Vitamin B-12 has the largest and most complex chemical structure of all the vitamins. It is a water-soluble vitamin with a key role in the normal functioning of the brain and nervous system, as well as in the metabolic utilization of fats and proteins and the cellular production of energy. Vitamin B-12 is also essential for proper red blood cell production, homocysteine metabolism, and the maintenance of a healthy gastrointestinal tract.