Vitamins are essential organic substances that are needed in small amounts in the diet for the normal function, growth, and maintenance of body tissues. Water-soluble vitamins consist of the B vitamins and vitamin C. With exception of vitamin B6 and B12, they are readily excreted in urine without appreciable storage, so frequent consumption becomes necessary. They are generally nontoxic when present in excess of needs, although symptoms may be reported in people taking megadoses of niacin , vitamin C, or pyridoxine (vitamin B6). All the B vitamins function as coenzymes or cofactors, assisting in the activity of important enzymes and allowing energy-producing reactions to proceed normally. As a result, any lack of water-soluble vitamins mostly affects growing or rapidly metabolizing tissues such as skin, blood, the digestive tract, and the nervous system . Water-soluble vitamins are easily lost with overcooking.