Absorption and malabsorption of vitamin B 12

Accounting for differences in the bioactivity and bioavailability of vitamers

Essentially all vitamins exist with multiple nutritionally active chemical species often called vitamers. Our quantitative understanding of the bioactivity and bioavailability of the various members of each vitamin family has increased markedly, but many issues remain to be resolved concerning the reporting and use of analytical data. Modern methods of vitamin analysis rely heavily on chromatographic techniques that generally allow the measurement of the individual chemical forms of vitamins. Typical applications of food analysis include the evaluation of shelf life and storage stability, monitoring of nutrient retention during food processing, developing food composition databases and data needed for food labeling, assessing dietary adequacy and evaluating epidemiological relationships between diet and disease. Although the usage of analytical data varies depending on the situation, important issues regarding how best to present and interpret the data in light of the presence of multiple vitamers are common to all aspects of food analysis. In this review, we will evaluate the existence of vitamers that exhibit differences in bioactivity or bioavailability, consider when there is a need to address differences in bioactivity or bioavailability of vitamers, and then consider alternative approaches and possible ways to improve the reporting of data. Major examples are taken from literature and experience with vitamin B₆ and folate.

Pernicious anemia with associated glossodynia

Glossitis and glossodynia are commonly seen by the dental practitioner. The awareness that these symptoms in conjunction with other oral signs indicate pernicious anemia is important for the clinician. Correct diagnosis can be made, and neurological complications avoided. Two case reports are presented and typical oral and systemic features of pernicious anemia are discussed.

Vitamin absorption in the in vivo intestine of normal and infected (Hymenolepis diminuta: Cestoda) rats

Uptake and serosal transfer of the vitamins thiamine, riboflavin and folic acid have been studied in vivo in normal and parasitized rats infected with Hymenolepis diminuta (Cestoda). Regional differences in intestinal uptake of all three vitamins in both uninfected and parasitized animals were not satistically significant. In the parasitized intestine mucosal uptake and serosal transfer of thiamine were significantly inhibited, with increased mucosal accumulation of the vitamin as luminal thiamine concentration increased. Apparent increased riboflavin mucosal uptake in parasitized animals, was not matched by the reduced serosal transfer, suggesting adsorption of the vitamin in the unstirred aqueous layers. Mucosal uptake of folic acid increased in the parasitized gut; serosal transfer and mucosal accumulation were not affected. These results, indicating vitamin malabsorption associated with infection by H. diminuta, are consistent with the parasite inhibiting mucosal passive transport mechanisms. This conclusion is supported by the changes in net water fluxes associated with vitamin uptake in the parasitized intestine.

Formation of transcobalamin II--vitamin B12 complex by guinea-pig ileal mucosa in organ culture after in vivo incubation with intrinsic factor--vitamin B12

The in vivo incubation of intrinsic factor--[57Co]vitamin B12 in an ileal loop of a guinea-pig followed by in vitro culturing of segments of the ileum for 180 min has been used to study the transepithelial transport of vitamin B12. Analysis of the solubilized supernate of mucosa following the in vivo phase demonstrated that 44% of the [57Co]vitamin B12 was bound to intrinsic factor (IF), 26% was free, and 16% was bound to transcobalamin II (TCII). Following culture, similar analysis demonstrated that 18% of the vitamin was now bound to IF, 49% was free, and 35% ws bound to TCII. In the culture medium, 54% of the [57Co]vitamin B12 was free and 37% was bound to TCII. The formation of TCII-[57Co]vitamin B12 did not occur if homogenized mucosa was incubated with free[57Co]vitamin B12, but it did form in cultures of ileal segments from animals given an excess of unlabelled vitamin to saturate all circulating TCH, and in the medium containing puromycin. Indirect immunofluorescence using chicken anti-TCII demonstrated that TCII was associated with the mucosal cells of both the ileum and jejunum. These studies demonstrate that following transepithelial flux of vitamin B12 through the ileal mucosa, the vitamin becomes coupled to TCII. This coupling requires a structurally intact mucosa and the source of the TCII appears to be the ileal mucosal cell rather than unsaturated TCII circulating in the blood.

[Drugs and vitamins]

Since most of the drugs are taken orally via the gastrointestinal tract, interactions with food ingredients may lead to alterations in the intake of essential dietary factors. With regard to vitamins the following effects have been demonstrated: early decomposition resp. inactivation, decreased absorption or a qualitatively and quantitatively modified metabolism. Examples of such changes are given with regard to water- and lipid-soluble vitamins.

Newer aspects of pernicious anemia

Although readily treatable with vitamin B12, pernicious anemia continues to captivate investigative endeavors of those interested in the pathophysiology and pathogenesis of this disorder. Notable advances have been made in understanding properties of intrinsic factor, vitamin B12-binding proteins, structure and de novo synthesis of vitamin B12, mechanism of action of vitamin B12-dependent enzymes in man, and metabolic consequences of reduced activities of these enzymes in pernicious anemia. Similarly, newer morphological observations have given information regarding pathogenesis of some of the cytological abnormalities found in megaloblasts, and recent cytochemical studies have shed light on abnormalities of nuclear and cytoplasmic constituents in vitamin B12-deficient cells. Both cellular and humoral factors may contribute to immune-mediated processes in pernicious anemia, although as yet, it has not been established with certainty that pernicious anemia is an autoimmune disorder. As we look ahead, it will be important to define the process or processes responsible for atrophic gastritis, which is the pathophysiological basis of pernicious anemia. Likewise, advances in biophysics used in the study of cell membranes, cell surface phenomena, and metallic ion transport may find applicability in the study of pernicious anemia and perhaps provide further insights into metabolic abnormalities responsible for the development of megaloblastosis.

Influence of age and stomach function on serum vitamin B12 concentration

Serum vitamin B12 (B12), maximal gastric acid output (MAO), and B12 absorption were determined in 82 subjects, age 32 to 85 years, who had normal absorption of radiocyanocobalamin. In 46 of the patients the gastric intrinsic factor (IF) was also measured. Serum B12 concentration and MAO varied widely in all age groups from abnormally low to definitely normal. The mean values for serum B12 and MAO, however, declined very similarly with advancing age owing to an increased incidence of low values for these two measurerment in the aged. There was no significant fall in the mean B12 absorption or IF secretion as a function of old age. Achlorhydric and hypochlorhydric patients invariably had lower mean serum B12 concentrations than those with adequate MAO. Conversely, patients with normal MAO all had normal serum B12 levels. Serum B12 concentration, although correlating with both MAO and IF secretion, showed a closer relationship to the former than to the latter. These findings suggest that the stomach, aside from producing the IF, plays an important role in maintaining a normal serum B12 concentration.

[Interactions between nutrition and drugs]

It is only recently that the question of interaction between nutrition and drugs has been investigated. Generally there are two ways by which such an interaction may occur. On the one hand the food-intake and its composition may influence the effects of drugs by altering their pharmacokinetics. In most cases the abosrption of drugs and thereby also their biological effects are decreased when they are taken together with food, by only a few drugs the opposite effects have been observed. Furthermore the biotransformation of drugs may be inhibited by giving a diet poor in proteins. A diet rich in meat or in vegetables may also influence the urine pH in the more acid or basic direction and in this way the renal excretion of drugs may be changed considerably since drugs are generally either weak organic acid or bases. On the other hand drugs may also interfere with the availability and utilization of certain nutrients e.g. vitamins, electrolytes or trace elements. Such interaction could be observed when giving for example antibiotics, alcohol, contraceptives, anticonvulsives or laxatives over a long period. Deficiency of certain nutritional factors or even diseases can then be the consequence of such interactions.

Tests of intestinal function

The most important tests for investigating the functions of the small bowel are faecal-fat analysis, the determination of xylose tolerance, vitamin-B12 absorption, and 14C-glycoholate transport, barium examination of the small bowel, and small bowel biopsy. With these tests, most disorders of digestion and of absorption can be adequately investigated and diagnosed.

Solubilized receptor for intrinsic factor-Vitamin B12 complex from guinea pig intestinal mucosa

the absorption of vitamin B(12) in many animals requires its prior association with intrinsic factor (IF) and attachment to a specific receptor in the intestine. Employing Triton X-100, we have solubilized from guinea pig ileum a factor that binds intrinsic factor-vitamin B(12) complex (IF-B(12)). This binding factor was soluble to the extent that it was not sedimented by centrifugation at 100,000 g for 1 h and was small enough to enter the included volume of a Sepharose 4-B column. Furthermore, the ileal extract contained no microfine particles of membrane upon electron microscopic search. When a portion of the extract was incubated with a mixture of gastric juice and (57)Co-labeled vitamin B(12), a portion of the radioactivity was excluded from a Sephadex G-200 column. When gastric juice from a patient with a congenital abnormality of IF that prevented its binding to intestine was substituted for normal human gastric juice, radioactivity was not excluded from the gel, indicating failure of this abnormal IF-B(12) to bind to the intestinal extract. These data suggested the presence of a specific binder of IF-B(12) in the ileal mucosal extract. The reactions of normal IF-B(12) with the solubilized binding factor and with the membrane-bound "receptor" had several characteristics in common, including calcium dependence, temperature independence, and pH optimum near neutral. Extracts from the distal intestine showed more activity than did those from the proximal. The solubilized binding facter seemed specific for IF-B(12) in that it was not blocked by prior incubation with excesses of either free vitamin B(12) or IF. Binding activity of the extract was decreased by incubation at pH 2.0, by heating to 56 degrees C, and by incubation with chymotrypsin and dithiothretiol. Incubation with trypsin, neuraminidase, and sulphydryl blockers did not affect it. The Triton X-100 extract of guinea pig ileal mucosa contains a specific binding factor that probably is the receptor for IF-B(12). This appears to be a protein with function dependent on peptide and disulphide linkages.