This specialized protein binds to B12, protecting it so it can be absorbed in the small intestine. An autoimmune lack of intrinsic factor causes pernicious anemia.

Vitamin B12 is synthesized exclusively by certain bacteria and archaea. Since plants cannot make it, vegans rely on supplements or fortified foods to avoid a deficiency.

B12 is essential for DNA synthesis, which is needed to form healthy red blood cells. A deficiency causes these cells to become abnormally large, leading to severe anemia.

While most water-soluble vitamins are rapidly excreted in urine, the liver can accumulate a two to five-year supply of B12. This severely delays deficiency symptoms.

Only certain bacteria and archaea possess the enzymes required to synthesize B12. Plants don't need this vitamin, so they never evolved the ability to produce it.

Without enough B12, red blood cells become abnormally large and fragile. This condition, called megaloblastic anemia, impairs oxygen delivery and causes fatigue.

Vitamin B12 is the only vitamin that contains a metallic trace element. A single cobalt ion sits at the very center of its complex corrin ring molecular structure.

While excess water-soluble vitamins are typically excreted in urine daily, a healthy human liver can hold a two to five-year backup supply of B12.