The water soluble vitamins comprise of the vitamin B complex in addition to vitamin C. The B vitamins are vitamin B1 (thiamine), vitamin B2 (riboflavin), vitamin B3 (niacin, nicotinic acid and nicotinamide), pantothenic acid, biotin, vitamin B6 (pyridoxine, pyridoxal and pyridoxamine), folic acid (folacin) and vitamin B12 (cobalamin). Generally the B vitamin complex are required for energy production and many are co-factors in the main energy yielding pathways. Three B vitamins (folic acid, vitamin B6 and vitamin B12) are also required for the correct metabolism of homocysteine. Vitamin C is one of the primary water soluble antioxidants in humans, where it interacts with vitamin E and glutathione. Humans are one of the few higher organisms that cannot synthesise vitamin C hepatically, due to a genetic defect of the enzyme gulonolactone oxidase. Because the water soluble vitamins are generally not stored within tissues to the same degree as the fat soluble vitamins, daily intakes are required to maintain plasma levels. However, because they are not stored and more readily excreted, toxicity is not such a concern as with their fat soluble counterparts.