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Diabetes Mellitus: Basic to Advanced

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1. Definition and Overview

• Globally: >500 million people have DM; prevalence continues to rise
• Leading cause of new blindness (ages 20-74), end-stage renal disease, and non-traumatic lower-limb amputation

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2. The Pancreas and Normal Glucose Physiology

Islets of Langerhans

How Insulin Is Secreted (Glucose-Stimulated Insulin Secretion)

Insulin's Metabolic Actions

• Inactivates phosphorylase → halts glycogenolysis
• Activates glucokinase → traps glucose-6-phosphate intracellularly
• Activates glycogen synthase → promotes glycogen storage (up to 100g in liver)
• Suppresses gluconeogenesis

• Stimulates GLUT4 translocation to cell surface → increases glucose uptake 15-fold
• Promotes glycogen synthesis (up to 2-3% muscle concentration)
• Stimulates protein synthesis

• Activates lipoprotein lipase → promotes fat storage
• Inhibits hormone-sensitive lipase → suppresses lipolysis and free fatty acid release

Insulin Receptor Signaling

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3. Classification of Diabetes Mellitus

Type 1 DM

• Autoimmune or idiopathic destruction of beta cells
• Absolute insulin deficiency
• ~5-10% of all DM cases
• Often presents before age 14 (juvenile DM) but can occur at any age

Type 2 DM

• Insulin resistance + progressive beta cell failure
• Initially non-insulin dependent; may require insulin later
• ~90-95% of all DM cases
• Strongly associated with obesity, physical inactivity, family history

MODY (Maturity-Onset Diabetes of the Young) / Monogenic Diabetes

• Single gene defects in beta cell function: GCK (glucokinase), HNF-1α, HNF-4α, HNF-1β, IPF-1, NeuroD1, KLF11, CEL, PAX4, INS, BLK, ABCC8, KCNJ11, APPL1
• Mutations in the K_ATP channel (ABCC8, KCNJ11) can cause either neonatal DM or MODY
• Autosomal dominant; does NOT require insulin initially

Gestational Diabetes Mellitus (GDM)

• Glucose intolerance developing in the 2nd or 3rd trimester
• Caused by pregnancy-related insulin resistance from placental hormones
• Affects ~16% of pregnancies worldwide (IDF, 2021)
• 35-60% risk of progressing to T2DM within 10-20 years
• ADA recommends lifelong screening every ≥3 years post-GDM

Secondary / Other Causes

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4. Pathophysiology

Type 1 DM

• Autoimmune destruction of beta cells, mediated by T cells and autoantibodies against islet cell antigens (ICA, anti-GAD65, anti-IA-2, anti-insulin, anti-ZnT8)
• HLA associations: HLA-DR3 and HLA-DR4 (especially DR3/DR4 heterozygotes) confer highest risk; HLA-DR2 is protective
• Viral triggers (enterovirus), molecular mimicry, and genetic susceptibility interact
• Results in absolute insulin deficiency → unchecked glucagon secretion → hepatic glucose overproduction + peripheral underutilization
• Without insulin: lipolysis is unchecked → excess free fatty acids → ketone body formation (acetoacetate, β-hydroxybutyrate) → diabetic ketoacidosis (DKA)

Type 2 DM - "Ominous Octet" (DeFronzo)

1. Insulin resistance in muscle - impaired GLUT4 translocation, reduced glucose uptake
2. Hepatic insulin resistance - failure to suppress gluconeogenesis
3. Beta cell failure - progressive loss of insulin secretory capacity (often ~50% lost by diagnosis)
4. Incretin defect - reduced GLP-1 secretion + response
5. Alpha cell dysfunction - hyperglucagonemia even in the fed state
6. Kidney - increased glucose reabsorption by SGLT2
7. Brain - insulin resistance centrally, impaired satiety signaling
8. Adipose tissue - excess lipolysis, ectopic fat deposition worsening insulin resistance

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5. Diagnosis

Diagnostic Criteria (ADA 2024)

• Any single criterion must be confirmed by repeat testing (except when symptoms + random ≥200 mg/dL)
• Renal threshold for glucosuria: ~200 mg/dL (11 mmol/L)

Additional Tests

• C-peptide: low/absent in T1DM; normal/elevated in T2DM
• Islet autoantibodies: confirm T1DM (anti-GAD65 is most sensitive)
• HbA1c: reflects mean glucose over ~3 months (weighted toward recent weeks); affected by conditions that alter RBC lifespan (hemolytic anemia → falsely low; iron deficiency → falsely high)

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6. Clinical Features

Acute Symptoms (Hyperglycemia)

• Polyuria - osmotic diuresis from glucosuria
• Polydipsia - secondary to dehydration from osmotic diuresis
• Polyphagia - cellular starvation despite elevated blood glucose
• Weight loss - catabolism of fat and protein (especially T1DM)
• Blurred vision - osmotic changes in lens
• Fatigue, weakness

T1DM vs T2DM Clinical Comparison

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7. Acute Complications

Diabetic Ketoacidosis (DKA)

• Predominantly T1DM (can occur in T2DM with stress)
• Triad: hyperglycemia + ketonemia/ketonuria + metabolic acidosis
• Precipitants: infections, missed insulin, new T1DM diagnosis, surgery
• Pathophysiology: absolute insulin deficiency → unchecked glucagon → lipolysis → free fatty acids → hepatic ketogenesis → anion gap metabolic acidosis
• Blood glucose: typically 300-600 mg/dL; pH <7.3; bicarbonate <18
• Treatment: IV fluids, insulin infusion, potassium replacement, monitor glucose/electrolytes hourly

Hyperosmolar Hyperglycemic State (HHS)

• Predominantly T2DM (elderly)
• Severe hyperglycemia (glucose often >600 mg/dL) + profound dehydration + altered consciousness
• No significant ketosis (residual insulin sufficient to suppress ketogenesis)
• Serum osmolality >320 mOsm/kg
• Mortality ~5-20%

Hypoglycemia

• Blood glucose <70 mg/dL (symptomatic threshold)
• Causes: excess insulin dose, missed meals, exercise, alcohol
• Symptoms: adrenergic (tremor, palpitations, sweating, anxiety) → neuroglycopenic (confusion, seizure, coma)
• Treatment: 15g fast-acting carbohydrate (rule of 15); glucagon IM if unconscious
• Hypoglycemia unawareness: autonomic neuropathy → loss of adrenergic warning signs; more common with recurrent hypoglycemia

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8. Chronic Complications

1. Non-enzymatic glycation of proteins (AGEs - Advanced Glycation End-products)
2. Polyol pathway activation (sorbitol accumulation)
3. Protein kinase C activation
4. Oxidative stress
5. Hexosamine pathway activation

Microvascular Complications

• Non-proliferative (NPDR): microaneurysms, blot hemorrhages, cotton-wool spots, venous beading, intraretinal microvascular abnormalities (IRMAs)
• Proliferative (PDR): neovascularization → vitreous hemorrhage → traction retinal detachment
• Diabetic Macular Edema (DME): can occur at any stage; leading cause of vision loss
• Management: tight glycemic + BP control; anti-VEGF injections (ranibizumab, bevacizumab) for DME/PDR; laser photocoagulation for PDR

• Leading cause of end-stage renal disease (ESRD) globally
• Stages: hyperfiltration → microalbuminuria (30-300 mg/day) → macroalbuminuria (>300 mg/day) → declining GFR → ESRD
• Pathology: Kimmelstiel-Wilson nodular glomerulosclerosis (pathognomonic), diffuse mesangial expansion, basement membrane thickening
• Management: ACE inhibitors or ARBs (first-line for proteinuria); SGLT2 inhibitors (dapagliflozin, empagliflozin) have proven renoprotective effects; tight glucose + BP control

• Distal symmetric polyneuropathy (DSPN): classic "stocking-glove" distribution; numbness, tingling, burning pain, loss of vibration and position sense
• Autonomic neuropathy: cardiovascular (orthostatic hypotension, resting tachycardia, cardiac denervation); GI (gastroparesis - delayed gastric emptying, constipation/diarrhea); urogenital (erectile dysfunction, neurogenic bladder); sudomotor dysfunction
• Mononeuropathies: CN III palsy (painful, pupil-sparing - distinguishes from aneurysm), carpal tunnel, foot drop
• Diabetic amyotrophy (Bruns-Garland): proximal asymmetric muscle weakness/pain; immune-mediated
• Management: tight glycemic control (slows progression); pregabalin/gabapentin, duloxetine, TCAs, capsaicin for pain

Macrovascular Complications

• Coronary artery disease: leading cause of death in T2DM; often "silent MI" due to autonomic neuropathy
• Cerebrovascular disease: 2-4x increased stroke risk
• Peripheral arterial disease: 10x increased risk of lower-limb amputation

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9. Management

Glycemic Targets

Non-Pharmacologic: Lifestyle

• Medical nutrition therapy: carbohydrate counting, Mediterranean/DASH diet, caloric restriction
• Physical activity: 150 min/week moderate aerobic + resistance training
• Weight loss: 5-7% body weight loss can prevent/delay T2DM (DPP trial)
• Self-monitoring of blood glucose (SMBG) or continuous glucose monitoring (CGM)

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Pharmacotherapy - Type 2 DM

Step 1: Metformin (first-line unless contraindicated)

• Mechanism: Activates AMPK → suppresses hepatic gluconeogenesis; modest improvement in insulin sensitivity
• Benefits: No hypoglycemia, weight neutral/mild loss, cheap, CV neutral/slightly beneficial, renal protective at low doses
• Contraindications: eGFR <30 mL/min (risk of lactic acidosis), active liver disease, contrast procedures
• HbA1c reduction: ~1-1.5%

Step 2: Add second agent based on comorbidities

• Mechanism: Mimic endogenous GLP-1 → glucose-dependent insulin secretion + glucagon suppression + gastric emptying delay + central satiety
• Benefits: Significant weight loss (5-15%), CV benefit (LEADER, SUSTAIN-6 trials), renal protection
• SE: Nausea/vomiting/diarrhea (common early), pancreatitis (rare), worsening retinopathy with rapid glucose lowering
• Recent 2025 meta-analysis: GLP-1 RAs effective and safe in pediatric obesity/T2DM (PMID: 40952752)

• Mechanism: Block SGLT2 in proximal tubule → prevent reabsorption of ~90% filtered glucose → glucosuria + osmotic diuresis
• Benefits: Weight loss, BP reduction, proven heart failure benefit (reduces hospitalization by ~35%), proven renal benefit (reduces progression to ESRD), cardiorenal protection independent of glucose-lowering
• SE: Urogenital infections (candida), Fournier's gangrene (rare), DKA (rare in T2DM), euglycemic DKA, polyuria, dehydration

• Mechanism: Inhibit dipeptidyl peptidase-4 → prevent degradation of endogenous GLP-1 and GIP → prolonged incretin effect
• Benefits: Weight neutral, low hypoglycemia risk, oral, well tolerated
• SE: Nasopharyngitis, pancreatitis (rare), joint pain (rare)
• HbA1c reduction: ~0.5-0.8%

• Mechanism: Activate PPARγ → improve peripheral insulin sensitivity + reduce hepatic glucose production
• Benefits: Durable effect, pioglitazone reduces CV events (PROactive trial)
• SE: Weight gain, fluid retention (contraindicated in CHF), bone fractures, bladder cancer risk (pioglitazone)

• Mechanism: Bind SUR1 subunit of K_ATP channel on beta cells → channel closure → depolarization → insulin secretion (glucose-independent)
• Benefits: Cheap, effective (HbA1c -1 to -1.5%)
• SE: Hypoglycemia (significant risk), weight gain; older generation (glibenclamide) highest hypoglycemia risk

• Dual agonism of GIP + GLP-1 receptors
• Superior HbA1c reduction and weight loss compared to semaglutide alone (SURPASS trials)
• Approved for T2DM (2022) and obesity (2023 in US)

Insulin Therapy in T2DM

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Pharmacotherapy - Type 1 DM

• Multiple Daily Injections (MDI): 1-2 basal injections + rapid-acting bolus with each meal
• Continuous Subcutaneous Insulin Infusion (CSII/insulin pump): Programmable basal rates + boluses
• Automated Insulin Delivery (AID/"closed loop"): CGM + pump + algorithm that auto-adjusts basal rate in real time; some deliver correction boluses (hybrid closed loop)
• Sensor-Augmented Pump (SAP): CGM + pump, with auto-suspend feature when glucose falls/predicted to fall

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10. Advanced Topics

Immunotherapy in T1DM

• Teplizumab (anti-CD3 antibody): FDA-approved (2022) to delay clinical T1DM in at-risk individuals (stage 2 T1DM); median delay of ~2 years
• Screens high-risk relatives with 2+ islet autoantibodies + dysglycemia

Beta Cell Replacement

• Pancreas transplantation: Primarily for T1DM patients undergoing simultaneous kidney transplant; insulin independence in ~80% at 1 year
• Islet transplantation (Edmonton Protocol): Infusion of donor islets into portal vein; requires immunosuppression; currently investigational for recurrent severe hypoglycemia
• Stem cell-derived beta cells: Multiple clinical trials underway; Vertex Pharmaceuticals (VX-880, VX-264) showing promising results in 2024-2025

Cardiovascular Risk Reduction (ASCVD)

• Statins: All T2DM patients aged 40-75 should receive moderate-to-high intensity statin (ADA 2024)
• BP target: <130/80 mmHg (ACE-I or ARB preferred, especially with proteinuria)
• SGLT2 inhibitors + GLP-1 RAs have independent CV benefits beyond glucose lowering and should be prioritized in patients with established ASCVD, HF, or CKD regardless of baseline HbA1c
• Aspirin: Low-dose for secondary prevention; primary prevention only if >10% 10-year ASCVD risk

Monogenic Diabetes

• MODY subtypes require genetic testing; some respond to sulfonylureas (HNF-1α/4α MODY) rather than insulin
• Neonatal DM from KCNJ11 or ABCC8 mutations often switches from insulin to high-dose sulfonylureas

Diabetes in Special Populations

• Pregnancy: Target HbA1c <6% if achievable without hypoglycemia; insulin is the standard (metformin and glyburide cross the placenta); close fetal monitoring; GDM screening at 24-28 weeks
• CKD: Dose-adjust/avoid metformin at eGFR <30; prefer linagliptin (no renal excretion); SGLT2i renoprotective down to eGFR 20 for cardiorenal indication
• Elderly: Relax HbA1c to 7.5-8.5%; avoid hypoglycemia risk (falls, fractures, dementia association); prefer DPP-4i, GLP-1 RA with caution re: GI effects

Emerging Therapies (2024-2026)

• Oral GLP-1 RAs: Oral semaglutide (Rybelsus) widely available; newer higher-dose oral formulations in trials
• Triple agonists (GLP-1/GIP/glucagon): Retatrutide and others in Phase 3 trials
• Closed-loop insulin delivery: Second-generation AID systems approaching full automation
• GLP-1 RAs in T1DM: Adjunct use to reduce insulin doses and glycemic variability (off-label, evidence building; PMID: 40952752 in pediatric T2DM)

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11. Monitoring and Screening

• HbA1c: Every 3 months if uncontrolled; every 6 months if at target
• Annual: Urine albumin-to-creatinine ratio (ACR), serum creatinine/eGFR, lipid panel, ophthalmologic exam (dilated fundus or retinal photography), foot exam (monofilament, vibration, pulses)
• Screening for T2DM: All adults ≥35 years; earlier if BMI ≥25 + 1 risk factor (family history, GDM, prediabetes, high-risk ethnicity)
• Thyroid function (TSH): Screen in T1DM (associated autoimmune disease)
• Celiac disease antibodies: Screen in T1DM

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Summary Table: Key Distinguishing Features

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• GLP-1 RAs are effective and safe in children/adolescents with obesity or T2DM [Systematic Review/Meta-Analysis, JAMA Pediatrics, 2025 - PMID: 40952752]
• GLP-1 RA-associated GI adverse events comprehensively characterized [Meta-Analysis, Gastroenterology, 2025 - PMID: 40499738]
• Digital interventions improve T2DM self-management outcomes [Systematic Review, J Med Internet Res, 2024 - PMID: 39037772]