Vitamin B12 is an essential water-soluble vitamin, often highlighted for its role in energy production, red blood cell formation, DNA synthesis and prevention of pernicious anaemia. But, beyond the fact that it can be lacking in plant-based diets and should be consumed regularly, what do we really know about how this crucial nutrient functions within the body?
With an important role in so many metabolic reactions, B12 deficiency was uncovered in the mid-18th century. A diagnosis once deemed fatal, it was soon discovered that a diet rich in liver could largely ameliorate the condition. Nowadays, B12 related fatalities are rare, however a ‘sub-clinical’ category of B12 deficiency is on the rise.
While B12 deficiency has primarily been identified in people who are under-nourished and in those with the autoimmune condition pernicious anaemia; more recent discoveries have identified several sub-clinical factors which can contribute to a reduction in B12 status. These include malabsorption issues – which may occur with increasing age or in those with gastrointestinal impairments, genetic factors; vegetarian and vegan diets and even certain medical interventions.
As such, with a broader understanding of the factors which can contribute to low B12 and with knowledge of the severe consequences which can result from deficiency, it has become more important than ever to raise our awareness around B12 status.
In this blog, we discuss why B12 is so important and what factors can contribute to low levels. Furthermore, we will explain the various forms of B12 and the current methods of B12 testing available.
So, why is B12 important?
B12 participates in the metabolism of every cell in the human body. It contributes to normal nervous system functioning through its role in the synthesis of myelin (the fatty layer which surrounds our nerve cells); supports the maturation of red blood cells in bone marrow, as well as acting as a cofactor in fatty acid and amino acid metabolism and DNA synthesis.
For this reason, deficiency can lead to widespread signs and symptoms, including:
- Pale skin
- Weakness, fatigue and light-headedness
- Constipation, diarrhoea or gas
- Loss of appetite
- Nerve issues like tingling, numbness and muscle weakness
- Impaired vision
- Hormonal and mood imbalances
- Sleep disturbance
- Reduced immunity
- Cognitive impairment
- Problems with balance
Absorption of B12
The absorption of B12 in the body is a highly complex process which can be divided into three key phases: the gastric phase, the intestinal phase and the mucosal phase.1 Food-bound B12 must be released through an extra step before it can be absorbed in the ileum; as supplements are already in their free-form, they do not require this stage.
|Gastric Phase||Free-form (supplement): B12 binds to a plasma carrier protein known both as haptocorrin or transcobalamin I. This glycoprotein is secreted by the salivary glands and protects B12 from the acidic environment of the stomach. The B12-haptocorrin complex then moves towards the duodenum for the next step.
Protein-bound B12 (food): unlike free-form B12, protein-bound B12 must first undergo proteolytic cleavage in the stomach to be released from its protein carrier before it can bind to haptocorrin.2,3 Proteolytic cleavage refers to the digestion of protein by pepsin in the stomach, which frees B12 for binding. Adequate stomach acid is vital to trigger the conversion of pepsinogen into the functionally active pepsin.
The parietal cells in the stomach release intrinsic factor which will bind to B12 in the duodenum.
|Intestinal Phase||As the B12-haptocorrin complex enters the second part of the duodenum, protease secreted by the pancreas degrades the haptocorrin complexed to the B12. It is here in the duodenum that the B12 binds to intrinsic factor to create a B12-instrinsic factor complex which will be carried toward the ileum of the small intestine.|
|Muscosal Phase||Upon entry to the ileum, the B12-intrinsic factor complex is absorbed via the enterocytes (cells which line the small intestine) and binds to transcobalamin II; making it active B12. Generally, around 50% of this active B12 is delivered directly to the liver for storage, while the rest is circulated for use in the body tissues.|
Factors which may contribute to low B12 status
Due to the complexity of B12 absorption and synthesis within the body, low levels can often be observed even when there is adequate dietary intake. Some factors which can contribute to low B12 status include:
- Age: B12 deficiency is very common among the senior population with estimates suggesting that at least 20% of people over the age of 50 may have low levels.4 Malabsorption due to reduced enzyme and stomach acid activity or a lack of cobalamin transport proteins are potential factors, as well as dietary insufficiency, prescription medications or pernicious anaemia.4
- Dietary preferences: vegan and vegetarian diets can easily be lacking in B12 as it is primarily bound to animal protein. While vegetarians may fare better as some B12 can be found in dairy and eggs, levels should be monitored to ensure adequacy if attaining B12 through diet alone.
- Parietal cell damage: intrinsic factor is secreted by the parietal cells and is needed for B12 absorption. Parietal cell damage occurs as a result of autoimmunity (leading to pernicious anaemia), hypochlorhydria, gastritis and in those with a history of high alcohol intake.
- Hypochlorhydria (low stomach acid): sufficient stomach acid is required to release B12 from food. Several factors such as old age, radiation for gastric cancer, the use of anti-secretory medications (PPI’s, H2 blockers), antacids, hypothyroidism and Helicobacter pylori infection can all contribute to impaired stomach acid production.5
- Nitrous oxide: can irreversibly convert active B12 into its inactive form, thus preventing use in the body. Nitrous oxide is used in a number of dental and surgical procedures.
- Intestinal malabsorption: B12 is absorbed in the small intestine and so conditions such as ulcerative colitis, Crohn’s or coeliac disease which can cause damage to these cells could impede B12 absorption.
- Genetics: genetic polymorphisms affecting genes including TCN, FUT2 and MTR/MTRR can disrupt the transport, use and recycling of B12 in the body.
- Medication: in addition to some of the medications mentioned above, other prescription drugs such as metformin and neomycin can impair B12 absorption.
- Surgical intervention: a significant loss in the number of cells which produce hydrochloric (stomach) acid and intrinsic factor can result from gastric bypass surgery. In addition, partial or full removal of the ileum drastically reduces the surface area available for absorption of B12.
Forms of B12
B12 belongs to the cobalamin family of compounds as it has a cobalt atom at its centre. The structure of B12 is quite intricate, consisting of a corrinoid ring with an upper and lower ligand which attaches to a cyano, hydroxy, methyl or adenosine group.
|6Form||Natural or synthetic?||Biologically active?||No. of conversion steps required||Sustained release?||Unique properties|
|Hydroxocobalamin||Natural||No||3||Very good||Detoxification of cyanide & nitric oxide|
|Methylcobalamin||Natural||Yes||0||Average||DNA, brain, nerves, blood, detoxification|
|Adenosylcobalamin||Natural||Yes||0||Average||DNA, brain, muscles, energy|
Conversion: cyanocobalamin -> hydroxocobalamin -> methylcobalamin <–> adenosylcobalamin.
This is both a synthetic and inactive form of B12; produced with the addition of a cyanide donor. The cyanide molecule has a strong attraction to cobalamin, making it a very stable form of the vitamin. With that said, the presence of cyanide can make absorption more difficult as a methyl donor is required to detoxify the cyanide from the body.
While cyanocobalamin has historically been the chief form of B12 used both medically and commercially, it has received criticism of late for the following reasons:
- Potential for toxicity as cyanide can be produced during cyano breakdown – of particular concern for those experiencing detoxification issues such as in smokers or patients with liver or renal damage
- Requires reduction across four metabolic steps before it can be used
- The conversion of cyanocobalamin to active B12 can be prevented by a number of genetic and metabolic abnormalities
Conversion: hydroxocobalamin -> methylcobalamin <–> adenosylcobalamin.
This form of B12 is one of the predominant forms found in food. While it is also inactive, it does not contain a cyanide molecule and in fact aids the excretion of cyanide in the urine. Interestingly, hydroxocobalamin is commonly used as an antidote in cases of cyanide poisoning.7
Hydroxocobalamin has been found to have a higher affinity to plasma protein, as well as a longer half-life than cyanocobalamin. In a study which investigated the nasal absorption of hydroxocobalamin in older adults, it was found to be so effective that researchers suggested it as a possible alternative to intramuscular injections in some individuals.8
It is safely administered to those experiencing tobacco amblyopia,9 a condition related to cyanide metabolism. Patients with B12 deficiency optic neuropathy are also prescribed hydroxocobalamin in the UK.9 Hydroxocobalamin has demonstrated a distinct ability to inhibit nitric oxide above other cobalamin forms.10 High levels of nitric oxide can contribute to oxidative stress. However, in pregnancy nitric oxide aids the control of fetoplacental circulation.11 For this reason, methylcobalamin and/or adenosylcobalamin may be a better option during pregnancy.
Conversion: methylcobalamin <-> adenosylcobalamin (the body can convert to either of these forms depending on need).
As one of the two active forms of B12, methylcobalamin is required for several essential processes including:
- Conversion of homocysteine to methionine (thus lowering homocysteine concentrations) as part of the methylation cycle
- Generation of S-adenosyl methionine (SAMe) – an important methyl donor involved with over 200 enzymes required for cell growth, maturation and specialisation
- Supplying methyl groups required for various reactions, including generation of acetyl-CoA
- The only cobalamin which can regulate sleep and wake cycles12
Studies have demonstrated an increased utilisation of methylcobalamin within the body when compared with cyanocobalamin,13 possibly due to the fact that it does not require conversion as it is already in its active form. While serum levels were similar in a comparative study between oral doses of cyanocobalamin and methylcobalamin, high doses of cyanocobalamin resulted in increased urinary excretion of B12, while methylcobalamin appeared to replenish the body’s stores.13
Methylcobalamin has been noted for its role in neuroprotection. This cobalamin form supports the restoration of myelin sheath which results in improved neural conductance; with ongoing investigations into its application in nerve injury models. Furthermore, methylcobalamin demonstrated a distinct analgesic effect in patients with trigeminal neuralgia. Intravenous injections resulted in a reduction of paroxysmal pain, continuous spontaneous pain and allodynia (central pain sensitisation).14 It is highly likely that these therapeutic applications are associated with the positive effect methylcobalamin has on nerve action and regeneration.15
Conversion: adenosylcobalamin <-> methylcobalamin
This is a less well-known form of active B12. As it is involved in the Kreb’s cycle (creation of ATP or cellular energy), it is often referred to as the mitochondrial B12. During the Kreb’s cycle, adenosylcobalamin aids the conversion of methylmalonyl-CoA to succinyl-CoA. If this cycle is not functioning properly, fatigue and increased cellular ageing can ensue.
Most liver stores of B12 are as adenosylcobalamin and only convert to methylcobalamin as needed. While this conversion is straightforward for most people, certain genetic abnormalities can reduce the body’s ability to synthesise adenosylcobalamin.
As well as energy production, adenosylcobalamin is required to reduce levels of methylmalonic acid which can cause nerve damage at high levels. Furthermore, adenosylcobalamin plays a role in the catabolism of important amino acids and hormones such as valine, threonine, isoleucine, methionine, thymine and cholesterol.
Adenosylcobalamin is a good form to consider in cases of chronic exhaustion, muscle weakness, liver damage and hepatitis, where low B12 is suspected. In an Italian study where B12 was administered in the treatment of viral hepatitis, adenosylcobalamin was found to have a significantly greater effect on normalising total bilirubin, serum glutamic oxaloacetic transaminase (SGOT), serum glutamic pyruvic transaminase (SGPT) and alkaline phosphatase values compared to cyanocobalamin.16
Getting tested: how to measure your B12 status
A serum blood test is the standard method to assess B12 levels. This looks at total B12 or haptocorrin (transcobalamin I); the inactive plasma B12 binding protein. Within the cells, B12 is generally bound to transcobalamin II and so levels of total B12 are not necessarily indicative of the levels being assimilated into cells for use by the body. For this reason, B12 levels could be within a normal range in the blood, but the body may be unable to use it.
Serum B12 levels between 150 and 400ng/L are considered borderline for deficiency, with less than 180ng/L potentially causing pernicious anaemia and/or peripheral neuropathies. In these cases, a holotranscobalamin (transcobalamin II) blood assay will often be suggested, which looks at active B12 levels only.
If signs of B12 deficiency are present but total B12 levels are within normal range, it can be useful to seek functional assessment. Testing for methylmalonic acid via the blood or urine (high when adenosylcobalamin is low) and/or homocysteine in the blood (high when methylcobalamin is low) in conjunction with total or holotranscobalamin can create a much clearer picture of overall B12 status.17
Finally, genetic testing can also be helpful in assessing what form of B12 is most suitable. For example, methylcobalamin may be particularly useful for those with certain MTR or MTRR polymorphisms (genetic mutations). The MTR enzyme uses methylcobalamin and zinc in one of the steps to convert homocysteine to methionine and so adequate levels are vital in order for this reaction to take place. The MTRR enzyme is involved in regenerating methylcobalamin after this reaction and so must also be considered.
- Vitamin B12 is an essential water-soluble vitamin which plays a role in the metabolism of almost every cell in the body; including normal nervous system functioning and DNA synthesis.
- Deficiency can lead to widespread symptoms including fatigue, blurred vision, numbness, tingling, muscle weakness, sleep disturbance and hormonal imbalance.
- The absorption of B12 is a complex process which can be broken into three key phases: the gastric phase, the intestinal phase and the mucosal phase.
- There are several factors which can contribute to low B12 status including age, dietary preference, use of certain medications and digestive integrity.
- Cyanocobalamin is an inactive form of B12, often used in supplements, which contains a cyanide molecule. It is therefore not recommended for those with liver or renal issues. Those who smoke should also be cautious as cyanide may not be eliminated effectively.
- Hydroxocobalamin is an inactive form of B12 found predominantly in food. This is a good option for B12 deficient patients with tobacco amblyopia (a condition related to cyanide metabolism), cyanide toxicity and/or B12 deficiency optic nerve atrophy.
- Methylcobalamin is an active form of B12 with a methyl group and is the only form able to cross the blood brain barrier without further metabolism. This form is a good option for the majority of the population due to its distinct neuroprotective effects. It is also recommended for those with certain genetic polymorphisms.
- Adenosylcobalamin is the mitochondrial form of B12 and co-factor for a metabolic enzyme involved in energy production. It is a good option for those experiencing severe fatigue or with certain genetic polymorphisms reducing their ability to synthesise adenosylcobalamin in the body.
- B12 status is most commonly tested via total B12 levels in blood serum. As this can be an unreliable indication of cellular B12 levels, it is recommended to consider holotranscobalamin, homocysteine, methylmalonic acid and/or genetic testing in conjunction with this, in order to create a more accurate picture of B12 status.
If you have any questions regarding the topics that have been raised, or any other health matters please do contact me (Amanda) by phone or email at any time.
email@example.com, 01684 310099
Amanda Williams and the Cytoplan Editorial Team
- Busti AJ and Herrington JD (2015) ‘The Mechanism of Absorption of Vitamin B12 (Cobalamin) in the GI Tract’, Retrieved from: https://www.ebmconsult.com
- Lieberman M et al (2013) ‘Marks’ Basic Medical Biochemistry : A Clinical Approach’, Wolters Kluwer Health, 7(40), pp 744-759
- Institute of Medicine (1998) ‘Standing Committee on the Scientific Evaluation of Dietary Reference Intakes; Panel on Folate OBV; Subcommittee on Upper Reference Levels of Nutrients; dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B₆, Folate, Vitamin B₁₂, Pantothenic Acid, Biotin, and Choline’, National Academy Press
- Andrès E et al (2004) ‘Vitamin B12 (cobalamin) deficiency in elderly patients’, CMAJ, 171(3), pp 251-259
- Fatima R and Aziz M (2018) ‘Achlorhydria’, StatPearls Publishing, Retrieved from: http://www.ncbi.nlm.nih.gov/pubmed/29939570
- Roth JR et al (1993) ‘Characterization of the cobalamin (vitamin B12) biosynthetic genes of Salmonella typhimurium’, J Bacteriol, 175(11), pp 3303-3316
- Thompson JP and Marrs TC (2012) ‘Hydroxocobalamin in cyanide poisoning’, Clin Toxicol, 50(10), pp 875-885
- Van Asselt DZ et al (1998) ‘Nasal absorption of hydroxocobalamin in healthy elderly adults’, Br J Clin Pharmacol, 45(1), pp 83-86
- BNF: British National Formulary – NICE (2018), Retrieved from: https://bnf.nice.org.uk/drug/hydroxocobalamin.html.
- Weinberg JB et al (2009) ‘Inhibition of nitric oxide synthase by cobalamins and cobinamides’, Free Radic Biol Med, 46(12), pp 1626-1632
- Lyall F, Young A and Greer IA (1995) ‘Nitric oxide concentrations are increased in the fetoplacental circulation in preeclampsia’, Am J Obstet Gynecol, 173(3), pp 714-718
- Mayer G, Kröger M and Meier-Ewert K (1996) ‘Effects of Vitamin B12 on Performance and Circadian Rhythm in Normal Subjects’, Neuropsychopharmacology, 15(5), pp 456-464
- Okuda K et al (1973) ‘Intestinal absorption and concurrent chemical changes of methylcobalamin’, J Lab Clin Med, 81(4), pp 557-567
- Xu G et al (2013) ‘A Single-Center Randomized Controlled Trial of Local Methylcobalamin Injection for Subacute Herpetic Neuralgia’, Pain Med, 14(6), pp 884-894
- Okada K et al (2009) ‘Methylcobalamin increases Erk1/2 and Akt activities through the methylation cycle and promotes nerve regeneration in a rat sciatic nerve injury model’, Exp Neurol, 222(2), pp 191-203
- Medina F and Vitali D (1968) ‘Controlled clinical research with cobamamide in the treatment of viral hepatitis’, Clin Ter, 46(2), pp 139-144
- Spence JD (2015) ‘Metabolic vitamin B12 deficiency: a missed opportunity to prevent dementia and stroke’, Nutr Res, 36(2), pp 109-116
Last updated on 20th October 2022 by cytoffice