Vitamin B is a complex of eight water soluble vitamins, active in cell metabolism. The name arises because it was once considered a single vitamin, much like Vitamin C or Vitamin D. Since later research has shown it is in fact a complex of chemically distinct vitamins that happen to often coexist in the same foods, the name has gradually declined in use, being replaced by the generic term "the B vitamins", the vitamin B complex, or by the specific names of each vitamin.