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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 soy
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 maintenance 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
maintenance 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 2-5mg 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 re-absorption than from dietary sources. Re-absorption 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 soy
products, seaweeds and algae have all been proposed as possible
sources of B12. However, analysis of fermented soy products,
including Tempeh, Miso, Shoyu and tamari, found no significant
B12.
Spirulina, an algae available as a dietary
supplement in tablet form contains 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 utilized 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 utilized by humans.
However, the bacteria produce B12 too far down the intestine for
absorption to occur, B12 not being absorbed through the colon
lining.
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.
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, vegetable stock, veggie
burger mixes, textured vegetable protein, soy milks, vegetable and
sunflower margarines, and breakfast cereals.
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