Insulin resistance is the condition in which muscle, liver, and adipose tissue become less responsive to insulin. The pancreas compensates by producing more, until it can't.
Mechanism: Driven by intramuscular and visceral fat, inflammation, chronic hyperinsulinemia, sleep deprivation, and sedentary behavior. The insulin receptors don't downregulate. The downstream signaling becomes blunted.
Use case: Measure via fasting insulin + glucose (HOMA-IR), or an OGTT with insulin. Reverse via weight loss (especially visceral), resistance training, sleep restoration, and dietary pattern change.
Caveats: A normal HbA1c can mask significant insulin resistance. The pancreas compensates for years before glucose breaks down. Fasting insulin catches it earlier.