Pure vitamin B12 (Cobalamine) is a water-soluble, dark red crystalline compound which is the most structurally complex of all the vitamins. Each molecule of B12 contains 183 atoms and this explains why it took biochemists so long to unearth its structure! Microbial synthesis is the only source of B12 from foods.

This occurs by bacteria in the digestive tract  or animals or by microbial fermentation of foods. You can only find B12 in plants, such as fermented soy products, seaweed and algae, such as spirulina. However, it’s argued that these sources should not be relied upon as safe sources of B12, as they are not readily available in humans.

The best sources of B12 are therefore organ meat, such as liver, shellfish, egg yolk, salmon, sardines, with milk, dairy products and other meat/fish providing moderate sources – see Table 1. To efficiently absorb B12 our bodies need to secrete a substance called ‘intrinsic factor’ for gastric cells in the stomach.


If the digestive system is impaired B12 absorption can suffer – this is commonly observed in the elderly of those with digestive problems. Vegans should supplement with B12 as, as we have indicated B12, is not a viable proposition from plant sources.

Why do we need vitamin B12?

In the body vitamin B12 acts as carrier of fragments of organic molecules, such as methyl groups and hydrogen atoms. This means that it can aid in the chemical transformation of key biological molecules such as carbohydrate, amino acids, fats and the replication of DNA – the molecule that stores our genetic code.

In particular vitamin B12 is required for the synthesis of brain neurotransmitters, the myelin sheaths around nerve cells, the synthesis of the important amino acid methionine (from its precursor homocysteine) and the general growth and division of cells, including those in bone marrow, which go onto form blood cells.

Sources of vitamin B12


B12 content (mcg/100g edible portion)

Fried Lamb’s liver


Liver Pate


Beef/lamb/pork (lean cuts, cooked)


Eggs, whole, raw


Baked Cod fillets


Cornflakes (fortified)


Cheddar cheese







How much B12 do I need?

The UK RNI for B12 is 0.0015mpgs per day (that’s equivalent to 1.5mcgs a day. 1mg = 1000mcgs). There is no increment recommended during pregnancy, but breast feeding mothers are encouraged to take 0.002mgs a day  (2.0mcgs). Vital Vitamins_2 What happens if i don’t get enough? The typical response would be pernicious or megoblastic anaemia (autoimmune diseases) and disorders of the nervous system. Physical symptoms can include fatigue, dizziness, numbness  and tingling  and reduced vision, with mental symptoms ranging from agitation and moodiness to hallucination and eventually full  blow psychosis.

Vitamin B12 – Did you know?

✱ B12 contains the biochemically rare element cobalt hence its other name  Cobalamine.

✱  It is used to treat Cyanide poisoning

✱ High doses administered to the elderly may help combat brain shrinkage associated  with  Alzheimer’s disease.

✱ Large does of B12 may help treat sleep disorders

✱ The liver is the main site for the storage  of B12

Can I get too much Vitamin B12?

B12 is relatively safe, even at high supplementation doses. Chemical studies have reported no adverse effects following administration of up to 6.0mgs per day of B12 for several weeks and up to 1.0mgs per day for several years (6000mcgs and 1000mcgs respectively). The UK Food Standards Agency suggests that the long- term supplemental intake of 2.0mgs per day should not produce any adverse.

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