Ketosis is one of the more misunderstood and controversial aspects of low-carbohydrate diets. If the diet is changed from a highly glycemic diet to a diet 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 the adult brain does not burn ketones, however the brain makes immediate use of this important substrate for lipid synthesis in the brain. After about 48 hours of this process, the brain starts burning ketones in order to more directly utilize 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.
When ketone levels rise sufficiently, they "spill" into your urine. Although Ketostix (small strips of card that you pee on) can be used to detect ketosis, they're not totally accurate. In other words, you can be in ketosis, but there might not be enough ketones in your urine to indicate ketosis. One other sign you're in ketosis is when your breath smells "fruity."
By restricting carbohydrates drastically to a mere fraction of that found in the typical American diet, the body goes into a different metabolic state called ketosis, whereby it burns its own fat for fuel. Normally the body burns carbohydrates for fuel -- this is the main source of fuel for your brain, heart and many other organs.
A person in ketosis is getting energy from ketones, little carbon fragments that are the fuel created by the breakdown of fat stores. When the body is in ketosis, you tend to feel less hungry, and thus you're likely to eat less than you might otherwise. As a result, your body changes.
