Once “keto-adapted,” many tissues increase their capacity to utilize ketones for fuel. Some tissues even prefer to use ketones for their cellular energy needs. For instance, the heart, as well as many areas of the brain, prefers to use ketone bodies as opposed to glucose. One interesting fact is that Dr. Stephen Cunnane found that the uptake of ketone bodies in the brain is proportional to their production.[12] Additional research has also demonstrated that as uptake of ketones increases, the uptake of glucose decreases in brain tissue.[13][14] In contrast, there are certain cells that cannot use fatty acids or ketone bodies, and are considered “obligate glucose users.” Red blood cells and specific parts of the brain are obligate glucose users. Don’t stress, however, because even though you may not be consuming a lot of carbohydrates, these cells can obtain glucose through a biological process called gluconeogenesis (the production of glucose from other non-glucose materials).
Electrolytes: To reiterate, maintaining electrolyte balances is critical on a ketogenic diet, in order to prevent side effects and the “keto-flu.” While this can be done exclusively through whole foods, some individuals may require additional sources. Sodium, magnesium, potassium, and in some cases, calcium, can all be replenished via supplementation.
Carbohydrate-restricted ketosis: This type of ketosis mimics the same biological alterations seen during lengthy fasts, but without the complete restriction of food. Carbohydrate-restricted ketosis is achieved primarily through a very low-carbohydrate ketogenic diet. This restriction, in turn, results in reductions of insulin and blood glucose levels similar to that of fasting, which again increases blood ketone levels. For most individuals, nutritional ketosis is much more sustainable than fasting or starvation ketosis. Fasting can still have its place in a keto diet for beginners. Many individuals following a ketogenic diet like to implement a regular fasting protocol such as intermittent fasting (IF) (12–20 hours daily) or every-other-day fasts (EOD), depending on their goals. This practice is not critical for success on a ketogenic diet, but it can enhance the level of ketone production, and thus magnify the benefits.
On the ketogenic diet, carbohydrates are restricted and so cannot provide for all the metabolic needs of the body. Instead, fatty acids are used as the major source of fuel. These are used through fatty-acid oxidation in the cell's mitochondria (the energy-producing parts of the cell). Humans can convert some amino acids into glucose by a process called gluconeogenesis, but cannot do this by using fatty acids.[57] Since amino acids are needed to make proteins, which are essential for growth and repair of body tissues, these cannot be used only to produce glucose. This could pose a problem for the brain, since it is normally fuelled solely by glucose, and most fatty acids do not cross the blood–brain barrier. However, the liver can use long-chain fatty acids to synthesise the three ketone bodies β-hydroxybutyrate, acetoacetate and acetone. These ketone bodies enter the brain and partially substitute for blood glucose as a source of energy.[56]

The ketogenic diet is a mainstream dietary therapy that was developed to reproduce the success and remove the limitations of the non-mainstream use of fasting to treat epilepsy.[Note 2] Although popular in the 1920s and '30s, it was largely abandoned in favour of new anticonvulsant drugs.[1] Most individuals with epilepsy can successfully control their seizures with medication. However, 20–30% fail to achieve such control despite trying a number of different drugs.[9] For this group, and for children in particular, the diet has once again found a role in epilepsy management.[1][10]

The ketogenic diet is calculated by a dietitian for each child. Age, weight, activity levels, culture, and food preferences all affect the meal plan. First, the energy requirements are set at 80–90% of the recommended daily amounts (RDA) for the child's age (the high-fat diet requires less energy to process than a typical high-carbohydrate diet). Highly active children or those with muscle spasticity require more food energy than this; immobile children require less. The ketogenic ratio of the diet compares the weight of fat to the combined weight of carbohydrate and protein. This is typically 4:1, but children who are younger than 18 months, older than 12 years, or who are obese may be started on a 3:1 ratio. Fat is energy-rich, with 9 kcal/g (38 kJ/g) compared to 4 kcal/g (17 kJ/g) for carbohydrate or protein, so portions on the ketogenic diet are smaller than normal. The quantity of fat in the diet can be calculated from the overall energy requirements and the chosen ketogenic ratio. Next, the protein levels are set to allow for growth and body maintenance, and are around 1 g protein for each kg of body weight. Lastly, the amount of carbohydrate is set according to what allowance is left while maintaining the chosen ratio. Any carbohydrate in medications or supplements must be subtracted from this allowance. The total daily amount of fat, protein, and carbohydrate is then evenly divided across the meals.[37]