COBALAMIN (Vitamin B12)

clamsVitamin B12 is the only vitamin with the metal ion cobalt, hence it’s name cobalamin.  The active forms in the body are methyl-B12 and 5-adenosyl cobalamin (B12).  The cyanocobalamin used in most supplements is easily converted to these active forms, however many integrative medicine physicians (myself included) prefer to give ample B12 in the form of methyl-B12. 

Methyl-B12 is required for the function of methionine synthase which donates the methyl-group from the methyl-B12 converting homocysteine to methionine.  This is usually the rate limiting step in the vital methylation cycle, that also requires methyl-folate (hence the recommendation to also take methyl-folate (folinic acid) when supplementing B12. SAMe (S-adenosylmethionine) is the key methyl-group donor needed for the formation of neurotransmitters, and for methylation of DNA and RNA.  This is also important in the production of energy from fats and proteins, and the synthesis of hemoglobin. 

Getting B12 from food into the blood stream and then into the cells of our body is a complex process involving adequate stomach acid, R-proteins, enzymes, and intrinsic factor along with calcium. Only 1% of dietary B12 is absorbed when there is a breakdown in any of these steps. For a more detailed explanation, see here…

Pernicious anemia, thought to affect 2-10% of adults over 60, is an auto-immune destruction of the stomach lining which prevents the formation of the intrinsic factor-B12 complex thus preventing absorption of B12.  Those who have decreased stomach acid (either due to age, use of antacids, H pylori infection, surgical removal of part of the stomach) and those on strict vegan diets are particularly at risk for B12 deficiency.



Symptoms are typically related to anemia or neurological. However, given the importance of B12 for methylation, deficiency will also put you at greater risk for heart disease and cancers.  The earliest way to detect a B12 deficiency is to measure an elevated homocysteine (indirect evidence of impaired methylation) or measure elevated MMA due to impaired methylmalonyl-CoA. A blood B12 level may only reflect the short term recent B12 in the diet.  Functional testing ( can also give a good indication of how much B12 is making it into the cells over the previous several months. 

Anemia:  Low B12 impairs methionine synthase thus preventing the regeneration of tetrahydrofolate and trapping folate in the unusable form. This is another reason for supplementing methyl-folate along with methyl-B12. With anemia comes shortness of breath, fatigue, weakness, irritability and a sore tongue, decreased appetite, and constipation. 

Neurological: Initial symptoms include numbness and tingling of the arms and legs, difficulty walking and memory loss, disorientation, and mood issues or dementia. Ataxia and a loss of position sense may be the first signs and degeneration of the nerves and spinal cord can follow and be permanent if left untreated. There are studies showing increased Alzheimer’s when B12 is low. 



Bacteria can synthesize B12 and B12 is highest in animal products including fish, meats, poultry, and shellfish, being highest in muscles and clams  (20-84 micrograms per 3 ounces).  Poultry eggs and dairy tends to have about 0.3-1.0 micrograms per serving.  

Since many are not absorbing well, passive diffusion of 1% means that 1000 micrograms (1 gram) in the diet would result in 10 micrograms entering the blood stream. The recommended daily allowance (RDA) is 0.5 micrograms for infants and 3 micrograms for adults, pregnancy, and lactation. These levels are clearly inadequate for most and I recommend at least 100 micrograms even for infants or children and often will give 500-1000 micrograms daily for those known to be deficient. 



There are no reports of toxicity, even when long term oral intake of 1000 micrograms (1 gram) daily or monthly by injection. Injections are generally not necessary unless one has pernicious anemia or an absorption issue. 

More about detection of cobalamin deficiency in vegetarians can be read here…

More about cobalamin deficiency…

A previous blog post of mine about B12 deficiency…

My YouTube video on B12 deficiency…



Dr. Paul






One comment

  • What are your thoughts in a child with autism? My child cannot tolerate methyl-B12, we tried injections and it resulted in head banging and self harm and in pill form he is unable to sleep. We tried hydrox B-12 injections but didn’t know if they were doing anything at all and did those for over a year.

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