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Memory does not have to go back very far to recall the great excitement which occurred when soon after the end of World War II it was announced almost simultaneously in England and the United States that vitamin B12 (cyanocobalamin) had been isolated. B12 turned out to be both the extrinsic factor and the erythrocyte-maturing factor of Castle. Cohn's long and disappointing efforts to isolate the important factor in liver are to be explained by several elements of the problem which we can now see clearly. In the first place, folic acid has much the same effect and in a general way occurs in the same foods as B12. The second major stumbling block was that vitamin B12 is needed in an amount of approximately 1μg each day. Foods in which it occurred in ridiculously small amounts were effective, but the chemists were forever losing it. Add the
Bean WB. Vitamin B12.. Arch Intern Med. 1964;114(6):861. doi:10.1001/archinte.1964.03860120173033
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