Evaluation of Holotranscobalamin as an Indicator of Vitamin B12 Absorption

Overview

Vitamin B12 is an essential nutrient that the body needs for cells to divide and function normally. Individuals may develop a deficiency of vitamin B12 by either limiting the amount in the diet or by decreased vitamin B12 intake into the body (absorption). Keeping adequate B12 blood levels is important for health. Vitamin B12 deficiency may increase one's risk for developing anemia and can even lead to neurological problems and paralysis if the deficiency is severe and lasts a long time. It is very important for doctors to have accurate tests to determine if people are absorbing vitamin B12 normally so that treatment can be started before severe clinical problems occur. The purpose of this research is to provide new information that may help scientists develop a better method to test for problems with absorbing vitamin B12. In this study, changes in the amounts of vitamin B12 bound to protein (transcobalamin) in the blood will be measured after doses of vitamin B12 are taken. If the amounts of this vitamin B12-protein complex (called holo-transcobalamin) change in response to taking a vitamin B12 supplement in normal individuals, it may be possible to use this information to develop a new sensitive test to identify individuals who have problems absorbing vitamin B12. This new vitamin B12 absorption test may be a better clinical test for vitamin B12 absorption than those now available for doctors to use.

Part I Screening: Individuals who meet initial screening criteria will be scheduled to have fasting blood samples drawn in the General Clinical Research Center (GCRC). All participants will sign informed consent forms approved by the UF Institutional Review Board prior to the initiation of this part of the study. Subjects' heights and weights will be measured and subjects will be asked to complete a medical history questionnaire. Blood analyses will include serum B12, blood chemistry, and hematological indices. Females will be given a pregnancy test. Subjects eligible for Part II of the study (Intervention) will have serum B12 concentrations that are ≥444 pmol/L; will be non-anemic; and will have normal general blood chemistry (all values within or near the normal range), and no history of chronic disease (e.g., pernicious anemia, cancer, diabetes, renal disease, hypertension, neurological abnormalities, gastrointestinal disorders). Any subject with deficient (\<148 pmol/L) serum B12 concentration \[18\] will not be eligible for participation and will be instructed to see their personal physician for complete follow-up and treatment. Part II Intervention: Changes in B12 status assessment indicators in response to B12 supplementation will be evaluated. Subjects will come to the GCRC the night before (Day 1) the intervention protocol. The next morning (Day 1) following an overnight fast, an indwelling catheter will be inserted from which repeated blood samples will be drawn throughout Day 1. A baseline fasting blood sample (#1) will be drawn. Subjects will then consume a light breakfast (including 1 piece of bread and 8 oz of orange juice) and will take a 9 ug dose of B12 (Dose 1). Thirty minutes after completion of the meal and consumption of the B12 supplement, a second blood sample (#2) will be drawn. Five subsequent hourly blood samples (# 3-7) will be drawn. After the 7th blood sample the subjects will consume one piece of bread and 8 ounces of orange juice and will take a second 9 ug dose of B12 (Dose 2) and have a blood sample (#8) drawn 30 minutes after consumption of the meal and the B12 dose. Five subsequent hourly blood samples (#9-13) will be drawn. The subjects will then consume one piece of bread and 8 ounces of orange juice and take a 9 ug dose of B12 (Dose 3). Another blood sample (#14) will be drawn 30 minutes after the meal and the B12 dose. One hour later, a final (Day 1) blood sample (#15) will be drawn followed by removal of the catheter. In addition to the bread and juice consumed with each B12 supplement, subjects will receive a mid-morning snack 2 hours after Dose 1 of the B12 supplement, lunch 3.5 hrs after Dose 1 of the B12 supplement, dinner at 4 hours after Dose 2 of the B12 supplement, and an evening snack at 3 hours after Dose 3. Low B12-containing foods will be provided in the GCRC. Water and non-caffeinated, non-caloric beverages will be allowed ad libitum. The subjects will remain in the GCRC overnight and a fasting blood sample (#16) will be drawn in the morning on Day 2. On Day 2 participants will be provided breakfast following the blood draw, after which they will be free to leave the GCRC. Subjects will be provided with take-away meals and snacks (lunch, dinner, snacks) consisting of low-B12 containing foods. Subjects will be instructed to return on the morning of Day 3 at which time they will have one am fasting blood sample (#17) drawn and will be provided with breakfast before they leave the GCRC. All overnight fasts will be from 10 pm the previous night. The following B12 indicators will be measured at each blood draw (a) holo-TC; (b) total-TC; (c) holo-HC and total HC; (d) serum B12; and (e) plasma albumin. Additional B12 status indicators (e.g. homocysteine, methylmalonic acid, RBC B12) will be measured at select blood draw intervals. A baseline folate analysis will be done.