A metabolic state in which the body relies significantly on ketone bodies for energy because glucose is scarce. It develops during fasting, starvation, prolonged endurance exercise, and very-low-carbohydrate diets, when the liver converts fatty acids into acetoacetate and beta-hydroxybutyrate. The brain, which normally demands a constant glucose supply, adapts over several days to use ketones efficiently. Nutritional ketosis is a controlled, mild state distinct from diabetic ketoacidosis, in which ketone levels rise far higher and blood becomes acidic. Interest in ketosis has grown with the popularity of ketogenic diets for weight loss and epilepsy management, though the long-term health effects remain under study.