also called protein kinase B, and its activating kinase PDK1. Phosphorylation by PDK1 and mTOR complex 2 activates AKT. In skeletal muscle and adipose cells, AKT phosphorylates TBC1D4, formerly called AS160, relieving its inhibitory influence on Rab proteins that control vesicle trafficking. Vesicles containing GLUT4 then move toward, dock at, and fuse with the cell membrane. Increasing the number of surface glucose transporters raises glucose uptake; this rapid response redistributes existing transporters rather than requiring synthesis of new ones. (rupress.org)

Insulin promotes glycogen accumulation by increasing substrate availability and regulating glycogen-metabolizing enzymes. In muscle, enhanced glucose entry increases glucose-6-phosphate, which both supplies glycogen synthesis and allosterically activates glycogen synthase. AKT phosphorylates and inhibits glycogen synthase kinase 3, reducing inhibitory phosphorylation of glycogen synthase. Glycogen-associated protein phosphatase 1 also promotes synthase activation through dephosphorylation. These mechanisms cooperate: inhibition of glycogen synthase kinase 3 alone does not account for the entire response. Insulin additionally favors reduced glycogen breakdown, so storage reflects coordinated changes in synthesis and degradation rather than activation of a single enzyme. (pmc.ncbi.nlm.nih.gov)

In the liver, insulin promotes glycogen storage and suppresses hepatic glucose production through both direct signaling and indirect effects on other tissues. Hepatocyte glucose transport principally uses GLUT2 rather than insulin-responsive GLUT4, so the hepatic response depends chiefly on changes in glucose phosphorylation, metabolic enzyme activity, and gene expression. AKT phosphorylates the transcription factor FOXO1, promoting its exclusion from the nucleus and reducing transcription of genes encoding glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. This regulates the capacity for gluconeogenesis over longer timescales. Rapid suppression of glucose production also involves altered glycogen metabolism and reduced delivery of gluconeogenic substrates from adipose tissue; FOXO1-dependent transcription is therefore not the sole mechanism. (pmc.ncbi.nlm.nih.gov)

In adipose tissue, insulin suppresses lipolysis—the hydrolysis of stored triglycerides into fatty acids and glycerol—and favors lipid retention. An important signaling mechanism involves phosphodiesterase 3B, which lowers cyclic AMP and consequently reduces protein kinase A signaling to the lipolytic machinery. This restrains activation of hormone-sensitive lipase and associated lipid-droplet proteins, although the complete antilipolytic response involves additional mechanisms and is not explained by a single linear AKT pathway. GLUT4-mediated glucose uptake also supplies glycerol-3-phosphate for esterifying fatty acids into triglycerides. Insulin promotes lipid synthesis as well; in liver, signaling involving mTOR complex 1 and SREBP-1c increases expression of lipogenic enzymes. (pmc.ncbi.nlm.nih.gov)

Growth-related signaling follows a partly distinct branch. Phosphorylated SHC and IRS-associated signaling complexes recruit GRB2 and SOS, activating RAS and the RAF–MEK–ERK MAPK signaling pathway. ERK regulates cytoplasmic proteins and nuclear transcription factors involved in proliferation and differentiation, whereas IRS–PI3K–AKT is central to the acute metabolic actions described above. The distinction is functional rather than absolute: PI3K–AKT also influences protein synthesis and cell growth through mTOR complex 1, and the branches share upstream components and regulatory interactions. (pmc.ncbi.nlm.nih.gov)

Tissue-Specific Resistance and Interorgan Metabolism

In skeletal muscle, resistance to insulin reduces the uptake of circulating glucose and its storage as glycogen, particularly after carbohydrate-containing meals. Impaired recruitment of the GLUT4 glucose transporter to the cell surface limits glucose entry, while reduced activation of glycogen synthesis further restricts storage. Because muscle is a major destination for insulin-stimulated glucose disposal, these defects change how ingested carbohydrate is distributed among organs. A reduced muscle response does not necessarily produce immediate hyperglycemia: increased insulin secretion can partly compensate, maintaining glucose concentrations while altering the metabolic conditions experienced by the liver. (jci.org)

The liver contributes a different disturbance: insufficient suppression of glucose production and impaired accumulation of hepatic glycogen after meals. Hepatic glucose production combines gluconeogenesis, which makes glucose from precursors such as lactate, glycerol, and glucogenic amino acids, with glycogenolysis, which releases glucose from existing glycogen stores. These are distinct fluxes; increased glucose output cannot automatically be attributed to increased glycogen breakdown. Insulin normally promotes the transition toward net glycogen storage, while rising glucose concentrations also inhibit glycogen breakdown. Hepatic resistance can weaken this transition, allowing endogenous glucose release to continue when dietary glucose is entering the circulation. Both direct insulin action within the liver and indirect regulation through other tissues contribute to the measured hepatic response. (jci.org)

In adipose tissue, an important manifestation is inadequate suppression of lipolysis—the breakdown of stored triglycerides into fatty acids and glycerol. Continued release increases substrate delivery to the liver. Glycerol provides carbon for gluconeogenesis, whereas fatty-acid oxidation supplies energy and generates acetyl-CoA, an activator of pyruvate carboxylase, an enzyme that supports gluconeogenesis from pyruvate. Thus, adipose resistance can increase hepatic glucose production without requiring every component of liver-cell insulin signaling to be defective. In 2015, Rachel Perry and colleagues demonstrated this adipose–liver mechanism in mouse and rat experiments: insulin-mediated suppression of lipolysis lowered hepatic acetyl-CoA and pyruvate-carboxylase flux, whereas high-fat-fed rats lost these responses. These experiments establish a mechanism in the studied models rather than a universal quantitative explanation for human hepatic resistance. (pubmed.ncbi.nlm.nih.gov)

Human meal studies illustrate how muscle resistance can also redirect carbohydrate toward hepatic lipid synthesis. In a 2007 study, Kitt Falk Petersen and colleagues compared young, lean, insulin-resistant participants with insulin-sensitive controls matched for age, weight, body mass index, and activity. After two high-carbohydrate mixed meals, net muscle glycogen synthesis was approximately 60% lower in the resistant group, while hepatic de novo lipogenesis and triglyceride synthesis were each more than twice as high. Postprandial glucose concentrations were similar, but insulin concentrations were higher in the resistant participants. The findings support altered nutrient partitioning: carbohydrate that is not stored efficiently in muscle remains available for hepatic metabolism, where substrate availability and compensatory hyperinsulinemia favor lipid production. (pmc.ncbi.nlm.nih.gov)

Excess hepatic glucose production can therefore coexist with increased lipid synthesis, a pattern often termed selective hepatic insulin resistance. This description does not establish that one insulin-signaling branch is resistant while another is fully preserved. New fatty-acid synthesis must also be distinguished from esterification of pre-existing fatty acids into triglycerides. In 2015, Daniel Vatner and colleagues independently varied fatty-acid and insulin availability in rats, including animals with reduced hepatic insulin-receptor signaling. Triglyceride synthesis from supplied fatty acids depended principally on fatty-acid delivery rather than insulin concentrations or intact hepatocyte insulin signaling. Consequently, continued adipose lipolysis can simultaneously supply glycerol and fatty-acid-derived signals for glucose production and fatty acids for triglyceride assembly; reduced muscle carbohydrate storage adds a separate route toward increased hepatic de novo lipogenesis. (pubmed.ncbi.nlm.nih.gov)

Cellular Mechanisms in Common Insulin Resistance

Ectopic lipid accumulation in skeletal muscle and liver can accompany impaired insulin action, but stored triglyceride is not equivalent to a signaling inhibitor. Triglyceride in lipid droplets can sequester fatty acids away from more biologically active intermediates. The “athlete’s paradox”—high intramuscular triglyceride despite high insulin sensitivity in endurance-trained people—illustrates this distinction. In a 2007 human experiment, exercise before an overnight lipid infusion increased muscle triglyceride synthesis and reduced diacylglycerol accumulation while protecting against lipid-induced resistance. Thus, lipid partitioning and turnover, rather than total triglyceride alone, help determine metabolic effects. (jci.org)

Diacylglycerols (DAGs) can activate particular protein kinase C (PKC) isoforms that interfere with insulin signaling. Julia Szendroedi and colleagues’ 2014 study used serial muscle biopsies during lipid infusion in healthy, lean volunteers: increased muscle DAG preceded PKCθ activation, increased IRS-1 phosphorylation at serine 1101, and reduced insulin-stimulated IRS-1 and AKT2 activation. In liver, PKCε can inhibit insulin-receptor kinase activity. A 2016 experiment identified receptor threonine 1160 as a PKCε target; mice with an alanine substitution at the corresponding mouse residue, threonine 1150, were protected from high-fat-diet-induced hepatic resistance. These findings identify tissue-specific molecular routes, rather than a uniform effect of every intracellular lipid pool. (pubmed.ncbi.nlm.nih.gov)

Ceramides, a class of sphingolipids, provide another experimentally supported route to impaired signaling. Cell experiments have shown inhibition of AKT through protein phosphatase 2A–mediated dephosphorylation or PKCζ-dependent interference with AKT recruitment to the cell membrane. Experiments manipulating membrane subdomains indicate that the dominant route differs between cellular systems. Their contribution is therefore context-dependent: Szendroedi’s human lipid-infusion study detected DAG–PKCθ changes without an association between resistance and muscle ceramide concentrations. Ceramide inhibition of signaling in cultured cells does not establish that ceramide accumulation is necessary for every form of common human resistance. (pmc.ncbi.nlm.nih.gov)

Adipose inflammation adds interactions between fat cells and recruited immune cells. In 2003, Stuart Weisberg and colleagues documented increased adipose macrophage accumulation in mice and humans, estimating macrophage proportions from below 10% in lean tissue to nearly 40% in obese human samples; these were study-specific observations, not population reference values. Chemokines such as CCL2 recruit monocytes through CCR2, and CCR2 disruption reduced macrophage accumulation and improved insulin sensitivity in high-fat-fed mice. In cultured adipocytes, fatty acids activated JNK and IKKβ stress-signaling pathways and increased secretion of the cytokine TNF-α. JNK inhibition or TNF-α neutralization attenuated resistance, demonstrating how lipid exposure and inflammatory signaling can reinforce one another experimentally. (jci.org)

Stress in the endoplasmic reticulum links protein-folding disturbances to metabolic signaling. Umut Özcan and colleagues’ 2004 cell and mouse experiments connected ER stress with JNK activation, IRS-1 serine phosphorylation, and impaired insulin-receptor signaling; reduced XBP1-dependent stress-response capacity also produced resistance in mice. Oxidative stress has similarly been tested by perturbation rather than association alone. In 2006, Nicholas Houstis, Evan Rosen, and Eric Lander found increased reactive oxygen species in adipocyte models exposed to TNF-α or dexamethasone. Multiple interventions lowering these species improved insulin responsiveness, with one also improving insulin sensitivity in obese mice. These experiments support causal contributions within their models, without establishing a single initiating sequence in human disease. (pubmed.ncbi.nlm.nih.gov)

Mitochondrial changes can coexist with lipid accumulation before diabetes develops. Kitt Falk Petersen and colleagues reported in 2004 that young, lean, insulin-resistant offspring of people with type 2 diabetes had approximately 30% lower muscle mitochondrial phosphorylation rates than matched insulin-sensitive controls, alongside increased intramuscular lipid. This observational comparison supported, but did not prove, a mitochondrial defect upstream of resistance. Conversely, the acute human lipid-infusion experiments produced resistance without detectable changes in several circulating inflammatory markers or muscle ceramides. Differences between chronic adipose inflammation, acute muscle lipid exposure, and genetically modified mice therefore matter when assigning causal roles: a mechanism demonstrated in one tissue or experimental setting need not be required in another. (pubmed.ncbi.nlm.nih.gov)

Common Determinants and Physiological Variation

Common insulin resistance reflects interacting influences of adiposity, activity, inherited susceptibility, and physiological state. Obesity is associated with reduced insulin sensitivity, but body mass alone does not describe metabolically important differences in fat distribution. Visceral fat surrounds abdominal organs, whereas subcutaneous fat lies beneath the skin; individuals with similar body mass index can have different proportions of these depots. In a 2011 study of 115 overweight or moderately obese adults, Tracey McLaughlin and colleagues found greater visceral fat and less abdominal subcutaneous and thigh fat in insulin-resistant participants despite nearly identical mean body mass index. This association does not establish that visceral fat alone causes resistance; it also implicates differences in peripheral fat-storage capacity. (pubmed.ncbi.nlm.nih.gov)

Physical inactivity and advancing age are associated with insulin resistance, but their contributions are not interchangeable. Age-related differences can partly reflect changes in adiposity and habitual activity rather than chronological age itself. In a 2009 clamp study, Francesca Amati and colleagues compared 70 younger and older adults grouped as endurance-trained athletes, normal-weight sedentary participants, or obese sedentary participants. Insulin sensitivity did not differ significantly between age groups within these categories; at either age, athletes were more sensitive than normal-weight sedentary participants, who were more sensitive than obese participants. The findings demonstrate why age associations require consideration of body composition and activity rather than being interpreted as an inevitable physiological decline. (pubmed.ncbi.nlm.nih.gov)

Family history of diabetes is a recognized risk marker, while common insulin resistance usually has multifactorial rather than single-gene inheritance. Environmental and behavioral exposures—including inactivity, smoking, and medication use—coexist with inherited susceptibility. (niddk.nih.gov) A genomic analysis by Luca Lotta and colleagues, published online in 2016 and in Nature Genetics in 2017, identified 53 genomic regions associated with a pattern of higher fasting insulin and triglycerides and lower HDL cholesterol. Collectively, the associated variants also tracked with lower peripheral fat mass, supporting limited subcutaneous storage capacity as one route to insulin resistance without greater overall adiposity. These common variants have individually small effects and are distinct from the rare severe genetic syndromes described separately. (pmc.ncbi.nlm.nih.gov)

Puberty produces a normally transient reduction in insulin sensitivity. Antoinette Moran and colleagues’ 1999 clamp study of 357 children aged 10–14 found that resistance increased at Tanner stage II, peaked at stage III, and approached prepubertal levels by stage V. Adiposity was independently associated with resistance but did not fully explain the pubertal change. (pubmed.ncbi.nlm.nih.gov) Pregnancy likewise produces substantial physiological variation: in Patrick Catalano and colleagues’ 1991 longitudinal study of six nonobese women, insulin sensitivity fell by 56% from before conception through 36 weeks’ gestation, while measured insulin release increased approximately three- to three-and-a-half-fold. (pubmed.ncbi.nlm.nih.gov) Sensitivity generally improves postpartum, but pre-existing resistance can remain; thus gestational adaptation is not equivalent to persistent pathological resistance, and gestational diabetes reflects inadequate compensation for the increased metabolic demand. (onlinelibrary.wiley.com)

Secondary resistance occurs with glucocorticoid excess, including Cushing syndrome, and with acromegaly, characterized by chronic growth-hormone excess. Human studies in acromegaly demonstrate impaired muscle glucose uptake and nonoxidative glucose metabolism even in patients with normal glucose tolerance. (niddk.nih.gov) Exogenous glucocorticoids can also impair insulin action. In a 2011 randomized study of 32 healthy men, two weeks of prednisolone reduced insulin-mediated suppression of endogenous glucose production at both tested doses; the higher dose additionally reduced peripheral glucose uptake by $34 \pm 6\%$. (pubmed.ncbi.nlm.nih.gov)

Medication effects differ within drug classes and need not depend entirely on weight gain. A 2013 controlled study administering olanzapine, aripiprazole, or placebo for nine days to healthy volunteers—10 per group—found reduced insulin sensitivity with both antipsychotics without weight gain, although their postprandial hormonal effects differed. (pubmed.ncbi.nlm.nih.gov) Certain antiretroviral medicines also have direct metabolic effects: a 2001 study of 10 HIV-negative men found that four weeks of indinavir reduced insulin-mediated glucose disposal without increased visceral adipose tissue. Such findings identify drug-specific effects, not a uniform property of all antipsychotic or antiretroviral medicines. (pubmed.ncbi.nlm.nih.gov)

Rare Severe Insulin Resistance Syndromes

Rare severe syndromes fall into three mechanistic groups: inherited defects of the insulin receptor or downstream signaling proteins; acquired receptor dysfunction caused by autoantibodies; and disorders of adipose-tissue development or distribution. Marked hyperinsulinemia, sometimes without initially elevated glucose, is a shared finding. Other characteristic features include acanthosis nigricans, a thickened, hyperpigmented, velvety change in skin folds, and ovarian hyperandrogenism, which can produce hirsutism and menstrual irregularity. These findings overlap across syndromes, but fat distribution, growth abnormalities, inheritance, and lipid profiles help distinguish their mechanisms. (jci.org)

Pathogenic variants in INSR, the gene encoding the insulin receptor, can impair receptor processing, insulin binding, or signal transmission. Type A insulin resistance usually becomes apparent around puberty or later, particularly in non-obese females with acanthosis nigricans, excessive androgen production, and irregular or absent menstruation. Males may have subtler manifestations and present later with diabetes. Inheritance is usually autosomal dominant, although autosomal recessive cases occur. Some heterozygous variants have dominant-negative effects: the abnormal receptor protein interferes with the function of receptor complexes containing the normal protein. Rare downstream defects provide another route to severe resistance; loss-of-function variants in AKT2, for example, have caused dominantly inherited insulin-resistant diabetes with partial loss of adipose tissue. (medlineplus.gov)

Donohue syndrome and Rabson–Mendenhall syndrome result from biallelic pathogenic INSR variants and follow autosomal recessive inheritance. They form a clinical continuum rather than two completely separate molecular disorders. Donohue syndrome occupies the most severe end, with prenatal growth restriction, profound postnatal growth failure, reduced subcutaneous fat, characteristic facial features, and organ enlargement. Extreme hyperinsulinemia can coexist with fasting hypoglycemia and postprandial hyperglycemia from birth. Rabson–Mendenhall syndrome generally permits longer survival and may include abnormal dentition, growth impairment, acanthosis nigricans, and genital enlargement. As insulin secretion becomes insufficient to overcome resistance, persistent hyperglycemia, ketoacidosis, and diabetic microvascular complications can develop. The degree of residual receptor function contributes to the differences in severity. (ncbi.nlm.nih.gov)

Type B insulin resistance is acquired rather than caused by an inherited receptor variant. Autoantibodies bind the insulin receptor and impair insulin action; depending on their functional properties, they can also stimulate receptor activity and cause hypoglycemia. The syndrome often presents in adulthood with severe hyperglycemia, weight loss, widespread acanthosis nigricans, and, in women, hyperandrogenism. It is associated with autoimmune diseases, particularly systemic lupus erythematosus and mixed connective-tissue disease. A National Institutes of Health prospective cohort reported in 2018 included 22 patients with confirmed insulin-receptor antibodies; 21 had fasting hyperglycemia at presentation, illustrating the predominant phenotype without excluding normoglycemic or hypoglycemic presentations. (jci.org)

Lipodystrophies are classified along two independent axes: generalized versus partial fat loss, and inherited versus acquired disease. Congenital generalized forms can result from recessive variants in AGPAT2 or BSCL2, whereas familial partial forms include dominantly inherited disorders involving LMNA or PPARG. Generalized disease involves near-total adipose deficiency; partial disease affects selected regions and may coexist with fat accumulation elsewhere. Acquired generalized and partial forms develop without the corresponding inherited pattern and may accompany immune abnormalities. In affected metabolic phenotypes, inadequate adipose storage capacity redirects lipid toward liver and muscle. Reduced leptin availability, particularly in generalized forms, also contributes to disturbed energy regulation. Consequences can include severe insulin resistance, diabetes, hypertriglyceridemia, and hepatic steatosis; their severity varies with subtype and extent of fat loss. (pmc.ncbi.nlm.nih.gov)

The lipid and liver phenotype is not uniform across severe syndromes. In a 2009 human study, Robert K. Semple and colleagues found that patients with receptor-level resistance caused by INSR variants or inhibitory receptor antibodies had low triglycerides and normal HDL cholesterol despite extreme hyperinsulinemia. Liver fat was normal in the studied patients with INSR variants. By contrast, two patients with an AKT2 defect had increased hepatic lipogenesis, liver fat, and metabolic dyslipidemia. Thus, the pronounced hypertriglyceridemia and fatty liver often encountered in lipodystrophy are not obligatory consequences of equally severe receptor dysfunction. (jci.org)

Pancreatic Compensation and Associated Clinical Conditions

Pancreatic beta cells can compensate for reduced tissue sensitivity by increasing insulin secretion. The resulting higher insulin exposure can maintain sufficient glucose uptake and suppression of hepatic glucose production, allowing normal blood glucose concentrations to coexist with resistance. Compensation is therefore assessed relative to insulin sensitivity, not simply by whether insulin concentrations are high. In 1993, Steven Kahn and colleagues studied 93 apparently healthy adults aged 18–44 years and found an approximately hyperbolic relationship between insulin sensitivity and several measures of beta-cell response. Where this relationship holds, the disposition index expresses their product: $DI = S_I \times \text{beta-cell response}$. A lower sensitivity requires a proportionately greater secretory response to maintain the same index; an apparently substantial insulin response can nevertheless be inadequate for the prevailing resistance. (pubmed.ncbi.nlm.nih.gov)

Impaired glucose regulation develops when beta-cell compensation is insufficient for metabolic demand. In the insulin-resistant pathway to type 2 diabetes, the important defect is thus a relative shortfall in insulin secretion, which need not initially be an absolute reduction below concentrations found in insulin-sensitive people. Glucose may rise into the prediabetic range and subsequently meet diabetes criteria, but insulin resistance does not imply inevitable progression: adequate compensation can preserve normal glucose regulation. Neither hyperinsulinemia nor normal glucose alone establishes how well beta cells are functioning, because their response must be interpreted in relation to insulin action. (pubmed.ncbi.nlm.nih.gov)

Metabolic syndrome describes a cluster of clinical abnormalities rather than a direct measurement or synonym of insulin resistance. Common definitions require at least three of five components: increased waist circumference, elevated triglycerides, reduced HDL cholesterol, elevated blood pressure, and elevated fasting glucose. Illustrative adult thresholds include blood pressure of 130/85 mmHg or higher, HDL cholesterol below 40 mg/dL in men or 50 mg/dL in women, and fasting glucose of at least 100 mg/dL; waist thresholds vary with population and definition. Insulin resistance is closely associated with this pattern of dyslipidemia and hypertension, but the syndrome can be identified without measuring insulin sensitivity. Its components collectively mark increased risk of coronary heart disease, stroke, and diabetes; their coexistence does not establish insulin resistance as the sole cause of cardiovascular disease. (nhlbi.nih.gov)

Metabolic dysfunction-associated steatotic liver disease (MASLD) links hepatic fat accumulation with cardiometabolic abnormalities. The 2024 joint EASL–EASD–EASO guidelines define MASLD by hepatic steatosis together with at least one cardiometabolic risk factor, within a classification that also distinguishes alcohol-related and other causes of steatotic liver disease. Insulin resistance is strongly associated with MASLD, and greater resistance correlates with its occurrence even among people with type 2 diabetes. Conversely, MASLD often precedes the development of diabetes and other cardiometabolic abnormalities. This relationship is not adequately represented by a single causal arrow: hepatic and systemic metabolic dysfunction coexist, while shared determinants such as visceral adiposity complicate interpretation of observational associations. (pmc.ncbi.nlm.nih.gov)

Polycystic ovary syndrome (PCOS) combines reproductive and metabolic disturbances, with insulin resistance contributing to its pathophysiology but not serving as a diagnostic requirement. Compensatory hyperinsulinemia can enhance ovarian androgen production and reduce sex hormone-binding globulin, increasing androgen availability; ovarian responsiveness to insulin may remain relatively preserved despite resistance in metabolic tissues. These interactions help connect insulin resistance with androgen excess and ovulatory dysfunction, but PCOS also involves altered gonadotropin regulation and intrinsic ovarian abnormalities rather than one established initiating cause. The 2023 international guideline recognizes increased risks of impaired fasting glucose, impaired glucose tolerance, and type 2 diabetes in PCOS regardless of age or body mass index. (pmc.ncbi.nlm.nih.gov)

Common insulin resistance is usually asymptomatic, including when increased insulin secretion maintains normal glucose. Increased thirst, frequent urination, blurred vision, and fatigue can occur when blood glucose becomes elevated; these symptoms reflect disturbed glucose regulation rather than tissue resistance itself. Their absence therefore does not demonstrate normal insulin sensitivity, while their presence does not specifically identify insulin resistance as the underlying mechanism. (niddk.nih.gov)

Measurement and Clinical Assessment

The hyperinsulinemic–euglycemic clamp is the reference method for measuring metabolic insulin sensitivity in humans. After an overnight fast, insulin is infused intravenously at a fixed rate, while a variable glucose infusion maintains blood glucose near a predetermined normal concentration. At steady state, the glucose infusion rate equals whole-body glucose disposal only if endogenous glucose production is completely suppressed. The disposal rate, commonly designated $M$, is usually expressed per kilogram of body weight or fat-free mass; lower disposal at comparable insulin concentrations indicates lower sensitivity. Protocol comparisons must account for insulin dose, achieved insulin concentration, and normalization method. Stable-isotope or radiolabeled glucose tracers quantify residual endogenous glucose production, allowing correction of disposal estimates and separate assessment of insulin-mediated suppression of hepatic glucose output. Thus, tracer-assisted clamps distinguish hepatic resistance from reduced peripheral uptake more clearly than glucose infusion alone. (pubmed.ncbi.nlm.nih.gov)

Dynamic tests estimate sensitivity from the response to a glucose challenge. In the frequently sampled intravenous glucose tolerance test, an intravenous glucose bolus is followed by repeated glucose and insulin measurements, usually over several hours; modified protocols add insulin to improve parameter estimation. Minimal-model analysis estimates an insulin sensitivity parameter, $S_I$, and glucose effectiveness, the capacity of glucose itself to promote disposal and suppress production without an incremental insulin effect. These are model-derived estimates, not direct equivalents of clamp measurements. Oral glucose tolerance test–derived indices incorporate fasting and post-load glucose and insulin. The Matsuda index, introduced by Masafumi Matsuda and Ralph DeFronzo in 1999, combines fasting values with mean concentrations during the test; its original validation reported a correlation of $r=0.73$ with clamp-measured whole-body glucose disposal. Oral indices also reflect intestinal glucose delivery and endogenous insulin secretion. (ncbi.nlm.nih.gov)

Fasting surrogates are simpler but measure neither glucose flux nor tissue responsiveness directly. David Matthews and colleagues introduced homeostasis model assessment in 1985, modeling the basal feedback relationship between insulin secretion and glucose regulation. Its original approximation for resistance is

where $I_f$ is fasting insulin in $\mu\mathrm{U}/\mathrm{mL}$ and $G_f$ is fasting glucose in $\mathrm{mmol}/\mathrm{L}$. With glucose in $\mathrm{mg}/\mathrm{dL}$, the denominator is 405. Higher values indicate greater estimated resistance. The later computerized HOMA2 model is not interchangeable with this approximation. QUICKI, introduced by Arie Katz and colleagues in 2000, uses

with insulin in $\mu\mathrm{U}/\mathrm{mL}$ and glucose in $\mathrm{mg}/\mathrm{dL}$; higher values indicate greater estimated sensitivity. Its original study reported $r=0.78$ with clamp sensitivity. (pubmed.ncbi.nlm.nih.gov)

Interpretation depends on measurement uncertainty and the population studied. Circulating insulin reflects both secretion and clearance, so fasting hyperinsulinemia is not an isolated measurement of tissue resistance; impaired beta-cell secretion can also obscure resistance. Insulin assays differ in calibration and analytical specificity. In a 2008 study, Susan Manley and colleagues tested 11 serum insulin assays in 97 participants across glucose-tolerance categories and found approximately twofold variation in HOMA estimates. There is no universally applicable HOMA-IR cutoff: thresholds depend on assay, age, adiposity, glucose tolerance, and the reference population or outcome used to define resistance. A percentile-based threshold identifies a position within a particular distribution, not a universal physiological boundary. (ncbi.nlm.nih.gov)

Clinical assessment of an insulin-resistant phenotype—such as central obesity, acanthosis nigricans, elevated triglycerides, or low HDL cholesterol—is distinct from diagnosing abnormal glucose regulation. Under American Diabetes Association criteria for nonpregnant individuals, prediabetes is defined by fasting plasma glucose of 100–125 mg/dL, glycated hemoglobin $\mathrm{HbA1c}$ of 5.7–6.4%, or two-hour glucose of 140–199 mg/dL after a 75-g oral glucose load. Corresponding diabetes thresholds are at least 126 mg/dL, 6.5%, and 200 mg/dL. Diabetes generally requires confirmatory testing unless unequivocal symptomatic hyperglycemia is present. These tests classify glycemia rather than quantify insulin sensitivity; normal results do not exclude compensated insulin resistance. (niddk.nih.gov)

Population Distribution and Epidemiological Interpretation

The prevalence of insulin resistance depends on how a continuous physiological characteristic is converted into a category. Population surveys generally use fasting surrogate indices, such as the homeostasis model assessment of insulin resistance (HOMA-IR), because clamp measurements are impractical for thousands of participants. These methods do not measure identical aspects of insulin action: fasting indices principally reflect the basal glucose–insulin relationship, whereas stimulated tests also capture responses to glucose or insulin administration. Insulin secretion, clearance, and laboratory assay differences affect surrogate values. Consequently, an epidemiological estimate requires the index, cutoff, age range, sampling frame, and inclusion criteria; a numerical threshold established in one population is not automatically transferable to another. Defining resistance as the highest quartile also classifies approximately one-quarter of the reference sample by construction, rather than identifying a universal biological boundary. (ncbi.nlm.nih.gov)

A concrete example comes from a 2021 analysis of the US National Health and Nutrition Examination Survey (NHANES). It included 6,247 adults aged 18–44 years from the 2007–2018 survey cycles, excluding participants with diabetes or preexisting cardiovascular disease, and defined insulin resistance as HOMA-IR $\geq 2.5$. Estimated prevalence was 44.8% (95% confidence interval, 42.0–47.6%) in 2007–2010 and 40.3% (36.4–44.2%) in 2015–2018. The overall trend did not meet the conventional statistical significance threshold ($P=0.07$); the numerical difference therefore did not establish a population-wide decline. These are estimates for a specified US young-adult population under a particular operational definition, not worldwide prevalence figures or estimates covering all adult ages. (pmc.ncbi.nlm.nih.gov)

Insulin-resistance prevalence is not interchangeable with that of obesity, prediabetes, metabolic syndrome, or diabetes mellitus. In the same NHANES sample, using the same age range and HOMA-IR definition, 56.6% of insulin-resistant participants had obesity, defined as body mass index $\geq 30\ \mathrm{kg/m^2}$: substantial overlap existed, but many classified as resistant did not have obesity. Prediabetes and diabetes are defined by abnormalities of glucose regulation rather than by a sensitivity threshold. Metabolic syndrome identifies a cluster of clinical abnormalities and does not necessarily require an insulin-resistance measurement. Counts of these conditions therefore describe overlapping but different populations and cannot substitute for direct estimates of resistance. (pmc.ncbi.nlm.nih.gov)

Cohort studies measure insulin-related indices at baseline and then ascertain subsequent incidence of disease, separating existing cases from new outcomes. In a 2008 Framingham Offspring Study report, Martin K. Rutter and colleagues examined 2,720 participants using fasting insulin, HOMA-IR, and a glucose-tolerance-test-derived sensitivity index; follow-up over 7–11 years identified 130 incident diabetes cases and 235 cardiovascular events. Evaluating multiple percentile thresholds allowed the investigators to compare sensitivity and specificity rather than assume a single optimal boundary. An earlier, 2005 analysis of 2,898 participants initially free of diabetes and cardiovascular disease found an age- and sex-adjusted hazard ratio of 1.9 (95% confidence interval, 1.2–2.9) for cardiovascular disease in the highest versus lowest HOMA-IR quartile. Such findings establish predictive associations, not proof that changing the index itself changes disease risk. (pubmed.ncbi.nlm.nih.gov)

Interpretation depends on confounding and the adjustment strategy. Adiposity, physical activity, smoking, socioeconomic circumstances, and related metabolic abnormalities can differ between exposure groups; a NHANES III mortality analysis explicitly adjusted for body mass index, waist-to-hip ratio, activity, education, blood pressure, lipids, and inflammatory markers. Medication exposure also matters because glucocorticoids, some antipsychotics, and some HIV medicines are associated with insulin resistance. Statistical adjustment cannot ensure that all relevant differences have been measured accurately. Cross-population comparisons therefore require compatible assays, fasting protocols, thresholds, age structures, and exclusion criteria, alongside appropriate survey weighting. Differences between demographic groups must be interpreted within these measurement and sampling contexts rather than treated as direct evidence of an intrinsic biological cause. (pmc.ncbi.nlm.nih.gov)

Intervention Research and Clinical Applications

Physical-activity experiments distinguish changes in insulin action from changes in body weight and blood glucose. In a 2020 study, 31 inactive adults with obesity completed 12 weeks of either moderate continuous exercise or high-intensity interval training while maintaining body mass. Hyperinsulinemic–euglycemic clamps showed approximately 20% greater peripheral insulin sensitivity the day after the final exercise session, despite unchanged weight and fat mass. After four days without exercise, sensitivity returned to pretraining levels, illustrating how measurement timing can separate recent-exercise effects from longer-lasting adaptations. Weight-loss studies address a different intervention: Faidon Magkos and colleagues reported in 2016 that approximately 5% diet-induced weight loss improved adipose, hepatic, and skeletal-muscle insulin sensitivity in people with obesity; further weight loss produced additional improvements in muscle sensitivity. These physiological endpoints are not equivalent to reduced diabetes incidence. (pubmed.ncbi.nlm.nih.gov)

The Diabetes Prevention Program, reported in 2002 by William C. Knowler and colleagues, was a randomized controlled trial involving 3,234 adults without diabetes but with elevated fasting and post-load glucose concentrations. Participants had a mean age of 51 years and mean body-mass index of 34.0. The intensive lifestyle intervention combined dietary modification, a target weight reduction of at least 7%, and at least 150 minutes of physical activity weekly. Over a mean 2.8 years, diabetes incidence was 11.0, 7.8, and 4.8 cases per 100 person-years in the placebo, metformin, and lifestyle groups, respectively. Relative to placebo, incidence was approximately 58% lower with lifestyle intervention and 31% lower with metformin. These figures describe prevention or delay of type 2 diabetes in a selected high-risk population—not percentages of insulin resistance reversed. The combined lifestyle intervention also did not isolate the separate contributions of exercise and weight loss. (pubmed.ncbi.nlm.nih.gov)

Insulin-sensitizing medicines differ in their tissue effects. Metformin reduces hepatic glucose production and improves glucose uptake and utilization without directly increasing insulin secretion. Thiazolidinediones, including pioglitazone, activate the nuclear receptor PPARγ, altering expression of genes involved in glucose and lipid metabolism and increasing responsiveness to insulin in peripheral tissues and the liver. Their effects are distinguishable from those of sulfonylureas, such as glimepiride, which primarily stimulate pancreatic beta-cell insulin release, and from administering exogenous insulin itself. Consequently, a reduction in glucose or glycated hemoglobin alone does not establish how much tissue sensitivity has changed. (dailymed.nlm.nih.gov)

Direct metabolic measurements can reveal drug effects that body-weight changes would not predict. In a study published in 2002, rosiglitazone treatment increased hepatic and peripheral insulin sensitivity and reduced nonesterified fatty-acid turnover in patients with type 2 diabetes, even though body weight and fat mass increased. Such findings distinguish insulin sensitivity from weight reduction as an experimental endpoint; they do not establish that every glucose-lowering medicine improves insulin action through the same mechanism. (pubmed.ncbi.nlm.nih.gov)

Disease-specific research in lipodystrophy has examined replacement of deficient leptin with the recombinant analogue metreleptin. In a 2018 nonrandomized crossover study by Rebecca J. Brown and colleagues, food intake was held constant during inpatient observation. Among 14 previously untreated participants, peripheral insulin sensitivity increased by 32% after 14 days of metreleptin; among eight previously treated participants, it decreased by 41% after withdrawal. The initiation cohort also showed improvements in hepatic sensitivity, fasting glucose, triglycerides, and liver fat. The controlled feeding design supported effects beyond reduced food intake, but the small, selected cohorts do not justify extrapolation to common obesity-associated resistance. (pubmed.ncbi.nlm.nih.gov)

For type B resistance caused by insulin-receptor autoantibodies, immune-directed treatment targets receptor dysfunction rather than ordinary multifactorial resistance. A 2018 prospective cohort study by Joanna Klubo-Gwiezdzinska and colleagues followed 22 patients treated with rituximab, cyclophosphamide, and pulsed glucocorticoids, followed by maintenance immunosuppression. Nineteen patients, or 86.4%, achieved remission after a median five months, with discontinuation of insulin and improvement in glucose regulation; three experienced recurrence during follow-up. The study used clinical remission rather than clamp-measured sensitivity as its endpoint and lacked a concurrent randomized control group. (pubmed.ncbi.nlm.nih.gov)

Development of the Concept and Research Methods

In January 1936, Harold Percival Himsworth distinguished “insulin-sensitive” from “insulin-insensitive” forms of diabetes mellitus in The Lancet. His distinction concerned differences in the blood-glucose response to administered insulin, rather than assuming that every case resulted solely from inadequate insulin supply. This established a physiological separation between deficient hormone availability and diminished responsiveness to the hormone. The terminology anticipated an important distinction underlying later classifications, but it should not be equated directly with modern type 1 and type 2 diabetes: Himsworth’s categories were based on physiological responses, not the autoimmune, genetic, and clinical criteria subsequently developed. (sciencedirect.com)

Quantitative infusion experiments subsequently made it possible to investigate insulin action separately from endogenous secretion. In 1970, Shiao-Wei Shen, Gerald Reaven, and John Farquhar described an insulin suppression method using steady infusions of insulin and glucose while pharmacologically suppressing endogenous insulin secretion. With comparable circulating insulin concentrations, a higher steady-state glucose concentration indicated greater resistance to insulin-mediated glucose uptake, termed “impedance” in their report. In 1979, Ralph DeFronzo, Jordan Tobin, and Reubin Andres published a systematic account of glucose clamp methods. The euglycemic insulin clamp reversed the experimental arrangement: insulin was elevated while glucose was held near its basal concentration through an adjustable infusion. The glucose requirement quantified insulin-supported disposal under controlled conditions; interpretation as total uptake depended on suppression of endogenous glucose production. The companion hyperglycemic clamp examined insulin secretion at a controlled elevated glucose concentration. (jci.org)

Mathematical modeling offered another route to separating interacting physiological processes. Richard Bergman, Y. Ziya Ider, Charles Bowden, and Claudio Cobelli’s 1979 “minimal model” treated measured insulin concentrations as an input and glucose disappearance after intravenous glucose administration as an output. Its initial experimental evaluation used conscious dogs, an important distinction from subsequent human applications. In 1981, Bergman, Lawrence Phillips, and Cobelli applied minimal-model analysis to intravenous glucose tolerance tests in 18 lean and obese people. The fitted insulin-sensitivity parameter, $S_I$, helped distinguish reduced insulin action from reduced pancreatic responsiveness even when both produced impaired glucose tolerance. These approaches made sensitivity a quantitative parameter of a dynamic system rather than a classification inferred from glucose concentration alone. (researchgate.net)

Reaven’s 1988 Banting lecture, published as “Role of insulin resistance in human disease,” extended attention beyond hyperglycemia. His “Syndrome X” formulation linked resistance to insulin-stimulated glucose uptake and compensatory hyperinsulinemia with glucose intolerance, elevated triglyceride-rich lipoproteins, reduced high-density lipoprotein cholesterol, and hypertension. The central proposal was that these abnormalities could cluster around impaired insulin action, including in people without diabetes, thereby connecting metabolic physiology with cardiovascular disease research. This was a mechanistic hypothesis about a cluster of abnormalities, not evidence that every component necessarily had a single cause; later metabolic-syndrome definitions were operational classifications rather than direct measurements of insulin responsiveness. (diabetesjournals.org)

Molecular biology supplied experimentally testable links between the whole-body phenotype and specific proteins. In 1985, Yousuke Ebina and colleagues reported the human insulin receptor complementary-DNA sequence, providing a structural basis for investigating transmembrane signaling. Human genetic evidence followed: in 1988, Yoshimasa and colleagues identified a point mutation in the receptor gene in a patient with type A insulin resistance. The substitution disrupted the cleavage site required to process the receptor precursor, directly connecting a molecular defect with severe impaired insulin action. Such patient-based findings established mechanisms for particular rare disorders; they did not establish receptor mutations as the usual explanation for common insulin resistance. (pubmed.ncbi.nlm.nih.gov)

Noninvasive tissue measurements also narrowed the gap between systemic physiology and intracellular metabolism. In 1990, Gerald Shulman and colleagues combined controlled glucose–insulin infusions with carbon-13 nuclear magnetic resonance spectroscopy in five people with diabetes and six age- and weight-matched healthy participants. Muscle glycogen synthesis averaged $78 \pm 28$, compared with $183 \pm 39$, micromoles of glucosyl units per kilogram of muscle per minute, with uncertainties reported as standard errors. This directly demonstrated reduced glycogen synthesis under the study’s matched hyperglycemic–hyperinsulinemic conditions. The measurement localized a metabolic impairment in living humans but did not, by itself, identify which upstream signaling or transport step caused it. (pubmed.ncbi.nlm.nih.gov)

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