Dr L: What is Ketosis? Jimmy Moore: Put simply, ketosis is a metabolic state that happens when you consume a customized low-carb, moderate-protein, high-fat diet that causes your body to switch from using glucose as its primary source of fuel to running mostly on ketones and fatty acids. Ketones themselves are produced when the body burns fat, and they’re used as an alternative fuel source when glucose isn’t available. In other words, your body changes from being a sugar-burner to a fat-burner. It's a completely normal and natural state that happens within a few days to a few weeks in most people who reduce their carbohydrate intake, moderate their protein consumption, and consume dietary fat, especially saturated fat, to satiety.