Is the current RDA of 2.4 mcg B12/day inadequate?
http://www.ncbi.nlm.nih.gov/pubmed/20071646
Quote:
Abstract
BACKGROUND:
Studies have questioned whether the current Recommended Dietary Allowance (RDA) of 2.4 microg vitamin B-12/d is adequate.
OBJECTIVE:
We examined the association between dietary vitamin B-12 intake and biomarkers of vitamin B-12 status.
DESIGN:
Dietary vitamin B-12 intake was estimated, and biomarkers of vitamin B-12 status were measured, in healthy men and women (n = 299; age range: 18-50 y) who were recruited from a Florida community. The National Cancer Institute Diet History Questionnaire was used. Plasma cobalamin, total transcobalamin, holo-transcobalamin, methylmalonic acid (MMA), total homocysteine (tHcy), and autoantibodies against intrinsic factor (IF) and Helicobacter pylori were analyzed in blood samples.
RESULTS:
Antibodies to H. pylori were detected in 12% of subjects (35/299), and negative results for IF antibodies were obtained for all subjects. The intake of vitamin B-12 correlated significantly with cobalamin, holo-transcobalamin, MMA, and tHcy. Subjects were divided into quintiles on the basis of their dietary vitamin B-12 intake (range: 0.42-22.7 microg/d), and biomarkers of vitamin B-12 status were plotted against estimated dietary vitamin B-12 intake. All biomarkers appeared to level off at a daily dietary vitamin B-12 intake between 4.2 and 7.0 microg.
CONCLUSION:
In persons with normal absorption, our data indicate that an intake of 4-7 microg vitamin B-12/d is associated with an adequate vitamin B-12 status, which suggests that the current RDA of 2.4 microg vitamin B-12/d might be inadequate for optimal biomarker status even in a healthy population between 18 and 50 y of age
http://www.ncbi.nlm.nih.gov/pubmed/16400049
Quote:
Abstract
BACKGROUND:
Recommended daily intakes of vitamin B-12 vary between 2 and 6 microg.
OBJECTIVE:
The objective was to examine the associations between vitamin B-12 intake and markers of vitamin B-12 deficiency.
DESIGN:
We studied 98 Danish postmenopausal women aged 41-75 y. Serum cobalamin, transcobalamin (TC) saturated with vitamin B-12 (holo-TC), TC saturation (holo-TC/total TC), methylmalonic acid (MMA), and total homocysteine (tHcy) were measured to assess vitamin B-12 status. Dietary intakes of vitamin B-12 were determined from 7-d weighed food records. Gastric pH measurement and the alkali-challenge test were performed with the use of Heidelberg radiotelemetric capsules.
RESULTS:
The total intake of vitamin B-12 ranged from 1.2 to 23.9 microg/d. All vitamin B-12-related variables, except gastric pH, correlated significantly with total vitamin B-12 intake. Those taking supplements (54%) had higher circulating concentrations of cobalamin and TC saturation and lower concentrations of MMA and tHcy than did those not taking supplements. All subjects were divided into quintiles according to their total vitamin B-12 intake. For all the variables analyzed, the curves appeared to level off at a daily vitamin B-12 intake of approximately 6 microg; the median (and 25th-75th percentiles) for cobalamin was 380 (270-480) pmol/L, for holo-TC was 119 (92-162) pmol/L, for MMA was 0.12 (0.14-0.17) micromol/L, and for tHcy was 9.75 (8.3-11.40) micromol/L (n = 58).
CONCLUSION:
A daily vitamin B-12 intake of 6 microg appeared to be sufficient to correct all the vitamin B-12-related variables measured in the postmenopausal Danish women in this study.
Note that these people weren't vegans, and that there's a limit to how much B12 one can absorb 'per consumption'; 2 mcg B12 taken 3 times a day results in more absorbed B12 than 6 mcg absorbed once a day.
