There are several reasons that the B12 requirements suggested for 'normal people' may not apply to vegans.
From http://www.vegan-straight-edge.org.uk/b12.htm:
"Lifestyle risks other than your heavy metal load. Meat or other animal products and refined carbohydrates (sugars) when used generously may more than double B12 needs persons who use drugs (eg Losec), chemicals, or beverages which destroy or remove B12 (e.g., tobacco, alcohol, caffeine, laxatives, etc.) egg albumin and egg yolk decrease B12 absorption
heavy metals present in food decrease dietary uptake a lack of calcium in food may decrease dietary uptake intestinal disorders or surgery affecting the intestines washing, cooking and light exposure of food containing B12
[adapted from; Thrash & Thrash, "NUTRITION FOR VEGETARIANS", 1982, "Heavy Metal Bulletin" Vol 2, Iss 3, Dec 1995 and Thorsons "Complete Guide To Vitamins and Minerals"] *B12 detoxifies cyanide in food and also tobacco smoke"
One more possible reason that vegans might need less B12:
"the greater quantities of fiber consumed by vegetarians tend to promote the settlement of bacteria in the lower areas of the small intestine and hence also the production of B12 in this region." http://www.veganforum.com/forums/sho...ighlight=fiber
And another one:
"The author contends that animal and dairy produce is a poor source of Vitamin B12 since the vitamin is contained in nutrient-deranged foodstuffs which will inevitably destroy the usability of the vitamin. Studies show that those following a typical animal-based diet require more vitamin B12 than those who do not. This is because the typical diet leads to digestive atrophy. Because B12 is peptide-bound in animal products and must be enzymatically cleaved from the peptide bonds to be absorbed, a weakened gastric acid and gastric enzyme secretions (due to a cooked food diet) causes an inability to efficiently extract vitamin B12 from external food. Nevertheless, raw food vegans who have a more powerful digestion actually get more B12 by reabsorption from the bile than they do from external food."
http://www.living-foods.com/articles/b12issue.html
And here's something I saw on notmilk.com:
Robert Cohen didn't provide any documentation - neither about his claim that most vegans do not take supplemental B12, nor that the enterohepatic circulation actually is all it takes to "keeps the adult vegan from developing B-12 deficiency disease"."In 1996, Victor Herbert determined that B-12 deficiency is rare among vegans, even though most do not take supplemental B-12. His landmark work was published in the American Journal of Clinical Nutrition, vol. 59(suppl), pp. 1213S- 1222S. Herbert wrote: "To a great extent, B-12 is recycled from liver bile in the digestive system...The enterohepatic circulation of vitamin B-12 is very important in vitamin B-12 economy and homeostasis...bodies reabsorb 3-5 mcg of bile vitamin B-12. Because of this, an efficient enterohepatic circulation keeps the adult vegan, who eats very little vitamin B-12, from developing B-12 deficiency disease..."
There's also some research performed on animals showing that if thy are fed a high protein diet, they need a lot more B12 in order to survive. If this applies to humans, the vegans who have a lower protein intake than the average meat eater may need less B12 than the average (Western) population, which consists mainly of meat eaters.
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