Vitamin B12 Functions and Responsibilities

Main Functions of Vitamin B12

Vitamin B12 serves as a cofactor for methionine synthase, which transfers methyl groups from methyl tetrahydrofolate to form methionine from homocysteine, and for methyl malonyl-CoA mutase in the synthesis of succinyl CoA, an important intermediate in the citric acid cycle, as stated by the Clinical Nutrition journal 1, 2
Vitamin B12 is essential for mitochondrial metabolism, immune response, DNA integrity, myelin sheath preservation around neurons, and synthesis of neurotransmitters—all critical for normal blood cell formation and neurological functions, according to Clinical Nutrition 1

Vitamin B12 preserves the myelin sheath around neurons, which is essential for proper nerve conduction, and supports synthesis of neurotransmitters necessary for normal neurological function, as reported by Clinical Nutrition 1, 2
Deficiency can lead to extensive demyelination in the central nervous system, particularly in the spinal cord, and focal demyelination in the brain's white matter, according to Nutrition Research Reviews 3, 4
Vitamin B12 maintains nerve conduction velocity, which is crucial for peripheral motor function, as stated by Nutrition Research Reviews 3

Neurological symptoms of vitamin B12 deficiency include paraesthesia, numbness, muscle weakness, abnormal reflexes, tendon jerks, spasticity, gait ataxia, myelopathies, and myelo-neuropathies, as reported by Nutrition Research Reviews 3, 4
Cognitive difficulties associated with vitamin B12 deficiency include concentration problems and short-term memory loss, while vision problems include blurred vision, optic atrophy, and visual field loss, according to BMJ 5
Hematological manifestations of vitamin B12 deficiency include anemia and macrocytosis, as stated by BMJ 5
Glossitis and unexplained fatigue are also symptoms of vitamin B12 deficiency, as reported by BMJ 5

Vitamin B12 is synthesized by fungi and microorganisms, including bacteria in the gastrointestinal tracts of animals, and humans are dependent on animal sources or fortification, as stated by Clinical Nutrition and Nutrition Research Reviews 1, 3
The absorption of vitamin B12 involves multiple steps, including release from proteins by gastric acid and pepsin, binding to R-protein from salivary glands, then to gastric intrinsic factor, followed by absorption in the terminal ileum, as reported by Clinical Nutrition 1, 2

Primary sources of vitamin B12 include ruminant meat, organs, milk, fish, shellfish, fortified cereals, and nutritional yeast, as stated by Clinical Nutrition 1
The daily requirement for healthy adults is 2.4 μg/day according to DRI, while EFSA recommends 4 μg/day for healthy adults, as reported by Clinical Nutrition 1
Requirements increase during pregnancy (5 μg/day) and lactation (4.5 μg/day), and physiological needs may increase with age, oxidative stress from prolonged illness, or use of certain medications, according to Clinical Nutrition 1

Assessment of vitamin B12 status is best performed using both direct markers (serum cobalamin or holotranscobalamin) and functional markers (methylmalonic acid and homocysteine), as stated by Clinical Nutrition and BMJ 1, 5