A dietary regimen very low in carbohydrates and high in fat that shifts the body’s primary fuel from glucose to ketone bodies derived from fat. Originally developed in the 1920s as a treatment for drug-resistant epilepsy in children, it remains a recognised medical therapy requiring professional supervision. The diet typically restricts carbohydrate to a small fraction of energy intake, forcing the liver to produce ketones that the brain can use. It has gained popularity for weight loss, though long-term effects are still debated. Side effects can include constipation, nutrient deficiencies, and raised blood lipids, so medical monitoring and careful food choices are important.