Cobalamin, also known as vitamin B12, is a water-soluble vitamin produced by bacteria that is vital for neurological function (brain, spinal cord and nerves), cardiovascular health and erythropoiesis (formation of red blood cells).
It can be obtained through the diet by eating meat or dairy (which I strongly advise against), soil containing feces (which I also discourage for hopefully obvious reasons) or through supplementation which I cover in another article: How to Supplement Vitamin B12 on a Plant-Based Diet.
Cobalamin gets its name from the cobalt atom found at the center of its molecular structure.
Cyanocobalamin is the protein-bound form of vitamin B12 most commonly found in supplements. When taken orally, it makes its way down to the stomach where it finds hydrochloric acid (HCl), gastric intrinsic factor (GIF) and R-binders (transcobalamin I).
HCl, commonly referred to as stomach acid, is produced by cells of the stomach lining. When cyanocobalamin is in presence of HCl, the protein bound to the B12 molecule is cleaved (taken off) and is then bound to an R-binder before moving onto the duodenum, the first section of the small intestine, for further cleavage.
HCl is vital for breaking up unchewed tablets of vitamin B12.
Transcobalamin I, also known as an R-binder, binds to the free B12 molecule in the stomach and is then cleaved off of the molecule once in the duodenum where it is in presence of proteases (enzymes secreted by the pancreas that break down proteins).
The R-binders are produced and secreted by the gastric mucosal cells as well as the salivary glands.
GASTRIC INTRINSIC FACTOR
GIF is a glycoprotein that is produced by cells in the stomach. It binds to cobalamin in the second section of the duodenum and carries it on its way down through the small intestine where it is absorbed by enterocytes (intestinal cells) of the ileum, the last section of the small intestine. Once in the enterocytes, the B12/GIF complex binds to transcobalamin II, with approximately half of these compounds being taken to the liver and the rest being delivered to other tissues throughout the body.
Without GIF it is very difficult to absorb B12. Only about 1-3% of the cobalamin ingested can be passively absorbed without GIF. This is why many patients who have GIF deficiency can not rely only on oral supplementation and may require vitamin B12 injections or sublingual administration.
Conditions that lead to GIF deficiency include: gastrectomy (total or partial removal of the stomach due to cancer or weight loss surgery in most cases), atrophic gastritis (a leading cause of B12 deficiency in adults over the age of 55), prolonged use of proton pump inhibitors (PPIs) such as omeprazole, and H2 inhibitors such as famotidine.
When megaloblastic anemia (anemia due to B12 and/or folate deficiency) is caused by insufficient production of GIF is called pernicious anemia and is a severe condition which requires immediate treatment.
THE LIVER AND INTESTINES
Enterohepatic circulation is the process that occurs between the small intestines and the liver. The liver makes bile which is secreted into the duodenum. It goes through the small intestine as it helps break up the fat that we ingest and is then absorbed in the ileum and taken back to the liver through the blood stream, where it is used again to make bile.
About 60% of the cobalamin stored in the human body is in our livers. Some of it is lost in this process of enterohepatic circulation, about 1.4 micrograms (mcg) being excreted per day through in bile with 0.7 mcg being reabsorbed in the intestines for a net loss of 0.7 mcg per day.
RED BLOOD CELLS
Erythrocytes, also known as red blood cells, are what carry oxygen from the lungs to different tissues in the body, making their proper size and function essential for optimal health. They are produced from bone marrow and vitamins B12 and B9 (folate) are necessary for their maturation.
They are initially very large and, as they mature, they get smaller and smaller until they reach their appropriate size. When there is not enough B12 available, the red blood cells stay large. This is why the form of anemia caused by B12 deficiency is known as macrocytic (large cell) anemia.
When there are large amounts of folate in one’s diet, the body does not necessarily need B12 to regenerate red blood cells because the folate can be used in DNA and red blood cell synthesis. This is the reason that folate can mask vitamin B12 deficiency and why proper B12 testing should be done in people on a plant based diet.
Vitamin B12 is important for the maintenance of myelin, a fatty substance that surrounds nerve cells, which is essential for proper functioning of the nervous system. Inadequate synthesis of myelin leads to neurological damage and can be the cause of spinal cord injury.
In pregnancy and breast feeding, vitamin B12 supplementation should be considered as it is vital for proper brain formation of the fetus and baby. This is why I absolutely recommend that all women who are pregnant, may become pregnant or nursing mothers supplement with vitamin B12 properly.
Vitamin B12 also has another role in protecting the nervous system by acting in the metabolism of an amino acid called homocysteine, whose accumulation in the body can cause great harm to the nervous system and arteries.
- Scott JM. Bioavailability of vitamin B12. Eur J Clin Nutr. 1997 Jan;51 Suppl 1:S49-53.
- L.L. Brunton, B.A. Chabner, B.C. Knollmann (eds.). Goodman and Gilman’s The Pharmacological Basis of Therapeutics. 12th ed. New York, NY: McGraw-Hill, 2011., p. 1335
- Messina M, Messina V. The Dietitian’s Guide to Vegetarian Diets. Gaithersburg, MD: Aspen Publishers, Inc., 1996.
- Okuda K, Yashima K, Kitazaki T, Takara I. Intestinal absorption and concurrent chemical changes of methylcobalamin. J Lab Clin Med. 1973 Apr;81(4):557-67.
- Obeid R, Fedosov SN, Nexo E. Cobalamin coenzyme forms are not likely to be superior to cyano- and hydroxyl-cobalamin in prevention or treatment of cobalamin deficiency. Mol Nutr Food Res. 2015 Jul;59(7):1364-72.
- Lieberman M, Marks AD. Tetrahydrofolate, Vitamin B12, and S-adenosylmethionine. Eds. In: Mark’s Basic Medical Biochemistry: A Clinical Approach. 3rd Ed. Wolters Kluwer/Lippincott Willaims & Wilkins. Philadelphia, PA. 2009.
- Petchkrua W, Little J, Burns SP. Vitamin B12 deficiency in spinal cord injury: a retrospective study. J Spinal Cord Med. 2003 Summer;26(2):116-21.
- Vitamin B12 structure depicted by Richard-59 [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons