Vitamin B12

The Energy Vitamin

Nicknamed the “energy vitamin,” vitamin B12 is essential for health. Likewise, the effects of vitamin B12 deficiency are extensive. Deficiencies manifest principally as hematological (blood), gastrointestinal, neurological (nervous system), and neuropsychiatric (brain) disorders.

B12 is involved in the metabolism taking place in every cell in the body.  It plays a vital role in DNA synthesis, hemopoiesis (formation of blood cells), and carbohydrate, fat, and protein metabolism. It is also essential for healthy brain function and has been found to significantly affect mood — and even delusions and paranoia — in some patients. [Lachner] And it plays a key role in the synthesis of myelin, the sheath that surrounds and protects nerves. Some studies suggest that B12 may even be able to exert control over inflammatory responses and modulate the immune system. [Wheatley] Both low and high B12 serum levels are associated with life-changing and life-threatening conditions, such as dementia, autism, cardiovascular disease, osteopenia, and some cancers.

B12 Deficiency Symptoms

The signs and symptoms of B12 deficiency are varied and some are atypical. Some of the more frequently found symptoms include numbness or tingling in the hands, legs, or feet; balance problems; swollen or inflamed tongue; loss of appetite; constipation; muscle weakness; pallor; fatigue; breathlessness; dizziness; visual disturbances; insomnia; memory loss; brain fog; and mood changes. [Chapter 9]

B12 Deficiency Detection

It is critical that a vitamin B12 deficiency is detected as early as possible. However, there is no longer a gold standard for assessing vitamin B12 deficiency. From approximately 1950-2003, the diagnostic search for B12 deficiency began with the Schilling test. The Shilling test delivered a mixture of water and radioactive B12 — both orally and by injection. The test identified abnormal IF-related absorption and also distinguished between gastric (stomach) and intestinal defects.

Many experts disagree on the optimal approach to B12 deficiency detection. Numerous studies have established that serum vitamin B12 has limited diagnostic value as a stand-alone marker for B12 deficiency. Levels of serum B12 do not always represent cellular B12 status; serum B12 can be normal when a patient is functionally deficient. Oh and Brown state, “…about 50 percent of patients with subclinical disease have normal B12 levels.” [Oh R. and Brown DL.] Likewise, serum B12 can be low when a patient is not deficient.  Low-normal cobalamin levels (250-350 ng/L) need not be pursued without clinical evidence of deficiency.

There are quite a few biomarkers to consider and typically a combination is used. These biomarkers include red blood cell count (RBC), mean cell volume (MCV), red cell distribution width (RDW), iron, folate, methylmalonic acid (MMA), homocysteine (Hcy), as well as intrinsic factor antibodies. IF antibody has a specificity for pernicious anemia of 95% — as long as the blood is not drawn within a few days after injection or consumption — but sensitivity is only 50-70%. Of these markers listed, MMA is considered by many to be the most sensitive and specific marker of intracellular B12 status. Low serum B12 and normal MMA lab results, in an untreated patient without clinical or absorption abnormalities, suggest a falsely low serum B12 level. Yet normal serum B12 and elevated MMA lab results, in an untreated patient without clinical or absorption abnormalities, suggests functionally low levels of B12 — even when serum B12 is normal.  A failure of MMA to normalize during the first week of therapy suggests an incorrect diagnosis. [Allen RH, Stabler SP, Savage DG, Lindenbaum J.] That is unless renal (kidney) failure or hypovolemia (low blood volume) or other causes of MMA elevation coexist.

Neutrophil segmentation is another highly sensitive measure of B12 deficiency (~93%), although it lacks specificity. Serum gastrin and pepsinogen I abnormalities tests can sometimes be valuable because of their high sensitivities for pernicious anemia (90%-92%), but they also lack specificity. Some physicians combine the specific but insensitive IF antibody test with the sensitive but nonspecific serum gastrin or pepsinogen level. [Carmel R.] Formiminoglutamic acid (FIGLU) is also considered by some to be valuable.  Holotranscobalamin (holoTC) is controversial.  Many claim results vary with the timing of the test in relation to injection or supplementation. (HoloTC is cobalamin (vitamin B12) attached to the transport protein transcobalamin, in the serum.)

Carmel recommends peak reticulocyte count (immature red blood cells) one week after starting treatment to determine if B12 deficiency is the correct diagnosis.

Although serum B12 has limited diagnostic value as a stand-alone marker for B12 deficiency, a number of studies show that high serum vitamin B12 signals defects in vitamin B12 uptake or elimination.  An elevated serum B12 can be an early warning sign.  At least one study found that cancer patients with elevated B12 levels had higher mortality than those with normal B12 levels. These findings may have clinical significance for assessing the prognosis of cancer patients. [Arendt JF, Farkas DK, Pedersen L, Nexo E, Sørensen HT] Elevated serum B12 is associated with a number of life-threatening conditions for which an early diagnosis is critical.

B12 deficiency is best diagnosed using a combination of tests since no test alone is completely reliable. Because of this, because B12 can interact with some medications, and because the research is unclear as to whether high doses of B12 are safe or not, work with Dr. Harlin to determine the best B12 dosage and supplement for you.

B12 in Nature 

The synthesis of B12 is achieved only by microorganisms.  Food sources of B12 are predominately animal products. B12 is found much less frequently in plant foods. The B12-like compounds found in fermented foods, mushrooms, and yeast have no bioactivity in human physiology and may not be bioavailable within our bodies. Regardless of where B12 compounds are found, the original source is always microorganisms.

These microorganisms live in the ground, in untreated groundwater, in a few algae, and in the intestinal tract of some humans and animals (primarily in those that are pasture-raised). In nature, only a relatively small number of microorganisms are known to encode a complete de novo biosynthetic pathway of vitamin B12. One of these bacteria is a probiotic called Lactobacillus reuteri CRL 1098, which is found naturally in the intestines of many humans. Preliminary studies propose that because the B12 producing organisms live primarily in the colon, humans don’t absorb the B12 made by their gut bacteria.  So B12 food sources and/or supplementation is recommended.


RDA Recommendations vs. Individual Requirements

The recommended daily intake (RDA) of B12  has been set at 2.4 mcg for adults. (An assumed absorption of 50% is included in the recommended intake.) Some researchers claim their studies indicate that 4-7 mcg/d is more closely associated with adequate B12 status — and this recommendation is for healthy young to middle-aged adults.  Other researchers recommend more than twice these levels. The RDA guidelines state that 10% to 30% of adults older than 50 years have B12 malabsorption syndromes and recommend that seniors supplement.  Due to the uncertainty of one’s individual B12 requirements, and because B12 absorption tends to decrease over time, it is best to check B12 biomarkers periodically.

Unfortunately, sufficient quantities of B12 are not found in a modern whole-food, plant-based diet — with our triple washed lettuces, and so forth. Our ancestors obtained their vitamin B12 by drinking water from streams and well water. However, water purification destroys B12 and the microorganisms that produce it. (So best not to put your B12 supplement into chlorinated tap water nor take it with tap water.)

The Body’s B12 Store

The recommended daily dose assumes there are sufficient levels stored in the body.  Unlike other water-soluble vitamins, B12 is stored in substantial amounts — mainly in the liver, which can store approximately 1-1.5 mg. [Quadros] In an adult, the total body B12 store is ~2500 mg., which is over 1000 times the recommended daily dose of 2.4 mcg.  Based on the math, one might assume that the body store could last for a few years without consuming B12.  However, this does not account for the natural half-life and degradation of B12.  How fast B12 stores are depleted — without consuming B12 — depends on how much is retained in the body and how efficiently it is reabsorbed after biliary secretion. Researchers estimate that B12 depletion, from a full body store, ranges from 6 months to a year.

Supplementation — A U-Shaped Curve

In cases of deficiency, 50, 500, or 1000 mcg per day for one to three months is recommended, depending on the degree of deficiency. [Sharabi] [Del Bo’] [Castelli] Note this is an interim dose until the deficiency is corrected.  A large study conducted in the Netherlands found that a daily dose of 500 mcg of vitamin B12 and 400 mcg of folic acid to seniors (65 or older) for 2-3 years was associated with a 77% higher risk of colorectal cancer than the control group.  (Oliai Araghi) We recommend that patients work with Dr. Harlin to determine appropriate B12 dosing.

Most vitamins follow a U-shaped curve; in other words, there is a sweet spot — too little or too much is suboptimal or even detrimental.  Multi-vitamin B supplements commonly far exceed daily requirements and some B vitamins can cause toxicity, despite being water-soluble.   For example, daily doses of 1000 mcg of folic acid supplementation have been associated with more than doubling the risk of prostate cancer. [Figueiredo] And high daily doses of folic acid can complicate the diagnosis of a B12 deficiency.

Because our bodies store B12, it is not necessary to take B12 daily.  Although, our ancestors likely ingested B12 every time they took a drink of water.  We recommend that our senior and plant-based vegan patients do supplement with B12 on a daily basis but at the recommended daily level.

“Normal” B12 Blood Serum Range

In the U.S., limits of “normal” blood serum B12 is set to 200 – 900 ng/L. At lower levels of normal range, many people suffer from fatigue, anemia, shortness of breath, palpitations, and nerve damage, while others are asymptomatic at even lower levels. Abnormally high vitamin B12 serum levels are associated with hematological malignancies; liver disease; kidney failure; increased liver, colon, and lung cancer; and all-cause mortality.

Reasons for Vitamin B12 Deficiency

Polymorphisms and possibly the Gut Microbiome too

There are many reasons for B12 deficiency beyond our B12 consumption, or lack of it. Over the past decade, our understanding of the genes involved in the complex pathways of B12 absorption, assimilation, plasma transport, and cellular metabolism has pushed our understanding of B12 disorders far beyond what can be gleaned from lab testing.  For instance, mutations in cblG and cblE loci can explain isolated hyperhomocysteinemia (elevated Hcy in the blood), without methylmalonic aciduria (without elevated MMA). That said, there are still facets of B12 metabolism that science has not fully elucidated.

This relatively new genetic information enables us to identify a patient’s genetic mutations, or SNPs (pronounced “snips”), along the complex B12 metabolic pathway.  This enables us, as precision medicine physicians, to make more informed treatment-plan decisions.

Another factor that likely plays a role in susceptibility to B12 deficiency is an individual’s gut microbiome. While some microbiota produce B12, others are capable of degrading B12 and may affect the bioavailability of the vitamin.

B12 Absorption in the Gut

Vitamin B12 deficiency as a result of insufficient dietary intake is a risk for vegans who do not take a B12 supplement. Yet in the general population, B12 deficiency is typically the result of poor absorption. B12 absorption in the gut is largely dependent on the healthy functioning of parietal cells in the stomach lining. Parietal cells produce hydrochloric acid and intrinsic factor (IF). Hydrochloric acid facilitates the release of B12 from food while IF facilitates the absorption of B12 into the bloodstream. IF is a protein that binds with B12. (There are other binding proteins involved in the absorption of B12 but none appear to be as crucial as IF.) Once bound with IF, B12 is resistant to further digestion. The B12-IF complex is absorbed only in the ileum — the last part of the small intestine. Because the number of specific receptor sites in the ileum is restricted, only small amounts, approximately 2 mcg, can be absorbed from each meal.

B12 is continually secreted in the bile. In healthy individuals, most of this B12 is reabsorbed and available for metabolic functions. When IF is absent or its ileal uptake fails, absorption of cobalamin fails.  Reabsorption of B12 from the bile acids also fails, which typically amounts to ~50% of the 1.4g/day of biliary cobalamin. [Carmel]

Transport Proteins Haptocorrin and Transcobalamin II

On traversing the brush border, vitamin B12 dissociates from IF and enters the bloodstream where it binds with other transport proteins: transcobalamin I, also known as haptocorrin (HC), or transcobalamin II (TCII).

It is estimated that approximately 75-80% of the circulating B12 is bound to HC and 20-25% to TCII.  Although a definitive role for HC is uncertain, its suggested functions include a role in B12 storage and in the removal of degraded B12 derivatives. TCII binds only intact B12.

Approximately 50 percent of the B12 is taken up by the liver and the remainder is transported to other tissues. [Chaper 9]

Cellular Absorption of B12

The TCII complex is the bioactive form. TCII mediates the transport of B12 across cells by binding to specific receptors that are thought to be present in all human cells.  Once the TCII-B12 is absorbed into a cell, the cobalamin (Cbl) may become methylated to make methylcobalamin (MeCbl) or it may move into the mitochondria and be converted to adenosylcobalamin (AdCbl).

Reasons for B12 Malabsorption in the Gut

There are many factors that affect vitamin B12 absorption. Some people have genetic mutations, or SNPs, which reduce their production of IF, and some people have a condition that destroys IF, such as an autoimmune disorder that causes antibodies to attack it. Research suggests that anti-parietal cell antibodies can be induced by Helicobacter pylori infections. Medications, such as Metformin and proton pump inhibitors (the purple pill), and levadopa reduce B12 absorption or bioavailability. Alcoholism can also reduce B12 absorption. Those who have had surgery to remove portions of their stomach, such as gastric bypass surgery, or who have gastrointestinal tract conditions, such as Crohn’s disease or celiac disease, also have an increased risk of insufficient IF and poor B12 absorption. Among the elderly, the high prevalence of B12 malabsorption from food is likely due to general gastric dysfunction. (Due to the prevalence of B12 deficiency in seniors,  a B12 supplement is generally recommended to those over 65.)

The Consequence of B12 Deficiency

Megaloblasts — enlarged red blood cells

Clinically, vitamin B12 has been best known for its role in red blood cell production. Vitamin B12 deficiency can lead to enlarged red blood cells called megaloblasts. Anemias are blood disorders that occur when the body has fewer red blood cells than normal. Pernicious anemia is a type of megaloblastic anemia in which the body isn’t able to make enough healthy red blood cells due to a vitamin B12 deficiency. Pernicious anemia can be due to an autoimmune condition in which the body’s immune system attacks the intrinsic factor protein or it attacks the parietal cells in the stomach lining, which make intrinsic factor. However, it is thought that pernicious anemia can be due to factors other than an autoimmune disorder.

It is estimated that pernicious anemia explains approximately 76% of patients with clinical B12 deficiency. [Carmel]

Insufficient Oxygen

Red bone marrow produces the main blood cells of the body: red cells, white cells, and platelets. Red blood cells are released from the marrow into the bloodstream where they travel throughout the body delivering oxygen to tissue. If the body does not receive a regular supply of the necessary nutrients, red blood cells may become malformed or die off at a faster rate than the bone marrow can replace them. Vitamin B12 plays a key role in the formation of new red blood cells. Early signs of a deficiency in healthy, fully-matured red blood cells are fatigue, pallor (pale skin), and lightheadedness.

B12 for Those with Reduced Intrinsic Factor

For those with reduced IF, oral B12 is not always effective and sublingual and intranasal supplementation have not been well studied in this population.  However, these two forms may be adequate for some. Sublingual B12 has been shown to be as effective as B12 supplementation in pill form for those without absorption issues. (Be aware that sublingual B12 supplements often contain sugar substitutes such as sorbitol, mannitol or sucralose and these varieties are not recommended.) Intranasal formulations of B12 are also available.

For individuals needing more reliable dosing, injectable dosing of 1000 mcg 3 times per month, for a few months, followed by monthly injections has been found to yield a reliable clinical effect. Hydroxocobalamin injections, used more commonly in Europe, can be spaced at twice the interval needed for cyanocobalamin, which is typically used in the U.S.

Interesting to note… studies have found even when the intrinsic factor-dependent pathway is impaired, between 1 and 4% of an oral dose of B12 is absorbed passively in the intestines — in other words, without a transport protein such as IF.

Types of Vitamin B12 in Supplement Form

Hydroxocobalamin, Methylcobalamin, Adenosylcobalamin, and Cyanocobalamin

There are four types of vitamin B12, or cobalamin (Cbl), in supplement form: hydroxocobalamin (OHCbl), methylcobalamin (MeCbl), adenosylcobalamin (AdCbl), and cyanocobalamin (CnCbl).  OHCbl, MeCbl, and AdCbl are all natural form of B12.  CnCbl is a synthetic (man-made) form. B12, or cobalamin, is the only vitamin that contains a metal ion, and its name derives from the positively charged cobalt ion. The molecule that is attached to the cobalt ion is called a donor.

Inactive and Bioactive Forms

OHCbl and CnCbl are inactive forms of B12 but each can be metabolized in the body to produce the bioactive forms, MeCbl and AdCbl.  Once inside the cell and inside the lysosome, the donor component of vitamin B12 is cleaved and cobalamin is released into the cytosol. Studies indicate that all forms of B12 are reduced to a core cobalamin molecule.  The cobalamin is then converted to either MeCbl, which takes place inside the cytosol, or to AdCbl, which takes place inside the mitochondria.

Most Effective Forms of B12

Clinical studies have shown that all forms of vitamin B12 supplements can improve B12 status.  Any of the four supplement forms of cobalamin can be converted to an active form, although research suggests that all forms of cobalamin are not equally effective from one person to the next.  It is likely that we all have genetic mutations at points along the B12 metabolic pathway.  The few human and animal studies that have compared the forms of cobalamin show better outcomes with the use of one (or more) of the natural forms, OHCbl, MeCbl, or AdCbl, over the synthetic form, CnCbl.

MeCbl and AdCbl are the bioactive forms of B12, but these molecules are somewhat unstable outside of the body. This is primarily due to their photosensitivity, which makes them difficult to produce. However, because of their significant therapeutic value, these two forms have become more readily available as supplements.

Although the normal proliferation of cells depends on adequate folate, and other vitamins too, B12 has a vital role in cell growth and development.  These two B12 structures, MeCbl and AdCbl, are both essential and have distinct metabolic functions. MeCbl and AdCbl are activated in two separate cellular compartments: MeCbl in the cytosol and AdCbl in the mitochondria. They are bioactive cofactors of different enzymes, as well.  (A cofactor is a helper molecule that assists in biochemical reactions.)

Note: Some researchers have found that oral cobalamin is more effectively absorbed on an empty stomach than when taken with food. (2.8-13.4 g vs. 1.8-7.5 of a 500-g dose). [Berlin H, Berlin R, Brante G.]  However Henderson, found that some patients with chronic pancreatitis malabsorbed vitamin B12 in the fasting state, but absorption was normal when food was given. [Henderson JT, Simpson JD, Warwick RR, Shearman DJ.]


Methylcobalamin is perhaps most famous for being a methyl donor. Despite what you may have read, credible researchers state that MeCbl is NOT a methyl donor.  Methyl groups are synthesized as part of the conversion of ingested vitamin B9, or folate, (think foliage or leafy greens) to 5-methyl tetrahydrofolate (MTHfolate).  To a lesser extent, MeCbl can obtain its methyl group from SAMe or betaine. (Methylation is a biochemical process involving the transfer of a methyl group between molecules and is required for numerous physiological functions.)

However, MeCbl has been shown to have other advantages over other forms of vitamin B12.  One study showed that the lysosomal reduction of B12 to cobalamin was 67 times slower with AdCbl supplementation compared to MeCbl supplementation.  Thus, AdCbl supplementation may result in a slower synthesis of intracellular AdCbl and MeCbl compared with MeCbl supplementation. (Banerjee R, Ragsdale SW)

MeCbl is active in the cytosol, the intercellular fluid inside a cell.  Inside the cytosol, a portion of available cobalamin participates in cyclic methylation reactions by acquiring the methyl group from 5-methyl tetrahydrofolate (MTHfolate), or occasionally from S-adenosylmethionine (SAMe), and is converted to MeCbl. MeCbl is an essential cofactor for the regeneration of tetrahydrofolate (THfolate) in folate metabolism.

Methionine synthase (MS) promotes methyl group transfer from MTHfolate to homocysteine to yield methionine. This reaction is also supported by MeCbl as a cofactor to MS.  The MeCbl created in the regeneration of THfolate donates its methyl group to homocysteine, thereby converting it to methionine.  In the process, MeCbl is reduced back to cobalamin.

These reactions are essential for the normal progression of the folate and methionine cycles, without which the expression of genes would be seriously hindered. The methionine product is a precursor of SAMe, the methyl donor for downstream reactions with proteins, hormones, neurotransmitters, and DNA.

Most dietary folate is metabolized to 5-methyl-tetrahydrofolate (MTHfolate) during its passage across the intestinal mucosa. In vitamin B12 deficiency, the vitamin B12-dependent methionine synthase enzyme is inactive and cytosolic folate is “trapped” as 5-methyl-tetrahydrofolate. Folate may accumulate in the serum, but in the cell, this leads to a functional folate deficiency. [Dietary Reference Intakes…]  Inadequate DNA synthesis is attributed in large measure to a block of tetrahydrofolate (THfolate) regeneration, which in turn is due to B12 deficiency.

A Word about Methyl Groups and Gene Expression

Methyl groups are perhaps best known for regulating gene expression. Gene expression means that only genes needed in a particular tissue are expressed. Proteins attach methyl groups to the bases of the DNA molecule in specific places as a type of tag that directly affects DNA function. These tags effectively turn genes on or off by affecting interactions between the DNA and other proteins. Studies indicate that a lack of methyl groups in the body can lead to incorrect gene activation and, as a consequence, may lead to cancer, cardiovascular disease, diabetes, neurological disorders, and a host of other diseases.

MeCbl and Folate Supplementation Can Normalize Homocysteine

Studies indicate that MeCbl may play a role in cardiovascular health. Elevated plasma homocysteine (Hcy) is considered a risk factor for cardiovascular disease and may also be associated with hypertension. Studies show the combination of MeCbl and folate supplementation can normalize elevated homocysteine. (Although the impact on cardiovascular disease risk is less clear.)

Note that some studies have found that cases of vitamin B12 deficiency with elevated serum folate are associated with increased homocysteine and methylmalonic acid (MMA) concentrations — suggesting a worsening of B12 status as serum folate status increases.  Some experts suggest that elevated serum folate may have been due to excess consumption of folic acid fortified foods (processed grains) and/or folic acid supplementation, which we recommend against.


AdCbl is active only in the mitochondria, the powerhouse in our cells. The Krebs cycle, also known as the citric acid cycle, is a series of chemical reactions that release stored energy derived from carbohydrates, fats, and proteins. It is an engine that generates chemical energy: ATP, NADH, and FADH2. The Krebs cycle takes place in the mitochondria.  Within this cycle, every product of a reaction is a potential substrate for another reaction. Each step of the Krebs cycle requires enzymes and each step contributes either directly or indirectly to energy production. For these enzymes to function, they need the B vitamins — B1, B2, B3, B5, and B12.

In the case of B12, cobalamin must be transported into mitochondria and adenosylated before it can be utilized. The generation of ATP (adenosine triphosphate) requires AdCbl and the adenosyl group that is used to assemble AdCbl is synthesized from ATP.  So, contrary to what you may have read, ingesting AdCbl in supplement form is not likely affects one’s mitochondrial AdCbl status.

AdCbl functions as a cofactor of the enzyme methylmalonyl-CoA mutase, which catalyzes the conversion of methylmalonyl-CoA to succinyl-CoA.  Succinyl-CoA is an intermediary of the Krebs cycle and can be readily incorporated there, as well as other metabolic processes. The first step in heme synthesis takes place in the mitochondria. Succinyl-CoA is necessary for the production of hemoglobin which is the substance that carries oxygen in red blood cells.

Conversion of methylmalonyl-CoA to succinyl-CoA reduces the dangerous buildup of methylmalonic acid (MMA).  Elevated MMA is a marker of inadequate intracellular AdCbl. Elevated MMA can cause headaches, fatigue, muscle weakness, weight loss, tremors, and nerve damage. (Elevated odd-chain fatty acids can also be a biomarker for inadequate AdCbl, or inadequate biotin — vitamin B8.)

Although essential for health, it is not necessary to consume vitamin B12 daily because AdCbl is stored in the liver and AdCbl can be converted in the other active form of B12, MeCbl.

Cyanocobalamin: The case against it

Cyanocobalamin (CnCbl) is a synthetic form of B12. In the U.S., it has been traditionally used in vitamin B12 supplements and in vitamin B12 shots.  No form of vitamin B12 is as inexpensive to produce as CnCbl, and its chemical stability allows it a particularly long shelf life. Inside the body, CnCbl is an inactive form of B12 and requires conversion to MeCbl or AdCbl before it can be utilized.

Studies comparing the effectiveness of the different forms of Cbl and OHCbl, MeCbl, and AdCbl exist but there are relatively few.

Poorer Cancer Survival Rates

A murine study showed that supplementation with a bioactive form of B12 had either significantly increased cancer survival rates or better outcomes, while CnCbl appeared to have little or no effect.  [Tsao C.S. Myashita K.]

More Excreted and Lower Amounts Stored in the Liver

Another murine study compared the B12 blood, urine, and liver status at set time periods after supplementation with CnCbl or MeCbl. The data showed that the blood level rise was the same for each of the supplements, but the CnCbl urinary excretion was three times higher than that of MeCbl. The study also found that MeCbl supplementation was associated with 13% more Cbl stored in the liver than CnCbl supplementation. [Okuda, K., Yashima, K.]

Lower Serum Binding Capacity

In the 1960s there were more than a few human studies comparing CnCbl to OHCbl, MeCbl. Similar to the murine studies, researchers found lower tissue retention and increased urinary excretion of B12 as a result of supplementation with CnCbl compared to one of the other forms. Some researchers found that CnCbl had a lower serum binding capacity.  Others concluded that the lower bioavailability of CnCbl was due to its lower efficiency in cellular uptake and metabolic activation.

Unlike Other Forms, No Effect on Sleep Quality

A small human study compared the effect of CnCbl to MeCbl on circadian rhythms. For those supplementing with MeCbl, researchers found a significant correlation between vitamin B12 plasma levels and individuals reported “sleep quality,” “concentration,” and “feeling refreshed.”  Individuals also reported that their sleep duration was reduced.  The researches found no correlation for those individuals supplementing with CnCbl. [Mayer G, Kröger M, Meier-Ewert K]

Potential Cyanide Accumulation

Researchers also have expressed concerns about potential cyanide accumulation in human tissues after long-term supplementation and/or intake from foods fortified with CnCbl. Overall, the CnCbl form of B12 seems to be an inferior choice despite its lower cost.


Relatively recent advances in molecular genetics have provided information on vitamin B12 binding proteins, their receptors, and enzymes involved in the intracellular processing of Cbl. This has enabled us to identify SNPs involved in pathways of absorption, cellular uptake and intracellular metabolism of Cbl.  Some of the genetic mutation information, or SNPs, is available to us through commercial testing services, but many known SNPs are not yet being reported.

Until we have a robust precision medicine approach to assessing the reasons behind a patient’s B12 deficiencies, individuals may find a trial-and-error approach to supplementation be the best approach — by supplementing with one of the three natural forms of B12 at a time.  Some may benefit most by supplementing with all three B12 forms.  We find that individuals with biomarkers indicative of impaired B12 assimilation can raise their B12 status by taking a lower dose two or three times a day. It is likely that B12 status is dependent on other nutrients, particularly folate and other B vitamins,  So we recommend checking multiple nutrient biomarkers when B12 status is checked.

On a regular basis, we find new research on B12 deficiency, the spectrum of its effects, and the methods for its diagnosis. Stay tuned.

    1. Lachner C, Steinle NI, Regenold WT. The neuropsychiatry of vitamin B12 deficiency in elderly patients. J Neuropsychiatry Clin Neurosci. 2012 Winter;24(1):5-15. doi: 10.1176/appi.neuropsych.11020052. PMID:22450609
    2. Rietsema WJ. Unexpected recovery of moderate cognitive impairment on treatment with oral methylcobalamin. Journal of the American Geriatrics Society 2014;62(8):1611-12 doi: 10.1111/jgs.12966.
    3. Wheatley, C. Cobalamin in inflammation III — glutathionylcobalamin and methylcobalamin/adenosylcobalamin coenzymes: the sword in the stone? How cobalamin may directly regulate the nitric oxide synthases. J Nutr Environ Med. 2007 Sep–Dec; 16(3–4):212–226. Article
    4. Oh R, Brown DL. Vitamin B12 deficiency. Am Fam Physician. 2003 Mar 1; 67(5):979-86.
    5. Green, R. Vitamin B12 deficiency from the perspective of a practicing hematologist. Blood. 2017 :blood-2016-10-569186. Article
    6. Paul Henry Golding. Holotranscobalamin (HoloTC, Active-B12) and Herbert’s model for the development of vitamin B12 deficiency: a review and alternative hypothesis. Springerplus. 2016; 5(1): 668. PMID: 27350907
    7. Martin G,Tentoni J, Cicchetti G. Megaloblastic anemia: rapid and economical study. Sangre (Barc). 1997, 42(3):235-238. PMID: 9381269
    8. Thompson WG, Cassino C, Babitz L, Meola T, Berman R, Lipkin, Jr M, Freedman M. Hypersegmented neutrophils and vitamin B12 deficiency. Acta Haematol 1989;81:186–191. Article
    9. Carmel R.  How I treat cobalamin (vitamin B12) deficiency.  Blood. 2008 Sep 15; 112(6): 2214–2221. PMID: 18606874
    10. Berlin H, Berlin R, Brante G. Oral treatment of pernicious anemia with high doses of vitamin B12 without intrinsic factor. Acta Med Scand. 1968; 184:247-258.
    11. Selhub, J., Morris, M. S. & Jacques, P. F. In vitamin B12 deficiency, higher serum folate is associated with increased total homocysteine and methylmalonic acid concentrations. Proc. Natl Acad. Sci. USA 104, 19995–20000 (2007).
    12. Allen RH, Stabler SP, Savage DG, Lindenbaum J. Diagnosis of cobalamin deficiency: usefulness of serum methylmalonic acid and total homocysteine concentrations. Am J Hematol. 1990;34:90- 98. Article.
    13. Miller, J. W. et al. Metabolic evidence of vitamin B-12 deficiency, including high homocysteine and methylmalonic acid and low holotranscobalamin, is more pronounced in older adults with elevated plasma folate. Am. J. Clin. Nutr. 90, 1586–1592 (2009).
    14. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Institute of Medicine (US) Standing Committee on the Scientific Evaluation of Dietary Reference Intakes and its Panel on Folate, Other B Vitamins, and Choline. Washington (DC): National Academies Press (US); 1998. Chapter 9.
    15. Arendt JF, Farkas DK, Pedersen L, Nexo E, Sørensen HT. Elevated plasma vitamin B12 levels and cancer prognosis: A population-based cohort study. Cancer Epidemiol. 2016 Feb;40:158-65. doi: 10.1016/j. PMID: 26724465
    16. Quadros, EV. Advances in the Understanding of Cobalamin Assimilation and Metabolism. Br J Haematol. 2010 Jan; 148(2): 195–204.PMID: 19832808
    17. Sharabi A, Cohen E, Sulkes J, Garty M. Replacement therapy for vitamin B12 deficiency: comparison between the sublingual and oral route. Br J Clin Pharmacol. 2003 Dec;56(6):635-8.PMID:14616423
    18. Del Bo’ C, Riso P, Gardana C, Brusamolino A, Battezzati A, Ciappellano S. Effect of two different sublingual dosages of vitamin B12 on cobalamin nutritional status in vegans and vegetarians with a marginal deficiency: A randomized controlled trial. Clin Nutr. 2018 Feb 15. pii: S0261-5614(18)30071-2. doi: 10.1016/j.clnu.2018.02.008. PMID:29499976
    19. Castelli MC, Friedman K, Sherry J, Brazzillo K, Genoble L, Bhargava P, Riley MG. Comparing the efficacy and tolerability of a new daily oral vitamin B12 formulation and intermittent intramuscular vitamin B12 in normalizing low cobalamin levels: a randomized, open-label, parallel-group study. Clin Ther. 2011 Mar;33(3):358-371.e2. doi: 10.1016/j.clinthera.2011.03.003. PMID:21600388
    20. Oliai Araghi S, Kiefte-de Jong JC, van Dijk SC, Swart KMA, van Laarhoven HW, van Schoor NM, de Groot LGPGM, Lemmens V, Stricker BHCh, Uitterlinden AG, van der Velde N. Folic acid and vitamin-B12 supplementation and the risk of cancer: long-term follow-up of the B-vitamins for the prevention of osteoporotic fractures (B-PROOF) trial. AACR Publication. 2018 Oct.  10.1158/1055-9965.EPI-17-1198. PMID:30341095
    21. Figueiredo JC, Grau MV, Haile RW, Sandler RS, Summers RW, Bresalier RS, Burke CA, McKeown-Eyssen GE, Baron JA. Folic acid and risk of prostate cancer: results from a randomized clinical trial. J Natl Cancer Inst. 2009 Mar 18;101(6):432-5. doi: 10.1093/jnci/djp019. Epub 2009 Mar 10. PMID:19276452
    22. Henderson JT, Simpson JD, Warwick RR, Shearman DJ. Does malabsorption of B12 occur in chronic pancreatitis? Lancet. 1972 Aug 5;2(7771):241-3. PMID: 4114504 Article
    23. Okuda K, Yashima K, Kitazaki T, Takara I. Intestinal absorption and concurrent chemical changes of methylcobalamin. J Trans. Res. 1973 Apr, Vol 81, Issue 4, 557–567. Article
    24. Chalmers JN, Shinton NK. Comparison of hydroxocobalamin and cyanocobalamin in the treatment of pernicious anemia. Lancet. 1965; Vol 2 (7426):1305–1308.
    25. Hertz H, Østergaard Kristensen HP, Hoff‐JØrgensen E. Studies on vitamin B12, retention: comparison of retention following intramuscular injection of cyanocobalamin and hydroxocobalamin. Eu J of Haematol. 1964 Mar; Vol 1, Issue 1. Article
    26. Mayer G, Kröger M, Meier-Ewert K. Nature. Effects of vitamin B12 on performance and circadian rhythm in normal subjects. Nature: Neuropharmacology. 1966 Nov, Vol 15, 456–464
    27. Paul C, Brady DM. Comparative bioavailability and utilization of particular forms of B12 supplements with potential to mitigate B12-related genetic polymorphisms. Integr Med. 2017 Feb; Vol 16(1); PMC5312744