Vitamin B12

Vitamin B₁₂, vitamin B12 or vitamin B-12, also called cobalamin, is a water-soluble vitamin that has a key role in the normal functioning of the brain and nervous system, and the formation of red blood cells. It is one of eight B vitamins. It is involved in the metabolism of every cell of the human body, especially affecting DNAsynthesis, fatty acid and amino acid metabolism.

Neither fungi, plants, nor animals (including humans) are capable of producing vitamin B₁₂. Only bacteria and archaeahave the enzymes needed for its synthesis. Some plant foods are a natural source of B₁₂ because of bacterial symbiosis. B₁₂ is the largest and most structurally complicated vitamin and can be produced industrially only through a bacterial fermentation-synthesis. This synthetic B₁₂ is used to fortify foods and sold as a dietary supplement.

Vitamin B₁₂ consists of a class of chemically related compounds (vitamers), all of which show pharmacological activity. It contains the biochemically rare element cobalt(chemical symbol Co) positioned in the center of a planar tetra-pyrrole ring called a corrin ring. The vitamer is produced by bacteria as hydroxocobalamin, but conversion between different forms of the vitamin occurs in the body after consumption.

A common synthetic form of the vitamin is cyanocobalamin, produced by chemically modifying bacterial hydroxocobalamin. Because of superior stability and low cost this form is used in many pharmaceuticals and supplements as well as for fortification of foods. In the body it is converted into the human physiological forms methylcobalaminand 5′-deoxyadenosylcobalamin. In this process a cyanide ion, (CN⁻), is produced, but the amount is very, very small (20 μg from 1,000 μg of cyanocobalamin) compared to what would cause a toxicity risk, and is in fact less than the amount of cyanide consumed daily from food (primarily fruit, nuts, seeds, legumes). Cyanide-free synthetic forms of the vitamin—hydroxocobalamin, methylcobalamin, and adenosylcobalamin—are being used in some pharmacological products and supplements, but their claimed superiority to cyanocobalamin is debatable.

Supplements of this B vitamin have been shown to be effective in pain management for a variety of conditions, including low-back pain and recurrent aphthous stomatitis, or mouth sores. Most people get enough B12 from their diet through animal products, such as eggs, milk, fish, and meat, and some fortified cereals . Vegans and some vegetarians should take B12 supplements to replace what they are not getting through their diet, but taking supplements beyond the recommended daily allowance of 2.4 micrograms for most adults is not suggested because a safe upper limit has not been established.

Vitamin B₁₂ was discovered from its relationship to the disease pernicious anemia, an autoimmune disease in which parietal cells of the stomach responsible for secretingintrinsic factor are destroyed; these cells are also responsible for secreting acid in the stomach. Because intrinsic factor is crucial for the normal absorption of B₁₂, its lack in the presence of pernicious anemia causes a vitamin B₁₂ deficiency. Many other subtler kinds of vitamin B₁₂ deficiency and their biochemical effects have since been elucidated.

How much vitamin B12 do I need?

The amount of vitamin B12 you need each day depends on your age. Average daily recommended amounts for different ages are listed below in micrograms (mcg):

What foods provide vitamin B12?

Vitamin B12 is found naturally in a wide variety of animal foods and is added to some fortified foods. Plant foods have no vitamin B12 unless they are fortified. You can get recommended amounts of vitamin B12 by eating a variety of foods including the following:

• Beef liver and clams, which are the best sources of vitamin B12.
• Fish, meat, poultry, eggs, milk, and other dairy products, which also contain vitamin B12.
• Some breakfast cereals, nutritional yeasts and other food products that are fortified with vitamin B12. To find out if vitamin B12 has been added to a food product, check the product labels.

Am I getting enough vitamin B12?

Most people in the United States get enough vitamin B12 from the foods they eat. But some people have trouble absorbing vitamin B12 from food. As a result, vitamin B12 deficiency affects between 1.5% and 15% of the public. Your doctor can test your vitamin B12 level to see if you have a deficiency.

Certain groups may not get enough vitamin B12 or have trouble absorbing it:

• Many older adults, who do not have enough hydrochloric acid in their stomach to absorb the vitamin B12 naturally present in food. People over 50 should get most of their vitamin B12 from fortified foods or dietary supplements because, in most cases, their bodies can absorb vitamin B12 from these sources.
• People with pernicious anemia whose bodies do not make the intrinsic factor needed to absorb vitamin B12. Doctors usually treat pernicious anemia with vitamin B12 shots, although very highoral doses of vitamin B12 might also be effective.
• People who have had gastrointestinal surgery, such as weight loss surgery, or who have digestive disorders, such as celiac disease or Crohn’s disease. These conditions can decrease the amount of vitamin B12 that the body can absorb.
• Some people who eat little or no animal foods such as vegetarians and vegans. Only animal foods have vitamin B12 naturally. When pregnant women and women who breastfeed their babies are strict vegetarians or vegans, their babies might also not get enough vitamin B12.

What happens if I don’t get enough vitamin B12?

Vitamin B12 deficiency causes tiredness, weakness, constipation, loss of appetite, weight loss, and megaloblastic anemia.  Nerve problems, such as numbness and tingling in the hands and feet, can also occur. Other symptoms of vitamin B12 deficiency include problems with balance, depression, confusion, dementia, poor memory, and soreness of the mouth or tongue. Vitamin B12 deficiency can damage the nervous system even in people who don’t have anemia, so it is important to treat a deficiency as soon as possible.

In infants, signs of a vitamin B12 deficiency include failure to thrive, problems with movement, delays in reaching the typical developmental milestones, and megaloblastic anemia.

Large amounts of folic acid can hide a vitamin B12 deficiency by correcting megaloblastic anemia, a hallmark of vitamin B12 deficiency. But folic acid does not correct the progressive damage to the nervous system that vitamin B12 deficiency also causes. For this reason, healthy adults should not get more than 1,000 mcg of folic acid a day.

Vitamin B12 Deficiency Symptoms

Vitamin B12 (cobalamin) deficiency can cause a wide range of hematologic, neurologic, psychiatric, gastrointestinal, and systemic symptoms. B12 is essential for DNA synthesis, red blood cell formation, and myelin maintenance in the nervous system. Here’s a comprehensive and detailed breakdown of the symptoms:

1. Hematologic Symptoms

Due to impaired DNA synthesis affecting rapidly dividing cells like red blood cells.

Megaloblastic Anemia

• Fatigue
• Pallor (pale skin)
• Shortness of breath
• Dizziness or lightheadedness
• Tachycardia (rapid heartbeat)
• Palpitations
• Glossitis (inflamed, smooth, red tongue)
• Cheilitis (cracks at corners of the mouth)

Note: The anemia is typically macrocytic (MCV >100 fL) and megaloblastic on blood smear.

2. Neurological Symptoms

These can occur even in the absence of anemia and may be irreversible if untreated.

Peripheral Nervous System

• Paresthesia (tingling/numbness) – Typically in the hands and feet.
• Burning sensation in feet or hands
• Loss of proprioception – Poor sense of joint position (e.g., difficulty walking in the dark).
• Loss of vibration sense
• Ataxia (unsteady gait)
• Muscle weakness
• Clumsiness

Spinal Cord Involvement

• Subacute combined degeneration of the dorsal columns and lateral corticospinal tracts:

  • Weakness
  • Spasticity
  • Gait abnormalities
  • Babinski sign

3. Neuropsychiatric Symptoms

Even in the absence of anemia or macrocytosis.

• Memory impairment
• Cognitive decline
• Dementia (may resemble Alzheimer’s)
• Depression
• Irritability
• Psychosis (“megaloblastic madness”)
• Delirium or confusion
• Hallucinations
• Paranoia

4. Gastrointestinal Symptoms

Due to effects on the mucosal lining and indirect effects of anemia.

• Glossitis – Red, swollen, painful tongue with a smooth surface.
• Glossodynia – Burning tongue pain.
• Cheilitis
• Loss of appetite
• Weight loss
• Diarrhea or constipation
• Nausea

5. Other Systemic Symptoms

• Infertility – Reversible with treatment.
• Low-grade fever – Rare.
• Jaundice – Due to ineffective erythropoiesis and hemolysis.
• Increased homocysteine and methylmalonic acid (MMA) – Can damage blood vessels and nerves.

6. Symptoms in Infants and Children

• Developmental delays
• Poor feeding
• Hypotonia (floppiness)
• Failure to thrive
• Regression of milestones
• Irritability
• Seizures (in rare, severe cases)

Causes of B12 Deficiency

For context, common causes include:

• Pernicious anemia (autoimmune destruction of intrinsic factor)
• Gastric surgery (e.g., bariatric surgery)
• Chronic gastritis or H. pylori
• Strict vegan diet (no animal products)
• Malabsorption syndromes (e.g., Crohn’s disease)
• Medications: Metformin, proton pump inhibitors, H2 blockers
• Nitrous oxide use (inactivates B12)

Diagnosis of B12 Deficiency

• Serum B12 level: <200 pg/mL is usually deficient.
• Methylmalonic acid (MMA) and homocysteine: Elevated in B12 deficiency.
• Complete blood count: Macrocytic anemia, possible pancytopenia.
• Peripheral smear: Hypersegmented neutrophils, macro-ovalocytes.
• Intrinsic factor antibody: To detect pernicious anemia.

What Foods Contain Rich Vitamin B12?

Vitamin B12 is found naturally only in animal-derived foods, as it’s produced by microorganisms and accumulates through the food chain. Here’s a detailed list of vitamin B12-rich foods, categorized by type and amount per serving:

Here’s a clean and detailed list of vitamin B12-rich foods:

Animal-Based Foods (Richest Natural Sources)

1. Organ Meats

• Beef liver: 70–85 mcg per 100g (extremely high content)
• Lamb liver: similarly high levels
• Kidneys (beef or lamb): around 55–60 mcg per 100g

Organ meats are by far the most concentrated source of vitamin B12.

2. Shellfish and Fish

• Clams: 98.9 mcg per 100g (among the richest sources)
• Oysters: 16 mcg per 100g
• Mussels: 20 mcg per 100g
• Crab: around 9.8 mcg per 100g
• Sardines (canned): 8.9 mcg per 100g
• Mackerel: 7.0 mcg per 100g
• Salmon: 3.2 mcg per 100g
• Tuna: 2.5 mcg per 100g

Seafood is generally a very good source of B12, especially bivalves.

3. Meat

• Beef: 1.5–2.5 mcg per 100g (depending on the cut)
• Lamb: 2.0 mcg per 100g
• Pork: 0.6–1.0 mcg per 100g
• Chicken (especially thigh): 0.3–0.6 mcg per 100g

4. Eggs and Dairy

• Eggs (whole): 0.6 mcg per large egg (mostly in the yolk)
• Milk (whole or skim): 1.1 mcg per cup (240 mL)
• Cheddar cheese: 0.9 mcg per 100g
• Swiss cheese: 3.3 mcg per 100g
• Plain yogurt (whole milk): 0.8 mcg per 170g (6 oz)

Fortified Plant-Based Sources (for Vegetarians and Vegans)

Plant-based foods do not naturally contain vitamin B12. However, several are fortified:

1. Fortified Foods

• Fortified breakfast cereals (check the label): typically 3–6 mcg per serving
• Fortified nutritional yeast: 5–8 mcg per tablespoon
• Fortified plant-based milks (soy, almond, oat): 1–3 mcg per cup
• Fortified tofu or plant-based meat substitutes: varies by brand and formulation

Synthetic Sources (Supplements)

• Cyanocobalamin and methylcobalamin are common supplement forms.

• Available as:

  • Tablets (oral or sublingual)
  • Lozenges
  • Nasal sprays
  • Injections (for severe deficiency or malabsorption)

Recommended Dietary Allowance (RDA)

• Adults (male and female): 2.4 micrograms per day
• Pregnant women: 2.6 micrograms
• Breastfeeding women: 2.8 micrograms