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vitamin B12 - there are 4 different forms.

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Vitamin B12 is the name given to a group of related compounds containing cobalt as the central
ion in a corrin ring. The cobalt ion can be coordinated to a methyl, 5�€™-deoxyadenosyl-,hydroxy- or cyano- group. Hydroxycobalamin and cyanocobalamin used in food supplements
are transformed in the human body by coordinating with other ligands into methylcobalamin
and 5-deoxyadenosylcobalamin. The latter two are actively involved in endogenous
metabolism. Thus, 5-deoxyadenosylcobalamin and methylcobalamin can be considered to
represent naturally occurring endogenous derivatives of vitamin B12.

Intake of vitamin B12 may originate from food, from food supplements and from other sources.
Exposure to cobalamin vitamers depends on the frequency of consumption of the foods containing these compounds.

Kidney (lamb) and liver (lamb, beef, calf, pork) for instance can contain 69-122 ug/100 g, heart (beef) and egg yolk 5-50 ug/100 g, and fish and milk 0.2-5 µg /100 g. 5-Deoxyadenosylcobalamin accounts for 50-70% of the total vitamin B12 in organ meat. In cows milk vitamin B12 is mainly present as 5-deoxyadenosylcobalamin (Castle and Hale, 1998)
Major dietary sources of vitamin B12, mainly in the forms of methyl-, deoxyadenosyl- and
hydroxycobalamin, include meat (e.g. >0.1 mg/kg in lamb), particularly liver (>0.1 mg/kg) and
fish (e.g. 0.03-0.1 mg/kg in salmon, 0.01-0.03 mg/kg in tuna) (EVM, 2003). Dietary vitamin
B12 only comes from animal sources, mainly from dairy products, fish and (red) meat.
Therefore individuals consuming large amounts of liver and some types of fish (sardines) may
have high intakes in contrast to individuals avoiding animal products.

1 ug or mcg=1/1000 of a milligram.

oysters and clams are great sources as well.

some cool info-

Effects of methylcobalamin on the proliferation of androgen-sensitive or estrogen-sensitive malignant cells in culture and in vivo.

Ultra-high dose methylcobalamin promotes nerve regeneration

Experimental study of antitumor effect of methyl-B12

Effects of vitamin B12 on plasma melatonin rhythm in humans: increased light sensitivity phase-advances the circadian clock Edited by AutonomousOne1980

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