This has particular implications for those who choose to follow a vegan diet, ie, a diet containing no animal products.

For its absorption from food vitamin B12 requires the "intrinsic factor". This is a substance secreted by certain cells in the stomach. In the absence of intrinsic factor, vitamin B12 in food or supplements will not be absorbed into the body. A lack of intrinsic factor may occur as a result of aging, surgical removal of part or all of the stomach or inflammatory conditions of the stomach. Rarely a congenital absence of intrinsic factor occurs and children born with this condition early exhibit evidence of vitamin B12 deficiency.

Actual absorption of vitamin B12 takes place at the lower end of the small bowel. Again, surgical removal or inflammatory conditions of this section of the bowel can prevent the absorption of vitamin B12 and lead to deficiency states.

So what happens to us if we have an inadequate intake of vitamin B12 either because of dietary lack or failure to absorb the vitamin? There are a couple of well known conditions, one being macrocytic (pernicious) anaemia, and the other being vitamin B12 neuropathy, that is, damage to the nervous system. These conditions may occur together but the neurological disorders are more often seen. (It is thought that the anaemia may not be seen as often due to the high folate content of the vegetarian diet. Folate is also involved in the production of normal red blood cells but it is not a substitute for vitamin B12.)

Neurological symptoms characteristic of vitamin B12 deficiency range from numbness and tingling sensations in the hands and legs, to memory loss and dementia. If untreated, nerve damage can be irreversible.

While vegetarians, and especially vegans, may be at higher risk of vitamin B12 deficiency because of reduced dietary intake, the problem is by no means exclusive to this group. Older people especially need to be aware of the possibility of reduced absorption due to a reduction in production of intrinsic factor.

How can we protect ourselves from vitamin B12 deficiency?

1. We can ensure an adequate dietary intake. This is not difficult for lact-ovo-vegetarians and non-vegetarians. These days, in many countries, there are vitamin B12 fortified products available that make it easier for vegans to obtain their requirement. Oral supplements are also available. Persons using supplements need to ensure that their chosen supplement contains an active form of the vitamin - either cyanocobalamin or hydroxocobalamin. Don't be persuaded by claims that are made for products such as certain mushrooms, algae, etc. These products do not contain any active vitamin B12. They contain instead analogues of the vitamin that are inactive and which may block the uptake of the active vitamin.

2. As we age we can ask our doctors periodically to check our blood levels of the vitamin. If low levels are detected treatment can be prescribed before any irreversible damage occurs.

3. Especially during pregnancy and lactation, women who have been vegans for some time are advised to ensure an adequate intake of vitamin B12 either by the use of fortified foods or a regular supplement. In the second half of pregnancy the foetus draws upon the mother's stores to meet its own requirement. During lactation vitamin B12 is secreted in the breast milk. If the pregnant or lactating mother's own stores are low and her dietary intake is inadequate, her infant is at significant risk of the very early onset of vitamin B12 deficiency disorders.

4 In summary, vitamin B12 is essential for life and health for humans. It is obtained from foods of animal origin. For the lacto-ovo-vegetarian that means dairy foods and eggs or from supplements which must contain an active form of the vitamin. For vegans, fortified products or supplements are recommended. Untreated vitamin B12 deficiency can lead to irreversible neurological disorders. Macrocytic anaemia may also occur.

Author :Kath King. BSC. Grad. Dip. Nutr and Diet.