Anaerobic metabolism uses only carbohydrates as fuel.
As the body breaks down glucose, a simple sugar molecule, in the first dozen steps, it produces a compound called pyruvate. When this compound enters the mitochondria, also called the powerhouse of the cell, aerobic metabolism occurs in which this compound is oxidized (loses electrons) and turned into carbon dioxide and water.
In case of anaerobic metabolism, pyruvate enters the fluid, or cytoplasm, of the cell and is turned into lactic acid via process called glycolysis. While aerobic metabolism uses fat, proteins and carbohydrates and can continue indefinitely, anaerobic metabolism uses only carbohydrate, namely glucose (and glycogen) and does not last long.