As cats get older, they need nutrition that promotes healthy immune function. This allows their immune system to respond quickly and appropriately to disease when necessary. Dietary antioxidants make up an important part of this. These include vitamins A, C and E, as well as beta-carotene. Together, they help to reduce cell damage caused by oxidation. In diets lacking these important antioxidants, damage can occur to the wall or membrane of cells in the body.