Eggs As a Source of Bioavailable Vitamin B12
Obesity and Metabolism Research Unit
2011 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.
This project, designed the use a novel method of measuring vitamin B12 bioavailability, was completed In FY 2011. In FY2010 14C-labelled vitamin B12 was synthesized by adding a 14C-labeled substrate to the growth medium of Salmonella enterica. The labeled vitamin was extracted and purified, and 350 nCi injected intramuscularly over 4 days into hens that subsequently produced eggs enriched in vivo with a very low amount of 14C-B12. The eggs were scrambled, cooked and fed to 10 human volunteers in FY2010. Baseline and post-ingestion blood, urine and stool samples were collected over a one-week period and assessed for 14C-B12 content in FY 2011 using accelerator mass spectrometry at Lawrence Livermore National Laboratories. Bioavailability was determined by measurement of the fraction of the total oral dose appearing in the stool and urine. Overall this research confirmed the value of eggs as a good source of vitamin B12, with one egg supplying approximately 30% of the average requirement, and 10% of the recommended intake, of absorbable vitamin. Because the amount of unlabeled B12 differed between batches of eggs, we also detected that the efficiency of B12 absorption was 50% from ˜1.5 ug vs. 20% from 2.6 ug. While the inverse relationship between B12 dose and absorption efficiency is well known, the pronounced fall in efficiency across this small difference in intake was unexpected and suggests that intake needs to be corrected for dose in any studies attempting to relate intake to status. The study also revealed that the vitamin unabsorbed in the small intestine is degraded in the lower intestine, and excreted in the urine. A final report was submitted to the American Egg Board in December, 2010, and two manuscripts are in preparation.