Ketosis and Weight Loss
A ketogenic diet forces the body to burn fats rather than carbohydrates. The mechanisms behind this behavior are explained below.
In western diets (and many others), most meals are sufficiently high in nutritive carbohydrates to evoke insulin secretion. The primary control for this insulin secretion is glucose in the blood stream, typically from digested carbohydrate. Insulin also controls ketosis; in the non-ketotic state, the human body stores dietary fat in fat cells (i.e., adipose tissue) and preferentially uses glucose as cellular fuel. Diets low in nutritive carbohydrates introduce less glucose into the blood stream and thus evoke less insulin secretion, which leads to longer and more frequent episodes of ketosis.
Low-carbohydrate diet advocates in general recommend reducing nutritive carbohydrates (commonly referred to as “net carbs,” i.e., grams of total carbohydrates reduced by the non-nutritive carbohydrates) to very low levels. This means sharply reducing consumption of desserts, breads, pastas, potatoes, rice, and other sweet or starchy foods. Some recommend levels less than 20 grams of “net carbs” per day, at least in the early stages of dieting (for comparison, a single slice of white bread typically contains 15 grams of carbohydrate, almost entirely starch). By contrast, the U.S. Institute of Medicine recommends a minimum intake of 130 grams of carbohydrate per day (the FAO and WHO similarly recommend that the majority of dietary energy come from carbohydrates).
If the diet is changed from one that is high in carbohydrates to one that does not provide sufficient carbohydrate to replenish glycogen stores, the body goes through a set of stages to enter ketosis. During the initial stages of this process, blood glucose levels are maintained through gluconeogenesis, and the adult brain does not burn ketones. However, the brain makes immediate use of ketones for lipid synthesis in the brain. After about 48 hours of this process, the brain starts burning ketones in order to more directly use the energy from the fat stores that are being depended upon, and to reserve the glucose only for its absolute needs, thus avoiding the depletion of the body’s protein store in the muscles.