The B vitamins are eight water-soluble vitamins that play necessary roles in cell metabolism. By tradition, the B vitamins were once thought to be a single vitamin, recognized to as vitamin B (a lot like how people known to vitamin C or vitamin D). Later research showed that they are chemically distinct vitamins that frequently coexist in the same foods. Supplements containing the complete eight are generally referred to as a vitamin B complex. Individual B vitamin supplements are referred to by the exacting name of each vitamin (e.g. B1, B2, B3).