"It was extremely difficult," he recalls. "You spend your entire life hearing that fat makes you fat and causes heart attacks and strokes. Now, all of a sudden, you're eating 200 grams of fat per day. There is a huge psychological component to conquer before you can become successful with the keto diet. In the beginning, it's like trying to convince people 1,000 years ago that the world is in fact round, not flat."
Josh Axe, a doctor of natural medicine and clinical nutritionist, estimates that about 25% of people who try a ketogenic diet experience these symptoms, with fatigue being the most common. “That happens because your body runs out of sugar to burn for energy, and it has to start using fat,” he says. “That transition alone is enough to make your body feel tired for a few days.”
When ketosis kicks in, the body switches from stealing protein for fuel to burning fat. “The body starts to rely more heavily on fat stores,” Notte explains. Molecules called ketones are released when body fat is metabolized, and some cells in the brain can use ketones for fuel. However, your brain needs more than 100 grams of carbohydrates a day, according to research, and while your body makes the switch to relying on fat—ketones—for energy, your brain can suffer.
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, 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. On the other hand, it is absolutely contraindicated in the treatment of other diseases such as pyruvate carboxylase deficiency, porphyria and other rare genetic disorders of fat metabolism. A person with a disorder of fatty acid oxidation is unable to metabolise fatty acids, which replace carbohydrates as the major energy source on the diet. On the ketogenic diet, their body would consume its own protein stores for fuel, leading to ketoacidosis, and eventually coma and death.
Other forms of ketogenic diets include cyclic ketogenic diets, also known as carb cycling, and targeted ketogenic diets, which allow for adjustments to carbohydrate intake around exercise. These modifications are typically implemented by athletes looking to use the ketogenic diet to enhance performance and endurance and not by individuals specifically focused on weight loss.
In terms of weight loss, you may be interested in trying the ketogenic diet because you’ve heard that it can make a big impact right away. And that’s true. “Ketogenic diets will cause you to lose weight within the first week,” says Mattinson. She explains that your body will first use up all of its glycogen stores (the storage form of carbohydrate). With depleted glycogen, you’ll drop water weight. While it can be motivating to see the number on the scale go down (often dramatically), do keep in mind that most of this is water loss initially.