Vitamin B12 or cyanocobalamin (not the active form of B12) is one of the most complex molecules required for the human body. B12 comes in different active varieties depending upon the reaction it catalyzes. Methylcobalamin catalyzes the conversion of homocysteine to methionine (by adding a methyl group) and adenosylcobalamin catalyzes the conversion of certain amino acids and three carbon fatty acid fragments into molecules funneled into energy production.
The term “cyano” comes from the method of purifying the vitamin (using cyanide) and cobalamin comes from the cobalt complexed in the center of the molecule. That cobalt ring is generically called a corrin ring. There is no cyanide in naturally occurring B12.
Vitamin B12 is made exclusively by bacteria and archaea, and can be found in organisms that have a symbiotic relationship with bacteria. Mollusks and organ meats such as liver are an excellent source of B12 as are muscle meats and varieties of other foods such as milk eggs and a host of seafoods including fish and shellfish. The requirements for B12 are fairly well known and the sources encompass primarily the organisms that harbor the vitamin producing bacteria. The Dietary Reference Intake for most adults is 2.4 µg per day. Most multivitamins contain 6µg.
What does B12 do? Again, B12 functions in methyl group transfers and has been found important in two human biochemical reactions:
1. Conversion of the amino acid Homocysteine to Methionine
2. Energy conversion in specific amino acids and odd chain fatty acid fragments
How humans evolved to require the water-soluble vitamin is still somewhat of a mystery and certainly beyond the scope of this blog. Needless to say, there are no artificial means of acquiring B12 outside of bacterial production. Why bacteria produce B12 in the first place is unknown as well. And human bacteria living lives in our colons do indeed produce B12 but it isn’t absorbed in appreciable quantities. Instead, it’s lost in the feces.
There are basically three ways to acquire B12 from the environment. One method is to consume soil containing bacteria which produce B12, a practice call “geophagia” and not good on the teeth. Another less savory method involves a practice called “coprophagia” where feces are consumed, presumably to acquire those lost vitamins and nutrients. Chimpanzee’s and numerous other animal species practice coprophagia, presumably to acquire the vitamin B12 content, but that’s speculation as well.
The final and most common approach to acquiring B12 is through the consumption of other animals, mollusks or algal species which themselves live in symbiosis with the bacteria that produce the B12.
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A Point of View
Modern Western society is awash in a sea of food affluence. For many of us, from the moment we arise in the morning to the time we fall asleep at night, the one rhythmic pattern occurring daily with anticipated consistency is food intake—and in many cases very high quality food intake. Even the smallest of excess calories consumed daily translates over time to excess energy being stored as fat in adipose tissue.
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Overeating has become the symptom of a cultural disease associated with conditioned food intake, not a mystical physiologic process involving genes gone wild. From one diet manual to the next, the book offerings to navigate this mess are fancied up versions of the same old thing, eventually returning the dieter to a conditioned system of eating behavior. The contention of this blog, is it's time to get off the merry-go-round of dieting and learn the ABC's of basic nutritional science. Teach your children what they need to know to navigate the gauntlet of foods in the 21st century. We encourage any experts in the field to contribute.
3 comments:
WG, thanks, very useful. One question: assuming someone takes one of those vitamin tablets providing 6 micrograms when the RDA is 2.4, does the excess B12 get flushed out of the body or will it last that person 2.5 days? Thanks.
Shefaly: Thanks for your interest. B12 strikes me as one of those necessary yet bizarre collisions between symbiosis and evolution. The digestion, absorption, transport and storage of B12 are the subject of my next blog. However, in short, the process has apparently evolved around a saturable (limited) process requiring the co-secretion of a specific protein (intrinsic factor) to bind and transport the B12 complex to the distal ileum for absorption at specific receptor sites. From there a transport protein called transcobalamin shuttles B12 to the necessary organs, tissues and cells. Another recently discovered method of absorption is described as a mass-absorption process with little known regarding the kinetics and mechanism.
The profile of a typical water soluble vitamin is one of minimal storage and short half lives in the human body. In this case we have the opposite; with minimal to no intake of B12, storage depots may take 3 to 5 years or more to result in a B12 deficiency. Our evolution has come to regard B12 as little bits of gold, holding on to the molecules with tenacity and allowing minimal waste.
To finally answer your question, the 6 μg vitamin compared to the Dietary Reference Intake of 2.4 μg would probably not overwhelm the absorption process as patients deficient in B12 are routinely given 1000 to 2000 μg daily. And there are no known toxic reactions to large doses of vitamin B12. In studies looking at radioactive tracer B12, the typical fractional absorption of B12 from the gastrointestinal tract (called FAB-12 if you’re interested in doing a search) is in the range of 12-30%. Thus a significant amount is lost in the stool.
Thanks, WG. That is a detailed and useful explanation.
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