Microbial assays for B-12 are unreliable. A common misconception in vegan circles is that fermented foods and spirulina contain B-12. This claim may, at times, be supported by lab tests for B-12 based on the USP (U.S. Pharmacopeia) assay methods.
Unfortunately, as explained in Herbert et al. [1984] and Herbert [1988], the USP assay method for B-12 is unreliable. The assay measures total corrinoids--that is, true B-12 plus analogues (forms of B-12 that are not metabolically active in the body)--and the analogues have the potential to block the absorption of true B-12 by occupying B-12 receptor sites. A preferred, reliable test that can differentiate between true B-12 and corrinoids is provided by differential radioassay. The assay problem must be considered in evaluating "old" studies on B-12.
Spirulina and tempeh contain mostly analogues of B-12. Herbert [1988] reports that tests on tempeh, a fermented soy product, and spirulina revealed that they contained almost no true B-12, i.e., the "B-12" they contained (per USP assay test) was predominantly analogues. Herbert [1988, p. 857] reports:
We suspect that people taking spirulina as a source of vitamin B-12 may get vitamin B-12 deficiency quicker because the analogues in the product block human mammalian cell metabolism in culture [i.e., in the lab] and we suspect that they will also do this in the living human.
The presence of analogues, rather than true B-12, in fermented foods makes them unreliable sources for B-12.
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# posted by madthumbs @ 10:36 AM
