Insulin and Diabetes: Glucose Regulation and Disease

Glucose is the primary fuel for every cell in the human body, and maintaining blood glucose within a narrow 4–7 mmol/L range is one of physiology's most intricate control problems. Insulin — a 51-amino-acid peptide hormone — is the master regulator. When the system breaks down, diabetes results: a disease affecting over 530 million people worldwide.

1. Glucose Homeostasis

Normal glucose range: Fasting: 3.9-5.5 mmol/L (70-100 mg/dL) Post-meal (2h): < 7.8 mmol/L (<140 mg/dL) HbA1c (3-month average): < 5.7% (below prediabetes threshold) Two main counter-regulatory hormones: Insulin (β-cells of islets of Langerhans, pancreas): Released when blood glucose RISES. Promotes glucose UPTAKE into muscle and fat. Suppresses liver glucose output. Net effect: LOWER blood glucose. Glucagon (α-cells, pancreas): Released when blood glucose FALLS. Stimulates liver glycogen breakdown (glycogenolysis) \to glucose release. Stimulates gluconeogenesis (de novo glucose synthesis from amino acids/glycerol). Net effect: RAISE blood glucose. Also involved: Cortisol, epinephrine (adrenaline), growth hormone — all raise glucose (counter-regulatory, activated by stress/hypoglycaemia) Glucose sensing in β-cells: Glucose enters β-cell via GLUT2 transporter \to Phosphorylated by glucokinase \to glycolysis \to ATP↑ \to ATP-sensitive K⁺ channels close \to cell depolarises \to Voltage-gated Ca²⁺ channels open \to Ca²⁺ influx \to Insulin-containing vesicles fuse with membrane \to exocytosis

2. How Insulin Works

Insulin's primary action in muscle and fat is to drive glucose uptake via GLUT4 transporter translocation — a process that is deficient in Type 2 diabetes:

Insulin signalling cascade (simplified): 1. Insulin binds to insulin receptor (IR) on cell surface IR = heterotetrameric tyrosine kinase (2α + 2β subunits) 2. Receptor autophosphorylation \to activates intrinsic kinase activity 3. IRS-1/2 (insulin receptor substrate) phosphorylated on tyrosine residues 4. PI3K (phosphatidylinositol 3-kinase) activated \to generates PIP3 at plasma membrane 5. PDK1 \to AKT (protein kinase B) phosphorylated and activated 6. AKT has multiple downstream effects: a) GLUT4 translocation: AKT phosphorylates AS160 \to releases vesicle inhibition \to GLUT4-containing vesicles translocate to plasma membrane \to GLUT4 inserts \to glucose uptake ↑ 10-40 fold in muscle/fat b) Glycogen synthesis: AKT inhibits GSK-3 \to activates glycogen synthase \to glucose\toglycogen c) Protein synthesis: AKT \to mTORC1 \to S6K1 \to ribosome biogenesis \to anabolic effects d) Anti-lipolysis: AKT \to PDE3B activation \to cAMP↓ \to HSL inhibited \to no fat breakdown

3. Type 1 Diabetes: Autoimmune Destruction

Type 1 diabetes (T1D) is an autoimmune disease in which the immune system destroys the insulin-producing β-cells of the pancreatic islets. No β-cells → no insulin → absolute insulin deficiency.

Prevalence: ~5-10% of all diabetes. Typically onset in childhood/adolescence (formerly "juvenile diabetes"), but can occur at any age.
Pathogenesis: Autoreactive T-cells (CD4+ and CD8+) infiltrate islets (insulitis), destroying β-cells. Autoantibodies (anti-insulin, anti-GAD65, anti-IA-2) are markers, not primary causative agents. Genetic risk: HLA-DR3/DR4 haplotypes confer ~40% of genetic risk. Environmental triggers (enteroviral infection?) contribute.
Without treatment: Diabetic ketoacidosis (DKA) — without insulin, cells can't use glucose → fat breakdown → ketones accumulate → metabolic acidosis → fatal within days to weeks.
Treatment: Lifelong insulin replacement. Multiple daily injections (MDI) or continuous insulin pump (CSII). No cure, though pancreas/islet transplantation can restore short-term normoglycaemia.

Discovery of insulin (1921): Frederick Banting and Charles Best at the University of Toronto extracted a pancreatic secretion and injected it into diabetic dogs — reversing their symptoms within hours. The first human injection (Leonard Thompson, January 1922) was transformative: a child comatose from T1D was saved within 24 hours. Banting and Macleod received the 1923 Nobel Prize in Physiology or Medicine. Before insulin, T1D patients were placed on starvation diets — they survived months, not years.

4. Type 2 Diabetes: Insulin Resistance

Type 2 diabetes (T2D) is characterised by insulin resistance (target tissues respond poorly to insulin) combined with progressive β-cell dysfunction. About 90-95% of all diabetes is T2D.

Natural history of T2D: Stage 1 — Insulin resistance: Muscle, liver, and fat cells respond less to normal insulin concentrations. Cause: excess lipid in muscle and liver cells disrupts IRS-1 signalling (ceramides, diacylglycerol activate inhibitory kinases IKKβ, JNK, PKCθ) \to AKT activation reduced \to GLUT4 translocation impaired Stage 2 — Compensatory hyperinsulinaemia: Pancreatic β-cells compensate by secreting MORE insulin. HbA1c remains near-normal for years. β-cell mass increases initially; markers of metabolic stress appear. Stage 3 — β-cell exhaustion/failure: Chronic glucose toxicity and lipotoxicity cause β-cell apoptosis. β-cell mass reduces ~50% by the time T2D is clinically diagnosed. Insulin secretion becomes insufficient \to fasting hyperglycaemia. T2D diagnostic criteria (WHO 2006): Fasting glucose \geq 7.0 mmol/L (126 mg/dL) OR 2h post-75g OGTT glucose \geq 11.1 mmol/L OR HbA1c \geq 48 mmol/mol (6.5%) Risk factors: obesity (central adiposity), physical inactivity, genetic predisposition (TCF7L2, PPARG, KCNJ11 variants), age, ethnicity (South Asian, African-Caribbean)

5. Complications and Biochemistry

Chronic hyperglycaemia causes tissue damage through several biochemical pathways:

Advanced glycation end-products (AGEs): Excess glucose non-enzymatically glycates proteins (Maillard reaction). AGEs cross-link collagen → stiffening of blood vessel walls, reduced kidney glomerular filtration, retinal changes.
Polyol pathway activation: Glucose → sorbitol (aldose reductase) → fructose. Accumulation of sorbitol causes osmotic stress in lens (cataract), nerves (neuropathy).
Oxidative stress: Mitochondrial superoxide overproduction from high glucose → activates all four pathways of hyperglycaemic damage.

Major complications: diabetic retinopathy (leading cause of blindness in working-age adults), diabetic nephropathy (leading cause of end-stage renal disease), diabetic neuropathy (burning pain, foot ulcers → amputation), cardiovascular disease (2-4× increased risk).

6. Therapies: Insulin, GLPs, and SGLT2s

Insulin analogues: Rapid-acting (lispro, aspart — peak 1h, clears in 3h; mimics meal response), long-acting (glargine, detemir — 24h flat profile; basal coverage). Human insulin structure modified at specific amino acids to tune pharmacokinetics.
Metformin: First-line T2D drug. Reduces hepatic glucose output by activating AMPK (AMP-activated protein kinase) — possibly via mitochondrial complex I inhibition. Extremely safe, cheap, reduces cardiovascular events.
GLP-1 receptor agonists (GLP-1 RAs): Semaglutide (Ozempic/Wegovy), liraglutide, tirzepatide. Mimic glucagon-like peptide-1 — stimulate insulin secretion glucose-dependently, suppress glucagon, slow gastric emptying, reduce appetite. Dramatic weight loss (15-22% body weight with semaglutide). Cardioprotective. Reduce MACE by 26% (LEADER trial, SUSTAIN-6).
SGLT2 inhibitors: Dapagliflozin, empagliflozin. Block SGLT2 co-transporter in kidney proximal tubule → glucose excreted in urine (glycosuria). Kidney-protective, heart failure-reducing effects independent of glucose lowering. Major cardiovascular and renal outcome benefits.

7. The Artificial Pancreas

A closed-loop insulin delivery system (artificial pancreas) automates glucose control by integrating continuous sensing and infusion:

Closed-loop system components: 1. Continuous Glucose Monitor (CGM): Glucose oxidase sensor in subcutaneous tissue Measures interstitial fluid glucose every 1-5 minutes Slight lag vs blood glucose (~5-15 min lag) Current accuracy (MARD): ~9% mean absolute relative difference 2. Insulin pump (CSII): Delivers rapid-acting insulin subcutaneously via catheter Basal rate adjustable every 5 minutes Bolus for meals (manual or algorithm-suggested) 3. Control algorithm: Proportional-Integral-Derivative (PID) or Model Predictive Control (MPC): Uses compartmental glucose-insulin model to predict future glucose and calculate optimal future insulin delivery Avoids hypoglycaemia (suspend below threshold) FDA-approved hybrid closed-loop systems (2016-2025): Medtronic MiniMed 780G \to 72-74% time-in-range (TIR 70-180 mg/dL) Tandem t:slim X2 + Dexcom \to ~70% TIR Insulet Omnipod 5 \to tubeless, smartphone-controlled "Bihormonal" systems: add glucagon microdoses to help prevent hypoglycaemia Next frontier: fully automated meal detection, closed-loop glucagon delivery