Dual isotope Schilling test for measuring absorption of food-bound and free vitamin B12 simultaneously.

A prototype food-bound vitamin B12 (food-B12) absorption test has been developed in which 57Co-B12 was incorporated in vitro into egg yolk (yolk-B12) and served to volunteers in 50-g cooked portions together with toast and coffee for breakfast. Six hours later, 1 mg nonlabeled B12 was given intramuscularly and 24-hour urine was collected for radioactivity measurement. In separate tests, the absorption of yolk-B12 and crystalline 57Co-B12 was equally poor in patients with pernicious anemia. However, in patients with simple gastric achlorhydria and those who had undergone gastric surgery, the assimilation of yolk-B12 was impaired greatly, whereas the absorption of crystalline radio-B12 was normal. Egg yolk labeled with 58Co-B12 was administered together with crystalline 57Co-B12 in a dual isotope test with results similar to those obtained when the tests were prepared separately. This yolk-58Co-B12 test with its ability to detect malabsorption of food-B12 may be considered as an addition to the first part of the Schilling test.

Vitamin B12 bioavailability from egg yolk and egg white: relationship to binding proteins.

Egg yolk has been reported to inhibit B12 absorption less than egg white suggesting that different vitamin B12 binding proteins may be present in egg white and egg yolk. Using gel-exclusion chromatography we found that the mean MR for the B12 binding protein derived from egg yolk was 125,000, whereas that derived from egg white was 97,750. Heat treatment of the apoprotein differentially reduced the binding capacity of egg yolk and egg white in a time-dependent manner with the greatest decrease in binding capacity occurring with egg white. In contrast, heat treatment of the holoenzyme delineated the egg yolk as the more labile. These studies suggest that egg yolk and egg white contain distinct R binders which could explain the differential B12 absorption from egg yolk and egg white.

Effect of cimetidine on the absorption of vitamin B12.

The effect of cimetidine on the absorption of orally administered crystalline or food (egg yolk-bound) vitamin B12 (B12) was studied in 13 patients. Absorption or crystalline B12 was normal and not significantly changed by cimetidine. In contrast, the uptake of food-bound B12 decreased in all patients, from a mean of 5.3% without the drug to 2.5% after it, a fall of 53% (p less than 0.0001). This impairment of B12 absorption raises the possibility that long-term, full-dose therapy with cimetidine may produce B12 deficiency similar to that seen in other hypochlorhydric states. Our data indicate that cimetidine-induced B12 malabsorption would not be detected by the standard Schilling test.

Vitamin B12 assimilation from chicken meat.

Chicken meat labeled in vivo with radio-B12 was ingested by normal volunteers. The absorption, measured by the fecal excretion method, was similar to that reported for crystalline radiocyanocobalamin and for mutton, but exceeded that from eggs. Parenteral injection of 1000 microgram of nonlabeled vitamin B12 did not interfere with the absorption of the radio-B12 from the meat. The urinary radioactivities, which were as low as those after oral administration of radioactive hydroxocobalamin and vitamin B12 coenzyme, suggested that the radio-B12 was present in meat in coenzyme form or was converted into the stable hydroxoform during the process of cooking and digestion. Patients with pernicious anemia showed insignificant urinary radioactivities in a standardized urinary excretion test using chicken meat whereas subjects with simple gastric achlorhydria and partial gastrectomy had subnormal values although their absorption of crystalline radiocyanocobalamin was normal. The subnormal serum vitamin B12 concentration seen in these latter subjects may, therefore, be due to impaired assimilation of vitamin B12 from food.

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.

Inhibitory effect of eggs on vitamin B12 absorption: description of a simple ovalbumin 57Co-vitamin B12 absorption test.

Ovalbumin and egg yolks, mixed separately in vitro with radiocyanocobalamin (57 Co-vitamin B12), were served to normal volunteers in a cooked form. Ovalbumin, and to a lesser degree, egg yolks were observed to inhibit vitamin B12 absorption. This observation explains the rather poor assimilation of vitamin B12 from eggs labelled in vivo with 57 Co-vitamin B12.

Vitamin B12 absorption from eggs.

The assimilation of 57Co B12 from in vivo labeled eggs was much inferior to that of a comparable amount of crystalline 57Co B12. Furthermore, the absorption varied with the form in which the eggs were served. Judged by the urinary excretion test and the plasma absorption of radioactivity the average absorption from boiled and fried eggs was more than twice that from scrambled whole eggs, but less than half that absorbed from crystalline 57Co B12.