Metabolic Disorders 3: Diabetes Mellitus - A Multifactorial Disease

Introduction

This section focuses on Diabetes Mellitus, one of the most common metabolic diseases globally, affecting over 5% of the population and representing an increasing health challenge. We will explore:

The normal metabolic states of fasting/starvation and the fed state, including hormonal regulation by insulin and glucagon.
The general concept of diseases affecting fuel utilization.
Different types of Diabetes Mellitus: Type 1, Type 2, Gestational Diabetes, and MODY.
The pathogenesis, genetics, potential triggers, clinical manifestations (including ketoacidosis), and treatment approaches for Type 1 Diabetes.
The characteristics, epidemiology, risk factors (obesity, ethnicity, genetics), and pathophysiology of Type 2 Diabetes, with a focus on insulin resistance and β-cell dysfunction.
The mechanisms of insulin resistance, including the role of TNF-α and adiponectin.
Gestational Diabetes Mellitus: its causes, risks, and management.
Diagnosis of diabetes, including blood glucose tests and HbA1c.
Acute and long-term complications of Diabetes Mellitus (macrovascular and microvascular).
The role of exercise in managing Type 2 Diabetes.

Normal Fuel Homeostasis: Fasting/Starvation and Fed States

The body tightly regulates fuel utilization depending on nutritional status.

A. Fasting/Starvation State (Post-Absorptive):

Hormonal Profile: Low insulin, high glucagon. Cortisol levels may also rise with prolonged fasting/stress.
Metabolic Shifts:
- Early Post-Absorptive State (4-16 hours after a meal):
  - Blood glucose levels begin to fall, triggering glucagon release from pancreatic α-cells.
  - Hepatic glycogenolysis (breakdown of liver glycogen to glucose) is the initial major source for maintaining blood glucose.
  - Lipolysis in adipose tissue is initiated, releasing free fatty acids (FFAs) into circulation as an alternative fuel source.
- Late Post-Absorptive State (Early Fasting, up to 3 days):
  - Hepatic glycogen stores become depleted.
  - Hepatic gluconeogenesis (synthesis of new glucose from non-carbohydrate precursors like amino acids, lactate, glycerol) becomes the primary source of blood glucose.
  - Cortisol stimulates muscle protein degradation, providing amino acids (e.g., alanine) for gluconeogenesis.
  - Glucagon and cortisol stimulate lipolysis in adipose tissue; FFAs are exported to the liver and other tissues for fuel, sparing glucose for the brain.
- Prolonged Fasting/Starvation State (Beyond 2-3 days):
  - The body aims to spare protein.
  - Ketogenesis: The liver converts FFAs into ketone bodies (acetoacetate, β-hydroxybutyrate).
    - Excessive acetyl-CoA from fatty acid oxidation, coupled with a lack of oxaloacetate (which is diverted to gluconeogenesis), leads to acetyl-CoA accumulation and shunting into ketone body synthesis.
  - The brain adapts to utilize ketone bodies as a major fuel source (70-80%), reducing its reliance on glucose.
  - This reduces the need for gluconeogenesis from amino acids, thus "sparing" muscle protein.
  - The kidney also becomes a major gluconeogenic tissue.
  - When fat stores are exhausted, protein degradation increases again, leading to irreversible organ damage.
Blood Levels During Fasting (Trends):
- Glucose: Decreases and then stabilizes at a lower level.
- Insulin: Decreases.
- Glucagon: Increases.
- Free Fatty Acids: Increase.
- Ketone Bodies: Increase significantly after a few days.

B. Fed State (Postprandial - 0-4 hours after a meal):

Hormonal Profile: High insulin (released from pancreatic β-cells in response to rising blood glucose and amino acids), low glucagon.
Metabolic Shifts (Mixed Diet):
- Carbohydrates: Absorbed primarily as glucose, causing a rise in blood glucose.
  - Insulin promotes glucose uptake into insulin-sensitive tissues:
    - Muscle: Increased glucose uptake via GLUT4 transporters (translocated to the cell membrane); increased glycolysis and glycogen synthesis.
    - Adipose Tissue: Increased glucose uptake via GLUT4; glucose is converted to glycerol-3-phosphate for Triacylglycerol (TAG) synthesis.
  - Liver: Insulin promotes glycogen synthesis and glycolysis. Excess glucose can be converted into fatty acids (via acetyl-CoA) and then TAGs, which are packaged into Very-Low-Density Lipoproteins (VLDLs) and exported.
  - Overall effect: Blood glucose concentration is reduced back to baseline.
- Proteins: Amino acids are absorbed and distributed to tissues for protein synthesis. Excess amino acids are degraded in the liver (carbon skeletons can be used for energy or gluconeogenesis/lipogenesis; nitrogen converted to urea).
- Fats: Dietary TAGs are transported via chylomicrons. Lipoprotein lipase (activated by ApoC-II on chylomicrons, activity enhanced by insulin) hydrolyzes TAGs in chylomicrons, releasing fatty acids for uptake by tissues (e.g., adipose tissue for storage, muscle for energy). Chylomicron remnants are taken up by the liver. Excess dietary fatty acids taken to the liver can be re-esterified into TAGs and packaged into VLDLs.

Diabetes Mellitus: "Starvation in a Sea of Plenty"

Diabetes Mellitus is a group of metabolic diseases characterized by chronic hyperglycemia (elevated blood glucose) resulting from defects in insulin secretion, insulin action, or both. Glucose entry into many cells is impaired, leading to a paradoxical state where cells are "starving" for glucose despite high levels in the blood.

Classification:

Type 1 Diabetes Mellitus (Insulin-Dependent Diabetes Mellitus - IDDM): ~10% of cases.
Type 2 Diabetes Mellitus (Non-Insulin-Dependent Diabetes Mellitus - NIDDM): ~90% of cases.
Gestational Diabetes Mellitus (GDM): Develops or is first recognized during pregnancy.
Maturity Onset Diabetes of the Young (MODY): Rare monogenic forms.

A. Type 1 Diabetes Mellitus (T1DM)

Primary Defect: Absolute failure of pancreatic β-cells to produce insulin, leading to insulin deficiency.
Pathogenesis (Type 1A - Immune-Mediated):
- Autoimmune destruction of β-cells.
- ~85% of patients have detectable autoantibodies against pancreatic islet cell components (Islet Cell Antibodies - ICA), Glutamic Acid Decarboxylase (GAD Antibodies - GADA), and/or Insulin Autoantibodies (IAAs). These antibodies may be detectable long before clinical presentation.
- The number of different autoantibodies present correlates with the risk of developing T1DM (e.g., ~10% of the general population may have one antibody, but the risk increases significantly with multiple antibodies). Concordance rate in monozygotic twins is <40%, suggesting environmental triggers are important.
Pathogenesis (Type 1B - Idiopathic):
- Rare form with β-cell destruction but no evidence of autoimmunity.
- Mainly occurs in individuals of African or Asian descent.
- Strongly inherited.
- Characterized by episodic ketoacidosis due to varying degrees of insulin deficiency, with periods of absolute insulin deficiency.
Onset: Mainly in the young ("juvenile onset diabetes"), but can occur at any age. β-cell destruction rate is usually rapid in children but can be slower in adults (Latent Autoimmune Diabetes in Adults - LADA).
Genetic Susceptibility: Associated with certain HLA (Human Leukocyte Antigen) haplotypes.
Potential Environmental Triggers:
- Viral infections (e.g., Coxsackie B, congenital Rubella, possibly COVID-19).
- Dietary factors (e.g., early exposure to cow's milk - bovine insulin, wheat peptides).
- Chemicals (e.g., Nitrosamines).
- Vitamin D deficiency.
Metabolic Consequences of Insulin Deficiency:
- Hyperglycemia:
  - Despite high blood glucose, insulin remains low.
  - Liver: Gluconeogenesis and glycogenolysis continue inappropriately (not suppressed by insulin). Glycolysis and glycogenesis are not stimulated. The liver actively contributes to hyperglycemia.
  - Peripheral Tissues (muscle, adipose): Glucose uptake via GLUT4 is severely impaired due to lack of insulin stimulation. (Slide notes "excessive uptake into peripheral tissues" which seems contradictory to impaired GLUT4; typically, insulin-dependent tissues have reduced uptake. Non-insulin dependent tissues like brain might take up more if glucose is very high, but the primary issue is lack of uptake into muscle/fat).
- Glycosuria and Osmotic Diuresis: When blood glucose levels exceed the renal threshold for reabsorption, glucose spills into the urine (glycosuria). Glucose in the urine acts as an osmotic agent, drawing water with it, leading to excessive urine production (polyuria), dehydration, and increased thirst (polydipsia).
- Muscle Wasting: Lack of insulin promotes muscle protein degradation to provide amino acids for hepatic gluconeogenesis.
- Ketoacidosis:
  - Low insulin leads to unrestrained lipolysis, releasing large amounts of FFAs.
  - In the liver, excessive FFA oxidation generates large amounts of acetyl-CoA.
  - Due to ongoing gluconeogenesis, oxaloacetate levels in the liver are low (diverted to glucose synthesis), preventing acetyl-CoA from entering the citric acid cycle efficiently.
  - Acetyl-CoA accumulation is shunted towards ketogenesis, leading to the production of ketone bodies (acetoacetate, β-hydroxybutyrate, acetone).
  - The body is essentially "stuck" in a state of starvation metabolism.
  - Excessive ketone body production leads to ketonemia (ketones in blood) and ketonuria (ketones in urine).
  - Ketone bodies are acidic, and their accumulation causes metabolic acidosis, known as Diabetic Ketoacidosis (DKA). This is a life-threatening condition that can lead to compensatory hyperventilation (Kussmaul breathing), dehydration, coma, and death if untreated.
Treatment:
- Insulin injections: Essential for survival. Various insulin preparations (rapid, short, intermediate, long-acting) are used in complex regimens to mimic physiological insulin profiles.
- Dietary management.
- Blood Glucose Monitoring: Frequent self-monitoring of blood glucose concentrations (using reagent strips/glucose oxidase and glucose meters) is required to adjust insulin doses and maintain glucose homeostasis, preventing both hyperglycemia and hypoglycemia.
- Pancreas or Islet Cell Transplant: An option for some patients, but requires immunosuppression.
- Immunomodulatory therapies (experimental): Aimed at preventing or halting autoimmune β-cell destruction.

B. Maturity Onset Diabetes of the Young (MODY)

Definition: A group of monogenic (caused by a single gene defect) forms of diabetes, typically with autosomal dominant inheritance.
Prevalence: Accounts for ~1-2% of diabetics.
Characteristics:
- Usually presents as a mild form of diabetes, often diagnosed in adolescence or young adulthood, but can occur at any age.
- Characterized by impaired glucose-stimulated insulin secretion from pancreatic β-cells, but generally without insulin resistance (at least initially).
- Patients may experience transient hyperglycemia.
Genetic Defects (MODY 1-6 and more): Due to mutations in specific genes crucial for β-cell function, insulin synthesis, or insulin secretion.
- Glucokinase (MODY 2): Inactivating mutations in the glucokinase gene. Glucokinase acts as the glucose sensor in β-cells, phosphorylating glucose to glucose-6-phosphate (the first step in glycolysis). Defective glucokinase leads to a higher threshold for glucose-stimulated insulin secretion (↓ ATP produced from glucose metabolism -> KATP+ channels remain open -> no depolarization -> ↓ insulin release), resulting in mild, stable hyperglycemia.
- Transcription Factors (MODY 1, 3, 4, 5, 6): Mutations in genes encoding transcription factors important for pancreatic development or β-cell function (e.g., HNF4A - MODY1, HNF1A - MODY3, IPF1 - MODY4, HNF1B - MODY5, NEUROD1 - MODY6) can lead to decreased transcription of β-cell genes involved in insulin synthesis, storage, and secretion.
- KATP+ Channel Mutations: Mutations in genes encoding subunits of the ATP-sensitive potassium (KATP+) channel (e.g., KCNJ11, ABCC8) can cause neonatal diabetes by preventing channel closure in response to ATP, thus inhibiting insulin secretion.

C. Gestational Diabetes Mellitus (GDM)

Definition: Glucose intolerance with onset or first recognition during pregnancy. Affects ~3-5% of pregnancies.
Timing: Hyperglycemia usually becomes apparent around 24-28 weeks of gestation.
Pathophysiology:
- Normal Pregnancy Insulin Resistance: Insulin resistance is a normal physiological phenomenon that emerges in the second trimester and progresses. It functions to:
  - Secure a continuous glucose supply to the developing fetus.
  - Avoid maternal hypoglycemia between meals.
- Hormonal Basis: Maternal and placental hormones contribute to this insulin resistance:
  - Human Placental Lactogen (HPL): Levels increase with fetal and placental growth. HPL stimulates lipolysis (increasing maternal FFAs) and has anti-insulin effects, increasing maternal blood glucose to ensure adequate nutrient supply to the fetus.
  - Tumor Necrosis Factor-α (TNF-α): Secretion by the placenta increases in the 2nd and 3rd trimesters and contributes to insulin resistance.
  - Other placental hormones (e.g., progesterone, estrogen, cortisol) also play a role.
- Development of GDM: If a pregnant woman cannot adequately increase her insulin secretion to overcome this physiological insulin resistance (due to underlying β-cell dysfunction or excessive insulin resistance), GDM develops.
Risks:
- For the Mother: Increased risk of pre-eclampsia, development of Type 2 diabetes later in life, and increased likelihood of requiring a Caesarean section.
- For the Child:
  - Macrosomia (large baby): Maternal hyperglycemia leads to fetal hyperglycemia, stimulating fetal insulin production. Insulin is a growth factor, causing the baby to "put on extra weight" (increased fat deposition).
  - Neonatal Hypoglycemia: After birth, the high fetal insulin levels persist for a while, but the maternal glucose supply is cut off, leading to a rapid drop in the newborn's blood glucose.
  - Increased risk of developing Type 2 diabetes later in life.
Management: Usually through diet control, exercise advice, and if necessary, anti-diabetic drugs (e.g., metformin) or insulin.

D. Type 2 Diabetes Mellitus (T2DM)

Prevalence: Most common form of diabetes (~90% of cases).
Primary Defects: A combination of:
1. Insulin Resistance: Target tissues (muscle, liver, adipose tissue) show a blunted response to normal or even high levels of insulin.
2. β-Cell Dysfunction: Pancreatic β-cells are unable to produce enough insulin to overcome the insulin resistance. This is often a progressive failure.
Pathogenesis:
- Genetic Predisposition: Strong genetic link (concordance rate >90% in monozygotic twins). It is a polygenic disease (not associated with HLA antigens like T1DM).
- Environmental Factors/Triggers: Lifestyle (sedentary behavior, unhealthy diet) and overeating are major triggers, often leading to obesity.
- Insulin Resistance Development:
  - Obesity: Particularly central (abdominal/visceral) obesity is strongly linked to insulin resistance. Waist circumference or waist:hip ratio are good indicators.
    - Ectopic Lipid Storage: Obesity results in the storage of lipids in non-adipose tissues like the liver (leading to non-alcoholic fatty liver disease - NAFLD) and skeletal muscle. This can impair insulin signaling.
    - Inflammation: Adipose tissue, especially in obesity, becomes a source of pro-inflammatory molecules. Chronic low-level inflammation is associated with insulin resistance and cardiovascular disease (CVD). Ectopic lipid accumulation in the liver can lead to reduced HDL cholesterol and increased triglycerides (TGs); the TG:HDL ratio and Total:HDL cholesterol ratio show strong correlations with insulin resistance.
  - Adipokines: Molecules released from adipocytes play a role:
    - Tumor Necrosis Factor-α (TNF-α): Secreted from adipocytes (especially visceral). Increased in obesity. TNF-α can impair insulin signaling by, for example, promoting serine phosphorylation of Insulin Receptor Substrate-1 (IRS-1) via activation of kinases like JNK. Phosphorylated IRS-1 (on serine residues) can no longer effectively bind to the insulin receptor or PI3-kinase, thus inhibiting downstream insulin action (e.g., ↓ glycogen synthesis, ↓ GLUT4 translocation in skeletal muscle).
    - Adiponectin: A hormone secreted by adipose tissue that normally enhances insulin sensitivity (e.g., by increasing glucose uptake and fatty acid oxidation in muscle/liver, decreasing hepatic gluconeogenesis). In obesity, TNF-α levels are increased, which reduces plasma adiponectin levels. Decreased adiponectin contributes to increased plasma FFAs and glucose, exacerbating insulin resistance and hyperglycemia. Polymorphisms in the adiponectin gene (e.g., SNP276, SNP45) are linked to plasma adiponectin levels, insulin resistance, and T2DM risk. Adiponectin is a strong biochemical predictor of T2DM.
  - Free Fatty Acids (FFAs): Elevated FFAs in obesity can also contribute to insulin resistance via serine phosphorylation of IRS-1.
  - Receptor Downregulation: Constant high glucose and insulin levels (due to overeating) can lead to downregulation of insulin receptors (e.g., via endocytosis).
- β-Cell Dysfunction and Failure:
  - Initially, β-cells compensate for insulin resistance by increasing insulin secretion (hyperinsulinemia). This may maintain normal glucose tolerance for some time.
  - Over time, particularly with genetic predisposition and the toxic effects of chronic hyperglycemia ("glucotoxicity") and high FFAs ("lipotoxicity"), β-cells begin to fail. Their ability to secrete insulin diminishes.
  - When β-cell function can no longer overcome insulin resistance, overt hyperglycemia and T2DM develop.
- Onset: Usually subacute, may take months or years to become clinically apparent, or may be asymptomatic for a long time. Ketoacidosis is rare (unless under major stress like severe infection) because there is usually enough residual insulin to prevent excessive ketogenesis.
Epidemiology:
- Major contributing factors: Age (mainly adults >40 years), obesity (increases risk 80-100 fold), ethnicity (2-4 times more common in South Asian, African, and Caribbean people living in the UK compared to general UK population).
- UK incidence ~6-10%; lifetime risk ~15-25%.
- Prevalence by ethnic groups (US data example): General Population 5.2%, African Descent 10.6%, Latin American Descent 10.2%, Native American Descent 12.2%, Pima Indians 50%.
Levels of Insulin Resistance: Can occur at:
- Pre-receptor: e.g., insulin autoantibodies, mutant insulin structure (rare).
- Receptor: e.g., blocking antibodies, decreased receptor number, decreased receptor affinity (altered structure).
- Post-receptor: Most common. Impaired insulin receptor tyrosine kinase activity or defects in downstream signaling molecules (e.g., IRS-1, PI3-kinase, Akt), leading to downregulation or impaired translocation/function of GLUT4.
Glucose Transporters (GLUTs):
- Facilitate glucose uptake into cells. Different GLUTs are tissue-specific and have different kinetic properties (Km).
  - GLUT2: Liver, pancreatic β-cells. High Km (low affinity, e.g., 20 mmol/l), so glucose uptake rate is proportional to extracellular glucose concentration over a wide physiological range. Important for glucose sensing in β-cells.
  - GLUT3: Brain, nerve tissue. Low Km (high affinity, e.g., 1.6 mmol/l), allowing relatively constant glucose uptake even at low blood glucose levels.
  - GLUT4: Muscle, adipose tissue. Insulin-sensitive. Km around 5 mmol/l. Translocates from intracellular vesicles to the plasma membrane in response to insulin.
- Faulty GLUT2 or KATP+ channels, or defects in insulin synthesis/release/storage, or issues with pre-receptor/receptor/post-receptor signaling can all contribute to reduced insulin action and/or secretion.

Diagnosis of Diabetes Mellitus

Signs and Symptoms: Polyuria (excessive urination), polydipsia (excessive thirst), unexplained weight loss (especially in T1DM), fatigue, blurred vision, recurrent infections. Many T2DM patients may be asymptomatic initially.
Diagnostic Tests:
- Elevated Fasting Blood Glucose (FBG):
  - FBG ≥ 7.0 mmol/L on two occasions indicates diabetes.
  - FBG 6.1 - 6.9 mmol/L indicates Impaired Fasting Glycaemia (IFG) - prediabetes.
  - FBG ≤ 6.0 mmol/L is normal.
- Elevated Random Blood Glucose: Random plasma glucose ≥ 11.1 mmol/L with symptoms suggests diabetes.
- Oral Glucose Tolerance Test (OGTT):
  - Patient ingests a standard glucose load (e.g., 75g anhydrous glucose) after an overnight fast.
  - Plasma glucose is measured at baseline (fasting) and 2 hours post-load.
  - Diagnosis:
    - Normal: Fasting < 6.1 mmol/L AND 2-hour < 7.8 mmol/L.
    - Impaired Fasting Glycaemia (IFG): Fasting 6.1 - 6.9 mmol/L AND 2-hour < 7.8 mmol/L.
    - Impaired Glucose Tolerance (IGT): Fasting < 7.0 mmol/L AND 2-hour 7.8 - 11.0 mmol/L.
    - Diabetes Mellitus: Fasting ≥ 7.0 mmol/L OR 2-hour ≥ 11.1 mmol/L.
- Elevated HbA1c (Glycated Haemoglobin):
  - Reflects average blood glucose control over the preceding 2-3 months.
  - HbA1c ≥ 48 mmol/mol (6.5%) is diagnostic of diabetes.
  - HbA1c 42-47 mmol/mol (6.0-6.4%) indicates high risk of diabetes (prediabetes).
Prediabetes (IFG or IGT): Indicates elevated blood glucose levels, but not yet meeting diabetic criteria. Individuals are at increased risk of developing T2DM, insulin resistance, and CVD. It is a mortality risk factor.

Comparison of Type 1 and Type 2 Diabetes Mellitus:

Feature	Type 1 DM	Type 2 DM
Onset	Usually < 20 years (can be any age)	Usually > 40 years (increasing in younger individuals)
Insulin Synthesis	Absent; immune destruction of β-cells	Preserved (initially), then impaired β-cell function; insulin resistance
Plasma Insulin Conc.	Low or absent	Low, normal, or high (depending on stage)
Genetic Susceptibility	Yes, assoc. with HLA antigens	Strong polygenic inheritance, not assoc. with HLA
Islet Cell Antibodies	Yes (GADA, ICA, IAA) at diagnosis	No
Obesity	Uncommon	Common
Ketoacidosis	Yes (common if untreated)	Possible after major stress, but rare

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Complications of Diabetes Mellitus

Chronic hyperglycemia and associated metabolic disturbances lead to long-term complications.

A. Acute Complications:

Diabetic Ketoacidosis (DKA): Mainly in T1DM (see above). Often occurs prior to diagnosis or during illness/stress (cortisol release).
Hyperosmolar Non-Ketotic Coma (HONK) / Hyperosmolar Hyperglycemic State (HHS): Mainly in T2DM, especially elderly. Severe hyperglycemia and dehydration without significant ketoacidosis. Often triggered by concurrent illness and inability to take normal diabetic therapy or maintain fluid intake. High mortality (e.g., 30%).
Hypoglycemic Coma:
- Causes: Insulin overdose (most common), sulphonylurea overdose (in T2DM), alcohol (decreases gluconeogenesis), increased exercise without a_Fjusting medication/food, missed meals.
- Symptoms: Due to decreased glucose delivery to the brain. Changes in neural firing rates lead to headache, dizziness, irritability, fatigue, confusion, blurry vision, hunger, sweating, tremors, and if severe, seizures and coma.

B. Long-Term Complications: Prevalence of all complications increases with the duration of diabetes and the degree of glycemic control. All are related to the damaging effects of chronic hyperglycemia.

Macrovascular Complications: Related to atherosclerosis (hardening and narrowing of large arteries) affecting coronary arteries (Coronary Heart Disease - CHD), cerebral arteries (stroke), and peripheral arteries (Peripheral Vascular Disease - PVD). Often linked to insulin resistance syndrome (metabolic syndrome).
- Pathophysiology of Atherosclerosis in Diabetes:
  - Hyperglycemia leads to increased formation of Advanced Glycation Endproducts (AGEs) both intra- and extracellularly.
  - AGEs can bind to and modify LDL receptors, decreasing LDL uptake by systemic cells and increasing LDL uptake by macrophages (via scavenger receptors), promoting foam cell formation.
  - Damage to endothelium and basement membrane of blood vessels occurs.
  - Vasculature becomes leaky, allowing invasion by inflammatory cells (monocytes transmigrate) and lipids into the arterial intima.
  - Increased pro-inflammatory and pro-fibrotic molecules.
  - Increased Reactive Oxygen Species (ROS) production.
  - Smooth muscle cells migrate and proliferate, matrix synthesis increases, leading to fibrous cap formation over a lipid pool. Rupture of this plaque cap can lead to thrombosis and acute events like myocardial infarction or stroke.
Microvascular Complications: Directly linked to prolonged exposure to high glucose concentrations. The exact mechanisms are complex but involve:
- Polyol Pathway Activation: Excess intracellular glucose in non-insulin dependent cells (e.g., nerve cells, retinal cells, kidney glomeruli) overwhelms glycolysis and is shunted down the polyol pathway.
  - Aldose reductase converts glucose to sorbitol, consuming NADPH.
  - Sorbitol dehydrogenase converts sorbitol to fructose.
- Consequences of Polyol Pathway Activity:
  - Decreased NADPH: NADPH is required for the regeneration of reduced glutathione (GSH), a major antioxidant. Reduced GSH leads to increased susceptibility to ROS damage.
  - Increased Sorbitol: Accumulation of sorbitol can cause osmotic stress in cells.
  - Increased Fructose: Fructose can be a precursor for AGE formation.
- Advanced Glycation Endproduct (AGE) Formation: Non-enzymatic glycation of proteins and lipids by glucose and its metabolites leads to the formation of AGEs. AGEs can cross-link proteins (e.g., collagen in basement membranes, making them thicker and less functional), alter protein function, bind to RAGE (Receptor for AGEs) on cells, and promote inflammation and ROS production.
- Redox Imbalance and Oxidative Stress.
- Protein Kinase C (PKC) Activation.
- Hexosamine Pathway Activation.
- Decreased Nitric Oxide (NO) bioavailability: Leads to impaired vasodilation and reduced blood flow through capillaries.
- Changes to Endothelial Cells, Basement Membrane, and ECM: Basement membranes of capillaries thicken, become leaky.
- Specific Microvascular Complications:
  - Diabetic Nephropathy (Kidney Disease):
    - Leading cause of end-stage renal failure (ESRF) in the Western world.
    - Leading cause of premature death in young diabetics.
    - ~40% of diabetics will develop nephropathy.
    - Hyperglycemia damages small blood vessels (glomeruli) in the kidney, leading to proteinuria (albuminuria), decreased Glomerular Filtration Rate (GFR), and eventual renal failure.
  - Diabetic Retinopathy (Eye Disease):
    - Damage to blood vessels in the retina.
    - Non-proliferative retinopathy: Microaneurysms, hemorrhages, exudates.
    - Proliferative retinopathy (more severe): Blood supply to the retina decreases. Damaged vessels leak and burst; new, fragile blood vessels grow (neovascularization) in response to growth factors (e.g., VEGF). These can bleed easily, obscure vision, and lead to scar tissue formation, which can pull and distort the retina, potentially causing retinal detachment and blindness. Laser surgery is a treatment.
  - Diabetic Neuropathy (Nerve Damage):
    - Most common complication. Chronic high blood glucose damages blood vessels supplying nerves (vasa nervorum), leading to ischemia and direct damage to nerve fibers. Leaky capillaries also contribute. Nerves shrivel as blood vessels disappear.
    - Peripheral Neuropathy: Affects sensory and motor nerves, typically in a "stocking-glove" distribution. Loss of temperature, pain, and touch sensation (increases risk of foot ulcers and amputations). Muscle weakness.
    - Autonomic Neuropathy: Affects nerves controlling involuntary functions. Can lead to defects in vasomotor responses (orthostatic hypotension), gastroparesis, erectile dysfunction, bladder dysfunction.

Treatment of Diabetes Mellitus

Type 1 DM:
- Diet: Balanced diet with carbohydrate counting.
- Insulin Therapy: Essential, via injections or insulin pump.
- Pancreas Transplant / Islet Cell Transplant: For selected patients.
- Immunomodulatory Therapies (research): To prevent or halt autoimmune attack.
Type 2 DM:
- Diet: Healthy eating, weight management.
- Exercise: Crucial for improving insulin sensitivity, weight control, lipid profiles, and blood pressure. Exercise increases GLUT4 expression on muscle cells.
- Oral Hypoglycemic Drugs and/or Injectable Medications: Various classes of drugs are used to lower blood glucose:
  - Metformin (increases insulin sensitivity, reduces hepatic glucose output).
  - Sulphonylureas (stimulate insulin secretion).
  - Thiazolidinediones (TZDs) (increase insulin sensitivity).
  - DPP-4 inhibitors (enhance incretin effect).
  - SGLT2 inhibitors (increase urinary glucose excretion).
  - GLP-1 receptor agonists (enhance insulin secretion, suppress glucagon, slow gastric emptying, promote satiety).
  - Insulin (may be required as β-cell function declines).

Learning Outcomes Summary

Understand the normal metabolic states (fed, fasting/starvation) and the roles of insulin and glucagon.
Differentiate between Type 1, Type 2, Gestational Diabetes, and MODY in terms of cause, pathophysiology, and typical onset.
Understand how insulin deficiency (T1DM) leads to hyperglycemia, ketoacidosis, and other metabolic derangements.
Understand the concepts of insulin resistance and β-cell dysfunction in the pathogenesis of T2DM, including the role of obesity, adipokines (TNF-α, adiponectin), and genetic predisposition.
Describe the diagnostic criteria for diabetes and prediabetes.
Understand the acute complications (DKA, HONK, hypoglycemia) and the pathophysiology of long-term macrovascular (atherosclerosis) and microvascular (retinopathy, nephropathy, neuropathy) complications of diabetes, particularly the role of chronic hyperglycemia and related pathways (polyol, AGEs).
Appreciate the general principles of diabetes management, including lifestyle (diet, exercise) and pharmacological interventions.