Good question, cells eat loads! The basic answer is that everything you eat, cells need to survive. They use sugar and proteins to make energy. But there are special cells called phagocytes, these cells ‘eat’ harmful bacteria, virus-infected cells and dust in and break them down as much as they can. These cells don’t do this for energy but as a way to make sure you don’t get ill.
When growing cells in the lab, you grow them in media that contains the sugars and other molecules they need to survive. These molecules are in different forms to how you eat them, a cell wouldn’t be able to do much with a pizza or an apple because they can’t digest them into their components (such as amino acids or sugars).