As a vegetarian (vegan most days) I know I have to supplement my diet with vitamin B 12.
[However,] I'm really puzzled about something.
The best sources (non-veg) of vitamin B 12 are mollusks, snapper, calf's liver, lamb, venison, etc.
If these animals are able to produce vitamin B 12 in their tissues, why aren't we?
-- Jennifer Armstrong
Saratoga Springs, NY
Vitamin B12 is created by bacteria in animals' digestive tracts -- including humans'.
However, since this occurs in our large intestine, it is past the point of absorption.
You may be wondering why Vitamin K (synthesized by bacteria in that same location) is absorbed whereas B12 is not.
Simple -- our colons contain Vitamin K receptors which aid in this absorption process (keep in mind, though, that we do not produce enough vitamin K to meet the Recommended Dietary Allowance, so we must get some from the diet.)
While we're on the subject of B12, I want to point out other important factors regarding its consumption and absorption.
First, the only known plant source of B12 is wakame, a form of kelp.
All other algae and seaweeds, which some people contend are good sources of B12, are absolutely not.
What those foods contain are B-12 analogues -- compounds that mimic B12 and compete with real B12 for absorption.
Vegans are better off supplementing B12 through fortification (best bets are many brands of soy milk and cereal) or synthetic supplementation (such as a B-12 vitamin)
It is not only vegans who need to be concerned with B12, though.
People with celiac disease need to be careful, as gluten intake damages the microvilli in their small intestines (this is important because it is precisely in the small intestine that nutrient absorption takes place.)
Similarly, individuals who undergo total gastrectomy are at high risk of developing B12 deficiencies, as they lack intrinsic factor (a glycoprotein produced by the stomach and required for B12 absorption.)
A large body of research has also established that B12 absorption capacity decreases with age, which is why many dietitians recommend that individuals over the age of 60 supplement B12.