Holo-Transcobalamin Concentration and Transcobalamin Saturation Reflect Recent Vitamin B12 Absorption Better than Does Serum Vitamin B12

BACKGROUND: We evaluated whether measurement of vitamin B(12)-saturated transcobalamin (holo-TC) concentrations or TC saturation (holo-TC:total TC) reflects active vitamin B(12) absorption in healthy individuals and patients after vitamin B(12) intake. METHODS: We obtained blood samples from 31 healthy individuals (age range, 25-57 years) before (days -1 and 0) and after (days 1, 2, and 6) oral administration of three 9-microg doses of vitamin B(12). The blood samples from seven patients (age range, 22-39 years) suspected to have decreased vitamin B(12) absorption were obtained before and 1 day after the vitamin B(12) intake. The blood samples were analyzed for vitamin B(12), total TC, and holo-TC. The TC saturation was calculated. RESULTS: Intraindividual variation was <13% for all measured values, as calculated from samples removed on day -1 and 0. In healthy individuals (n = 31) after intake of vitamin B(12), the maximum median (range) increase (as percentages and absolute values) was in TC saturation [52 (-2% to 128)% and 0.04 (0-0.23) as a fraction], closely followed by holo-TC concentrations [39 (0-108)% and 34 (0-149) pmol/L]. All but one healthy individual had an increase of >/=15% in these markers. Serum vitamin B(12) showed a smaller increase [14 (-8 to 51)% and 36 (-27 to 290) pmol/L]. After vitamin B(12) intake, three patients with Crohn disease had the lowest increases in holo-TC concentration (3, 7, and 14 pmol/L) and in TC saturation (0.004, 0.01, and 0.01) among patients and 30 healthy individuals. CONCLUSION: Holo-TC concentrations and TC saturation reflect normal vitamin B(12) absorption better than does serum vitamin B(12).