Multivitamin/mineral food supplements containing vitamin B12 may also contain analogues of vitamin B12

Vitamin B12 Supplementation Adequacy in Australian Vegan Study Participants

In our initial analysis of the Australian Vegan Study we estimated the mean daily intake of vitamin B12 of each participant and compared this to the Recommended Dietary Intake (RDI). However, the proportion of vitamin B12 that can be absorbed from large doses typically contained in oral supplements is considerably lower than the amount absorbed from food. In this analysis we took into account the estimated absorption from supplements in order to compare adequacy of vitamin B12 intake to the RDI. A cross-sectional online survey was used to obtain information from women (N = 1530) of reproductive age on a vegan diet in Australia. Vitamin B12 intake from food was estimated using a validated food frequency questionnaire and detailed questioning was used to estimate supplemental intake. We used published data on dose-dependent absorption rates to estimate amount of the vitamin absorbed to enable comparison to the RDI. Supplementation practices varied widely. Based on estimated amount absorbed, 39% of participants had an estimated total intake of vitamin B12 below RDI equivalency, compared to 26% based on mean daily intake. The potential absorption of vitamin B12 needs to be considered when estimating adequacy of intake and recommending supplementation.

Schilling test: physiologic basis for and use as a diagnostic test

With the advent of binding assays for vitamin B12 in blood, the Schilling test, which involves administration of radioactive B12 to a patient and subsequent urine collection for 24 to 48 h, fell into disuse in many laboratories. However, the test is still the only way to actually measure whether vitamin B12 is being absorbed through the terminal ilium. By administering radioactive vitamin B12, along with a preparation of intrinsic factor (IF), lack of functional IF may also be demonstrated. The Schilling test requires that attention be paid to a number of parameters, including the amount of radioactive vitamin B12 administered and the completeness of the urine collection. These factors, and others required for correct performance of the test, are discussed in this article.

Determination of metabolically active B12 and inactive B12 analog titers in human blood using several microbial reagents and a radiodilution assay

Metabolically active B12 analogs and inactive B12 analogs were measured in plasma, red blood cells (RBC), and pooled pernicious anemia serum. B12 values by Lactobacillus leichmannii, Escherichia coli, Euglena gracilis, and radioisotope dilution method (RIDA) as assays for total B12 (active analogs + inactive analogs) were compared to Ochromonas malhamensis values as index of only metabolically active B12. B12 values above those with O malhamensis distinguished inactive analogs from active B12. Inactive analogs contribute 85, 97, 135, and 163% above active B12 activity in normal plasma when E gracilis, L leichmannii, RIDA, and E coli, respectively, were used for B12 analysis. RIDA B12 determinations for active B12 in plasma showed that 44% of the B12 measured was still due to inactive analogs when compared to O malhamensis B12 activity. Inactive B12 analogs contributed 21, 151, and 224% above O malhamensis active B12 in RBC when E gracilis, L leichmannii, and E coli, respectively, were used.

The role of vitamin B12 and folate in carcinogenesis

The roles of vitamin B12 and folate in carcinogenesis are largely extensions of and linked to their roles in normal metabolism, particularly 1-carbon unit metabolism. A possible key area may be hypomethylation to "switch on" genes and methylation to "switch them off." Some vitamin analogues may act as antivitamins in these reactions, as may some vitamin-binding proteins. Others may act as specific delivery proteins. Using appropriate radioactive substrates and suspensions of vitamin-dependent normal and malignant cells, it may be possible to work out their positive and negative control of DNA synthesis.

Substances with biological activity of vitamin B12 formed during cultivation of Propionibacterium freudenreichii in the presence of precursors (short communications)

The papers of Kolhouse et al. and Cooper et al. described the occurrence of vitamin B12 analogues of unknown origin in blood serum. Some of these analogues may be derived from slaughter cattle raised on feed supplemented with vitamins and minerals, as was observed by Allen. Herbert et al. found vitamin B12 analogues in multivitamin preparations produced in U.S.A., and Kanazawa et al. in human liver, red cells and brain. It is not clear so far, if and how do vitamin B12 analogues interfere with vitamin B12 metabolism. When P. freudenreichii was cultivated in the presence of o-phenylenediamine and 5,6-dimethylbenzimidazole, which may be considered precursors as well as antimetabolites of vitamin B12, the stimulation of biosynthesis of substances with biological activity of vitamin B12 took place. Various signs show that these substances are probably vitamin B12 analogues. During stimulated and nonstimulated production of vitamin B12 by P. freudenreichii, two substances with vitamin B12 biological activity have always been obtained. Their relation was not stable and differed according to the conditions of cultivation. Every attempt to stimulate the biosynthesis of vitamin B12 resulted in the suppression of production of the substance with higher molecular weight, even if the biosynthesis of cobalamin (lower molecular weight) was increased. In our note we want to pay attention to the character of substances arising in the stimulated biosynthesis.

Disorders of cobalamin metabolism

Recent developments in our knowledge of the biochemistry and metabolism of cobalamin have given us some insight into clinical disorders. N2O, which easily induces cobalamin deficiency, both in vivo and in vitro, has greatly contributed to the investigation of the cobalamin deficient state, especially in relation to folate and amino acid metabolism. Demonstration of the cobalamin analog in human serum and a new enzyme which requires cobalamin as a coenzyme has led to recent increased interest in this field. The disorders of cobalamin metabolism will be summarized briefly as well as those areas currently of particular interest.

Solid-phase vitamin B12 assays using polyacrylamide-bound intrinsic factor and polyacrylamide-bound R-binder

A new solid phase vitamin B12 assay is described using intrinsic factor to measure microbiologically-available B12 and R-binder to measure total B12. The solid phase reagent consists of intrinsic factor coupled to polyacrylamide beads and salivary R-binder coupled to polyacrylamide beads. The assay is simple to perform and separates completely sera from controls and patients with megaloblastic anaemia due to B12 deficiency.