Low serum vitamin B12 levels in patients receiving ascorbic acid in megadoses: studies concerning the effect of ascorbate on radioisotope vitamin B12 assay

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Abstract
Serum vitamin B12 levels were found to be low in four of 18 hospitalized patients with traumatic spinal cord injury, who had been receiving 2 g daily of oral ascorbic acid as a urine acidifier for varying periods of time. Bone marrow studies, on two of the patients with low B12 levels, showed normoblastic erythropoiesis and a normal deoxyuridine suppression test. One of the two, however, had hypersegmentation of the granulocyte nuclei. Three control subjects showed no change in assayable B12 levels after 2 weeks of such ascorbate supplementation. In vitro studies demonstrated that in the absence of added KCN the addition of sodium ascorbate to normal serum, liver homogenate, or crystalline B12 solution before, but not after, the samples were heated reduced the amount of radioassayable B12. This study demonstrates that serum B12 determinations performed without added KCN may be low due to the presence of ascorbate in the serum, and that such low levels do not necessarily reflect tissue deficiency of the vitamin. Therefore, to adequately interpret a low serum B12 level, it is important to know if the patient has been taking large doses (in the range of 0.5 g four times daily) of ascorbic acid, by prescription or as self-medication. In situations where neither heat alone nor ascorbate alone will destroy B12, both together will unless the B12 is protected against such destruction by conversion to a stable form, such as cyanocobalamin. We now add cyanide before the heating step in B12 radioassay, to prevent ascorbate destruction of vitamin B12, unless the assay is carried out at pH 9.3.

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