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

• DOI: 10.1093/ajcn/31.2.253
• Publication Date: 2018-01-17T09:54:26Z
• Authors: V Herbert; E Jacob; K-T J Wong; J Scott; RD Pfeffer
• Keywords: Adult; Intrinsic Factor; Male; Cyanides; Chemical Phenomena; Dose-Response Relationship, Drug; Ascorbic Acid; Hydrogen-Ion Concentration; Middle Aged; Urine; 3. Good health; Chemistry; Vitamin B 12; 03 medical and health sciences; 0302 clinical medicine; Humans; False Negative Reactions; Spinal Cord Injuries

Serum vitamin B,2 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 B,2 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 B,2 levels after 2 weeks of such ascorbate supplementa- tion. In vitro studies demonstrated that in the absence of added KCN the addition of sodium ascorbate to normal serum, liver homogenate, or crystalline B,2 solution before, but not after, the samples were heated reduced the amount of radioassayable B,2. This study demonstrates that serum B,2 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 B,2 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 B,2, both together will unless the B,2 is protected against such destruction by conversion to a stable form, such as cyanocobalamin. We now add cyanide before the heating step in B,2 radioassay, to prevent ascorbate destruction of vitamin B,2, unless the assay is carried out at pH 9.3. Am. J. Clin. Nutr. 31: 253-258, 1978.