Cancer cells require a lot of glucose for energy—more than normal cells. This is because cancer cells metabolize, or break down, sugars using a different and less efficient process than that of normal, healthy cells. Cells typically use oxygen to burn glucose for energy. Because cancer cells grow in excess and become densely packed, however, they often survive in a low-oxygen environment and have adapted to breaking down sugars in the absence of oxygen—a process called anaerobic metabolism. Unfortunately, anaerobic metabolism is much less efficient than breaking down sugars aerobically, or with oxygen. As a result, cancer cells may need as much as 40 times more glucose than normal cells that function with sufficient levels of oxygen to generate the same amount of energy.