Vitamin B12 is a dark-red crystalline hygroscopic substance, freely soluble in water and alcohol but insoluble in acetone, chloroform and ether. Cyanocobalamin, which has a molecular weight of 1,355, is the most complex structure and heaviest compound of all vitamins. Oxidizing and reducing agents and exposure to sunlight tend to destroy its activity. Losses of vitamin B12 during cooking are usually not excessive because it is stable at temperatures lower than 250°C.
Passage of vitamin B12 through the intestinal wall is a complex procedure that requires intervention of certain carrier compounds able to bind the vitamin molecule (McDowell, 2000). To summarize the vitamin B12 absorption for most species studied, the following is required: (1) adequate quantities of dietary vitamin B12, (2) normal stomach for breakdown of food proteins for release of vitamin B12, (3) normal stomach for production of intrinsic factor for absorption of vitamin B12 through the ileum, (4) normal pancreas (trypsin) required for release of bound vitamin B12 prior to combining the vitamin with the intrinsic factor and (5) normal ileum with receptor and absorption sites. Additional factors that diminish vitamin B12 absorption include deficiencies of protein, iron, vitamin B12, thyroid removal and dietary tannic acid (Hoffmann-La Roche, 1984).
Intrinsic factor concentrates prepared from one animal's stomach do not in all cases increase B12 absorption in other species of animal or in man. There are structural differences in the vitamin B12 intrinsic factor among species. Species differences exist likewise for vitamin B12 transport proteins (Polak et al., 1979). Intrinsic factor has been demonstrated in man, monkey, pig, rat, cow, ferret, rabbit, hamster, fox, lion, tiger and leopard. It has not, at present, been detected in dog, guinea pig, horse, sheep, chicken and a number of minor species.
Storage of vitamin B12 is found principally in liver; other sources are kidney, heart, spleen and brain. The main excretion routes for absorbed vitamin B12 are urinary, biliary and fecal. Urinary excretion of the intact vitamin B12 by kidney glomerular filtration is minimal. Biliary excretion via feces is the major excretory route. The majority of cobalamin excreted in bile is reabsorbed; at least 65% to 75% is reabsorbed in the ileum by means of the intrinsic factor mechanism.
