The switch from glucose to ketone fuel
A ketogenic diet keeps carbohydrate low enough (commonly under about 50 grams per day, sometimes lower) that liver glycogen empties and circulating insulin falls. With glucose scarce, the liver converts fatty acids into ketone bodies (beta-hydroxybutyrate, acetoacetate, and acetone), which the brain and muscle can use as an alternate fuel. This metabolic state, nutritional ketosis, typically establishes over several days to a couple of weeks.
Early on many people experience the so-called keto flu (fatigue, headache, irritability), largely attributed to fluid and electrolyte shifts as falling insulin causes the kidneys to excrete sodium and water.
Where keto helps and where evidence is still developing
The strongest established medical use is drug-resistant epilepsy, where ketogenic diets have decades of clinical support. For weight loss and type 2 diabetes glucose control, short-to-medium-term trials show benefits that largely reflect calorie reduction and lower insulin demand; long-term adherence and outcome data are thinner.
LDL-cholesterol response varies widely between individuals, sometimes rising substantially. This is general educational information, not medical advice; anyone with diabetes, on glucose-lowering or blood-pressure medication, or with liver, kidney, or pancreatic conditions should consult a qualified clinician before starting keto.