Vitamin B12 is a dark-red crystalline hygroscopic substance, freely soluble in water and alcohol but insoluble in acetone, chloroform and ether. Cyanocobalamin has a molecular weight of 1,355 and 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 feed processing are usually not excessive, because vitamin B12 is stable at temperatures as high as 250°C.
Of the total vitamin B12 produced by rumen microorganisms, only 1% to 3% is absorbed. As in most species, the absorptive site for ruminants is the lower portion of the small intestine. Substantial amounts of B12 are secreted into the duodenum and then reabsorbed in the ileum.
Passage of vitamin B12 through the intestinal wall is a complex procedure and requires intervention of certain carrier compounds able to bind the vitamin molecule (McDowell, 2000). For vitamin B12 absorption in most species studied, the following is required: (a) adequate quantities of dietary vitamin B12 or cobalt, (b) functional abomasum (true stomach) for breakdown of food proteins for release of vitamin B12, (c) functional abomasum for production of intrinsic factor for absorption of vitamin B12 through the ileum, (d) functional pancreas (trypsin secretion) required for release of bound vitamin B12 prior to combining the vitamin with the intrinsic factor and (e) functional ileum with receptor and absorption sites. Gastric juice defects are responsible for most cases of food-vitamin B12 malabsorption in monogastrics (Carmel, 1994), but for ruminants the lack of cobalt for vitamin B12 synthesis is the major problem (McDowell, 1997). Factors that diminish vitamin B12 absorption include deficiencies of protein, iron and vitamin B6; thyroid removal; and dietary tannic acid (Hoffmann-La Roche, 1984). The absorption of vitamin B12 is limited by the number of intrinsic factor-vitamin B12 binding sites in the ileal mucosa, so that not more than about 1 to 1.5 µg of a single oral dose of the vitamin in humans can be absorbed (Bender, 1992). The absorption is also slow; peak blood concentrations of the vitamin are not achieved for some 6 to 8 hours after an oral dose.
Intrinsic factor concentrates prepared from the stomach of one animal species do not in all cases increase B12 absorption in other animal species or in humans. There are structural differences in the vitamin B12 intrinsic factors among species. Similarly, there are species differences for vitamin B12 transport proteins (Polak et al., 1979). Intrinsic factor has been demonstrated in human, monkey, pig, rat, cow, ferret, rabbit, hamster, fox, lion, tiger and leopard. It has not, at present, been detected in the guinea pig, horse, sheep, chicken and various other species. It is now established that the dog stomach produces only small amounts of intrinsic factor, with larger amounts produced by the pancreas (Batt et al., 1991).
Storage of vitamin B12 is found principally in the liver. Other storage sites include the kidney, heart, spleen and brain. Andrews et al. (1960) reported that the proportion of liver cobalt that occurs as vitamin B12 varied with the cobalt animal status. Under grazing conditions with adequate cobalt in the pasture, most liver cobalt can be accounted for as vitamin B12, but in cobalt deficiency only about one-third of the liver cobalt is present in this form. This indicates that in cobalt deficiency, liver vitamin B12 is depleted faster than other forms of cobalt.
Even though vitamin B12 is water soluble, Kominato (1971) reported a tissue half-life of 32 days, indicating considerable tissue storage. Cattle and sheep with normal liver stores can receive a cobalt-deficient diet for months without showing vitamin B12 deficiency signs.
In ruminants, both cobalt and vitamin B12 are mainly excreted in feces, although variable amounts are excreted in urine (Smith and Marston, 1970). Lactating cows on a normal diet excrete 86% to 87.5% of all excreted cobalt in the feces (mainly with the bile), 0.9% to 1.0% with urine, and 11.5% to 12.5% with milk.
