Ketosis

Ketosis is the metabolic state in which the liver converts fatty acids into ketone bodies, chiefly beta-hydroxybutyrate and acetoacetate, and tissues including the brain use them as fuel. It develops when carbohydrate availability is low, typically after a day or two of fasting or on a very low carbohydrate diet.

It is a normal adaptation rather than a special state. The brain cannot use fatty acids directly, so ketones are how the body keeps it supplied when glucose is scarce, and gluconeogenesis provides the remaining glucose that some tissues still require.

It should not be confused with diabetic ketoacidosis, which is a medical emergency involving far higher ketone levels alongside high blood glucose and acidosis, and which occurs when insulin is absent rather than merely low.

For weight loss, ketogenic diets work the way other diets work, by reducing intake, and trials matching calories and protein generally find no metabolic advantage. The rapid first week is largely glycogen and its associated water leaving.

For performance, the picture is fairly consistent: fat oxidation rises reliably, and high intensity performance tends to suffer, partly because carbohydrate use becomes less efficient. That is a reasonable trade for some endurance contexts and a poor one for most training.

It has genuine clinical uses, most established in drug resistant epilepsy, where ketogenic diets are a recognised treatment. Exogenous ketone supplements are a different thing again and have not shown consistent performance benefits.

In practice, choose a ketogenic diet if you find it suits your appetite and preferences, expect the usual adaptation period, and do not expect a metabolic advantage the calorie ledger does not support.