Ketosis

Ketosis occurs when carbohydrate availability is low enough that the liver converts fatty acids into ketone bodies — beta-hydroxybutyrate, acetoacetate and acetone — as an alternative fuel, particularly for the brain, which cannot burn fatty acids directly.

It matters because it is a normal adaptive state, reached through very-low-carbohydrate eating or prolonged fasting. It should not be confused with diabetic ketoacidosis, a dangerous condition in which ketones and blood glucose rise together in the absence of insulin — nutritional ketosis operates at a fraction of those concentrations and in a completely different context.

In practice entering it takes several days and often brings a transient period of fatigue and headache as the body adapts. Ketone levels can be measured in blood, breath or urine, though being in ketosis is not itself a fat-loss guarantee: energy balance still decides that.