Eggs As a Source of Bioavailable Vitamin B12
Obesity and Metabolism Research Unit
2010 Annual Report
1a.Objectives (from AD-416)
The objective of the research is to determine the bioavailability of vitamin B12 from chicken eggs. The term bioavailability refers to the percentage of a nutrient, in this case vitamin B12, within a food that is actually absorbed into the body during the digestive process. This is contrasted with the percentage of the nutrient that passes through the digestive tract and is excreted without being absorbed by the body. The hypothesis to be tested is that a high percentage of the vitamin B12 in egg is bioavailable, thus indicating that eggs are a good source of dietary vitamin B12.
1b.Approach (from AD-416)
Egg enriched with vitamin B12 labeled with a radioactive tag (carbon 14) will be cooked and fed to healthy human volunteers. The amount of the carbon-14 labeled vitamin B12 that is excreted in the stook (unabsorbed vitamin B12) will be compared with the amount of carbon-14 labeled vitamin B12 that appears in the blood and urine (absorbed vitamin B12). From these measurements, the fraction or percentage of vitamin B12 absorbed (bioavailability) will be calculated. The study will be carried out in 10 human volunteers and the mean bioavailability of vitamin B12 from egg will be calculated and reported. Documents Trust with the American Egg Board. Log 32483. Formerly 5306-51520-006-07T (6/09).
The bioavailability of vitamin B12 from eggs has been in question as there are binders in egg yolk and in albumin that were thought to impair or block absorption of the vitamin. We measured absorption of the vitamin from eggs prepared by labeling chicken with 14C-B12 in vivo. The eggs were scrambled and fed to 10 human volunteers. Each received 2 ug B12 in the eggs labeled with approx. 30 nCi 14C. Serum, urine and fecal samples were collected for the next 8 days and 14C measured by accelerator mass spectrometry. The isotope appeared in serum 4 h after dosing and peaked at 8 h providing new information on rate of absorption of B12. Surprisingly from 5 to 62% of the dose appeared in urine, a novel finding that emerged from the fact that the B12 was labeled in its lower ring; apparently substantial amounts of the vitamin are degraded by intestinal bacteria, a novel finding, and the breakdown products appear in urine. Thus bioavailability estimates must include loss of the label in both feces and urine. Preliminary estimates are that one third of the vitamin is absorbed from eggs but final analyses are pending.