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DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline
Absorption from Food. The approximate percentage absorption of B12 from a few foods is presented in Table 9-1. These values apply to normal, healthy adults. No studies were found on the absorption of B12 from dairy foods or from red meat other than mutton and liver. The absorption efficiency of B12 from liver reportedly was low because of its high B12 content. Although evidence indicates that a B12 content of 1.5 to 2.5 µg/meal saturates ileal receptors and thus limits further absorption (Scott, 1997), absorption of as much as 7 µg in one subject (18 percent) was reported from a serving of liver paste that contained 38 µg of B12 (average absorption was 4.1 µg or 11 percent) (Heyssel et al., 1966).
Assumptions Used in this Report. Because of the lack of data on dairy foods and most forms of red meat and fish, a conservative adjustment for the bioavailability of naturally occurring B12 is used in this report. In particular, it is assumed that 50 percent of dietary B12 is absorbed by healthy adults with normal gastric function. A smaller fractional absorption would apply, however, if a person consumed a large portion of foods rich in B12. Different levels of absorption are assumed under various conditions, as shown in Table 9-2. Crystalline B12 appears in the diet only in foods that have been fortified with B12, such as breakfast cereals and liquid meal replacements.
B12 is continually secreted in the bile. In healthy individuals most of this B12 is reabsorbed and available for metabolic functions. El Kholty et al. (1991) demonstrated that the secretion of B12 into the bile averaged 1.0 ± 0.44 nmol/day (1.4 µg/day) in eight cholecystectomized patients, and this represented 55 percent of total corrinoids. If approximately 50 percent of this B12 is assumed to be
TABLE 9-1 Percentage Absorption of Vitamin B12 from Foods by Healthy Adults