这东西根本就是细菌合成的, 和吃什么东西没有什么鸟关系.
(下面文字的资料来源是维基百科 http://en.wikipedia.org/wiki/Vitamin_B12 )
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Vitamin B-12 is the name for a class of chemically-related compounds, all of which have vitamin activity. It is structurally the most complicated vitamin. Biosynthesis of the basic structure of the vitamin can only be accomplished by bacteria, but conversion between different forms of the vitamin can be accomplished in the human body. A common form of the vitamin, cyanocobalamin, does not occur in nature, but is used as a supplement and food additive, due to its stability. It is converted to other forms of the vitamin which are actually used in chemical reactions in the body.
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Vitamin B-12 cannot be made by plants or animals[5] as only bacteria have the enzymes required for its synthesis. The total synthesis of B-12 was reported by Robert Burns Woodward[6] and Albert Eschenmoser,[7][8] and remains one of the classic feats of organic synthesis.