Cobalamin, commonly referred to as vitamin B 12 , is the only nutrient not directly available in plants. This is because vitamin B 12 is synthesized by microorganisms, bacteria, fungi and algae, but not by plants or animals. Animals consume these microorganisms along with their food, so this vitamin can be found in their meat, organs and by-products (eggs and dairy products). Vitamin B 12 deficiency can lead to irreversible neurological disorders, gastrointestinal problems and megaloblastic anemia. Among other populations, vegans who do not supplement with a reliable source of vitamin B 12 or infants of vegan mothers who do not consume a regular, reliable source of vitamin B 12 are at risk for deficiency.

The body can store vitamin B 12 for approximately three to five years, but after that, without repletion or with an inability to absorb it, deficiency symptoms may occur; the deficiency may also be asymptomatic. Because of this time lag and because serum tests for B 12 levels can be skewed by other variables, irreversible damage can occur before a deficiency is detected.

In a vegan diet, vitamin B 12 can be found in fortified vegetable milks, cereals or nutritional yeast. However, these are not reliable means to meet B 12 requirements. While there are claims that fermented foods, spirulina, chlorella, certain mushrooms and sea vegetables, among other foods, can provide B 12 , the vitamin is generally not biologically active. These inactive forms act as B 12 analogues, binding to B 12 receptors, preventing absorption of the functional version and, therefore, promoting deficiency. The most reliable method of preventing deficiency for vegans or anyone else at risk is to take a B 12 supplement.