Insulinn.
Insulin resistance and type 2 diabetes are related but distinct: resistance is impaired tissue response; diabetes is the threshold reached when compensatory secretion can no longer maintain normal blood glucose.
The definition
Insulin is a peptide hormone secreted by pancreatic beta cells in response to rising blood glucose. It binds its receptor on skeletal muscle, liver, and adipose tissue, triggering a signalling cascade that drives glucose uptake, glycogen synthesis, and fat storage. Chronically impaired insulin signalling, termed insulin resistance, underpins type 2 diabetes and a spectrum of related metabolic disorders.
The mechanism
Insulin release is triggered by glucose entry into pancreatic beta cells via GLUT1 and GLUT2 transporters. As intracellular glucose is metabolised, ATP levels rise, closing ATP-sensitive potassium (KATP) channels. The resulting plasma membrane depolarisation opens voltage-gated calcium channels; calcium influx stimulates exocytosis of pre-formed insulin granules into the portal circulation. 1 The system is concentration-sensitive: small increments in blood glucose produce proportionate increases in secreted insulin.
At target cells, insulin binds its receptor's extracellular domain, activating the receptor's intrinsic tyrosine kinase. 12 The kinase phosphorylates insulin receptor substrate (IRS) proteins, which recruit phosphoinositide 3-kinase (PI3K) to generate the lipid messenger PIP3 and activate the serine/threonine kinase Akt. Akt is the master mediator of metabolic insulin action: in skeletal muscle and adipose tissue it drives GLUT4-containing vesicles to the plasma membrane for glucose entry; in the liver it phosphorylates the FOXO1 transcription factor, excluding it from the nucleus and suppressing gluconeogenic enzymes PEPCK and G6Pase. 3
Skeletal muscle is the primary site of insulin-stimulated glucose disposal, accounting for approximately 80% of whole-body uptake under insulin-stimulated conditions. 2 Think of GLUT4 vesicles as a fleet of delivery vehicles held in depot until insulin signals the dispatch: without the signal the cargo sits idle; once signalled, the entire fleet reaches the membrane within minutes.
Post-Meal Response — Insulin after a meal — a sharp rise as glucose enters the blood, then a return to baseline.
In practice
The signalling sequence is most visible in the hours around structured exercise.
Worked example
A person completes a compound resistance-training session. During the work, muscle contractions activate an insulin-independent pathway via AMPK and RAC1, moving GLUT4 to the membrane without insulin. In the hours that follow, cellular insulin sensitivity remains elevated as muscle rebuilds GLUT4 stores and repairs tissue. A carbohydrate-rich meal consumed in this window is preferentially directed into glycogen rather than stored as fat.
Knowing the mechanism lets you engineer the timing: exercise shifts glucose fate before you eat, not just after.
Why it matters
Insulin resistance, defined as impaired biological response of target tissues to insulin stimulation, is often detectable a decade or more before type 2 diabetes is clinically diagnosed. 3 One well-supported mechanistic account identifies ectopic lipid accumulation in skeletal muscle and liver as a root driver: excess intracellular fat generates diacylglycerol (DAG), which activates novel protein kinase C (PKC) isoforms that directly impair insulin receptor kinase activity. 2 This model competes with accounts centred on ceramide accumulation and chronic low-grade inflammation; the current evidence does not compel a single explanation.
The clinical stakes extend well beyond blood sugar. Insulin resistance underpins polycystic ovary syndrome, non-alcoholic fatty liver disease, cardiovascular disease, and is implicated in certain cancers, making it one of the highest-impact metabolic dysfunctions in contemporary populations. 3 For the performance-focused individual, the practical priority is maintaining insulin sensitivity rather than managing resistance after it develops.
Questions of record
What does insulin do in the body?
Insulin is a signalling hormone that coordinates how cells access glucose. It binds receptors on skeletal muscle, liver, and adipose tissue, activating a cascade that moves GLUT4 transporters to cell membranes for glucose uptake, promotes glycogen synthesis, and in the liver suppresses new glucose production. 12
What is insulin resistance and how does it develop?
Insulin resistance is a state in which target tissues respond inadequately to normal circulating insulin levels. One leading mechanism involves ectopic lipid accumulation generating diacylglycerol, which activates PKC isoforms that block insulin receptor kinase activity. 32 It often precedes type 2 diabetes by a decade or more.
How does exercise improve insulin sensitivity?
Exercise activates GLUT4 translocation via an insulin-independent AMPK and RAC1 pathway, enabling glucose uptake during the session. 4 A single bout improves insulin sensitivity for 24-48 hours afterwards; sustained training programmes produce lasting gains by increasing total GLUT4 protein expression in skeletal muscle. Both aerobic and resistance training contribute.
What is the difference between insulin resistance and type 2 diabetes?
Insulin resistance is a physiological state of impaired tissue response to insulin; type 2 diabetes is the clinical condition that follows when the pancreas can no longer produce enough insulin to compensate for that resistance. 3 Insulin resistance is present years before a diabetes diagnosis and is independently associated with cardiovascular risk.