The studies covered by Table 9-7 suggest that the B12 requirement is higher than the amounts reported to be consumed by the subjects and more than that provided by the treatments that were described. In three studies (Baker and Mathan, 1981; Jathar et al., 1975; Winawer et al., 1967), all adults required more than 1 µg/day of B12 by mouth. Two studies (Narayanan et al., 1991; Stewart et al., 1970) give evidence that 1.5 µg/day of dietary B12 is not sufficient to maintain hematological status and serum B12 in half of the subjects studied. The meager data provided by the studies of vegetarians indicate that the B12 average requirement should probably be at least 1.5 µg/day, but a higher average requirement is not ruled out.
Various studies have indicated losses of 0.1 to 0.2 percent/day of the B12 pool (e.g., Amin et al., 1980; Boddy and Adams, 1972; Heyssel et al., 1966; Reizenstein et al., 1966) regardless of the size of the pool. A loss of 0.2 percent appears to be typical for individuals who do not reabsorb biliary B12 because of pernicious anemia (Boddy and Adams, 1972). A person with a B12 pool of 1,000 µg and a loss of 0.1 percent would excrete 1 µg of B12 daily, and a person with a 3,000-µg pool would excrete 3 µg daily. If only 50 percent of dietary B12 is absorbed, the amounts required daily to replenish the pools are 2 and 6 µg of B12, respectively. The higher value would lead to less efficient use of B12, but the larger store of B12 would cover a longer period of inadequate B12 intake or absorption.
With a 0.1 percent loss, the period of protection afforded by the B12 pool can be estimated if the lowest pool size consistent with health is also known. If it is assumed that this value is 300 µg (derived from Bozian and coworkers ), there is no absorption of B12 from food or supplements, and the enterohepatic circulation is intact, then stores of 1 mg would be expected to meet the body’s needs for 3 years, 2 mg for about 5 years, and 3 mg for about 6 years. A 1.5 percent loss would reduce these estimates to 2, 3.6, and 4 years (see Appendix N for the method used to obtain these values).
The extent of the supply of reserve B12 may be an important consideration when persons approach the age of 50 and the risk increases for food-bound B12 malabsorption secondary to atrophic gastritis (see “Factors Affecting the Vitamin B12 Requirement” and section “Adults Ages 51 Years and Older”). Because the absorption of B12 from fortified foods, oral supplements, or the bile does not