In short, most animal products only contain B12 because the animals were given supplements and so are not a more natural source of this vitamin than if you take a supplement yourself
The scientific papers reviewed for this article involve enzyme systems, cultured cells, rats or mice
Watson, E., MacNeil, L
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NCI concluded that the data demonstrated no benefit.[2] In 1972, the American Cancer Society (ACS) published a statement summarizing the negative assessments of Gerson's treatment.[10] Another statement published by ACS in 1991 concerned various "metabolic therapies" (defined as treatments that depend on changing metabolism through diet, enemas, and supplements given at clinics in Tijuana, Mexico) and reemphasized the lack of scientific evidence on the efficacy of the Gerson regimen.[11] Gerson died in 1959, leaving behind no systematic way to continue offering his treatment