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Vitamin B12

Jump to: Introduction : Functions : Dietary Sources : Required Intakes

Introduction

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 2-5mg 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.

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, 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 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. 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, Vecon vegetable stock, veggieburger mixes,
textured vegetable protein, soya milks, vegetable and sunflower
margarines, and breakfast cereals.

Required Intakes

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, µg/day. (1000 µg = 1mg)

Age

RNI

1. to 6 months

0.3 µg

1. to 12 months

0.4 µg

1. to 3 yrs

0.5 µg

1. to 6 yrs

0.8 µg

1. to 10 yrs

1.0 µg

11 to 14 yrs

1.2 µg

15 + yrs

1.5 µg

Breast feeding women

2.0 µg

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.

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