Dr. MJ Bazos MD,
B12 is a member of the vitamin B complex. It contains cobalt, and so is also
known as cobalamin. It is exclusively synthesised by bacteria and is found
primarily in meat, eggs and dairy products. There has been considerable
research into proposed plant sources of vitamin B12. Fermented soya products,
seaweeds, and algae such as spirulina have all been suggested as containing
significant B12. However, the present consensus is that any B12 present in plant
foods is likely to be unavailable to humans and so these foods
should not be relied upon as safe sources. Many vegan foods are supplemented
with B12. Vitamin B12 is necessary for the synthesis of red blood cells, the
maintenance of the nervous system, and growth and development in children.
Deficiency can cause anaemia. Vitamin B12 neuropathy, involving the degeneration
of nerve fibres and irreversible neurological damage, can also occur.
B12's primary functions are in the formation of red blood cells and the
maintenence of a healthy nervous system. B12 is necessary for the rapid
synthesis of DNA during cell division. This is especially important in tissues
where cells are dividing rapidly, particularly the bone marrow tissues
responsible for red blood cell formation. If B12 deficiency occurs, DNA
production is disrupted and abnormal cells called megaloblasts occur. This
results in anaemia. Symptoms include excessive tiredness,
breathlessness, listlessness, pallor, and poor resistance to infection. Other
symptoms can include a smooth, sore tongue and menstrual disorders. Anaemia may
also be due to folic acid deficiency, folic acid also being necessary for DNA
B12 is also important in
maintaining the nervous system. Nerves are surrounded by an insulating fatty
sheath comprised of a complex protein called myelin. B12 plays a vital role in
the metabolism of fatty acids essential for the maintainence of myelin.
Prolonged B12 deficiency can lead to nerve degeneration and irreversible
occurs, it is more commonly linked to a failure to effectively absorb B12 from
the intestine rather than a dietary deficiency. Absorption of B12 requires the
secretion from the cells lining the stomach of a glycoprotein, known as
intrinsic factor. The B12-intrinsic factor complex is then absorbed in the ileum
(part of the small intestine) in the presence of calcium. Certain people are
unable to produce intrinsic factor and the subsequent pernicious anaemia is
treated with injections of B12.
Vitamin B12 can be stored in small
amounts by the body. Total body store is 2-5mg in adults. Around 80% of this is
stored in the liver. Vitamin B12 is excreted in the bile and is effectively
reabsorbed. This is known as enterohepatic circulation. The amount of B12
excreted in the bile can vary from 1 to 10ug (micrograms) a day. People on diets
low in B12, including vegans and some vegetarians, may be obtaining more B12
from reabsorption than from dietary sources. Reabsorption is the reason it can
take over 20 years for deficiency disease to develop in people changing to diets
absent in B12. In comparison, if B12 deficiency is due to a failure in
absorption it can take only 3 years for deficiency disease to occur.
The only reliable
unfortified sources of vitamin B12 are meat, dairy products and eggs. There has
been considerable research into possible plant food sources of B12. Fermented
soya products, seaweeds and algae have all been proposed as possible sources of
B12. However, analysis of fermented soya products, including tempeh, miso, shoyu
and tamari, found no significant B12.
Spirulina, an algae available as a
dietary supplement in tablet form, and nori, a seaweed, have both appeared to
contain significant amounts of B12 after analysis. However, it is thought that
this is due to the presence of compounds structurally similar to B12, known as
B12 analogues. These cannot be utilised to satisfy dietary needs. Assay methods
used to detect B12 are unable to differentiate between B12 and it's analogues,
Analysis of possible B12 sources may give false positive results due to the
presence of these analogues.
Researchers have suggested that
supposed B12 supplements such as spirulina may in fact increase the risk of B12
deficiency disease, as the B12 analogues can compete with B12 and inhibit
The current nutritional
consensus is that no plant foods can be relied on as a safe source of vitamin
B12. Bacteria present in the large intestine are able to synthesise B12. In the
past, it has been thought that the B12 produced by these colonic bacteria could
be absorbed and utilised by humans. However, the bacteria produce B12 too far
down the intestine for absorption to occur, B12 not being absorbed through the
Human faeces can contain
significant B12. A study has shown that a group of Iranian vegans obtained
adequate B12 from unwashed vegetables which had been fertilised with human
manure. Faecal contamination of vegetables and other plant foods can make a
significant contribution to dietary needs, particularly in areas where hygiene
standards may be low. This may be responsible for the lack of aneamia due to B12
deficiency in vegan communities in developing countries.
Good sources of vitamin B12 for
vegetarians are dairy products or free-range eggs. ½ pint of milk (full fat
or semi skimmed) contains 1.2 µg. A slice of vegetarian cheddar cheese
(40g) contains 0.5 µg. A boiled egg contains 0.7 µg. Fermentation in
the manufacture of yoghurt destroys much of the B12 present. Boiling milk can
also destroy much of the B12.
are recommended to ensure their diet includes foods fortified with vitamin B12.
A range of B12 fortified foods are available. These include yeast extracts,
Vecon vegetable stock, veggieburger mixes, textured vegetable protein, soya
milks, vegetable and sunflower margarines, and breakfast cereals.
The old Recommended Daily
Amounts (RDA's) have now been replaced by the term Reference Nutrient intake
(RNI). The RNI is the amount of nutrient which is enough for at least 97% of
Intakes for Vitamin B12, µg/day. (1000 µg = 1mg)
0 to 6 months
7 to 12
1 to 3 yrs
4 to 6
7 to 10 yrs
11 to 14
15 + yrs
feeding women 2.0
Pregnant women are not
thought to require any extra B12, though little is known about this. Lactating
women need extra B12 to ensure an adequate supply in breast milk.
B12 has very low toxicity and high
intakes are not thought to be dangerous.