When the liver turns fat into ketone bodies
When carbohydrate is scarce, such as during fasting or a very low-carbohydrate diet, the body breaks down fatty acids through beta-oxidation, generating large amounts of acetyl-CoA. Normally acetyl-CoA enters the citric acid cycle, but when that cycle is limited (partly because oxaloacetate is diverted) the liver routes excess acetyl-CoA into ketogenesis instead.
This produces the main ketone bodies: acetoacetate and beta-hydroxybutyrate (BHB), plus a small amount of acetone. These water-soluble fuels travel in the blood to tissues, importantly the brain, which cannot use fatty acids directly but can run substantially on ketones.
Nutritional ketosis versus ketoacidosis
The mild, regulated rise in ketones during fasting or low-carb eating, often called nutritional ketosis, is a normal adaptation and is distinct from diabetic ketoacidosis, a dangerous, uncontrolled accumulation of ketones and acid that occurs mainly when insulin is severely lacking. The two should not be confused; the difference is the degree of regulation and the ketone concentrations involved.
This is general educational information about metabolism, not medical advice. People with diabetes or other conditions should consult a qualified clinician before pursuing ketogenic states.