Because the ketogenic diet alters the body's metabolism, it is a first-line therapy in children with certain congenital metabolic diseases such as pyruvate dehydrogenase (E1) deficiency and glucose transporter 1 deficiency syndrome,[35] which prevent the body from using carbohydrates as fuel, leading to a dependency on ketone bodies. The ketogenic diet is beneficial in treating the seizures and some other symptoms in these diseases and is an absolute indication.[36] However, it is absolutely contraindicated in the treatment of other diseases such as pyruvate carboxylase deficiency, porphyria, and other rare genetic disorders of fat metabolism.[9] Persons with a disorder of fatty acid oxidation are unable to metabolise fatty acids, which replace carbohydrates as the major energy source on the diet. On the ketogenic diet, their bodies would consume their own protein stores for fuel, leading to ketoacidosis, and eventually coma and death.[37]

It seems strange that a diet that calls for more fat can raise “good” cholesterol and lower “bad” cholesterol, but ketogenic diets are linked to just that. It may be because the lower levels of insulin that result from these diets can stop your body from making more cholesterol. That means you’re less likely to have high blood pressure, hardened arteries, heart failure, and other heart conditions. It's unclear, however; how long these effects last.

On a ketogenic diet, you’re generally eating a diet that’s high in fat (roughly 70 percent of your total calories come from fat), moderate in protein (about 20 percent of your calories), and low in carbohydrate (about 5 percent of calories). By limiting carbohydrates (to usually less than 45 grams for the average person), your body lacks the glucose (from carbs) that it normally uses for energy, so it eventually switches over to burning fat as its primary fuel source instead; through a metabolic process called ketosis, the liver converts the fat into fragments of fatty acids called ketones, which power the brain and other organs and tissues.
A short-lived increase in seizure frequency may occur during illness or if ketone levels fluctuate. The diet may be modified if seizure frequency remains high, or the child is losing weight.[19] Loss of seizure-control may come from unexpected sources. Even "sugar-free" food can contain carbohydrates such as maltodextrin, sorbitol, starch, and fructose. The sorbitol content of suntan lotion and other skincare products may be high enough for some to be absorbed through the skin and thus negate ketosis.[31]
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]
Avoid sweetened drinks. Drinks that are sweetened, whether artificially or naturally, will have little nutritional value and add lots of empty calories to your diet. Do not drink fruit juice or other sweetened drinks. Even natural sugars will increase your daily calorie intake and prevent weight loss. If you chose to drink juice, do not exceed 4 oz per day (1/2 cup). Instead of drinking sweetened beverages throughout your day, drink unsweetened, calorie-free beverages. Some beverages to avoid include:
The Mayo Clinic Diet is a long-term weight management program created by a team of weight-loss experts at Mayo Clinic. The Mayo Clinic Diet is designed to help you reshape your lifestyle by adopting healthy new habits and breaking unhealthy old ones. The goal is to make simple, pleasurable changes that will result in a healthy weight that you can maintain for the rest of your life.
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