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 |
| 0 to 6 months |
0.3 µg |
| 7 to 12 months |
0.4 µg |
| 1 to 3 yrs |
0.5 µg |
| 4 to 6 yrs |
0.8 µg |
| 7 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. |
