Vitamin B12 status and absorption using holo-transcobalamin in young men and women

• Main Author: von Castel-Roberts, Kristina
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01.01.2006
• Subjects: Nutrition
• ISBN: 1109874049; 9781109874044

Vitamin B12 (B12) status of young adults has been considered adequate based on estimated intakes that met the RDA; however, few studies in the US have evaluated B12 status of young adults using a panel of B12 biomarkers. Vitamin B12 deficiency impairs neurological function and increases other health-related risks. Early detection and determination of whether B12 deficiency is due to dietary insufficiency, a genetic abnormality, or malabsorption are critical to effective treatment. The aims of the first study were to compare B12 status using numerous biomarkers in young adult non-supplement users consuming vegetarian and omnivorous diets, determine the level of intake associated with optimal B12 status, and determine if the transcobalamin (TC) 776C→G polymorphism affected B12 metabolism. Blood samples were collected for determination of holo-TC, B12, methylmalonic acid (MMA), and homocysteine (Hcy) (n = 388). Dietary B12 intake was assessed using a food frequency questionnaire. A surprisingly high incidence of B12 deficiency was observed in both vegetarians and omnivores. Relative to omnivores, vegetarians had a higher rate of B12 deficiency, with lower B12 and higher MMA concentrations. Vitamin B12 status improved with B12 intake above the RDA. No differences were detected between TC 776C→G genotypes for any biomarkers. In the second study the magnitude and patterns of post-absorption changes in several B12 biomarkers were assessed. Subjects (n = 21) had blood drawn at 17 intervals over three days with administration of three 9 μg doses of B12 at 6 hour intervals on day one. Mean B12, holo-TC, TC saturation, and the ratio of holo-TC to B12 increased significantly from baseline at hour 24 only. In conclusion, a high incidence of impaired B12 status was observed in otherwise healthy young adults. The data suggest that further assessment of the adequacy of the B12 RDA is warranted. Measurement of multiple B12 biomarkers may provide a more accurate assessment of B12 status than measurement of one biomarker alone. Holo-transcobalamin appears to be a sensitive indicator of B12 absorption and a holo-TC based absorption test should involve measurement at 0 and 24 hours. No effect of the TC 776C→G polymorphism was detected.