Ketosis is a metabolic state characterized by elevated levels of ketone bodies in the blood or urine. It involves the body producing ketone bodies out of fat and using them for energy instead of carbs. You can get into ketosis by following a very low carb, high fat ketogenic diet. The ketogenic diet is a high-fat, adequate-protein, low-carbohydrate diet that in medicine is used mainly to treat hard-to-control (refractory) epilepsy in children. The diet forces the body to burn fats rather than carbohydrates.https://en.wikipedia.org/wiki/Ketogenic_diet. A ketogenic diet can help you lose weight. In the short term, you can lose weight quickly, because it reduces the body’s stores of glycogen and water. https://www.healthline.com/nutrition/what-is-ketosis
For ketosis to start, you generally need to eat fewer than 50 grams of carbs per day and sometimes as little as 20 grams per day. However, the exact carb intake that will cause ketosis varies between individuals.
It takes an average of 3 days to go into ketosis if you eat 20–50 grams of carbs per day, but sometimes can take more than a week.https://pubmed.ncbi.nlm.nih.gov/28599043/, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5987302/.