Ketosis is the state in which the liver produces ketone bodies (beta-hydroxybutyrate, acetoacetate, acetone) from fat as the primary fuel source.
Mechanism: Triggered by sustained low-carb intake (<50g/day), fasting, or exogenous ketone supplements. The brain readily uses ketones once adapted. Ketones have signaling roles beyond fuel. Anti-inflammatory, neuroprotective.
Use case: Clinical: epilepsy, glioblastoma adjunct, severe insulin resistance. Performance: variable; some athletes adapt well, some lose top-end power. Test ketones via blood meter (most accurate) or breath device.
Caveats: Adaptation takes 2-6 weeks. Initial 'keto flu' is electrolyte imbalance. Supplement sodium, potassium, magnesium. Long-term effects on lipids and arterial health are individual.