Nutritional ketosis is a metabolic state achieved when your body burns fat as its primary source of fuel rather than carbohydrates. In order to reach ketosis, you must reduce your intake of carbohydrates to around 5 percent of your total calorie intake, or roughly 20 grams of carbohydrates per day.

For most of us, our bodies are accustomed to using glucose, or sugar, for energy. When you reduce your carbohydrate intake, your body has to search for other sources of energy. It first turns to glycogen, which is excess glucose that has been stored in the liver and muscles. Each gram of glycogen is bound to 3 to 4 grams of water, so as the glycogen is released, the water goes with it. This is why it's common to lose water weight at the beginning of a low-carb or ketogenic diet.

Once stores of glycogen are depleted, your body will begin burning fat. When this happens, the liver produces molecules called ketones, which are then used as fuel. Dr. Richard Veech, one of the world's experts on ketosis, has shown that the heart and brain work 25 percent more efficiently when fueled by ketones rather than blood sugar, which may account for the increased mental clarity that is a hallmark of the ketogenic diet. Some parts of the brain do require glucose, but the body can produce glucose from protein through a process known as gluconeogenesis.

Ketosis should not be confused with ketoacidosis, which is a life-threatening condition marked by a high concentration of ketone bodies that can occur in people with diabetes. Ketoacidosis is very rare in people who do not have diabetes.