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Vitamin B-12 is an essential nutrient, but humans cannot depend on the absorption of bacterially-produced B-12 from the gut. Thus, our B-12 must come from food.1

The B-12 synthesized by bacteria is mainly concentrated in the bodies of predatory organisms, so meat, milk, eggs, fish, and shellfish all contain B-12. 2 Comparatvely, the B-12 in eggs seems to be the most poorly absorbed among these sources.2

Some plant foods, such as blue-green algae (cyanobacteria), contain large amounts of B-12, 2 but the B-12 compounds in algae appear to be inactive in mammals.3 Most of the blue-green algae used for human supplements predominately contain pseudovitamin B-12, which is inactive in humans.2 Cyanocobalamin, also used in most supplements, is readily converted to the coenzyme forms of cobalamin in the human body.4

The major signs of B-12 deficiency are megaloblastic anemia and neuropathy.5 Since the bioavailability of crystalline B-12 is not altered in people with atrophic gastritis, the Institute of Medicine has recommended that adults over the age of 50 should get most of their recommended dietary allowance (RDA) of B-12 from foods that are fortified with crystalline B-12 or B-12 supplements.5

While the RDA of B-12 for adults is 2.4 mcg/day in the United States and Japan; the daily loss of the vitamin is estimated to be between 2 and 5 mcg/day.5 Bor et al.6 reported that a daily B-12 intake of 6 mcg/day appears to be sufficient to maintain a steady-state concentration of plasma B-12 and B-12 related metabolic markers. However, B-12’s bioavailability significantly decreases with increasing intake of vitamin B-12 per meal. 2

Studies with vegans have shown that despite the physiological recycling and conservation that become increasingly efficient as B-12 intake falls, the likelihood is high that B-12 deficiency will eventually develop. This may only develop after 20 years or more in vegans who do not consume any animal-based products or take B-12 supplements.7

In summary, the inability of humans to absorb bacterially-produced B-12 in the colon, and the evidence that vegans will eventually show negative B-12 balance8,9 indicate an evolutionary history of animal-based foods in human diets.

References

1. Herbert V (1988) "Vitamin B-12: plant sources, requirements, and assay." American Journal of Clinical Nutrition, vol. 48, pp. 852-858.

2. Watanabe F. Vitamin B12 sources and bioavailability Exp Biol Med 232:1266 –1274, 2007.

3. Watanabe F, Takenaka S, Kittaka-Katsura H, Ebara S, Miyamoto E. Characterization and bioavailability of vitamin B12 -compounds from edible algae. J Nutr Sci Vitaminol 48:325–331, 2002.

4. Ball GFM. Vitamin B12 In: Bioavailability and Analysis of Vitamins in Foods. London: Chapman & Hall, pp 497–515, 1998.

5. Institute of Medicine. Vitamin B12 In: Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6 Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: Institute of Medicine, National Academy Press, pp 306–356, 1998.

6. Bor MV, Lydeking-Olesen E, Møller J, Nexø E. A daily intake of approximately 6 mcg vitamin B-12 appears to saturate all the vitamin B-12-related variables in Danish postmenopausal women. Am J Clin Nutr 83:52–58, 2006.

7. Herbert V (1994) "Staging vitamin B-12 (cobalamin) status in vegetarians." American Journal of Clinical Nutrition, vol. 59 (suppl.), pp. 1213S-1222S.

8. Majchrzak D, Singer I, Männer M, Rust P, Genser D, Wagner KH, Elmadfa I. B-vitamin status and concentrations of homocysteine in Austrian omnivores, vegetarians and vegans. Ann Nutr Metab. 2006;50(6):485-91.

9. Herrmann W, Schorr H, Obeid R, Geisel J. Vitamin B-12 status, particularly holotranscobalamin II and methylmalonic acid concentrations, and hyperhomocysteinemia in vegetarians. Am J Clin Nutr. 2003 Jul;78(1):131-6.

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Article Source: ArticlesBase.com - Recommended Sources of the Essential Vitamin B-12

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