Glycemic variability and alterations preceding diabetes

The relationship between glycemia and pancreatic β-cell function cannot be interpreted solely through the thresholds used to diagnose prediabetes and diabetes. In fact, glycemia does not rise suddenly the moment it exceeds a certain value; rather, it tends to change gradually as the systems responsible for its control lose efficiency. β-cells play a central role in this balance: they detect the increase in glucose and regulate insulin secretion so that hepatic production is contained and circulating glucose can be utilized by tissues. As long as this response remains adequate, glycemia can be maintained within apparently normal limits even in the presence of insulin resistance. When, however, insulin secretion is no longer proportional to metabolic needs, glycemic values begin to rise, initially slightly and subsequently until the onset of diabetes.

To correctly assess β-cell function, it is not enough to observe the absolute amount of insulin present in the blood. An insulin-resistant subject may produce much more than an insulin-sensitive person and still have an insufficient pancreatic response relative to their needs. Insulin secretion must therefore be interpreted in relation to the glycemic stimulus and insulin sensitivity. For this purpose, parameters such as the disposition index are used, which describes the pancreas's ability to compensate for insulin resistance. A reduction in this index indicates that β-cells are no longer able to adequately increase secretion, even when insulinemia (the concentration of insulin present in the blood at a given moment) still appears elevated.

The “San Antonio” metabolism study clearly showed that the deterioration of β-cell function spans the entire spectrum of glucose tolerance. By analyzing normotolerant subjects, people with impaired tolerance, and patients with type 2 diabetes, researchers observed a progressive inverse relationship between two-hour glycemia and pancreatic function. The decline was already evident among individuals who were formally normoglycemic: two-hour values between 101 and 119 mg/dL were associated with lower β-cell function compared to lower values, with further worsening between 121 and 139 mg/dL. The impairment then continued into glucose intolerance and reached maximum severity in overt diabetes (Gastaldelli et al., 2004) [17].

Normal glucose tolerance, therefore, does not represent a metabolically uniform condition. Two people with glycemia below the diagnostic threshold may possess very different β-cell reserves. One may respond rapidly to the glucose stimulus, while the other may have already lost a significant portion of their compensatory capacity. Glycemia is still classified as normal, but the pancreas may be forced to work under less favorable conditions and with progressively reduced margins of adaptation.

This pathophysiological continuity has also been observed in obese youth. In 407 children and adolescents with normal glucose tolerance, the increase in two-hour glycemia from values below 100 mg/dL toward values between 120 and 139 mg/dL was accompanied by a more pronounced early glycemic response. In the first thirty minutes of the OGTT, however, insulin secretion did not increase proportionally. This resulted in a progressive reduction of the insulinogenic index (which measures the pancreas's ability to increase insulin secretion in response to rising glycemia), present even when participants were compared at the same level of insulin sensitivity. This indicated that the impairment of the β-cell response was not simply a consequence of insulin resistance, but a functional alteration recognizable already within normoglycemia (Yeckel et al., 2005) [18].

The early phase of insulin secretion is particularly important because it limits the rise in glycemia immediately after glucose enters the circulation. A rapid release of insulin helps suppress hepatic glucose production and prepares peripheral tissues to utilize the glucose coming from the meal. When this response becomes weak or delayed, glycemia rises more rapidly. The pancreas may attempt to compensate through more prolonged secretion, but the delayed response does not always succeed in completely correcting the initial excursion.

The relationship between fasting glycemia and pancreatic function confirms this phenomenon. In 531 non-diabetic Japanese and Mexican-American subjects, insulin secretion was assessed via C-peptide (an indicator of pancreatic insulin production), reducing the influence exerted by hepatic insulin clearance. In participants with normal glucose tolerance, the disposition index decreased exponentially with the increase in fasting glycemia between 70 and 125 mg/dL. Each 28 mg/dL increment was associated with a 63% reduction in β-cell function corrected for insulin resistance. The deterioration primarily affected the secretion of the first thirty minutes, while the late phase did not show a significant relationship with basal glycemia (Abdul-Ghani et al., 2008) [19].

Even apparently modest glycemic variations can therefore reflect much broader functional alterations. Glycemia represents the final result of the interaction between hepatic production, peripheral utilization, insulin secretion, insulin sensitivity, and other regulatory mechanisms. When it begins to rise, even while remaining in the non-diabetic range, it may indicate that a portion of the pancreatic reserve has already been lost.

A study of 527 subjects with normoglycemia and normal glucose tolerance documented a reduction in β-cell function of up to approximately 40% in a portion of the participants. The impairment was associated with age, body weight, BMI (a parameter that relates weight and height), visceral fat, waist circumference, triglycerides, and cholesterol. Glycemia during the OGTT was also inversely correlated with pancreatic function. The product of fasting glycemia and one-hour glycemia showed the best ability to identify β-cell dysfunction, while one-hour glycemia above 110 mg/dL was associated with an almost sevenfold greater probability of presenting functional impairment (Vilchis-Flores et al., 2019) [20].

The vulnerability of β-cells thus fits into a context in which age, adiposity, lipid profile, and nutritional load interact. The accumulation of visceral fat increases insulin demand and is accompanied by a less favorable metabolic environment, while excessive energy availability can subject the pancreas to persistent stimulation. These observational data do not necessarily prove a direct causal relationship, but they show that the loss of pancreatic efficiency can be present long before diagnosis.

The weight of β-cell dysfunction appears particularly evident in populations of Indian Asian origin, in whom diabetes can appear at a young age and with relatively contained levels of adiposity. In a study of 1,264 subjects without known diabetes, the oral disposition index was reduced by 38% in isolated fasting hyperglycemia and by 32% in isolated glucose intolerance compared to normal tolerance. Variations in insulin resistance were, however, more modest. Even after correction for age, sex, BMI, waist circumference, and family history, β-cell function remained the parameter most associated with mild dysglycemia (Staimez et al., 2013) [21].

Similar results were obtained in young Indian Asians with prediabetes or early-onset type 2 diabetes. In diabetic patients, the oral disposition index was almost ten times lower compared to normotolerant controls. In prediabetes, both reduced insulin sensitivity and pancreatic impairment contributed; in overt diabetes, however, only β-cell function maintained a significant independent association (Mohan et al., 2013) [22].

Insulin resistance naturally retains an important role, as it increases the amount of insulin required to obtain the same biological response and therefore the work required of the pancreas. However, it does not inevitably lead to hyperglycemia. Many people remain normoglycemic because their β-cells manage to adequately increase secretion. Progression toward diabetes depends to a large extent on the ability to sustain such compensation over time.

This interpretation was confirmed in a longitudinal study of 302 normoglycemic women three months after childbirth. Over the following five years, 89 developed prediabetes or diabetes. These women initially presented with greater adiposity, insulin resistance, and already reduced pancreatic function, but the main predictor of progression was the continuous deterioration of the ISSI-2, an index that integrates insulin secretion and sensitivity. Its predictive contribution was much higher than that of HOMA-IR, the Matsuda index, BMI, and waist circumference (Retnakaran et al., 2023) [23].

The debate remains open regarding the initial order of events. The traditional model considers hyperinsulinemia a compensatory response to insulin resistance, followed by the progressive failure of β-cells. An alternative hypothesis proposes instead that excessive insulin secretion may initially contribute to the development of resistance. By examining clinical, genetic, experimental, and pharmacological studies, a review concluded that the available evidence predominantly supports the presence of early β-cell dysfunction as the main determinant of dysglycemia, while acknowledging possible heterogeneity among individuals and clinical subgroups (Esser et al., 2020) [24].

Overall, the rise in glycemia becomes evident when insulin secretion is no longer adequate for metabolic demand. Glycemia can therefore be considered a mirror, often a late one, of residual pancreatic capacity. Its increase while fasting, after one hour, or after two hours from a load does not only indicate that more glucose has remained in circulation, but suggests that the compensation system has already begun to lose efficacy. For this reason, formally normal values do not necessarily guarantee the integrity of β-cells, just as elevated insulinemia does not automatically demonstrate efficient pancreatic function. Prevention should therefore intervene before overt hyperglycemia, by reducing the metabolic load and preserving the adaptive capacity of β-cells for as long as possible.

The concentration of insulin in the blood, as we saw in the previous chapter, is commonly interpreted as a direct consequence of the amount of hormone secreted by pancreatic β-cells. In reality, it is the result of the balance between two distinct but closely integrated processes: on one hand, pancreatic secretion, and on the other, the rate at which insulin is removed before entering circulation. This latter process is called insulin clearance and constitutes one of the main determinants of metabolic homeostasis. After being released into the portal vein, insulin first reaches the liver, which is its primary organ of elimination. Under physiological conditions, the first hepatic pass removes a very significant portion of the hormone, thereby regulating the amount that reaches systemic circulation and, consequently, muscle, adipose tissue, and other target organs (Tillil et al., 1988) [31]; (Meier et al., 2005) [32].

To understand the importance of this mechanism, it is useful to imagine the liver as an extremely sophisticated filter. Every molecule of insulin secreted by the pancreas must pass through it before reaching the rest of the body. The liver does not eliminate a fixed amount of the hormone but continuously modulates its activity based on metabolic needs. As early as the 1980s, it was demonstrated that insulin clearance is not a static parameter: as the glucose load increases, pancreatic secretion rises, but at the same time, the rate at which insulin is removed by the liver decreases. This adaptation allows for the maintenance of sufficiently high systemic insulin concentrations during periods of greater glucose availability, regardless of whether the glucose is taken orally or administered intravenously. In other words, the body simultaneously regulates both the production and elimination of the hormone to maintain optimal control of glycemia (Tillil et al., 1988) [31].

The regulation of clearance also depends on the specific way the pancreas secretes insulin. Contrary to what one might imagine, the hormone is not released continuously, but through rhythmic pulses that occur every few minutes. Studies conducted via direct sampling of the hepatic vein have shown that approximately 80% of secreted insulin is extracted during the first pass through the liver and that the liver modifies its extraction capacity almost instantaneously, following the rhythm of pancreatic secretion. What changes during an increase in glycemia is not so much the frequency of the pulses, but their amplitude: each "burst" contains a larger amount of insulin, and the mass of these pulses represents the main determinant of hepatic clearance. This reveals a very close functional integration between the pancreas and the liver, in which the latter does not play the simple role of an organ dedicated to insulin degradation, but acts as a true regulator of the hormone's distribution throughout the entire body (Meier et al., 2005) [32].

For many years, the reduction of insulin clearance was considered exclusively a compensatory response to insulin resistance. According to this interpretation, when tissues become less sensitive to the action of insulin, the body reduces its elimination to maintain high circulating concentrations and compensate for the reduced biological efficacy of the hormone. In recent years, however, this view has been profoundly re-evaluated. Numerous experimental and clinical findings suggest an opposite perspective: reduced clearance could represent one of the initial events in metabolic pathogenesis, preceding the onset of insulin resistance. In this model, lower hepatic elimination leads to chronic exposure of peripheral tissues to high concentrations of insulin. Persistent hyperinsulinemia would progressively alter insulin signaling: this mechanism could explain the simultaneous presence of reduced glucose utilization and persistent lipid synthesis observed in type 2 diabetes (Watada and Tamura, 2017) [33]; (Bergman et al., 2019) [34].

Experimental observations further reinforce this new paradigm. In a large study conducted on ninety metabolically healthy dogs, metabolic insulin clearance was found to be the main determinant of insulin sensitivity, even surpassing the contribution of adiposity and insulin secretion. Animals characterized by lower clearance had higher basal insulin concentrations and significantly reduced insulin sensitivity, while the secretory response of β-cells was not an independent predictor of insulin resistance. The authors therefore proposed that reduced insulin elimination induces chronic hyperinsulinemia, which continuously exposes peripheral tissues to the hormone, progressively promoting desensitization. This model overturns the traditional paradigm, suggesting that the reduction in clearance may precede and directly contribute to the development of insulin resistance, rather than simply representing a consequence of it (Ader et al., 2014) [35].

Clinical evidence in humans is equally convincing. In a prospective cohort including 570 Native American adults initially free of diabetes and followed for nearly eight years, reduced metabolic insulin clearance was associated with greater adiposity, fasting hyperinsulinemia, increased compensatory secretion, and reduced insulin sensitivity. More importantly, the decrease in clearance proved to be an independent predictor of the onset of type 2 diabetes even after adjustment for adiposity, insulin secretion, insulin sensitivity, and other metabolic risk factors. Subjects with the lowest capacity to eliminate insulin therefore had a significantly higher probability of developing the disease in subsequent years. Furthermore, the association observed with high levels of alanine aminotransferase (an enzyme used as an indicator of possible liver cell damage) suggests that fatty liver may compromise insulin extraction, further contributing to chronic hyperinsulinemia (Shah et al., 2021) [36].

Ethnic differences observed in numerous populations also seem to confirm the central role of insulin clearance. Systematic reviews dedicated to African populations have highlighted that peripheral insulin levels are frequently higher than those observed in European populations. However, when secretion is assessed through C-peptide measurement, which allows for the distinction between pancreatic production and hepatic elimination, it emerges that much of the hyperinsulinemia can be explained by reduced insulin extraction rather than actual hypersecretion by β-cells. In sub-Saharan African populations, a pathophysiological model has even been proposed in which hyperinsulinemia, sustained by the combination of increased secretion and reduced hepatic clearance, represents the initial event that precedes obesity, insulin resistance, and subsequent deterioration of β-cell function. These observations indicate how insulin clearance can represent an important determinant of individual susceptibility to diabetes, helping to explain part of the metabolic differences observed between different ethnic groups (Ladwa et al., 2019) [37]; (Goedecke and Mendham, 2022) [38].

The importance of insulin clearance also emerges clearly in some specific clinical conditions. In women affected by polycystic ovary syndrome, for example, marked hyperinsulinemia has traditionally been attributed exclusively to insulin resistance. However, when comparing women with PCOS and women with prediabetes who were carefully matched for age and body mass index, it emerged that peripheral insulin sensitivity was substantially similar in the two groups. The main difference lay in the capacity to eliminate insulin: patients with PCOS had significantly reduced metabolic clearance, which was closely correlated with the levels of hyperinsulinemia observed during the oral glucose tolerance test. This suggests that the excess circulating insulin depends to a large extent on altered hormone elimination and not exclusively on increased pancreatic secretion (Amato et al., 2015) [39].

Taken together, these findings indicate that insulin clearance represents much more than a simple hormone elimination mechanism. It constitutes one of the main regulators of tissue exposure to insulin and contributes significantly to metabolic physiology. Its reduction can lead to chronic hyperinsulinemia even in the absence of significant pancreatic hypersecretion, favoring the onset of insulin resistance over time and increasing the risk of type 2 diabetes. The concentration of insulin in the blood, therefore, should never be interpreted exclusively as an index of β-cell function, but as the result of the dynamic interaction between pancreatic secretion and hepatic clearance. Understanding this balance means profoundly changing the way we interpret the origin of hyperinsulinemia and, consequently, the pathophysiology of type 2 diabetes itself.

Hyperinsulinemia is often interpreted as the direct result of increased insulin secretion by pancreatic β-cells. This explanation, however, is incomplete. The concentration of the hormone in the blood actually depends on the balance between the amount secreted and the rate at which it is cleared, primarily by the liver. Consequently, high insulin levels can result from increased pancreatic production, reduced clearance, or a variable combination of both processes. This distinction is fundamental because the two conditions do not necessarily describe the same metabolic phase or the same pathophysiological mechanism.

The complexity becomes clear when observing fasting insulin, often used as an indirect indicator of insulin resistance. In a cohort of 470 Hispanic-American subjects, its concentration was found to be independently influenced by insulin metabolic clearance, insulin sensitivity, BMI, waist circumference, and fasting glycemia. Among all these factors, clearance showed the strongest quantitative association, even greater than that of insulin sensitivity. The parameters considered explained approximately 52% of the variability in fasting insulin levels, demonstrating that this value cannot be automatically interpreted as a measure of insulin resistance alone (Goodarzi et al., 2011) [40].

The ability to clear insulin also varies considerably under seemingly physiological conditions. In an experimental study that directly measured first-pass hepatic extraction, the proportion of insulin removed by the liver ranged from 22.4% to 77.2%, with an average of about 50%. Greater hepatic extraction was associated with better insulin sensitivity, while lower extraction corresponded to higher fasting insulin concentrations. The wide dispersion observed among healthy subjects suggests that clearance represents a highly individual physiological characteristic, likely influenced by genetic, environmental, and metabolic factors (Asare-Bediako et al., 2018) [41].

This variability can acquire pathological significance as early as the very first stages of weight gain. In ten healthy men subjected to an intentional increase of two BMI points, from 21.8 to 23.8 kg/m², insulin levels and insulin resistance increased even though body weight remained within the range considered normal. Basal hyperinsulinemia was certainly dependent on a modest increase in secretion but, above all, on a reduction in clearance. After a meal, an oral glucose load, or intravenous stimulation, however, the increase in insulin was entirely explained by the lower elimination of the hormone, as C-peptide and pancreatic secretion remained substantially unchanged. This indicates that reduced clearance may represent one of the first metabolic adaptations to an increase in adipose mass, preceding the marked hypersecretion typical of overt obesity (Erdmann et al., 2008) [42].

The relative contribution of secretion and clearance, however, does not remain constant, but changes as a function of the degree of adiposity and metabolic condition. In 291 subjects divided by BMI classes, fasting hyperinsulinemia increased progressively with body weight and was supported primarily by increased secretion, while the reduction in clearance provided a more limited contribution. After a meal, however, secretion increased only in overweight subjects and did not grow further in those with obesity. In the latter, the further increase in circulating insulin depended mainly on the progressive decrease in clearance. In parallel, the postprandial glycemic response worsened with increasing BMI, suggesting that high insulin levels were no longer sufficient to fully compensate for the loss of sensitivity (Erdmann et al., 2009) [43].

More recent studies have further reinforced the hypothesis that reduced clearance may constitute an initial event in hyperinsulinemia associated with obesity. In two independent cohorts, comprising over five hundred participants in total, all subjects with obesity presented a decrease in insulin clearance, which was progressively more marked as insulin resistance increased. In the early levels of metabolic impairment, however, secretion remained substantially unchanged. The increase in circulating insulin therefore did not seem to necessarily require greater pancreatic production, but certainly resulted from a reduction in clearance (Fu et al., 2022) [44].

When type 2 diabetes appears, the interaction between secretion and clearance becomes even more complex. In 348 patients with diabetes, basal insulin was generally higher compared to controls, but this increase depended primarily on the reduction in clearance and not on greater secretion. In the postprandial period, by contrast, insulin concentrations were lower and did not increase in relation to BMI as they did in non-diabetic subjects. As the disease progressed, the early secretory response decreased progressively, reflecting the deterioration of β-cells. Late secretion could instead increase considerably, but it was counterbalanced by an increase in clearance, so that peripheral insulin did not fully reflect the amount actually secreted (Erdmann et al., 2012) [45].

These observations show why the simple measurement of plasma insulin can be misleading. A high concentration does not necessarily demonstrate hypersecretion, just as a relatively low value does not exclude an intense pancreatic response if the hormone is cleared rapidly. To understand the origin of hyperinsulinemia, it is therefore necessary to distinguish the contribution of secretion from that of clearance.

The liver occupies a central position in this balance: in patients with simple hepatic steatosis or non-alcoholic steatohepatitis, hepatic insulin clearance was reduced by approximately 30% compared to subjects without hepatic fat accumulation. The impairment was already present in simple steatosis and did not worsen in relation to the severity of fibrosis and inflammation, suggesting that the hepatic defect appears early with the accumulation of lipids. Systemic clearance, however, decreased progressively with the worsening of inflammation and cellular damage. Since insulin secretion did not differ significantly between simple and non-alcoholic steatosis, hyperinsulinemia was more associated with the reduced elimination of the hormone than with an increase in its production (Bril et al., 2014) [46].

In another study on a Portuguese cohort, “PREVADIAB2”, including 1,019 participants, the transition from normoglycemia to prediabetes and type 2 diabetes was accompanied by a progressive increase in BMI, insulin resistance, insulin levels, and hepatic steatosis. Subjects with the worst metabolic profile simultaneously showed a lower ability to clear insulin. The correlation was particularly evident in normoglycemic and less compromised subjects (Patarrão et al., 2024) [47].

The possible causal relationship between reduced clearance and hyperinsulinemia has also been investigated using animal models which, as early as two months of age, presented a marked decrease in hepatic insulin elimination and hyperinsulinemia, in the absence of obesity, steatosis, increased lipolysis, or systemic insulin resistance. Only in the following months did a reduction in the insulin receptor, hepatic insulin resistance, and steatosis appear. The observed temporal sequence therefore places the reduction in clearance and the resulting hyperinsulinemia at the beginning of the cascade, before the main alterations traditionally considered responsible for insulin resistance (Ghadieh et al., 2019) [48].

Overall, these findings show that hyperinsulinemia does not constitute a single phenomenon nor can it be automatically attributed to excessive β-cell activity. In the early stages of weight gain, the decrease in clearance can precede hypersecretion and increase systemic exposure to insulin. With increasing obesity and insulin resistance, secretion and clearance can contribute to different degrees in the basal state and after meals. Finally, in advanced stages of diabetes, the deterioration of β-cell function further modifies this balance. Reduced insulin clearance should therefore not be considered merely a compensatory response, but a possible initial regulator of hyperinsulinemia and, in some conditions, one of the mechanisms capable of favoring the subsequent appearance of insulin resistance.

For many years, hyperinsulinemia was interpreted almost exclusively as a compensatory response to insulin resistance: when tissues become less sensitive to the action of insulin, pancreatic β-cells increase the secretion of the hormone to keep glycemia within normal limits. This explanation retains a significant degree of validity, but it does not fully describe the relationship between the two phenomena. Experimental evidence shows, in fact, that prolonged exposure to high concentrations of insulin can, in turn, reduce the ability of tissues to respond to the hormone. Hyperinsulinemia and insulin resistance should therefore not be considered as two events necessarily arranged in a rigid sequence, but as components of a self-reinforcing circuit: the increase in insulin may initially support glycemic control, but over time it can contribute to the progressive desensitization of target tissues.

One of the most significant demonstrations comes from a study conducted on young, healthy subjects in whom physiological hyperinsulinemia was induced for 72–96 hours while maintaining normal glycemia. After a few days, systemic glucose utilization was reduced by 20–40%. The defect did not concern glucose utilization, which remained substantially within normoglycemic parameters, but rather its use for energy purposes, specifically its transformation and storage as glycogen (the body's glucose reserve). The ability of insulin to suppress hepatic glucose production also remained preserved, indicating that the initial alteration primarily affected peripheral tissues, particularly skeletal muscle. Isolated hyperinsulinemia also reduced the subsequent secretory response of β-cells to a hyperglycemic stimulus. When, however, elevated insulin was associated with mild chronic hyperglycemia, the pancreatic response increased significantly, but the selective defect in glycogen synthesis persisted. These data demonstrate that a persistently high concentration of insulin can directly induce insulin resistance even in initially healthy individuals (Del Prato et al., 1994) [49].

Human muscle cells cultured in vitro confirm this model. After three days of prolonged exposure to insulin, subsequent acute stimulation resulted in significantly lower glucose uptake. In parallel, the phosphorylation of IRS-1 and AKT decreased—two essential steps in the insulin cascade, which we will see in the following chapter. The reduction of TNF-α (a molecule that promotes the body's inflammatory response) and the absence of significant changes in various lipid and mitochondrial genes further indicated that hyperinsulinemia, on its own, was sufficient to induce insulin resistance without reproducing the entire inflammatory and lipotoxic environment of diabetes (Turner et al., 2020) [50].

Desensitization can begin at the level of the insulin receptor itself. Chronic exposure to the hormone reduces the receptor's ability to autophosphorylate (initiating signal transmission within the cell) after a new stimulation, even without necessarily decreasing the number of receptors present on the cell membrane. After temporary removal of the insulin, the structure and autophosphorylation appeared to return to normal; however, a subsequent brief exposure rapidly restored the resistant phenotype. This behavior suggests the existence of a sort of molecular memory of hyperinsulinemia, through which the receptor remains predisposed to respond inadequately even after the temporary disappearance of the chronic stimulus (Catalano et al., 2014) [51].

In other contexts, hyperinsulinemia not only modifies receptor functionality but also reduces its expression. A meta-analysis including over 450 human skeletal muscle samples highlighted an inverse relationship between fasting insulin and INSR mRNA expression (the gene containing the information necessary to produce the insulin receptor). In myotubes (immature cells that will give rise to muscle fibers) subjected to chronic insulin exposure, the INSR gene, the total amount of receptor protein, and the portion present on the cell surface decreased. Simultaneously, AKT phosphorylation and glucose uptake were attenuated. Some of these alterations persisted even after several hours of insulin removal. Chronic excess insulin can therefore reprogram the expression of the main components of its own signaling pathway, progressively reducing the cell's ability to perceive the hormone (Cen et al., 2022) [52].

A similar process was observed in Hep G2 liver cells. After sixteen hours of insulin exposure, the amount of receptor decreased in a dose-dependent manner. The functional alterations, however, were even more marked: receptor phosphorylation dropped to 22%, and the interaction between IRS-1 and PI3K (two essential steps in the insulin cascade) was reduced by 66%. The amount of PI3K remained unchanged, indicating that the problem was not a lack of the enzyme, but an inability to recruit it correctly into the signaling complex. Hyperinsulinemia therefore simultaneously compromised the receptor, IRS-1, and the transition to PI3K, interrupting the cascade in its very early stages (Yuan et al., 2002) [53].

A further form of insulin resistance has been observed in cerebral microvascular endothelial cells. In this case, the total number of receptors increased, but their functionality decreased: autophosphorylation was compromised, a greater portion remained blocked on the membrane, and internalization after insulin stimulation was reduced. Inhibition restored receptor phosphorylation, signaling, and internalization. Such alterations could reduce insulin transport across the blood-brain barrier (a selective filter that protects the brain from potentially harmful substances) and contribute to the cerebral insulin resistance associated with obesity (Watson et al., 2022) [54].

Overall, the evidence shows that chronic hyperinsulinemia can act on multiple levels: it modifies the conformation and autophosphorylation of the receptor, reduces its expression or accelerates its degradation, alters IRS-1 and PI3K recruitment, attenuates AKT, and compromises glucose transport and storage. These effects involve muscle, liver, adipose tissue, kidney, and cerebral endothelium, although they assume different characteristics in different tissues.

Thus, a pathophysiological vicious cycle emerges: reduced insulin clearance or increased secretion initially increases systemic exposure to the hormone; persistent stimulation progressively induces insulin resistance; the reduced effectiveness of insulin makes it necessary to maintain even higher hormone concentrations to control glycemia; the further hyperinsulinemia amplifies the desensitization mechanisms. In this perspective, hyperinsulinemia does not only represent the body's attempt to compensate for an existing defect, but can become one of the factors that sustain and accelerate its progression.