First, a little background: Eric Westman, MD, director of the Duke Lifestyle Medical Clinic, explained to Health in a previous interview that in order to successfully follow the keto diet, you need to eat moderate amounts of protein, reduce your carb intake, and increase fats. When you reduce your carb consumption, your body turns to stored fat as its new fuel source—a process called ketosis. To stay in ketosis, followers of the keto diet must limit their carbs to 50 grams a day, Dr. Westman says.
If you’ve decided to move forward in trying the keto diet, you will want to stick to the parameters of the eating plan. Roughly 60 to 80 percent of your calories will come from fats. That means you’ll eat meats, fats, and oils, and a very limited amount of nonstarchy vegetables, she says. (This is different from a traditional low-carb diet, as even fewer carbs are allowed on the keto diet.)
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.
At the start of a keto diet for beginners, it is not uncommon for an individual to experience rapid weight loss. This may concern some people, but it is worth noting that not ALL weight lost is body fat. When we lower our carbohydrate intake our insulin lowers, causing us to excrete more water. Additionally, reducing carbohydrate intake causes the body to break down glycogen stores (to initially maintain blood sugar levels); glycogen is the storage form of glucose that is located in our liver and skeletal muscle. The loss of both water and glycogen accounts for most of the weight loss in the first few days. Don’t be discouraged! Soon after, total body water and glycogen levels will balance out.
The easiest macro to calculate in the ketogenic diet is fat. Once you've got your carbs and protein set, simply fill the rest of your daily calorie needs with fat sources. If you find yourself wanting to gain a bit of weight, add approximately 500 calories, or 55 grams. If you want to lose weight, cut down on your fat intake by 200-500 calories, or 22-55 grams.