Most defenders of veganism will argue that this problem can easily be solved by taking supplemental vitamin B12. However, the authors refer to some interesting research on vitamin B12 supplements:
"The present review reveals that there is only poor evidence available of vegetarians consuming vitamin B12 supplements and/or vitamin B12-fortified food and beverages. However, supplements, fortified food and beverages normally contain the less efficient cyanocobalamin form of vitamin B12, which when it enters the bloodstream must be converted to methylcobalamin(62), the only form of vitamin B12 that has a methyl donor that is required to neutralise homocysteine(63). It takes 4–9 weeks for this conversion to take place(64), assuming there are no disruptions by genetic factors, age-related problems and metabolic obstacles that may be present. Furthermore, research suggests that vitamin B12 that is not dissolved in the mouth will not (up to 88 %) be absorbed(65), due to the lack of R-binder mostly obtained from saliva, which is required to start the absorption process. The aforementioned study indicates that supplementation with cyanocobalamin can be poorly absorbed, which will have little or no effect on raising vitamin B12 levels."Indeed, Crane et al. (reference 65 in above passage) found that oral supplementation of B12 did not raise serum B12 unless the tablet could be readily dissolved. Reference 64 is a paper entitled "The coenzyme forms of vitamin B12: towards an understanding of their therapeutic potential" by Kelly [here].
"The compound most commonly referred to as vitamin B12 is CN-Cbl; however, this molecule does not occur naturally in plants, micro-organisms, or animal tissues. CN-Cbl has a cyanide molecule at the metal-carbon position and its cobalt atom exists at an oxidative state of +3, not the biologically active +1 state. In order to be utilized in the body, the cyanide molecule must removed and eliminated through phase II detoxification. It is thought that glutathione (GSH) might be the compound performing the function of decyanation in vivo, since glutathionylcobalamin (GS-Cbl) has been isolated from mammalian tissue. If, in fact, GSH is needed as a cofactor to activate CN-Cbl to the coenzyme forms of vitamin B12, clinical situations characterized by decreased tissue levels of GSH might be expected to result in functional deficiency of vitamin B12, even in the presence of adequate plasma or tissue levels of the cobalamin moiety (typically labs are looking only for a cobalamin moiety and do not differentiate between CN-Cbl and the active forms of vitamin B12)."
Apparently, one can take CN-Cbl, a.k.a. cyanocobalamin through supplements and fortified foods, and thereby raise your blood B12 level, yet remain functionally B12 deficient because CN-Cbl is inactive. Further, we do not use the basic cobalamin molecule produced by micro-organisms, but the methylcobalamin (MetCbl) and adenosylcobalamin (AdeCbl) found only in animal tissues:
"Although the basic cobalamin molecule is only synthesized by micro-organisms, all mammalian cells can convert it into the coenzymes AdeCbl and MetCbl. OH-Cbl, MetCbl, and AdeCbl are the three forms of cobalamin most frequently isolated from mammalian tissue. However, only MetCbl and AdeCbl actually function as cofactors in human enzymes. AdeCbl is the major form in cellular tissues, where it is retained in the mitochondria. MetCbl predominates in blood plasma and certain other body fluids, such as cerebral spinal fluid, and, in cells is found in the cytosol."
As already stated, apparently the body can convert CN-Cbl to the active forms, but the process takes 1-2 months, and may be disrupted by genetic factors, age-related problems, and metabolic issues. Kelly lists a number of problems that may develop in the absence of sufficient active B12, including liver disease, sleep disturbances, elevated homocysteine and methylmalonic acid (common in vegans), anemia, male infertility (low sperm concentrations, counts and motility, which have been reported common in Adventist vegetarians), eye disorders, anorexia, and cancer.
"While information is very limited, both AdeCbl and MetCbl might eventually be shown to have a supportive role in the prevention or treatment of cancer. A significant body of experimental evidence suggests a deficiency of vitamin B12 can enhance the activity of various carcinogens . Experimental results also indicate a link between alterations in the intracellular metabolism of cobalamin and the increased growth of human melanoma cells ."Who is investigating the role of B12 deficiency in the genesis and promotion of cancer?
In view of all of this data, Obersby et al. conclude that in the absence of improved delivery systems for "vegan" B12,
"The present study confirmed that an inverse relationship exists between plasma tHcy and serum vitamin B12, from which it can be concluded that the usual dietary source of vitamin B12 is animal products and those who choose to omit or restrict these products are destined to become vitamin B12 deficient."In other words, in the absence of ideology and technology, it appears that humans have an evolved nutritional requirement for animal products to obtain the metabolically active forms of vitamin B12. If you eat a vegan diet, you should supplement with methylcobalamin, which is better retained and utilized than cyanocobalamin, although it is unknown at this time whether this will adequately address all vitamin B12 requirements in the absence of dietary animal products which also contain adenosylcobalamin.