| Vitamin B12
Vitamin 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
Vitamin 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 synthesis.
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 neurological damage.
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.
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.
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 metabolism.
nutritional consensus is that no plant foods can be relied on as a safe
source of vitamin B12.
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
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.
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
Nutrient Intakes for Vitamin B12, µg/day. (1000 µg = 1mg)
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 the population.
|0 to 6 months
|7 to 12 months
|1 to 3 yrs
|| 0.5 µg
|4 to 6 yrs
|| 0.8 µg
|7 to 10 yrs
|11 to 14 yrs
|| 1.2 µg
|15 + yrs
|Breast feeding women
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.