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DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline
Results of other studies of patients with pernicious anemia are presented in Table 9-6. The short-term study by Hansen and Weinfeld (1962) used relatively high B12 doses to restore normal status but did not assess maintenance requirement. The long-term studies by Bastrup-Madsen et al. (1983) and Lindenbaum et al. (1990) used different dosages and methods of reporting that make it impossible to draw precise conclusions. Nonetheless, the results indicate that 0.8 to 1.0 µg/day of B12 IM will maintain normal hematological, serum B12, and serum metabolite status in nearly half of the individuals over time and that 1.7 µg will maintain it in all individuals. The study conducted by Best and colleagues (1956) was designed to determine the effective dosage of intrinsic factor concentrates, not to estimate the B12 requirement, but it suggests that 1.4 µg of B12 exceeds the requirement for absorbed B12 in most of the subjects tested. The often-cited study of Sullivan and Herbert (1965) was interpreted as providing evidence that 0.1 µg/day of B12 was not sufficient for treating pernicious anemia and maintaining adequate B12 status. Similarly, the 0.6 to 0.7 µg/day of B12 supplied IM in the study by Will and coworkers (1959) was also judged too low to maintain a normal serum B12 concentration.
The study by Darby and colleagues (1958), which indicates an average requirement in such patients of approximately 1.5 µg, is supported by the supplementary data from the other studies described in Table 9-6. These studies provide support for a physiological average requirement of 1.0 µg/day of B12 after adjustment for the extra loss of B12 by subjects with pernicious anemia (0.5 µg/ day) (Step 2 in Box 9-2). Adjusting for incomplete absorption of B12 from food of 50 percent (Step 3) converts this value to an EAR for B12 of 2.0 µg/day.
Studies of Individuals with Low B12Intake. Studies of individuals with low B12 intake were examined to determine whether these reports (Table 9-7) supported the findings for subjects with pernicious anemia. Because B12 is not a component of plant foods, diets containing little or no animal food may lead to B12 deficiency. Deficiency develops slowly because of efficient reabsorption of biliary B12. It is also possible but not certain that vegans consume some B12 from animal products that contaminate plant food or from bacterial action. Studies of vegetarians generally have not analyzed the B12 content of the food, and accurate data are not available for some of the foods (e.g., certain algae) consumed by vegetarians. Without actual analyses it is not clear what B12 content should be assumed for vegans.