Vitamins are complex substances that regulate body processes and act as coenzymes in enzyme reactions. They are named alphabetically and classified as either fat-soluble or water-soluble. Fat-soluble vitamins like A, D, E and K can be stored in the body, while water-soluble vitamins like C and the B vitamins cannot be stored and must be replenished regularly through diet. Deficiencies in vitamins can cause a variety of symptoms and even permanent damage if left untreated. The document provides details on the functions, dietary sources, deficiency symptoms and risk factors for various vitamins.