No plant material can synthesize vitamin B12. Apart from reports that some algae can synthesize vitamin Bi2 its origin in the food chain seems to be exclusively due to its biosynthesis by microorganisims. Thus, most vitamin B12 enters the human food chain from biosynthesis by microorganisms in herbivorous animals. Meat and products such as milk, cheese, or eggs introduce vitamin Bi2 into the human food chain. Chickens ingest food contaminated with microbes and introduce the vitamin via their meat and eggs. Vegetarians who have milk or eggs (lacto ovo vegetarians) as part of their diet and thus a source of some, albeit reduced, dietary vitamin B12 still have reduced vitamin B12 status. Yet other communities who for religious or other reasons are strict vegetarians (vegans) have no source of vitamin B12 and are at high risk of deficiency. This risk can be reduced in some of these communities where fermented food is eaten, in which bacteria have introduced vitamin B12; also, it has been suggested that in some circumstances the food is contaminated by bacteria. However, vegans and in particular babies born to and weaned by strict vegan women are established to be at risk of vitamin B12 deficiency and such babies have been reported on several occasions to show the signs and symptoms of the neuropathy associated with such deficiency.
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