Introduction
B12 is a water-soluble vitamin that is needed in DNA synthesis in the production of erythrocytes (Red blood cells). It has an important role in the metabolism of carbohydrates, proteins and lipids. It also has a role in recycling Folate (Vitamin B9) and is involved with Homocysteine recycling (along with folate and Pyridoxine (B6)).
B12 is synthesised by bacteria and is absent from all plants. It can be obtained from animal products such as red meat, fish, poultry, milk, and eggs. It is particularly concentrated within liver and kidneys. It can also be found in some fortified breakfast cereals. The lack of B12 in plants means that vegans are at risk of B12 deficiency unless they take Vitamin supplements to increase their B12 concentration.
B12 Absorption
For B12 to be absorbed by the body utilises a protein known as Intrinsic Factor (IF) which is excreted by the stomach's parietal cells. IF binds onto B12 creating an IF-B12 Complex which can be absorbed into the blood by through the Distal Ileum (Ileum) of the intestines. B12 can be absorbed unbound (Free B12) across the intestinal membrane but this only accounts for around 3% of all transported B12 the rest (around 97%) is bound to IF. The limiting factor of B12 absorption into is the number of receptors located on the Ileal mucosa.
As the IF-B12 Complex passes through the Ileal mucosa B12 is transferred to a plasma transport protein called Transcobalamin (TCI, TCII and TCIII). While bound to TCII B12 can be stored in the liver and other tissues.
Image: Generalised flow diagram showing how B12 is absorbed
B12 deficiency
Symptoms
Deficiency of B12 can be characterised by anaemia and have fatigue, constipation, weight loss, diarrhoea and some neurological symptoms.
Neurological problems caused by B12 deficiency include:
Problems with Vision
Memory loss/ dementia
Confusion
Mood Swings
Pins and Needles (known as paraesthesia)
Loss of physical co-ordination, known as ataxia - can affect your whole body and cause difficulty speaking or walking
Damage to parts of the nervous system (peripheral neuropathy), particularly in the legs
Megaloblastic anemia
Megaloblasts are large, immature, nucleated cells and the forerunner cell that becomes a normal red blood cell. The lack of B12 causes disruptions in the process that allows megaloblasts to become red blood cells and so there are too few normal red blood cells within the blood and instead megaloblasts can be seen.
It has been shown that B12 deficiency can cause infertility and recurrent Fetal loss (Bennett M. Vitamin B12 deficiency, infertility and recurrent fetal loss. J Reprod Med. 2001;46(3):209-212.)
Causes
While there are many different causes of B12 deficiency the most common is known as Pernicious anemia.
Pernicious anemia is defined as an autoimmune condition where atrophy of cells in the stomach occurs there is a lack of IF thus preventing the absorption of B12.
This intrinsic factor deficiency can also be caused by gastric and weight loss surgery, known as bariatric surgery. this is not classed as Pernicious anaemia but is a cause for B12 deficiency.
B12 deficiency can also occur if the patient's ilium has been altered sue to surgery, specifically an ileostomy. As the Ilium is the part of the intestines that can absorb B12 its removal or any damage can affect this function. One study in 2014 shows that there is minimal risk of B12 deficiency following this type of operation but is still something to consider.
Other causes include:
Parasites – most commonly Fish Tapeworms (Diphyllobothrium) are known to cause B12 deficiency. The parasites Giardia duodenalis and Giardia lamblia have also known n to have an effect on B12 absorption.
Medicines – certain medicines have been shown to reduce B12 absorption in some way:
Contraceptive Pill
Ibuprofen (pain killer)
Colchicine (used to treat gout)
Cimetidine (used to treat peptic ulcers)
Omeprazole and Lansoprazole (proton pump inhibitors)
Phenobarbital, Pregabalin, Primidone and Topiramate (epilepsy medication)
Metformin (Type 2 Diabetes)
B12 Analysis
Analysis to determine B12 concentration is typically done via Immunoassay.
A two stage competitive assay is used where B12 analytes bind to a specific antigen. This 1st stage simply singles out the B12 antigen allowing other antigens to be removed via a wash stage. The second stage allows for binding of as secondary labelled antibody which will fluoresce. The amount of fluorescence measured is proportional to the concentration of B12. The fluorescence can be activated by different methods depending on the assay, The main ways seem to be due to a pH change or electrical stimulation.
Image: Example of immunoassay stages for B12 measurement.
Stage 1 - Binding of B12 antigens to specific antibodies, Other antigens are washed away.
Stage 2- Binding of Flourecence labelled antibodies to B12 antigens, Unbound antibodies are washed away.
Flouresence - Bound Flourecence labelled antibodies emit light at a certain wavelength to be measured.
What results indicate
On their own B12 results don’t mean a great deal as all it can really say is if the patient is deficient or not, the result will not inform you on the severity of any associated anaemia or neuropathy conditions the patient might have. as is the case with any result it is important to look at the whole picture for the patient, hopefully you will have access to results of other tests which can help explain the B12 result.
Final Thoughts
B12 should be considered with the other vitamin folate, in fact when a B12 test is requested folate is normally added on automatically and vice-versa. The reason for this is because B12 and folate are interrelated and a deficiency of either vitamin produces the same symptoms.