The information below is taken from 'Plant Based Nutrition and Health' by Stephen Walsh (endorsed by the Vegan Society).
All vitamin B12 is produced by bacteria whether it is obtained from fortified foods,supplements, meat, milk or eggs. B12 for use in supplements and fortified foods is produced by fermentation using carefully selected bacteria which have not been genetically modified. This form of B12 is highly absorbable...
Vitamin B12 deficiency usually becomes apparent as neurological symptoms, partiucarly if folate intake is high. The classic symptoms include numbness and tingling, lack of energy, blurred vision, loss of balance and limb control, poor memory, sore tongue and personality changes such as delusion and paranoia. In infants, B12 deficiency manifests as loss of energy, appetite and alertness and can progress to coma and death. Onset is also more rapid than in adults. All symptoms are reversable if caught early enough but damage can become permanent, particularly in children...
A minority of primates have a second stomach, which acts as a bacterial fermenter (similar to the rumen of grazing animals) and provides an on-board source of B12... All other primates, like humans, require an external supply of B12 from bacteria outside their body. Vegans do not deliberately eat insects, and neither soil nor faeces are particularly attractive to most modern humans. Fortunately we now have technology to havest B12 directly from bacteria, thus removing the need for an intermediary and avoiding the risk of infection. Fortified foods and supplements provide a very reliable and well established source of B12 with no unwanted extras.
Some vegans, particularly those following raw food diets, are reluctant to use supplements or fortified foods. Unfortunately, none of the commonly suggested alternatives such as spirulina, nori or blue-green algae has been shown to contain sufficient true B12 to be a realistic source.
..It goes on to recommend 3 micrograms over the course of a day from fortified foods, or, 10 micrograms once daily from a supplement, or, 2000 micrograms once a week from a supplement.
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