This article needs more reliable medical references for verification or relies too heavily on primary sources .(October 2015) |
Insulin resistance | |
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Specialty | Endocrinology |
Insulin resistance (IR) is a pathological condition in which cells in insulin-sensitive tissues in the body fail to respond normally to the hormone insulin or downregulate insulin receptors in response to hyperinsulinemia.
Insulin is a hormone that facilitates the transport of glucose from blood into cells, thereby reducing blood glucose (blood sugar). Insulin is released by the pancreas in response to carbohydrates consumed in the diet. In states of insulin resistance, the same amount of insulin does not have the same effect on glucose transport and blood sugar levels. There are many causes of insulin resistance and the underlying process is still not completely understood. Risk factors for insulin resistance include obesity, sedentary lifestyle, family history of diabetes, various health conditions, and certain medications. Insulin resistance is considered a component of the metabolic syndrome. There are multiple ways to measure insulin resistance such as fasting insulin levels or glucose tolerance tests, but these are not often used in clinical practice. Insulin resistance can be improved or reversed with lifestyle approaches, such as weight reduction, exercise, and dietary changes.
There are a number of risk factors for insulin resistance, including being overweight or obese or having a sedentary lifestyle. [1] Various genetic factors can increase risk, such as a family history of diabetes, and there are some specific medical conditions associated with insulin resistance, such as polycystic ovary syndrome. [1]
The U.S. National Institute of Diabetes and Digestive and Kidney Diseases states that specific risks that may predispose an individual to insulin resistance can include:
In addition some medications and other health conditions can raise the risk. [1]
Dietary factors are likely to contribute to insulin resistance. However, causative foods are difficult to determine given the limitations of nutrition research. Foods that have independently been linked to insulin resistance include those high in sugar with high glycemic indices, low in omega-3 and fiber, and which are hyperpalatable which increases risk of overeating. [2] Overconsumption of fat- and sugar-rich meals and beverages have been proposed as a fundamental factor behind the metabolic syndrome epidemic.
Diet also has the potential to change the ratio of polyunsaturated to saturated phospholipids in cell membranes. The percentage of polyunsaturated fatty acids (PUFAs) is inversely correlated with insulin resistance. [3] It is hypothesized that increasing cell membrane fluidity by increasing PUFA concentration might result in an enhanced number of insulin receptors, an increased affinity of insulin to its receptors, and reduced insulin resistance. [4]
Vitamin D deficiency has also been associated with insulin resistance. [5]
Sedentary lifestyle increases the likelihood of development of insulin resistance. [6] In epidemiological studies, higher levels of physical activity (more than 90 minutes per day) reduce the risk of diabetes by 28%. [7]
Studies have consistently shown that there is a link between insulin resistance and circadian rhythm, with insulin sensitivity being higher in the morning and lower in the evening. A mismatch between the circadian rhythm and the meals schedule, such as in circadian rhythm disorders, may increase insulin resistance. [8] [9] [10]
Insufficient sleep has been shown to cause insulin resistance, and also increases the risk of developing metabolic diseases such as type 2 diabetes and obesity. [11] [12] [13]
Some medications are associated with insulin resistance including corticosteroids, protease inhibitors (type of HIV medication), [14] and atypical antipsychotics. [15]
Being exposed to light during sleep has been shown to cause insulin resistance and increase heart rate. [16]
Many hormones can induce insulin resistance including cortisol, [17] growth hormone, and human placental lactogen. [18]
Cortisol counteracts insulin and can lead to increased hepatic gluconeogenesis, reduced peripheral utilization of glucose, and increased insulin resistance. [19] It does this by decreasing the translocation of glucose transporters (especially GLUT4) to the cell membrane. [20] [21]
Based on the significant improvement in insulin sensitivity in humans after bariatric surgery and rats with surgical removal of the duodenum, [22] [23] it has been proposed that some substance is produced in the mucosa of that initial portion of the small intestine that signals body cells to become insulin resistant. If the producing tissue is removed, the signal ceases and body cells revert to normal insulin sensitivity. No such substance has been found as yet, and the existence of such a substance remains speculative.[ citation needed ]
Leptin is a hormone produced from the ob gene and adipocytes. [24] Its physiological role is to regulate hunger by alerting the body when it is full. [25] Studies show that lack of leptin causes severe obesity and is strongly linked with insulin resistance. [26]
Polycystic ovary syndrome [27] and non-alcoholic fatty liver disease (NAFLD) are associated with insulin resistance. Hepatitis C also makes people three to four times more likely to develop type 2 diabetes and insulin resistance. [28]
Multiple studies involving different methodology suggest that impaired function of mitochondria might play a pivotal role in the pathogenesis of insulin resistance. [29] [30] Mitochondrial dysfunction may result from the formation of reactive oxygen species, genetic factors, aging, and reduced mitochondrial biogenesis. [31] Important questions remain unsolved to date, however. [32] If confirmed by rigorous studies, a link between mitochondrial disorders and reduced insulin sensitivity might pave the way to new therapeutic approaches. [33]
Acute or chronic inflammation, such as in infections, can cause insulin resistance. TNF-α is a cytokine that may promote insulin resistance by promoting lipolysis, disrupting insulin signaling, and reducing the expression of GLUT4. [34]
Several genetic loci have been identified as associated with insulin insensitivity. These include variations in loci near the NAT2, GCKR, and IGFI genes, which are linked to insulin resistance. Further research has indicated that loci near these genes are correlated with insulin resistance. However, it is estimated that these loci only account for 25–44% of the genetic component of insulin resistance. [35]
In normal metabolism, the elevated blood glucose instructs beta (β) cells in the Islets of Langerhans, located in the pancreas, to release insulin into the blood. The insulin makes insulin-sensitive tissues in the body (primarily skeletal muscle cells, adipose tissue, and liver) absorb glucose which provides energy as well as lowers blood glucose. [36] The beta cells reduce insulin output as the blood glucose level falls, allowing blood glucose to settle at a constant of approximately 5 mmol/L (90 mg/dL). In an insulin-resistant person, normal levels of insulin do not have the same effect in controlling blood glucose levels.
When the body produces insulin under conditions of insulin resistance, the cells are unable to absorb or use it as effectively and it stays in the bloodstream. Certain cell types such as fat and muscle cells require insulin to absorb glucose and when these cells fail to respond adequately to circulating insulin, blood glucose levels rise. The liver normally helps regulate glucose levels by reducing its secretion of glucose in the presence of insulin. However, in insulin resistance, this normal reduction in the liver's glucose production may not occur, further contributing to elevated blood glucose. [37]
Insulin resistance in fat cells results in reduced uptake of circulating lipids and increased hydrolysis of stored triglycerides. This leads to elevated free fatty acids in the blood plasma and can further worsen insulin resistance. [38] [39] [40] Since insulin is the primary hormonal signal for energy storage into fat cells, which tend to retain their sensitivity in the face of hepatic and skeletal muscle resistance, insulin resistance stimulates the formation of new fatty tissue and accelerates weight gain. [2]
In states of insulin resistance, beta cells in the pancreas increase their production of insulin. This causes high blood insulin (hyperinsulinemia) to compensate for the high blood glucose. During this compensated phase of insulin resistance, beta cell function is upregulated, insulin levels are higher, and blood glucose levels are still maintained. If compensatory insulin secretion fails, then either fasting (impaired fasting glucose) or postprandial (impaired glucose tolerance) glucose concentrations increase. Eventually, type 2 diabetes occurs when glucose levels become higher as the resistance increases and compensatory insulin secretion fails. [41] [42] The inability of the β-cells to produce sufficient insulin in a condition of hyperglycemia is what characterizes the transition from insulin resistance to type 2 diabetes.
Insulin resistance is strongly associated with intestinal-derived apoB-48 production rate in insulin-resistant subjects and type 2 diabetics. [43] Insulin resistance often is found in people with visceral adiposity, hypertension, hyperglycemia, and dyslipidemia involving elevated triglycerides, small dense low-density lipoprotein (sdLDL) particles, and decreased high-density lipoprotein (HDL) cholesterol levels. With respect to visceral adiposity, a great deal of evidence suggests two strong links with insulin resistance. First, unlike subcutaneous adipose tissue, visceral adipose cells produce significant amounts of proinflammatory cytokines such as tumor necrosis factor-alpha (TNF-a), and Interleukins-1 and −6, etc. In numerous experimental models, these proinflammatory cytokines disrupt normal insulin action in fat and muscle cells and may be a major factor in causing the whole-body insulin resistance observed in patients with visceral adiposity. Much of the attention on production of proinflammatory cytokines has focused on the IKK-beta/NF-kappa-B pathway, a protein network that enhances transcription of inflammatory markers and mediators that may cause insulin resistance. Second, visceral adiposity is related to an accumulation of fat in the liver, a condition known as non-alcoholic fatty liver disease (NAFLD). The result of NAFLD is an excessive release of free fatty acids into the bloodstream (due to increased lipolysis), and an increase in hepatic breakdown of glycogen stores into glucose (glycogenolysis), both of which have the effect of exacerbating peripheral insulin resistance and increasing the likelihood of Type 2 diabetes mellitus.[ citation needed ]
The excessive expansion of adipose tissue that tends to occur under sustainedly positive energy balance (as in overeating) has been postulated by Vidal-Puig to induce lipotoxic and inflammatory effects that may contribute to causing insulin resistance and its accompanying disease states. [44]
Also, insulin resistance often is associated with a hypercoagulable state (impaired fibrinolysis) and increased inflammatory cytokine levels. [45]
At the molecular level, a cell senses insulin through insulin receptors, with the signal propagating through a signaling cascade collectively known as PI3K/Akt/mTOR signaling pathway. [46] Recent studies suggested that the pathway may operate as a bistable switch under physiologic conditions for certain types of cells, and insulin response may well be a threshold phenomenon. [47] [46] [48] The pathway's sensitivity to insulin may be blunted by many factors such as lipolysis of free fatty acids, [49] causing insulin resistance. From a broader perspective, however, sensitivity tuning (including sensitivity reduction) is a common practice for an organism to adapt to the changing environment or metabolic conditions. [50] Pregnancy, for example, entails significant metabolic changes, during which the mother must decrease the insulin sensitivity of her muscles to conserve more glucose for both the maternal and fetal brains. This adaptation can occur by elevating the response threshold, thereby delaying the onset of sensitivity. This is achieved through the secretion of placental growth factor, which interferes with the interaction between insulin receptor substrate (IRS) and PI3K. This concept forms the basis of the adjustable threshold hypothesis of insulin resistance. [47]
Insulin resistance has been proposed to be a reaction to excess nutrition by superoxide dismutase in cell mitochondria that acts as an antioxidant defense mechanism. This link seems to exist under diverse causes of insulin resistance. It also is based on the finding that insulin resistance may be reversed rapidly by exposing cells to mitochondrial uncouplers, electron transport chain inhibitors, or mitochondrial superoxide dismutase mimetics. [51]
A fasting serum insulin level greater than 29 microIU/mL or 174 pmol/L indicates insulin resistance. [52] [ failed verification ] The same levels apply three hours after the last meal.
During a glucose tolerance test (GTT), which may be used to diagnose diabetes mellitus, a fasting patient takes a 75 gram oral dose of glucose. Then blood glucose levels are measured over the following two hours.
Interpretation is based on WHO guidelines. After two hours a glycemia less than 7.8 mmol/L (140 mg/dL) is considered normal, a glycemia of between 7.8 and 11.0 mmol/L (140 to 197 mg/dL) is considered as impaired glucose tolerance (IGT), and a glycemia of greater than or equal to 11.1 mmol/L (200 mg/dL) is considered diabetes mellitus.
An oral glucose tolerance test (OGTT) may be normal or mildly abnormal in simple insulin resistance. Often, there are raised glucose levels in the early measurements, reflecting the loss of a postprandial peak (after the meal) in insulin production. Extension of the testing (for several more hours) may reveal a hypoglycemic "dip," that is a result of an overshoot in insulin production after the failure of the physiologic postprandial insulin response.[ citation needed ]
The gold standard for investigating and quantifying insulin resistance is the "hyperinsulinemic euglycemic clamp," so-called because it measures the amount of glucose necessary to compensate for an increased insulin level without causing hypoglycemia. [53] It is a type of glucose clamp technique. The test is rarely performed in clinical care, but is used in medical research, for example, to assess the effects of different medications. The rate of glucose infusion commonly is referred to in diabetes literature as the GINF value. [54]
The procedure takes about two hours. Through a peripheral vein, insulin is infused at 10–120 mU per m2 per minute. In order to compensate for the insulin infusion, glucose 20% is infused to maintain blood sugar levels between 5 and 5.5 mmol/L. The rate of glucose infusion is determined by checking the blood sugar levels every five to ten minutes. [54]
The insulin sensitivity is determined by the rate of glucose infusion during the last thirty minutes of the test. If high levels (7.5 mg/min or higher) are needed, the patient is considered insulin-sensitive. Conversely, very low levels (4.0 mg/min or lower) indicate insulin resistance. Levels falling between 4.0 and 7.5 mg/min are not conclusive and suggest "impaired glucose tolerance," which is an early indication of insulin resistance. [54]
This fundamental technique can be greatly enhanced through the utilization of glucose tracers. Glucose can be labeled with either stable or radioactive atoms. Commonly employed tracers include 3-3H glucose (radioactive), 6,6 2H-glucose (stable), and 1-13C glucose (stable). Prior to initiating the hyperinsulinemic phase, a 3-hour tracer infusion allows for the determination of the basal rate of glucose production. Throughout the clamp, the plasma tracer concentrations facilitate the computation of whole-body insulin-stimulated glucose metabolism, as well as the production of glucose by the body. (i.e., endogenous glucose production). [54]
Another measure of insulin resistance is the modified insulin suppression test developed by Gerald Reaven at Stanford University. The test correlates well with the euglycemic clamp, with less operator-dependent error. This test has been used to advance the large body of research relating to the metabolic syndrome. [54]
Patients initially receive 25 μg of octreotide (Sandostatin) in 5 mL of normal saline over 3 to 5 minutes via intravenous infusion (IV) as an initial bolus, and then, are infused continuously with an intravenous infusion of somatostatin (0.27 μg/m2/min) to suppress endogenous insulin and glucose secretion. Next, insulin and 20% glucose are infused at rates of 32 and 267 mg/m2/min, respectively. Blood glucose is checked at zero, 30, 60, 90, and 120 minutes, and thereafter, every 10 minutes for the last half-hour of the test. These last four values are averaged to determine the steady-state plasma glucose level (SSPG). Subjects with an SSPG greater than 150 mg/dL are considered to be insulin-resistant. [54]
Given the complicated nature of the "clamp" technique (and the potential dangers of hypoglycemia in some patients), alternatives have been sought to simplify the measurement of insulin resistance. The first was the Homeostatic Model Assessment (HOMA), [55] and more recent methods include the Quantitative insulin sensitivity check index (QUICKI) [56] and SPINA-GR, a measure for insulin sensitivity. [57] All of these calculated markers employ fasting insulin and glucose levels to calculate insulin resistance, and all correlate reasonably with the results of clamping studies.
Maintaining a healthy body weight and engaging in regular physical activity can help mitigate the risk of developing insulin resistance. [1]
The primary treatment for insulin resistance is exercise and weight loss. [59] Both metformin and thiazolidinediones improve insulin sensitivity. Metformin is approved for prediabetes and type 2 diabetes and has become one of the more commonly prescribed medications for insulin resistance. [60]
The Diabetes Prevention Program (DPP) showed that exercise and diet were nearly twice as effective as metformin at reducing the risk of progressing to type 2 diabetes. [61] However, the participants in the DPP trial regained about 40% of the weight that they had lost at the end of 2.8 years, resulting in a similar incidence of diabetes development in both the lifestyle intervention and the control arms of the trial. [62] In epidemiological studies, higher levels of physical activity (more than 90 minutes per day) reduce the risk of diabetes by 28%. [63]
Furthermore, physical training has also generally been seen to be an effective antagonist of insulin resistance in obese or overweight children and adolescents (under the age of 19). [64] As per the 2016 systematic review and meta-analysis conducted by Marson et al., aerobic exercise is associated with fasting insulin reduction; however, resistance and combined exercise are not. [64] The authors caution against demeaning the importance of resistance and combined exercise, as this type of training is generally less researched than aerobic training. [64] Overall, physical training can be used in both adolescents and adults to prevent the progression of insulin resistance and future possible metabolic and cardiovascular disease.
Resistant starch from high-amylose corn, amylomaize, has been shown to reduce insulin resistance in healthy individuals, in individuals with insulin resistance, and in individuals with type 2 diabetes. [65]
Some types of polyunsaturated fatty acids (omega-3) may moderate the progression of insulin resistance into type 2 diabetes, [66] [67] [68] however, omega-3 fatty acids appear to have limited ability to reverse insulin resistance, and they cease to be efficacious once type 2 diabetes is established. [69]
The concept that insulin resistance may be the underlying cause of diabetes mellitus type 2 was first advanced by Professor Wilhelm Falta and published in Vienna in 1931, [70] and confirmed as contributory by Sir Harold Percival Himsworth of the University College Hospital Medical Centre in London in 1936; [71] however, type 2 diabetes does not occur unless there is concurrent failure of compensatory insulin secretion. [72]
Some scholars go as far as to claim that neither insulin resistance, nor obesity really are metabolic disorders per se, but simply adaptive responses to sustained caloric surplus, intended to protect bodily organs from lipotoxicity (unsafe levels of lipids in the bloodstream and tissues): "Obesity should therefore not be regarded as a pathology or disease, but rather as the normal, physiologic response to sustained caloric surplus... As a consequence of the high level of lipid accumulation in insulin target tissues including skeletal muscle and liver, it has been suggested that exclusion of glucose from lipid-laden cells is a compensatory defense against further accumulation of lipogenic substrate." [73]
Other prevailing thoughts that insulin resistance can be an evolutionary adaptation include the thrifty gene hypothesis. This hypothesis raises the point that if there is a genetic component to insulin resistance and Type 2 diabetes, these phenotypes should be selected against. [74] Yet, there has been an increase in mean insulin resistance in both the normoglycemic population as well as the diabetic population. [75]
J.V. Neel posits that in ancient human ancestors, during periods of heightened famine, genes facilitating increased glucose storage would have conferred an advantage. However, in today's modern environment, this is no longer the case. [74]
Evidence contradicts Neel's hypothesis in studies of the Pima Indians, which suggest that individuals with higher insulin sensitivity tended to have higher weights, whereas those with insulin resistance tended to weigh less on average within this demographic. [76]
Modern hypotheses propose that insulin metabolism serves as a socio-ecological adaptation, with insulin serving as the mechanism for allocating energy to different body components and insulin sensitivity being an adaptation to regulate this energy allocation. The Behavioral Switch Hypothesis suggests that insulin resistance leads to two methods of altering reproductive and behavioral strategies. These strategies are termed "r to K" and "soldier to diplomat." The "r to K" strategy involves directing insulin through the placenta to the fetus, resulting in weight gain in the fetus but not the mother, indicating an increase in parental investment (K strategy). In the "soldier to diplomat" strategy, the insensitivity of skeletal muscle to insulin could redirect glucose to the brain, which does not require insulin receptors. This has been shown to enhance cognitive development across various studies. [77]
Metabolic syndrome is a clustering of at least three of the following five medical conditions: abdominal obesity, high blood pressure, high blood sugar, high serum triglycerides, and low serum high-density lipoprotein (HDL).
Abdominal obesity, also known as central obesity and truncal obesity, is the human condition of an excessive concentration of visceral fat around the stomach and abdomen to such an extent that it is likely to harm its bearer's health. Abdominal obesity has been strongly linked to cardiovascular disease, Alzheimer's disease, and other metabolic and vascular diseases.
Hyperglycemia or hyperglycaemia is a condition where unusually high amount of glucose is present in blood. It is defined as blood glucose level exceeding 6.9 mmol/L after fasting for 8 hours and 10 mmol/L 2 hours after eating.
Ketosis is a metabolic state characterized by elevated levels of ketone bodies in the blood or urine. Physiological ketosis is a normal response to low glucose availability. In physiological ketosis, ketones in the blood are elevated above baseline levels, but the body's acid–base homeostasis is maintained. This contrasts with ketoacidosis, an uncontrolled production of ketones that occurs in pathologic states and causes a metabolic acidosis, which is a medical emergency. Ketoacidosis is most commonly the result of complete insulin deficiency in type 1 diabetes or late-stage type 2 diabetes. Ketone levels can be measured in blood, urine or breath and are generally between 0.5 and 3.0 millimolar (mM) in physiological ketosis, while ketoacidosis may cause blood concentrations greater than 10 mM.
Type 2 diabetes (T2D), formerly known as adult-onset diabetes, is a form of diabetes mellitus that is characterized by high blood sugar, insulin resistance, and relative lack of insulin. Common symptoms include increased thirst, frequent urination, fatigue and unexplained weight loss. Other symptoms include increased hunger, having a sensation of pins and needles, and sores (wounds) that heal slowly. Symptoms often develop slowly. Long-term complications from high blood sugar include heart disease, stroke, diabetic retinopathy, which can result in blindness, kidney failure, and poor blood flow in the lower-limbs, which may lead to amputations. The sudden onset of hyperosmolar hyperglycemic state may occur; however, ketoacidosis is uncommon.
Adipose tissue is a loose connective tissue composed mostly of adipocytes. It also contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular endothelial cells and a variety of immune cells such as adipose tissue macrophages. Its main role is to store energy in the form of lipids, although it also cushions and insulates the body.
Adiponectin is a protein hormone and adipokine, which is involved in regulating glucose levels and fatty acid breakdown. In humans, it is encoded by the ADIPOQ gene and is produced primarily in adipose tissue, but also in muscle and even in the brain.
Resistin also known as adipose tissue-specific secretory factor (ADSF) or C/EBP-epsilon-regulated myeloid-specific secreted cysteine-rich protein (XCP1) is a cysteine-rich peptide hormone derived from adipose tissue that in humans is encoded by the RETN gene.
Hyperinsulinemia is a condition in which there are excess levels of insulin circulating in the blood relative to the level of glucose. While it is often mistaken for diabetes or hyperglycaemia, hyperinsulinemia can result from a variety of metabolic diseases and conditions, as well as non-nutritive sugars in the diet. While hyperinsulinemia is often seen in people with early stage type 2 diabetes mellitus, it is not the cause of the condition and is only one symptom of the disease. Type 1 diabetes only occurs when pancreatic beta-cell function is impaired. Hyperinsulinemia can be seen in a variety of conditions including diabetes mellitus type 2, in neonates and in drug-induced hyperinsulinemia. It can also occur in congenital hyperinsulinism, including nesidioblastosis.
In chemistry, de novo synthesis is the synthesis of complex molecules from simple molecules such as sugars or amino acids, as opposed to recycling after partial degradation. For example, nucleotides are not needed in the diet as they can be constructed from small precursor molecules such as formate and aspartate. Methionine, on the other hand, is needed in the diet because while it can be degraded to and then regenerated from homocysteine, it cannot be synthesized de novo.
The Randle cycle, also known as the glucose fatty-acid cycle, is a metabolic process involving the cross inhibition of glucose and fatty acids for substrates. It is theorized to play a role in explaining type 2 diabetes and insulin resistance.
The fatty-acid-binding proteins (FABPs) are a family of transport proteins for fatty acids and other lipophilic substances such as eicosanoids and retinoids. These proteins are thought to facilitate the transfer of fatty acids between extra- and intracellular membranes. Some family members are also believed to transport lipophilic molecules from outer cell membrane to certain intracellular receptors such as PPAR. The FABPs are intracellular carriers that “solubilize” the endocannabinoid anandamide (AEA), transporting AEA to the breakdown by FAAH, and compounds that bind to FABPs block AEA breakdown, raising its level. The cannabinoids are also discovered to bind human FABPs that function as intracellular carriers, as THC and CBD inhibit the cellular uptake and catabolism of AEA by targeting FABPs. Competition for FABPs may in part or wholly explain the increased circulating levels of endocannabinoids reported after consumption of cannabinoids. Levels of fatty-acid-binding protein have been shown to decline with ageing in the mouse brain, possibly contributing to age-associated decline in synaptic activity.
Metabolic imprinting refers to the long-term physiological and metabolic effects that an offspring's prenatal and postnatal environments have on them. Perinatal nutrition has been identified as a significant factor in determining an offspring's likelihood of it being predisposed to developing cardiovascular disease, obesity, and type 2 diabetes amongst other conditions.
The portal hypothesis describes a possible mechanism for some of the health effects of obesity, particularly the metabolic syndrome. It says that obesity results in increased circulation of free fatty acids and thus, via Randle's effect, in insulin resistance.
Lipotoxicity is a metabolic syndrome that results from the accumulation of lipid intermediates in non-adipose tissue, leading to cellular dysfunction and death. The tissues normally affected include the kidneys, liver, heart and skeletal muscle. Lipotoxicity is believed to have a role in heart failure, obesity, and diabetes, and is estimated to affect approximately 25% of the adult American population.
A number of lifestyle factors are known to be important to the development of type 2 diabetes including: obesity, physical activity, diet, stress, and urbanization. Excess body fat underlies 64% of cases of diabetes in men and 77% of cases in women. A number of dietary factors such as sugar sweetened drinks and the type of fat in the diet appear to play a role.
Diabetes mellitus, often known simply as diabetes, is a group of common endocrine diseases characterized by sustained high blood sugar levels. Diabetes is due to either the pancreas not producing enough insulin, or the cells of the body becoming unresponsive to the hormone's effects. Classic symptoms include polydipsia, polyuria, weight loss, and blurred vision. If left untreated, the disease can lead to various health complications, including disorders of the cardiovascular system, eye, kidney, and nerves. Diabetes accounts for approximately 4.2 million deaths every year, with an estimated 1.5 million caused by either untreated or poorly treated diabetes.
The Disposition index (DI) is a measure for the loop gain of the insulin-glucose feedback control system. It is defined as the product of insulin sensitivity times the amount of insulin secreted in response to blood glucose levels. "Metabolically healthy" Insulin resistant individuals can maintain normal responses to blood glucose due to the fact that higher levels of insulin are secreted as long as the beta cells of the pancreas are able to increase their output of insulin to compensate for the insulin resistance. But the ratio of the incremental increase in plasma insulin associated with an incremental increase in plasma glucose provides a better measure of beta cell function than the plasma insulin response to a glucose challenge. Loss of function of the beta cells, reducing their capacity to compensate for insulin resistance, results in a lower disposition index.
Diabetes mellitus (DM) is a type of metabolic disease characterized by hyperglycemia. It is caused by either defected insulin secretion or damaged biological function, or both. The high-level blood glucose for a long time will lead to dysfunction of a variety of tissues.
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