When dietary carbohydrate is broken down into the energy substrate glucose, thereby raising blood glucose levels, the pancreas is stimulated to secrete insulin (the hormone that stores fat and inhibits ketone production). However, when carbohydrate intake is restricted, insulin remains suppressed, and the body’s primary fuel source shifts from glucose to fat, priming the body to enter a state of ketosis. When fat oxidation/breakdown is increased to a certain extent, ketones are made in the liver through a process known as ketogenesis (i.e., keto + genesis = ketone formation). When carbohydrate intake is restricted, blood glucose and insulin levels decrease, which allows fat stores to be broken down rapidly for energy. Most cells in the body can utilize either fatty acids or ketones for fuel, including the brain, which has shown to be more efficient in the presence of ketones rather than glucose.
Achieving optimal ketosis hinges on finding the right balance of macronutrients (or “macros” in keto-speak); these are the elements in your diet that account for the majority of your calories, a.k.a. energy—namely, fat, protein, and carbohydrates. By the way, it’s often “net grams” of carbohydrates that are counted toward your daily intake; “net” deducts the amount of fiber in a food from its carbohydrate total.