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DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline
TABLE 9-3 Change in Percentage Retention of Vitamin B12 with Increasing Intramuscular Dose
Vitamin B12 Dose (µg)
SOURCE: Chanarin (1969).
Early detection of vitamin B12 deficiency depends on biochemical measurements. Lindenbaum and colleagues (1990) reported that metabolites that arise from B12 insufficiency are more sensitive indicators of B12 deficiency than is the serum B12 value. This was found in patients with pernicious anemia or previous gastrectomy who experienced early hematological relapse: serum methylmalonic acid (MMA), total homocysteine, or both were elevated in 95 percent of the instances of relapse whereas the serum B12 value was low (less than 150 pmol/L [200 pg/mL]) in 69 percent. Similarly, serum B12 was found to be an insensitive indicator in a review of records of patients with clinically significant B12 deficiency. Five deficient individuals had neurological disorders that were responsive to B12 and had elevated serum MMA and homocysteine values even though their serum B12 values were greater than 150 pmol/L (200 pg/mL) and anemia was absent or mild. In a recent series of 173 patients, 5.2 percent of those with recognized B12 deficiency had serum B12 values in the normal range. Similar findings were reported elsewhere (e.g., Carmel, 1988; Pennypacker et al., 1992; Stabler et al., 1996). At present, the techniques developed to measure serum MMA and homocysteine (capillary gas chromatography and mass spectrometry) are costly and may be beyond the scope of routine laboratories. Conditions that may warrant assessment of B12 status because they may result in B12 deficiency are summarized in Table 9-4.