Cobalamin (Vitamin B12): Functions, Deficiency & Sources

What is Cobalamin?

Cobalamin is the chemical name for vitamin B12, a water-soluble vitamin and one of the most important micronutrients in the B-vitamin group. It is the only vitamin that contains the trace element cobalt, which is reflected in its name. Since the human body cannot produce cobalamin on its own, it must be obtained through food or dietary supplements.

Cobalamin exists in several chemical forms, including methylcobalamin, adenosylcobalamin, hydroxocobalamin, and cyanocobalamin. Methyl- and adenosylcobalamin are the biologically active forms found in nature, while cyanocobalamin is the synthetic form most commonly used in supplements.

Biological Functions

Cobalamin performs several vital functions in the human body:

• Red blood cell formation: Cobalamin is essential for the maturation of red blood cells in the bone marrow. A deficiency leads to megaloblastic anemia, in which red blood cells become abnormally large and dysfunctional.
• Nervous system health: Cobalamin is required for the synthesis and maintenance of the myelin sheath, the protective coating around nerve fibers. A deficiency can cause irreversible neurological damage.
• DNA synthesis: The vitamin is involved in the replication and repair of DNA, making it fundamental to healthy cell division and growth.
• Homocysteine metabolism: Cobalamin catalyzes the conversion of homocysteine into methionine. Elevated blood homocysteine levels are associated with an increased risk of cardiovascular disease.
• Energy metabolism: As a cofactor of methylmalonyl-CoA mutase, cobalamin plays a role in the breakdown of certain fatty acids and amino acids for energy production.

Dietary Sources

Cobalamin is found naturally only in animal-derived foods, as it is produced by microorganisms (bacteria) and stored in animal tissue. The richest dietary sources include:

• Liver (beef, veal) – the highest natural concentration of any food
• Clams, oysters, and other shellfish
• Fish (e.g., herring, mackerel, salmon)
• Meat (beef, pork, poultry)
• Eggs and dairy products (cheese, yogurt, milk)

Plant-based foods do not contain bioavailable cobalamin. Vegetarians and especially vegans therefore rely on fortified foods or dietary supplements to meet their needs.

Recommended Daily Intake

The World Health Organization (WHO) and national health authorities provide the following recommended daily intake values for cobalamin:

• Adults: 2.4 µg (micrograms) per day (WHO/NIH)
• Pregnant women: 2.6 µg per day
• Breastfeeding women: 2.8 µg per day
• Children: between 0.4 µg and 2.4 µg per day, depending on age

Because cobalamin can be stored in the liver (reserves lasting several years), a deficiency often does not become apparent until after a prolonged period of inadequate intake or impaired absorption.

Cobalamin Deficiency

Causes

A cobalamin deficiency can arise from several different causes:

• Insufficient dietary intake: vegan or strict vegetarian diets without supplementation
• Malabsorption: e.g., pernicious anemia (an autoimmune condition in which the intrinsic factor needed for intestinal absorption is absent), Crohn's disease, celiac disease, or following gastric surgery
• Drug interactions: long-term use of metformin (for diabetes) or proton pump inhibitors can reduce cobalamin absorption
• Older age: reduced stomach acid production in older adults impairs the release of cobalamin from food

Symptoms

A cobalamin deficiency can manifest through the following symptoms:

• Fatigue, exhaustion, and general weakness
• Pale skin and mucous membranes
• Tingling or numbness in the hands and feet (peripheral neuropathy)
• Memory problems, difficulty concentrating, and confusion
• Depressive mood and emotional instability
• Inflammation of the tongue and oral mucosa (glossitis)
• In severe cases: neurological damage and subacute combined degeneration of the spinal cord

Risk Groups

• Vegans and strict vegetarians
• Older adults (aged 65 and above)
• Individuals with gastrointestinal disorders
• Long-term metformin users
• Individuals who have undergone bariatric surgery

Diagnosis

A cobalamin deficiency is typically identified through a blood test. Common diagnostic markers include:

• Serum cobalamin (vitamin B12 in the blood) – normal range: 200–900 pg/ml
• Holotranscobalamin (HoloTC) – the active, bioavailable fraction; considered an early and sensitive marker of deficiency
• Methylmalonic acid (MMA) in blood or urine – elevated in functional cobalamin deficiency
• Homocysteine – elevated in both cobalamin and folate deficiency

Treatment and Supplementation

Treatment of cobalamin deficiency depends on the underlying cause and severity:

• Oral supplementation: For mild deficiency or prevention, high-dose cobalamin supplements (500–2000 µg daily) can be effective even without intrinsic factor, as a small proportion is absorbed passively.
• Intramuscular injections: For severe deficiency or malabsorption (e.g., pernicious anemia), hydroxocobalamin or cyanocobalamin injections are the preferred treatment.
• Dietary adjustments: Increasing the intake of cobalamin-rich animal foods for mild, diet-related deficiency.

Toxicity and Safety

Cobalamin is considered very well tolerated. As a water-soluble vitamin, excess amounts are excreted by the kidneys, and no toxic effects from high oral doses have been reported. However, unexpectedly elevated serum cobalamin levels without supplementation may indicate an underlying condition (such as liver disease or certain blood cancers) and should be investigated by a healthcare professional.

References

1. World Health Organization (WHO) and Food and Agriculture Organization (FAO): Vitamin and Mineral Requirements in Human Nutrition. 2nd edition. WHO Press, Geneva, 2004.
2. Green R. et al.: Vitamin B12 deficiency. Nature Reviews Disease Primers, 3, 17040, 2017. doi:10.1038/nrdp.2017.40
3. National Institutes of Health (NIH), Office of Dietary Supplements: Vitamin B12 – Fact Sheet for Health Professionals. Updated 2023. Available at: ods.od.nih.gov