Vitamin B₁₂ (also known as cobalamin) is a large and complex molecule that has a tetrapyrrole ring that binds to cobalt, an essential human micronutrient. We do not produce Vitamin B₁₂, it is produced by certain microrganisms. We therefore depend on microbes for our daily cobalamin supply; we get 5-30 micrograms from the food we eat, and a small amount is produced by the microbes that live in our intestines. A normal human has 2-5 mg of Vitamin B₁₂ in the body, most of it is in the liver and kidneys. The amount of cobalamin required to maintain normal levels is 2-5 micrograms per day and if we do not get vitamin B₁₂, deficiency symptoms will develop after several years. Cobalamin deficiency leads to neurological abnormalities and anemia. Therefore, vitamin B₁₂ appears to be important for brain function and blood production. Researchers are still investigating exactly how vitamin B₁₂ works at the cellular level.