Vitamin B12: Physiology, Dietary Sources, and Requirements

Vitamin B₁₂, a cobalamin, is required for only two enzyme reactions in humans and animals: The conversion of methylmalonyl CoA to succinyl CoA and addition of a methyl group to homocysteine to form methionine. Vitamin B₁₂ does not occur in plants, and the only natural sources are animal source foods, a low intake of which can cause deficiency. Vitamin B₁₂ release from food and its subsequent absorption are complex processes, which can be impaired by many factors leading to depletion of body stores over time. Several laboratory tests can be used to assess vitamin B₁₂ status but the mechanisms underlying the neurological and cognitive symptoms of deficiency are not yet understood.