This study has been commented on another site, provoding some interesting details:
Polish Clinical Trial (2012)A group of 20 omnivores agreed to follow a vegan diet for 5 years. Half the group ate B12 fortified foods and the other half did not. Neither group took B12 supplements. The amount of B12 received via fortified foods was not measured. After 5 years, B12 levels in the group using fortified foods went from about 340 to 310 pg/ml. B12 levels went from about 290 to 220 pg/ml in the group not using fortified foods. Only two participants had B12 levels fall below 200 pg/ml, traditionally considered the cutoff for B12 deficiency, and they were both from the unfortified foods group (12).
So:
After 5 years on a vegan diet without supplementation or fortified food, the average B12 levels went from about 340 to
310 pg/ml. The normal B12 range for adults is often referred to as from 200 to 900 pg/mL (but the fact that some people with higher levels than 200 also may have symptoms of B12 deficiency is being discussed).
Among those not using fortified foods, the average levels went down from 290 to 220 - again, after five years.
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