More on Cyanocobalamin Safety/Toxicity
Cobalamin (vitamin B-12) is synthesized only by certain microbes (bacteria and archaeon), and five forms of vitamin B-12 have been found in foods, according to the British Journal of Nutrition: adenosylcobalamin and hydroxocobalamin are the predominant forms, and other forms found include methylcobalamin, cyanocobalamin, and sulphitocobalamin. Most of these forms are considered synthetic substances, with additional compounds subsequently bonded to the already-synthesized cobalamin.
Methylcobalamin is produced commercially from cyanocobalamin, which is initially produced by fermentation and conversion processes. Adenosylcobalamin is directly produced by bacteria, while methylcobalamin requires an additional process of stripping off the cyanide group (CN, a carbon and a nitrogen atom with a triple bond) and adding a methyl group (CH3). Fermentations producing cyanocobalamin, methylcobalamin, and adenosylcobalamin do not utilize genetically engineered bacteria. The cyanocobalamin form is safe, stable, and bioavailable, and has been used for the vast majority of public health studies over the past century.
The amount of cyanide in cyanocobalamin is trivial, less than in an almond or apple; cyanide tends to be ubiquitous in the food supply in extremely tiny amounts. Cyanide is also produced by our own cellular processes (no great surprise since it's simply a single carbon and nitrogen molecule bonded together), but is quickly absorbed into the bloodstream, converted into thiocyanate (a less toxic form), and excreted in the urine or converted to carbon dioxide and exhaled.
In fact, the Institute of Medicine has not set an Upper Limit for vitamin B-12 because “no adverse effects have been associated with excess vitamin B-12 intake from food and supplements in healthy individuals” (and both supplementation and food fortification primarily use cyanocobalamin). That same IOM report features this quote: “the amount of cyanide in cyanocobalamin is considered to be physiologically insignificant.” It would take about 2,500 doses of 1,000 microgram cyanocobalamin tabs to approach a dangerous amount of cyanide.
The adenosylcobalamin form is
predominant in foods as well as in the human body, whereas methylcobalamin is
typically in the body at only half the amount of the adenosyl form. Both are
coenzyme forms with separate enzyme systems, and both are needed by the body.
Cyanocobalamin feeds both pathways and is an inexpensive, stable, safe, and
effective form of vitamin B-12 that does not require genetically engineered
bacteria to produce.