DM
Diabetes Mellitus
Diabetes mellitus is a group of metabolic disorders characterized by chronic hyperglycemia resulting from impaired insulin secretion, impaired insulin action, or both. Its endocrine basis centers on dysfunction of pancreatic islet cells and insulin-regulated glucose metabolism, with long-term effects on blood vessels, nerves, kidneys, eyes, and other organs.
RegionAbdomen
SystemEndocrine System
Diabetes mellitus is a group of metabolic disorders characterized by persistent hyperglycemia caused by abnormalities of insulin secretion, insulin action, or both. Although its manifestations involve virtually every organ system, its endocrine basis is closely related to the pancreatic islets, particularly the insulin-producing beta cells.
Insulin is essential for coordinated regulation of carbohydrate, lipid, and protein metabolism. It promotes glucose uptake in insulin-sensitive tissues, suppresses excessive hepatic glucose production, encourages energy storage, and inhibits breakdown of stored metabolic fuels. When insulin is absent or its actions are inadequate, blood glucose rises and normal metabolic regulation becomes disrupted.
The two major forms are type 1 diabetes mellitus, characterized primarily by autoimmune destruction of pancreatic beta cells and severe insulin deficiency, and type 2 diabetes mellitus, characterized by insulin resistance combined with progressive impairment of beta-cell function. Other forms include gestational diabetes and diabetes caused by specific genetic, pancreatic, endocrine, or drug-related conditions.
Endocrine Pancreas
The endocrine component of the pancreas consists of the pancreatic islets, also called the islets of Langerhans.
These small clusters of endocrine cells are distributed throughout the pancreatic parenchyma among the much more abundant exocrine acini.
Pancreatic Islet Cells
Pancreatic islets contain several endocrine cell populations that participate in regulation of nutrient metabolism.
| Cell Type | Major Hormone | Major Function |
|---|---|---|
| Beta cells | Insulin | Promotes nutrient storage and lowers blood glucose |
| Alpha cells | Glucagon | Promotes hepatic glucose production |
| Delta cells | Somatostatin | Modulates secretion of other islet hormones |
| PP cells | Pancreatic polypeptide | Participates in gastrointestinal and pancreatic regulation |
Beta Cells
Beta cells are the pancreatic endocrine cells responsible for insulin synthesis and secretion.
Loss or dysfunction of these cells is central to the pathophysiology of diabetes mellitus.
Insulin
Insulin is a peptide hormone that coordinates nutrient utilization and storage, particularly after food intake.
Its secretion increases when circulating glucose concentrations rise, allowing tissues to respond appropriately to an increased supply of metabolic fuel.
Insulin Synthesis
Insulin is initially synthesized within beta cells as preproinsulin. Removal of its signal peptide produces proinsulin.
Proinsulin is subsequently processed into biologically active insulin and C-peptide, which are stored within secretory granules and released together.
Insulin Synthesis Pathway
Preproinsulin → Proinsulin → Insulin + C-peptide
C-Peptide
C-peptide is released in approximately equimolar amounts with endogenous insulin.
Because administered insulin preparations do not contain endogenous C-peptide, measurement of C-peptide can provide useful information about a patient's own pancreatic beta-cell insulin production.
Glucose-Stimulated Insulin Secretion
Rising blood glucose stimulates insulin secretion from pancreatic beta cells.
Glucose metabolism increases the intracellular ATP-to-ADP ratio, influencing ATP-sensitive potassium channels, membrane electrical activity, calcium entry, and exocytosis of insulin-containing granules.
Insulin Receptor
Insulin acts through the insulin receptor, a transmembrane receptor with intrinsic tyrosine kinase activity.
Activation initiates intracellular signaling pathways that regulate glucose transport, metabolism, protein synthesis, gene expression, and other cellular processes.
Glucose Uptake
In skeletal muscle and adipose tissue, insulin promotes movement of GLUT4 glucose transporters to the cell membrane.
This increases cellular glucose uptake and helps reduce postprandial blood glucose concentrations.
Insulin and the Liver
The liver is a major target of insulin-mediated metabolic regulation.
Insulin suppresses hepatic glucose production while promoting glycogen synthesis and other processes associated with nutrient storage.
Insulin and Skeletal Muscle
Skeletal muscle represents an important site of insulin-stimulated glucose disposal.
Insulin promotes glucose uptake, glycogen formation, amino acid uptake, and protein synthesis within skeletal muscle.
Insulin and Adipose Tissue
Insulin promotes glucose utilization and triglyceride storage in adipose tissue while inhibiting excessive breakdown of stored triglycerides.
Loss of insulin action therefore promotes release of free fatty acids into the circulation.
Major Actions of Insulin
| Tissue or Process | Major Insulin Effect |
|---|---|
| Skeletal muscle | Increases glucose uptake and glycogen synthesis |
| Adipose tissue | Increases glucose uptake and promotes lipid storage |
| Liver | Suppresses glucose production and promotes glycogen synthesis |
| Lipolysis | Inhibits breakdown of stored triglycerides |
| Protein metabolism | Promotes protein synthesis and limits catabolism |
Glucagon
Glucagon is produced by pancreatic alpha cells and acts as an important physiological counter-regulator of insulin.
When blood glucose falls, glucagon secretion can increase and stimulate hepatic processes that restore circulating glucose.
Glucagon and Hepatic Glucose Production
Glucagon promotes hepatic glycogenolysis and gluconeogenesis.
These actions increase glucose release from the liver into the bloodstream.
Insulin-Glucagon Balance
Normal glucose homeostasis depends partly on coordinated changes in insulin and glucagon secretion.
After meals, insulin action predominates and promotes nutrient utilization and storage. During fasting, reduced insulin and increased glucagon activity support mobilization of stored fuels and hepatic glucose production.
Pathophysiology of Diabetes Mellitus
Diabetes develops when insulin availability or effectiveness becomes insufficient to maintain normal glucose homeostasis.
The specific mechanism varies substantially between different forms of diabetes.
Major Types of Diabetes Mellitus
| Type | Principal Mechanism |
|---|---|
| Type 1 diabetes mellitus | Autoimmune beta-cell destruction causing severe insulin deficiency |
| Type 2 diabetes mellitus | Insulin resistance with progressive beta-cell dysfunction |
| Gestational diabetes mellitus | Hyperglycemia first diagnosed during pregnancy that meets criteria for gestational diabetes |
| Other specific types | Genetic, pancreatic, endocrine, medication-related, or other defined causes |
Type 1 Diabetes Mellitus
Type 1 diabetes mellitus results primarily from immune-mediated destruction of pancreatic beta cells.
Progressive loss of beta-cell mass produces severe or absolute insulin deficiency, making exogenous insulin necessary for metabolic survival.
Autoimmune Beta-Cell Destruction
In most cases of type 1 diabetes, an autoimmune process targets pancreatic beta cells.
Genetic susceptibility interacts with environmental and immunological factors, eventually producing sufficient beta-cell destruction to cause clinically apparent hyperglycemia.
Pancreatic Islets in Type 1 Diabetes
Early autoimmune disease can be associated with inflammatory infiltration of pancreatic islets, sometimes termed insulitis.
As disease progresses, insulin-producing beta-cell mass becomes markedly reduced.
Islet Autoantibodies
Autoantibodies associated with autoimmune type 1 diabetes can include antibodies directed against several beta-cell-related antigens.
Examples include:
- Glutamic acid decarboxylase
- Insulin
- IA-2
- Zinc transporter 8
Absolute Insulin Deficiency
Severe insulin deficiency prevents normal insulin-mediated glucose utilization and removes important restraints on hepatic glucose production, lipolysis, and ketone production.
This metabolic state explains the tendency toward diabetic ketoacidosis in type 1 diabetes.
Type 2 Diabetes Mellitus
Type 2 diabetes mellitus is characterized by a combination of insulin resistance and progressive impairment of beta-cell function.
The relative contribution of these abnormalities varies among individuals and over the course of the disease.
Insulin Resistance
Insulin resistance refers to reduced biological responsiveness of target tissues to a given concentration of insulin.
Important insulin-sensitive tissues include skeletal muscle, adipose tissue, and the liver.
Compensatory Hyperinsulinemia
During early insulin resistance, pancreatic beta cells may increase insulin secretion to compensate for reduced tissue responsiveness.
This can maintain glucose concentrations near the normal range for a period of time.
Beta-Cell Dysfunction in Type 2 Diabetes
As beta-cell function becomes inadequate relative to the degree of insulin resistance, glucose regulation deteriorates.
Progressive beta-cell dysfunction is therefore a central component of established type 2 diabetes.
Liver in Type 2 Diabetes
Insulin resistance within the liver contributes to inappropriate hepatic glucose production.
Failure to adequately suppress gluconeogenesis and glycogenolysis contributes particularly to fasting hyperglycemia.
Skeletal Muscle in Type 2 Diabetes
Reduced insulin responsiveness in skeletal muscle decreases effective postprandial glucose uptake.
Because skeletal muscle represents a major site of insulin-mediated glucose disposal, this abnormality contributes substantially to hyperglycemia.
Adipose Tissue in Type 2 Diabetes
Insulin resistance in adipose tissue reduces insulin-mediated suppression of lipolysis.
Increased circulating free fatty acids can further influence hepatic and peripheral metabolism.
Type 1 Versus Type 2 Diabetes
| Feature | Type 1 Diabetes | Type 2 Diabetes |
|---|---|---|
| Primary mechanism | Autoimmune beta-cell destruction | Insulin resistance and beta-cell dysfunction |
| Endogenous insulin | Severely reduced or absent after substantial beta-cell loss | Often present, especially early in disease |
| C-peptide | Usually low with established severe beta-cell loss | May be normal or elevated early, declining with beta-cell failure |
| Autoantibodies | Often present in autoimmune disease | Not characteristic |
| Ketoacidosis | Important acute risk | Less common but can occur |
Gestational Diabetes Mellitus
Gestational diabetes mellitus refers to diabetes diagnosed during pregnancy that is not clearly overt diabetes predating pregnancy.
Pregnancy produces physiological changes in insulin sensitivity, and pancreatic beta cells must increase insulin secretion to maintain glucose homeostasis.
Other Specific Types of Diabetes
Diabetes can also result from specific abnormalities affecting insulin production or action.
Examples include:
- Monogenic defects affecting beta-cell function
- Diseases of the exocrine pancreas
- Endocrine disorders that antagonize insulin action
- Selected medications
- Genetic syndromes associated with diabetes
Pancreatic Disease and Diabetes
Extensive pancreatic disease can impair endocrine function by reducing functional islet tissue.
Chronic pancreatitis, pancreatic surgery, pancreatic neoplasms, and other destructive pancreatic processes can therefore produce diabetes.
Hyperglycemia
Hyperglycemia is the defining metabolic abnormality of diabetes mellitus.
It develops when glucose entry into the circulation exceeds effective glucose uptake and storage by peripheral tissues.
Hepatic Glucose Production
Inadequate insulin activity allows excessive hepatic glucose production through glycogenolysis and gluconeogenesis.
This mechanism contributes substantially to fasting hyperglycemia.
Reduced Peripheral Glucose Uptake
Impaired insulin action reduces glucose uptake by insulin-sensitive tissues, particularly skeletal muscle and adipose tissue.
This contributes prominently to postprandial hyperglycemia.
Glucosuria
When the filtered glucose load exceeds the kidney's capacity for glucose reabsorption, glucose appears in the urine, producing glucosuria.
Osmotic Diuresis
Glucose retained within the renal tubular fluid exerts an osmotic effect that increases urinary water loss.
This process is called osmotic diuresis.
Polyuria
Osmotic diuresis produces increased urine volume, resulting in polyuria.
Polydipsia
Water loss from excessive urination increases thirst and can produce polydipsia.
Polyphagia
Some patients develop increased hunger, or polyphagia, despite elevated circulating glucose concentrations.
This can occur because abnormalities of insulin action impair normal cellular nutrient handling and metabolic signaling.
Classic Features of Hyperglycemia
| Feature | Mechanism |
|---|---|
| Polyuria | Glucose-mediated osmotic diuresis |
| Polydipsia | Water loss and increased thirst |
| Glucosuria | Filtered glucose exceeds renal reabsorptive capacity |
| Weight loss | Caloric loss and increased catabolism, particularly with severe insulin deficiency |
| Polyphagia | Altered cellular nutrient utilization and metabolic signaling |
Protein Metabolism
Insulin normally promotes protein synthesis and suppresses excessive protein breakdown.
Severe insulin deficiency therefore shifts metabolism toward protein catabolism, providing amino acids that can contribute to hepatic gluconeogenesis.
Lipid Metabolism
Insulin normally suppresses lipolysis in adipose tissue.
When insulin activity is severely deficient, increased triglyceride breakdown releases large quantities of free fatty acids into the circulation.
Ketone Body Formation
The liver can convert fatty acid-derived substrates into ketone bodies.
Marked insulin deficiency combined with counter-regulatory hormone activity can produce excessive ketone formation.
Diabetic Ketoacidosis
Diabetic ketoacidosis (DKA) is an acute metabolic complication characterized by hyperglycemia, increased ketone production, and metabolic acidosis.
It is particularly associated with severe insulin deficiency and is therefore most characteristic of type 1 diabetes, although it can occur in other forms.
Pathophysiology of DKA
Severe insulin deficiency promotes lipolysis and hepatic ketogenesis while simultaneously allowing hyperglycemia to worsen.
Glucosuria produces osmotic diuresis and substantial fluid and electrolyte loss, while accumulation of acidic ketone bodies causes metabolic acidosis.
Ketone Bodies
The major ketone bodies include:
- Acetoacetate
- Beta-hydroxybutyrate
- Acetone
Features of Diabetic Ketoacidosis
DKA can produce:
- Hyperglycemia
- Dehydration
- Metabolic acidosis
- Ketosis
- Nausea and vomiting
- Abdominal pain
- Deep respiratory compensation
- Altered mental status in severe cases
Hyperosmolar Hyperglycemic State
Hyperosmolar hyperglycemic state (HHS) is another severe acute complication of diabetes, occurring most commonly in people with type 2 diabetes.
It is characterized by profound hyperglycemia, hyperosmolarity, and dehydration, usually without the degree of ketoacidosis characteristic of DKA.
Why Ketosis Is Often Less Prominent in HHS
Many patients with HHS retain sufficient insulin activity to suppress extensive lipolysis and ketogenesis while still having insufficient insulin action to maintain normal glucose concentrations.
DKA Versus HHS
| Feature | DKA | HHS |
|---|---|---|
| Common association | Type 1 diabetes | Type 2 diabetes |
| Ketosis | Prominent | Usually limited |
| Metabolic acidosis | Characteristic | Not the dominant abnormality |
| Hyperglycemia | Present | Often profound |
| Dehydration | Present | Often severe |
| Hyperosmolarity | Variable | Prominent |
Diagnosis of Diabetes Mellitus
Diabetes is diagnosed using standardized measurements of blood glucose or glycated hemoglobin.
Testing can include fasting plasma glucose, an oral glucose tolerance test, glycated hemoglobin, or random plasma glucose in an appropriate clinical setting.
Fasting Plasma Glucose
Fasting plasma glucose measures circulating glucose after an appropriate period without caloric intake.
Persistent elevation can indicate impaired glucose regulation or diabetes depending on the measured concentration.
Oral Glucose Tolerance Test
The oral glucose tolerance test evaluates the body's ability to regulate blood glucose after a standardized oral glucose load.
Hemoglobin A1c
Hemoglobin A1c (HbA1c) reflects glycation of hemoglobin and provides an estimate of average glycemic exposure over the preceding several months.
Its interpretation can be affected by conditions that alter erythrocyte lifespan or hemoglobin composition.
Random Plasma Glucose
A markedly elevated random plasma glucose concentration in a person with classic symptoms of hyperglycemia or hyperglycemic crisis can support the diagnosis of diabetes.
Chronic Complications
Persistent hyperglycemia can progressively damage blood vessels and tissues throughout the body.
Long-term complications are commonly divided into microvascular and macrovascular disease.
Microvascular Complications
Major microvascular complications include:
- Diabetic retinopathy
- Diabetic nephropathy
- Diabetic neuropathy
Diabetic Retinopathy
Diabetic retinopathy results from chronic injury to the retinal microvasculature.
Progressive vascular abnormalities can impair retinal function and threaten vision.
Diabetic Nephropathy
Diabetic nephropathy involves progressive injury to renal glomeruli and associated structures.
It can lead to albuminuria, declining renal function, and chronic kidney disease.
Diabetic Neuropathy
Diabetic neuropathy encompasses several patterns of peripheral and autonomic nerve injury associated with diabetes.
Distal symmetric sensory and sensorimotor neuropathy is a common presentation.
Macrovascular Complications
Diabetes substantially increases the burden of atherosclerotic vascular disease.
Major clinical manifestations can involve the coronary, cerebral, and peripheral arterial circulations.
Coronary Artery Disease
Atherosclerotic disease of the coronary arteries increases the risk of myocardial ischemia and myocardial infarction in people with diabetes.
Cerebrovascular Disease
Diabetes is associated with increased risk of atherosclerotic cerebrovascular disease and ischemic stroke.
Peripheral Arterial Disease
Peripheral arterial disease can reduce blood flow to the lower limbs and contribute to impaired tissue healing and ischemic complications.
Diabetic Foot Disease
Foot complications in diabetes frequently result from interactions among peripheral neuropathy, vascular disease, altered biomechanics, infection, and impaired wound healing.
Loss of protective sensation can allow repetitive tissue injury to progress without being recognized.
Autonomic Neuropathy
Diabetes can damage autonomic nerves supplying multiple organ systems.
Potential manifestations include abnormalities of cardiovascular reflexes, gastrointestinal motility, bladder function, sweating, and sexual function.
Mechanisms of Chronic Tissue Injury
Long-term exposure to hyperglycemia produces multiple biochemical and vascular abnormalities that contribute to tissue injury.
These include altered protein glycation, oxidative stress, abnormal intracellular signaling, endothelial dysfunction, and microvascular structural changes.
Advanced Glycation End Products
Persistent hyperglycemia promotes nonenzymatic glycation of proteins and formation of advanced glycation end products (AGEs).
AGE accumulation can alter extracellular matrix proteins, vascular function, cellular signaling, and inflammatory responses.
Diabetic Microangiopathy
Chronic diabetes can produce characteristic abnormalities of small blood vessels, including basement membrane thickening and endothelial dysfunction.
These changes contribute to retinal, renal, and neural complications.
Pancreatic Morphology in Diabetes
The anatomical and histological changes of the pancreas differ between types of diabetes.
Type 1 diabetes is associated with loss of pancreatic beta cells, while type 2 diabetes is associated with functional beta-cell abnormalities and can demonstrate structural changes within the pancreatic islets.
Islet Amyloid in Type 2 Diabetes
Pancreatic islets in type 2 diabetes can contain deposits of amyloid derived largely from islet amyloid polypeptide, also known as amylin.
Islet amyloid deposition is associated with beta-cell dysfunction and loss in established disease.
Counter-Regulatory Hormones
Several hormones oppose important metabolic actions of insulin and help protect against hypoglycemia.
These include:
- Glucagon
- Epinephrine
- Cortisol
- Growth hormone
Stress and Hyperglycemia
Acute illness and physiological stress can increase concentrations of counter-regulatory hormones.
These hormones increase hepatic glucose production and can reduce effective insulin action, worsening hyperglycemia in susceptible individuals.
Endocrine Disorders and Secondary Diabetes
Excess production of hormones that antagonize insulin action can contribute to diabetes.
Examples include disorders associated with excessive glucocorticoids, growth hormone, or catecholamines.
Cortisol Excess
Excessive glucocorticoid activity promotes hepatic glucose production and reduces insulin sensitivity.
Hyperglycemia and diabetes can therefore occur in patients with significant glucocorticoid excess.
Growth Hormone Excess
Growth hormone has important anti-insulin metabolic effects.
Excess growth hormone, as occurs in acromegaly, can produce substantial insulin resistance and impaired glucose tolerance.
Catecholamine Excess
Excessive catecholamine activity can increase hepatic glucose production, alter pancreatic hormone secretion, and promote metabolic conditions that favor hyperglycemia.
Insulin Treatment
Exogenous insulin replaces or supplements endogenous insulin activity.
It is essential in type 1 diabetes and is also used in many patients with type 2 diabetes, gestational diabetes, and other forms of diabetes when clinically indicated.
Hypoglycemia
Hypoglycemia is an important potential complication of glucose-lowering treatment, particularly insulin and medications that stimulate insulin secretion.
The brain depends heavily on circulating glucose, so severe hypoglycemia can produce neurological dysfunction.
Autonomic Response to Hypoglycemia
Falling glucose concentrations activate counter-regulatory and autonomic responses.
Adrenergic manifestations can include tremor, palpitations, anxiety, and sweating.
Neuroglycopenia
Neuroglycopenia refers to impaired brain function caused by inadequate glucose availability.
Manifestations can include difficulty concentrating, behavioral changes, confusion, seizures, and loss of consciousness in severe cases.
Key Features of Diabetes Mellitus
| Feature | Key Point |
|---|---|
| Defining abnormality | Chronic hyperglycemia |
| Primary endocrine organ | Pancreatic islets |
| Major glucose-lowering hormone | Insulin |
| Insulin-producing cells | Pancreatic beta cells |
| Glucagon-producing cells | Pancreatic alpha cells |
| Type 1 mechanism | Autoimmune beta-cell destruction |
| Type 2 mechanism | Insulin resistance with progressive beta-cell dysfunction |
| Major type 1 acute complication | Diabetic ketoacidosis |
| Major type 2 acute complication | Hyperosmolar hyperglycemic state |
| Microvascular complications | Retinopathy, nephropathy and neuropathy |
| Macrovascular complications | Atherosclerotic cardiovascular, cerebrovascular and peripheral arterial disease |
Anatomical and Physiological Importance
Diabetes mellitus demonstrates the systemic importance of the relatively small endocrine component of the pancreas. Pancreatic beta cells detect changes in nutrient availability and release insulin into the circulation, allowing distant tissues such as skeletal muscle, adipose tissue, and the liver to coordinate glucose utilization and storage.
In type 1 diabetes, destruction of beta cells removes this essential endocrine signal and produces severe insulin deficiency. In type 2 diabetes, insulin remains available but target tissues become resistant to its actions while beta cells progressively lose their ability to compensate. Although the initiating mechanisms differ, both pathways ultimately disrupt normal glucose homeostasis and produce chronic hyperglycemia.
The anatomical consequences extend far beyond the pancreas. Persistent metabolic abnormalities progressively affect the microvasculature of the retina, renal glomeruli, peripheral nerves, and other tissues, while simultaneously accelerating disease in larger arteries. Diabetes mellitus therefore represents both an endocrine disorder of metabolic regulation and a systemic disease capable of producing widespread structural and functional complications.
Published on September 29, 2026
Last updated on September 29, 2026
Last updated on September 29, 2026