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1995-03-13
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VegSocUK Information Sheet
THE VEGETARIAN SOCIETY
___________________________________
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 also
occur.
FUNCTIONS
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.
When deficiency 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
2-5 mg 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 10 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.
DIETARY SOURCES
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, a 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 metabolism.
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 colon lining.
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. A half pint of milk (full fat or semi skimmed) contains 1.2 micrograms.
A slice of vegetarian cheddar cheese (40g) contains 0.5 micrograms. A
boiled egg contains 0.7 micrograms. Fermentation in the manufacture of
yoghurt destroys much of the B12 present. Boiling milk can also destroy
much of the B12.
Vegans 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.
REQUIRED INTAKES
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VegSocUK Information Sheet
Page 7 of 8
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.
Reference Nutrient Intakes for Vitamin B12, micrograms/day. (1000 micrograms = 1 mg).
Age RNI
0 to 6 months 0.3 micrograms
7 to 12 months 0.4 micrograms
1 to 3 yrs 0.5 micrograms
4 to 6 yrs 0.8 micrograms
7 to 10 yrs 1.0 micrograms
11 to 14 yrs 1.2 micrograms
15 + yrs 1.5 micrograms
Breast feeding women 2.0 micrograms
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.
___________________________________
//
This article is copyright to the Vegetarian Society (UK), but may be freely
copied for non-commercial use provided it is kept intact, not altered
and these lines are included.
For futher information contact: The Vegetarian Society, Parkdale, Dunham Road,
Altrincham, Cheshire WA14 4QG, England. Tel: (England) 061 928 0793
email: vegsoc@vegsoc.demon.co.uk
//
[The text of this file was obtained from the Vegetarian Society (UK) in
March 1995.]