Ketosis is a normal process that happens when your body doesn't have enough carbs to burn for energy. When the body does not have enough glucose for energy, it burns fat and makes substances called ketones, which it can use for fuel (see: Wikipedia). Ketones are acids that build up in the blood and are eliminated in urine. In small amounts, they serve to indicate that the body is breaking down fat, but high levels of ketones can poison the body, leading to a process called ketoacidosis.