Here's one way we know that tumors love sugar: In order to see where cancer might have metastasized, we often use a PET scan. That gives us an indication of whether the cancer is in its original site, or if it has spread to the liver or bones or another site. To get those images, the radiologist injects a dye so we can see what lights up, indicating where the tumor is and where it may have gone. That dye is actually just sugar. The cancer likes to eat sugar because it's very active and it needs food. So if we're using sugar to see where cancer has gone, you can imagine that a diet high in sugar might be bad.