Ketosis

Ketosis is the metabolic state in which ketone bodies — beta-hydroxybutyrate (BHB), acetoacetate (AcAc), and acetone — are synthesized by the liver at rates exceeding peripheral utilization, producing measurably elevated circulating ketone concentrations. It is a normal physiological response to low carbohydrate availability and distinct from the pathological state of ketoacidosis.

Operational thresholds

Blood BHB concentration is the most reliable clinical marker of ketosis. Normative ranges are approximately: non-ketotic postprandial state <0.1 mmol/L; overnight fasted 0.1-0.3 mmol/L; nutritional ketosis 0.5-3.0 mmol/L; starvation ketosis (10+ day fast) 3-6 mmol/L; diabetic ketoacidosis 5-25+ mmol/L with concurrent acidemia. Urine ketone strips detect acetoacetate but not BHB, and become unreliable after several weeks of ketosis as BHB/AcAc ratio shifts. Breath acetone meters provide noninvasive estimates correlating imperfectly with blood BHB.

Dietary induction

The ketogenic diet restricts carbohydrate to approximately 20-50 g/day and provides 70-80% of energy from fat, producing ketosis within 2-4 days in most individuals. Initial "keto flu" (headache, fatigue, irritability, mild dehydration, electrolyte disturbance) reflects the transition period as peripheral tissues upregulate ketone-oxidizing enzymes and renal electrolyte handling adjusts to reduced insulin. Adaptation over 2-4 weeks produces stable ketone utilization, improved cognitive adjustment, and often improved exercise tolerance.

Metabolic adaptations

Chronic ketosis produces several adaptations documented by Cahill, Owen, and colleagues in the classical prolonged-fasting studies of the 1960s-70s: (1) brain glucose utilization falls from ~120 g/day to ~30-40 g/day as BHB replaces 60-70% of cerebral energy substrate; (2) muscle glycolysis is suppressed in favor of beta-oxidation; (3) whole-body protein catabolism is reduced after initial gluconeogenic demands are met; (4) insulin sensitivity changes variably, often improving in metabolically impaired subjects but with a transient insulin resistance pattern in glucose tolerance tests.

Therapeutic ketosis

The ketogenic diet has established efficacy in pediatric drug-resistant epilepsy (Epilepsy Foundation and NINDS consensus), with seizure reduction in approximately 30-50% of children. Emerging investigational applications include GLUT1 deficiency syndrome, certain mitochondrial disorders, and neurodegenerative disease research. Popular application for weight loss produces results comparable to other low-calorie diets when total energy intake is matched, with some evidence of superior short-term satiety.

Ketosis and physical performance

Endurance performance in ketosis-adapted athletes is broadly maintained for sub-maximal aerobic exercise but typically reduced at high intensities that depend on glycolytic flux. Muscle glycogen is not eliminated in chronic ketosis but is utilized more slowly during exercise. The Volek and Phinney keto-adaptation research program has documented both favorable and unfavorable metabolic responses depending on adaptation duration and athletic discipline.

Ketosis vs. ketoacidosis

Nutritional and fasting ketosis are self-limiting because intact insulin signaling caps hepatic ketogenesis through feedback at adipose lipolysis. In type 1 diabetes or severe insulin deficiency, this brake is absent, and hepatic ketogenesis runs unchecked, producing progressive acidemia, osmotic diuresis, and electrolyte derangement. DKA requires insulin replacement, fluid resuscitation, and electrolyte management and can be fatal if untreated.