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BOX 9-1 Assumptions Made in Estimating the Amount of Vitamin B12, Needed for Maintenance of Hematological Status and Serum Vitamin B12 Values

  • Maintenance of hematological status requires a relatively stable hemoglobin value upon administration of B12 and a normal mean cell volume, not just a reticulocyte response.

  • Normal serum B12 is ≥ 150 pmol/L (200 pg/mL).

  • Because B12 is not absorbed from the bile, the estimated extra loss of B12 by a person with pernicious anemia in remission is 0.4 nmol/day (0.5 µg/day) based on data from Bozian et al. (1963), El Kholty et al. (1991), Heyssel et al. (1966), and Reizenstein (1959).

  • The average fractional absorption of B12 from food by healthy individuals is approximately 50 percent (see “Absorption”).

provided information on levels of B12 intake needed to maintain hematological status. In some cases, neurological manifestations may be the earliest clinical sign of low B12 values (Beck, 1991; Karnaze and Carmel, 1990; Lindenbaum et al., 1988; Martin et al., 1992). Assumptions that were integral to the application of this method are shown in Box 9-1.

In brief, this method involves estimating the amount of B12 required daily to maintain hematological and serum B12 status of individuals with pernicious anemia in remission; subtracting the amount of endogenous B12 lost from the bile in excess of that lost by a healthy individual; and, because the value is to be used for individuals with normal ability to absorb B12 from food, correcting for bioavailability. The result is shown in Box 9-2.

BOX 9-2 Steps Used to Estimate the Vitamin B12 Requirement by Using Data Obtained from Subjects with Pernicious Anemia

Step 1

Estimate the average intramuscular requirement for maintenance of person with pernicious anemia

1.5 µg/day

Step 2

Subtract estimate of extra losses due to lack of reabsorption of biliary B12

– 0.5 µg/day

Subtotal

Estimate average requirement of normal person for absorbed B12

1.0 µg/day

Step 3

Correct for bioavailability (50 percent)

÷ 0.5

Result

Average requirement of normal person for B12 from food: Estimated Average Requirement (EAR)

2.0 µg/day



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