The ketogenic diet is a very low-carb, high-fat diet. This decrease in carbs puts the human body in a metabolic state called ketosis. Once you are in ketosis, your body is very effective at burning fat and supplies high levels of energy to your body and brain. It typically takes a few days to reach a state of ketosis. The keto diet is based on fatty foods such as meat, fish, eggs, butter, nuts, healthy oils, avocados and low-carb vegetables.