The body converts carbohydrates to sugar (glucose) which is used for fuel, left-over fuel is stored as fat. If carbs are restricted by a low-carb diet, fasting, or even prolonged exercise, blood sugar will fall and the body will begin to burn stored fat as a means of fuel. This physiological state is called Ketosis.
During ketosis, fatty acids are released from fat deposits and circulated through the bloodstream. When they enter the liver they are converted into ketones. The ketones are then delivered to tissues and organs by the bloodstream where they are metabolized for energy.
Certain types of ketones are captured by the kidneys and discharged from the body through urine. Ketone urine test strips can be used to detect ketone levels. If the levels are above 1.0 mmol/l, the body is in ketosis and burning fat.