Please Scroll Down to See Forums Below 
CYCLEON
New member
A good study on B12, why u need it and weather oral or inject is for you - 2000mcg ED orally VS 1000mcg every month injected, wiht oral having better results at that dosage
Tables of Numbers
The average intake of B12 is approximately 5 µg/day in the USA.49
U.S. Dietary Reference Intake:
Age
µg
0-5 months
0.4
6-11
0.5
1-3 yrs
0.9
4-8
1.2
9-13
1.8
14-50
2.4
> 50*
2.4
Pregnancy
2.6
Lactation
2.8
*Since 10-30% of older people may malabsorb food-bound B12, it is advisable for those older than 50 years to meet their RDA mainly through fortified foods or supplements.
Serum B12 is measured in both pmol/l and pg/ml. For ease in reading this article, all B12 measured in pmol/l will be converted to pg/ml using a conversion factor of 1 pmol/l = 1.35 pg/ml.
pg/ml
pmol/l
normal serum range*
200-900
148-666
normal serum average
450
333
normal breast milk contains55
180-300
*B12 deficiency symptoms (including neurological) have been found in levels as high as 350 pg/ml.98
normal MCV range95
80-96 fl
normal sMMA56
73-271 nmol/l
normal uMMA93
2-6 µg/mg creatinine
--------------------------------------------------------------------------------
Vitamin B12: A Pesky Molecule
Vitamin B12 deficiency in industrialized countries is rare.55 B12 is a complicated vitamin with a unique digestion, wide array of deficiency symptoms, and a number of "analogues" (molecules that appear to be B12, but actually are not) that possibly interfere with its function.
The B12 Molecule
B12 is a co-enzyme: it is needed for enzymes to do their job of changing one molecule into another. As vitamins go, B12 is large. One part of its structure is known as the corrin nucleus. The corrin resembles the heme of hemoglobin. In hemoglobin, the heme holds an atom of iron. In B12, the corrin holds an atom of cobalt. The corrin plus other atoms make up the part of B12 known as a cobalamin. In order to be true B12, the cobalamin must have one of a number of attachments. Depending on the attachment, cobalamin becomes cyanocobalamin, hydroxocobalamin, aquocobalamin, nitritocobalamin, methylcobalamin, or adenosylcobalamin (also called 5'-deoxyadenosylcobalamin).35 Only 2 cobalamins are active as co-enzymes in the human body: adenosylcobalamin and methylcobalamin. However, the body has the ability to convert most other cobalamins into one of these active forms. Cyanocobalamin is the form most often found in vitamin tablets because it is one of the most stable forms of cobalamin and the body readily converts it into one of the useable co-enzymes.47 There are many molecules that contain a corrin nucleus but that are not cobalamins. And some cobalamins might not be useable by the human body.
Any molecule resembling B12, but which is not active in the body, is considered an analogue of B12. These B12 analogues all contain a corrin nucleus and, along with true B12, are all known as corrinoids. About 1/3 of the corrinoids in the typical person are B12 analogues, while the rest are B12.48 In this article, unless otherwise noted, "B12" refers only to molecules that are thought to be physiologically active for humans.
Summary of the B12 Molecule & Its Analogues
Corrinoids -- molecules with a corrin nucleus
Non-cobalamins: B12 analogues
Cobalamins
B12: cyano-, methyl-, hydroxo-, adenosyl-, aquo-, nitrito-
Possibly more B12 analogues
In rare cases, some infants cannot convert cyanocobalamin to an active form and thus cannot rely on cyanocobalamin supplements.47
Digestion & Absorption of B12
Microorganisms, primarily bacteria, are the only known organisms that manufacture B12. These bacteria often live in bodies of water and soil. Animals get B12 by eating food and soil contaminated with these microorganisms. These bacteria also live inside animals' digestive tracts. Plants do not require B12 for any function, and therefore have no mechanisms to produce or store B12. In animals, B12 is normally attached to a protein (very large molecules made up of amino acids) either for transport or storage.
B12 is generally found in all animal products. When humans eat animal foods, the B12 is bound to protein. When the protein-B12 complex reaches the human stomach, the stomach secretes acids and enzymes that detach the B12 from the protein. Then, in a process unique to B12, another protein, R protein (also called cobalophilin or haptocorrin) picks up the B12 and transports it through the stomach and into the small intestine. R protein is found in many fluids in the human body including saliva and stomach secretions. R protein picks up all corrinoids in addition to true B12.45 The stomach cells also produce a protein called intrinsic factor (IF).
When the B12-R protein complex gets to the small intestine, B12 is liberated from the R protein by enzymes that are made in the pancreas.35 B12 then attaches to IF that has also made its way into the small intestine. The IF then carries the B12 to the last section of the small intestine, the ileum. The cells lining the ileum contain receptors for the B12-IF complex. Calcium is thought to be needed by the B12-IF receptor in order to take the B12 into the intestinal cell. The B12-IF complex protects B12 against bacterial and digestive enzyme degradation.64
Free B12
In supplements, B12 is not bound to protein. In large doses obtained only through supplements, B12 can bypass the absorption process described above. Instead, passive diffusion can account for much absorption of the free B12.35 (Passive diffusion normally accounts for 1-3% of B12 absorbed when obtained through normal food sources.35)
Enterohepatic Circulation
About 60% of the total amount of B12 in the body is stored in the liver and 30% is stored in the muscles.64 The body has a special circuit between the digestive tract and the liver. Bile, which is made in the liver and needed to digest fat, is secreted into the beginning of the small intestine. It is then reabsorbed at the end of the small intestine and taken back to the liver where it is used again. This circuit is called enterohepatic circulation. Omnivores normally eat about 2-6 µg of B12/day and their liver normally excretes 5-10 µg/day via their bile.48 Healthy omnivores reabsorb about 3-5 µg B12 from the bile.48 A (noninfant) vegan with B12 absorption problems will develop B12 deficiency in 1-3 years because absorption problems will block the enterohepatic circulation.48 Adult vegans decrease their bile excretion to as low as 1 µg/day and reabsorb almost 100% of it, thus delaying B12 deficiency for 20-30 years.48
Enterohepatic circulation can help remove B12 analogues (especially noncobalamin corrinoids) that find their way into circulation by dumping them in the intestine where they will not be picked up by IF, and will therefore be excreted.47
Pernicious Anemia
As mentioned above, without IF, only about 1-3% of the ingested B12 is absorbed.37 This is typically not adequate and results in a macrocytic anemia. When macrocytic anemia is caused by a lack or malfunction of IF, it is known as pernicious anemia (PA). PA can occur in inflammation of the stomach, when most or all of the stomach has been removed, or when the last part of the ileum has been removed.37
Transport in the Blood
After B12 is absorbed into the intestinal cells, it attaches to another R protein, transcobalamin 2 (TC2). TC2 is made in the intestinal cells48 and transports B12 to all body tissues through the blood. All body tissues have receptors for TC2. Once the B12-TC2 complex arrives at the cell where it is needed, B12 is released from TC2 in the form of hydroxocobalamin. It is then turned into methylcobalamin or adenosylcobalamin35 and used for their respective enzymes. TC2 normally contains about 20% of B12 in the blood, also called serum B12 (sB12) or plasma B12. When intake of B12 into the intestine slows, B12-TC2 levels fall rapidly.48 If TC2 lacks B12, the vitamin will not be delivered, regardless of whether the total sB12 is low, normal, or high.46 TC2 is depleted of B12 within days after absorption stops making the measurement of the B12-TC2 complex the best screening test for early negative B12 balance. B12-TC2 falls below normal long before sB12 falls below normal. TC1 and TC3 (also called haptocorrin46) are the proteins that normally store the other 80% of the B12 in the blood.48
Functions of B12
Homocysteine Clearance
Homocysteine (Hcy) is a nerve and vessel toxin (promoting heart attacks, thrombic strokes, and vessel blockages) at elevated levels.48 When B12 is no longer delivered to certain brain cells, Hcy builds up.48 Thus, the body has a need to turn Hcy into other molecules, one of which is methionine (an amino acid). If it cannot do this, Hcy levels build up in the blood. Methylcobalamin (a form of B12) is needed by the enzyme that converts Hcy into methionine. If someone is B12 deficient, Hcy levels will increase.
Elevated Hcy can also happen with deficiencies in vitamin B6 or folate.48 RDA amounts of the deficient vitamin can reduce Hcy levels to normal if a vitamin deficiency is the cause.48
Folate and DNA
The vitamin folate (also called folic acid) comes into play in B12 deficiency. Folate is needed to produce DNA. In creating methylcobalamin (used in the Hcy to methionine reaction mentioned above), B12 takes a methyl group from one form of folate. In so doing, it produces a form of folate needed to make DNA. If there is no B12 available, this form of folate can become reduced (known as the methyl-folate trap) and DNA cannot be produced.74 However, if there is enough incoming folate through the diet, the body can use the new folate to produce DNA. In a B12 deficiency, the accumulation of 5-methyl THF (a form of folate) and folic acid can occur.99 This could explain some of the high folate levels found in vegans.
Folate and DNA play a critical role in rapidly producing cells. For example, the production of red blood cells involves dividing large, inactive cells into smaller, active cells. Because of the methyl-folate trap, people with a B12 deficiency can sometimes have large red blood cells. People with a folate deficiency can also have large red blood cells. This problem is known as megaloblastic anemia or macrocytic anemia (depending on how the large red blood cells are measured). Megaloblastic anemia is measured by determining the quantity of large red blood cells. Macrocytic anemia is determined by the average volume of the red blood cells, known as mean corpuscular volume (MCV). In B12 deficiency, there may be high enough folate levels to allow red blood cell division to continue, preventing macrocytic anemia.
To add insult to injury, an iron deficiency (which results in small red blood cells) can counteract the large red blood cells making it appear as though the blood cells are normal in the face of multiple nutritional deficiencies.46
B12 helps transport and store folate in the cells. When serum B12 is low, folate is unable to be stored in the cells and it starts to accumulate in the serum, increasing serum folate levels.39
Intestinal cells are also rapidly dying and being replaced. Ironically, a B12 deficiency can make itself worse because it can prevent the production of the intestinal cells needed to absorb B12.
Methylmalonic Acid (MMA)
There is one metabolic pathway in which B12 is the only co-enzyme: the conversion of one molecule, methylmalonyl-CoA, to another molecule, Succinyl-CoA. When B12 is not available, methylmalonyl-CoA levels increase. Because it is toxic, methylmalonyl-CoA is converted to methylmalonic acid (MMA) which then accumulates in the blood and urine. Since this reaction only requires B12 as a co-enzyme, MMA levels are excellent indicators of B12 status. Rare genetic defects can also cause high MMA levels.
Measuring B12 in the Body and in Food
Traditionally, B12 has been measured by "feeding" B12 to certain bacteria and seeing if those bacteria are able to thrive on it. This is known as microbiological assay. However, many bacteria used in these assays also thrive on noncobalamin corrinoids.48 B12 has also been measured through radioassay (which has other names, such as competitive binding assay) by seeing whether it binds to R protein. It is now known that R protein can bind to noncobalamin corrinoids. So, these methods that were once thought to measure only B12 are now known to measure B12 analogues also. Despite this, some laboratories continue to rely on these methods to measure the B12 content of foods and the body (although it is becoming less common). Radioassays using IF as the binding agent have proven more accurate because IF does not measure noncobalamin corrinoids. However, problems have also been found even when using IF radioassays (as will be discussed in more detail later). IF from pigs is normally used for these radioassays, and there could be a slight possibility that pig IF is selective for B12 analogues that human IF would not select.
Determining B12 vs. Folate Deficiency: dU Suppression Test
To determine whether someone is deficient in B12, folate, or both, a deoxyuridine (dU) suppression test can be performed. To make DNA, cells require a molecule called deoxythymidine. If cells are given dU, they must convert dU into deoxythymidine before they can use it to make DNA. Cells with slowed DNA synthesis can be fed dU in combination with B12 or folate, or both. The deficient vitamin will produce the most noticeable increase in DNA synthesis.46
Stages of B12 Deficiency
von Schenck et al.103 (1997) reported that the average adult stores 3000 µg B12, while losing only about 3 µg/day. Herbert48 (1994) reports that when one stops eating B12, they pass through 4 stages of B12 deficiency as follows:
Depletion Stages:
Stage 1 Serum depletion: Shown by low amounts of B12 on TC2; also can be measured by lower than normal B12 in red blood cells since only young red blood cells contain B12.
Stage 2 Cell depletion: Shown by low TC1+TC3 and low B12 in red blood cells; low total serum B12 levels (i.e., TC1 + TC2 + TC3) are a relatively late indicator.
Deficiency Stages:
Stage 3 Biochemical deficiency: Shown by slowed DNA synthesis, elevated serum Hcy and MMA.
Stage 4 Clinical deficiency: Megaloblastic anemia, nerve damage, etc.
Because liver cells store more B12 than bone marrow or nervous tissue, liver cells can be in Stage 2 while bone marrow (which makes red blood cells) and nervous tissue is already in Stage 3 and possibly even 4.48
Herbert says that vegans can stabilize in Stage 2 for years because depleted stores trigger increased absorption, making more efficient the absorption of the trace amount of B12 from bacterial contamination in the small intestine and B12 secreted in the bile. Stage 2 will move on to Stage 3 much sooner in people with B12 absorption problems. The slight negative balance of vegans will eventually deplete stores and Stage 3 will be entered.
Symptoms
Neurological Symptoms
Neurological symptoms are the biggest worry in B12 deficiency because they can be irreversible. However, if caught early enough, they can be reversed in many cases.
One theory of why nerve problems occur in B12 deficiency is from a lack of methionine (from B12 not converting Hcy back into methionine) which creates a lack of S-adenosylmethionine (SAM).35 SAM is required for the production of phosphatidylcholine36 which is part of the myelin (the fatty material that insulates many nerves). Phosphatidylcholine improves nerve transmission.36
Another theory is that the altered nerve function of B12 deficiency is possibly due to the body's inability to convert methylmalonyl-CoA (a 3 carbon molecule) to succinyl-CoA (a 4 carbon molecule). This inability results in a build up of propionyl-CoA (a 3 carbon molecule). Fatty acids are normally made by adding 2 carbons at a time to an even numbered carbon molecule. In an overabundance of 3 carbon molecules, large amounts of unusual 15-carbon and 17-carbon fatty acids could be produced and incorporated into nerve sheets, causing altered nerve function.103
Early Signs of B12 Deficiency
According to Crane et al.18 (1994, USA), the usual vegan patient has no clinical symptoms or signs of inadequate B12. Early manifestations are unusual fatigue, faulty digestion (no appetite or nausea) and loss of menstruation. Other symptoms are nervousness, numbness and tingling of the hands and feet, mild depression, striking behavioral changes, paranoia, hyperactive reflexes, fever of unknown origin,18 frequent upper respiratory infections,19 impotence, impaired memory,49 infertility,55 sore tongue, and diarrhea.60
6 Ways to Get B12 Deficiency
Herbert48 (1994) reports the ways to get B12 deficiency as follows:
Inadequate dietary intake.
Inadequate absorption:
Loss of IF: This is genetically predetermined and age-dependent (sometimes as early as 45 yrs). It is the most common cause of B12 deficiency in nonvegetarians. Because of their lower stores, vegans will more rapidly express a genetically predisposed B12 deficiency.
Loss of gastric acid and/or protein digesting enzymes which break the protein-B12 bonds in food. According to Ho49 (1999), this can be caused by stomach surgery, atrophy or inflammation of the stomach, medications that suppress acid secretion, or a stomach infection by H. pylori or anaerobic bacteria.
Pancreatic disease that reduces free calcium in the ileum. This can be improved by giving calcium and/or bicarbonate.
Unhealthy ileum.
The oral, diabetic drug metformin ties up free calcium in the intestines producing B12 malabsorption.
Autoimmunity to IF: Circulating antibodies to IF indicate eventual PA if not treated. A chronic B12 deficiency damages immune function and the antibodies may disappear as B12 deficiency progresses.
Inadequate utilization: Defects in B12 enzymes, transport proteins, or storage proteins.
Increased requirement during pregnancy or hyperthyroidism.
Increased excretion caused by alcoholism.
Increased destruction: Megadoses of vitamin C can create free radicals which can damage B12 and IF. Nitrous oxide anesthesia in people who are low in B12 (nitrous oxide can change the cobalt atom of B1235).
Others causes of B12 deficiency are tapeworms,35 hypothyroidism (this relationship is possibly autoimmune),53 Giardia lamblia infection, chronic use of gastric acid secretion suppressors, or drugs affecting absorption (including cimetidine, metformin, potassium chloride, and cholestyramine).98
Alcoholism & AIDS
B12 deficiency can be masked by alcoholism because excess B12 is released into the blood from the damaged liver.48 AIDS can cause B12 deficiency as shown through macrocytic anemia and neurological problems, but without elevated Hcy levels.
Correcting & Preventing Deficiency
According to Herbert, measuring MMA or Hcy levels will not prevent deficiency. Instead, depletion must be measured by way of low B12-TC2 levels. Depletion precedes deficiency by months to years in over 95% of the cases.48 It only takes .1 µg of B12 to start reversing deficiency symptoms, though the response can be improved with more.47
Small Amounts of Animal Products Not Enough to Restore Optimal B12 Status
van Dusseldorp et al.102 (1999, Netherlands) investigated whether moderate consumption of animal products is sufficient for achieving normal B12 function in 73 adolescents (in good health) who had received a macrobiotic diet until 6 years of age and had then switched to a LOV or omnivorous diet. 94 nonvegetarian (NV) adolescents from birth were used as a reference. In macrobiotics, dairy products supplied on average 0.95 µg B12/day. Additionally, they consumed fish, meat, or chicken 2-3 times/week. Serum B12 was significantly lower and MMA, folate, and MCV were significantly higher in macrobiotics. Of macrobiotics, 21% had abnormal MMA levels, 10% had abnormal total homocysteine, and 15% had abnormal MCV (> 89 fl). Authors concluded that a substantial number of the formerly strict macrobiotic adolescents still had impaired B12 function. Thus, moderate consumption of animal products is not sufficient for restoring normal B12 status in adolescents with inadequate B12 intake during the early years of life. They might need B12 intakes higher than recommended to obtain normal B12 status.
Supplements, Fortified Foods, & Animal Products
Tucker et al.98 (2000, USA) examined the B12 status of 2999 subjects in the Framingham Offspring Study. Average sB12 for the entire group was 473 pg/ml. 39% of subjects had sB12 < 348, 17% < 250, and 9% < 200 pg/ml. There was a significant trend towards lower B12 levels with increased age. In contrast with previous reports, sB12 was associated with B12 intake. Supplement users were significantly less likely than non-supplement users to have B12 < 250 pg/ml (8% vs. 20%). Among non-supplement users, those who consumed fortified cereal over 4 times/week were significantly less likely to have B12 < 250 pg/ml. Those in the highest one-third vs. lowest one-third of dairy intake were significantly less likely to have B12 < 250 pg/ml (13% vs. 24%). There was no difference between meat intake groups. Tucker et al. concluded that the use of supplements, fortified cereal, and milk appears to protect against lower SB12. This could be because additional sources may have increased total intake and/or some sources may be more bioavailable. Meat, eggs, and seafood may not be as protective as dairy because cooking may destroy B12. There was a clear and strong increase in B12 levels with greater B12 intake up to about 10 µg/day.
Supplements
In 1988, Herbert cautioned that large amounts of B12 may eventually be found to be harmful.47 However, Hathcock & Troendle40 (1991) point out that there appears to be little or no question that B12 intakes of 500-1000 µg/day are safe. The cobalt and the cyanide contribution of 1000 µg/day of cyanocobalamin are toxicologically insignificant.40
Crane et al.18 (1994, USA) noted that tablets of one vitamin company dissolved slowly in water and acid. They then conducted a study to see if vegan patients who had not responded to oral B12 tablets could improve their B12 response by chewing the tablets. 7 participants chewed the tablets of 100 µg (once a week for 6 weeks) and their average levels went from 116 to 291 pg/ml. Of the 9 who didn't chew, levels increased from 123 to 139 pg/ml. These 9 then chewed 500 µg/day for 10 days and their levels rose to normal with a final average of 524 ± 235 pg/ml. 3 participants could not raise their levels orally and required injections to maintain sB12 above 300 pg/ml. Crane et al. recommend that vegans chew, or let dissolve in the mouth, a 100-500 µg B12 tablet once a week or more often.
Supplements should not be left in the light as prolonged light causes irreversible destruction of cyanocobalamin.89
Oral B12 Even for People with Malabsorption
Intramuscular injections (IMI) of B12 are the typical way to treat B12 deficiency. The injections can be painful and expensive. Kuzminski et al.56 (1998, USA) studied 33 newly diagnosed B12 deficient patients (almost all had malabsorption) to receive cyanocobalamin as either 1 mg intramuscularly on days 1, 3, 7, 10, 14, 21, 30, 60, and 90, or 2000 µg orally on a daily basis for 120 days. Results were:
oral
injection
Pretreatment:
serum B12 (pg/ml)
93
95
serum MMA (nmol/l)
3850
3630
serum homocysteine (µmol/l)
37.2
40.0
After 4 months:
serum B12
1005
325
serum MMA
169
265
serum homocysteine
10.6
12.2
The higher serum B12 and lower serum MMA levels at 4 months in the oral group were significantly higher than the injected group. Kuzminski et al. concluded that 2000 µg/day of oral cyanocobalamin was as effective as 1000 µg injected intramuscularly each month, and may be superior.
Delpre & Stark28 (1999, Israel) studied patients with B12 deficiency to see if B12 can be absorbed by holding a tablet under the tongue, known as sublingual. 5 patients had PA, 7 were vegetarians, and 2 had Crohn's disease (which can prevent the absorption of B12 in the ileum). The patients held two 1000 µg B12 tablets (equaling 2000 µg/day) under their tongues for 30 minutes until completely dissolved. This was done for 7 to 12 days. Average sB12 levels went from 127.9 ± 42.6 to 515.7 ± 235 pg/ml. All the patients' sB12 normalized. There were no side effects and all patients preferred this to IMI. Bodamer & Scaglia10 (1999) pointed out that in addition to the B12 absorption problems listed in Delpre & Stark's28 study, some people have inborn errors in B12 metabolism and people such as this might not benefit from oral cyanocobalamin therapy. Such people will need more careful medical attention.
Norberg75 (1999, Sweden) points out that investigations in the 1950s and 60s showed that oral B12 is absorbed by an alternative pathway not dependent on IF or an intact distal ileum. Approximately 1% of an oral dose in the range of 200-2000 µg/day was absorbed by the alternative pathway. Based on this research, oral treatment, rather than IMI, has been in use for the majority of B12 deficiency cases in Sweden since the early 70s.
In a literature review encouraging the use of oral cobalamin therapy over injections for patients with PA, Lederle58 (1991) reported that Swedish investigators recommend 2000 µg B12 twice a day or injections to replenish B12 stores of patients with PA. After initial therapy, doses of 1000 µg/day appear to be enough.58
Please note that the above doses are for people with B12 malabsorption. People without malabsorption problems or current B12 deficiency should only need doses much closer to the RDA.
Tables of Numbers
The average intake of B12 is approximately 5 µg/day in the USA.49
U.S. Dietary Reference Intake:
Age
µg
0-5 months
0.4
6-11
0.5
1-3 yrs
0.9
4-8
1.2
9-13
1.8
14-50
2.4
> 50*
2.4
Pregnancy
2.6
Lactation
2.8
*Since 10-30% of older people may malabsorb food-bound B12, it is advisable for those older than 50 years to meet their RDA mainly through fortified foods or supplements.
Serum B12 is measured in both pmol/l and pg/ml. For ease in reading this article, all B12 measured in pmol/l will be converted to pg/ml using a conversion factor of 1 pmol/l = 1.35 pg/ml.
pg/ml
pmol/l
normal serum range*
200-900
148-666
normal serum average
450
333
normal breast milk contains55
180-300
*B12 deficiency symptoms (including neurological) have been found in levels as high as 350 pg/ml.98
normal MCV range95
80-96 fl
normal sMMA56
73-271 nmol/l
normal uMMA93
2-6 µg/mg creatinine
--------------------------------------------------------------------------------
Vitamin B12: A Pesky Molecule
Vitamin B12 deficiency in industrialized countries is rare.55 B12 is a complicated vitamin with a unique digestion, wide array of deficiency symptoms, and a number of "analogues" (molecules that appear to be B12, but actually are not) that possibly interfere with its function.
The B12 Molecule
B12 is a co-enzyme: it is needed for enzymes to do their job of changing one molecule into another. As vitamins go, B12 is large. One part of its structure is known as the corrin nucleus. The corrin resembles the heme of hemoglobin. In hemoglobin, the heme holds an atom of iron. In B12, the corrin holds an atom of cobalt. The corrin plus other atoms make up the part of B12 known as a cobalamin. In order to be true B12, the cobalamin must have one of a number of attachments. Depending on the attachment, cobalamin becomes cyanocobalamin, hydroxocobalamin, aquocobalamin, nitritocobalamin, methylcobalamin, or adenosylcobalamin (also called 5'-deoxyadenosylcobalamin).35 Only 2 cobalamins are active as co-enzymes in the human body: adenosylcobalamin and methylcobalamin. However, the body has the ability to convert most other cobalamins into one of these active forms. Cyanocobalamin is the form most often found in vitamin tablets because it is one of the most stable forms of cobalamin and the body readily converts it into one of the useable co-enzymes.47 There are many molecules that contain a corrin nucleus but that are not cobalamins. And some cobalamins might not be useable by the human body.
Any molecule resembling B12, but which is not active in the body, is considered an analogue of B12. These B12 analogues all contain a corrin nucleus and, along with true B12, are all known as corrinoids. About 1/3 of the corrinoids in the typical person are B12 analogues, while the rest are B12.48 In this article, unless otherwise noted, "B12" refers only to molecules that are thought to be physiologically active for humans.
Summary of the B12 Molecule & Its Analogues
Corrinoids -- molecules with a corrin nucleus
Non-cobalamins: B12 analogues
Cobalamins
B12: cyano-, methyl-, hydroxo-, adenosyl-, aquo-, nitrito-
Possibly more B12 analogues
In rare cases, some infants cannot convert cyanocobalamin to an active form and thus cannot rely on cyanocobalamin supplements.47
Digestion & Absorption of B12
Microorganisms, primarily bacteria, are the only known organisms that manufacture B12. These bacteria often live in bodies of water and soil. Animals get B12 by eating food and soil contaminated with these microorganisms. These bacteria also live inside animals' digestive tracts. Plants do not require B12 for any function, and therefore have no mechanisms to produce or store B12. In animals, B12 is normally attached to a protein (very large molecules made up of amino acids) either for transport or storage.
B12 is generally found in all animal products. When humans eat animal foods, the B12 is bound to protein. When the protein-B12 complex reaches the human stomach, the stomach secretes acids and enzymes that detach the B12 from the protein. Then, in a process unique to B12, another protein, R protein (also called cobalophilin or haptocorrin) picks up the B12 and transports it through the stomach and into the small intestine. R protein is found in many fluids in the human body including saliva and stomach secretions. R protein picks up all corrinoids in addition to true B12.45 The stomach cells also produce a protein called intrinsic factor (IF).
When the B12-R protein complex gets to the small intestine, B12 is liberated from the R protein by enzymes that are made in the pancreas.35 B12 then attaches to IF that has also made its way into the small intestine. The IF then carries the B12 to the last section of the small intestine, the ileum. The cells lining the ileum contain receptors for the B12-IF complex. Calcium is thought to be needed by the B12-IF receptor in order to take the B12 into the intestinal cell. The B12-IF complex protects B12 against bacterial and digestive enzyme degradation.64
Free B12
In supplements, B12 is not bound to protein. In large doses obtained only through supplements, B12 can bypass the absorption process described above. Instead, passive diffusion can account for much absorption of the free B12.35 (Passive diffusion normally accounts for 1-3% of B12 absorbed when obtained through normal food sources.35)
Enterohepatic Circulation
About 60% of the total amount of B12 in the body is stored in the liver and 30% is stored in the muscles.64 The body has a special circuit between the digestive tract and the liver. Bile, which is made in the liver and needed to digest fat, is secreted into the beginning of the small intestine. It is then reabsorbed at the end of the small intestine and taken back to the liver where it is used again. This circuit is called enterohepatic circulation. Omnivores normally eat about 2-6 µg of B12/day and their liver normally excretes 5-10 µg/day via their bile.48 Healthy omnivores reabsorb about 3-5 µg B12 from the bile.48 A (noninfant) vegan with B12 absorption problems will develop B12 deficiency in 1-3 years because absorption problems will block the enterohepatic circulation.48 Adult vegans decrease their bile excretion to as low as 1 µg/day and reabsorb almost 100% of it, thus delaying B12 deficiency for 20-30 years.48
Enterohepatic circulation can help remove B12 analogues (especially noncobalamin corrinoids) that find their way into circulation by dumping them in the intestine where they will not be picked up by IF, and will therefore be excreted.47
Pernicious Anemia
As mentioned above, without IF, only about 1-3% of the ingested B12 is absorbed.37 This is typically not adequate and results in a macrocytic anemia. When macrocytic anemia is caused by a lack or malfunction of IF, it is known as pernicious anemia (PA). PA can occur in inflammation of the stomach, when most or all of the stomach has been removed, or when the last part of the ileum has been removed.37
Transport in the Blood
After B12 is absorbed into the intestinal cells, it attaches to another R protein, transcobalamin 2 (TC2). TC2 is made in the intestinal cells48 and transports B12 to all body tissues through the blood. All body tissues have receptors for TC2. Once the B12-TC2 complex arrives at the cell where it is needed, B12 is released from TC2 in the form of hydroxocobalamin. It is then turned into methylcobalamin or adenosylcobalamin35 and used for their respective enzymes. TC2 normally contains about 20% of B12 in the blood, also called serum B12 (sB12) or plasma B12. When intake of B12 into the intestine slows, B12-TC2 levels fall rapidly.48 If TC2 lacks B12, the vitamin will not be delivered, regardless of whether the total sB12 is low, normal, or high.46 TC2 is depleted of B12 within days after absorption stops making the measurement of the B12-TC2 complex the best screening test for early negative B12 balance. B12-TC2 falls below normal long before sB12 falls below normal. TC1 and TC3 (also called haptocorrin46) are the proteins that normally store the other 80% of the B12 in the blood.48
Functions of B12
Homocysteine Clearance
Homocysteine (Hcy) is a nerve and vessel toxin (promoting heart attacks, thrombic strokes, and vessel blockages) at elevated levels.48 When B12 is no longer delivered to certain brain cells, Hcy builds up.48 Thus, the body has a need to turn Hcy into other molecules, one of which is methionine (an amino acid). If it cannot do this, Hcy levels build up in the blood. Methylcobalamin (a form of B12) is needed by the enzyme that converts Hcy into methionine. If someone is B12 deficient, Hcy levels will increase.
Elevated Hcy can also happen with deficiencies in vitamin B6 or folate.48 RDA amounts of the deficient vitamin can reduce Hcy levels to normal if a vitamin deficiency is the cause.48
Folate and DNA
The vitamin folate (also called folic acid) comes into play in B12 deficiency. Folate is needed to produce DNA. In creating methylcobalamin (used in the Hcy to methionine reaction mentioned above), B12 takes a methyl group from one form of folate. In so doing, it produces a form of folate needed to make DNA. If there is no B12 available, this form of folate can become reduced (known as the methyl-folate trap) and DNA cannot be produced.74 However, if there is enough incoming folate through the diet, the body can use the new folate to produce DNA. In a B12 deficiency, the accumulation of 5-methyl THF (a form of folate) and folic acid can occur.99 This could explain some of the high folate levels found in vegans.
Folate and DNA play a critical role in rapidly producing cells. For example, the production of red blood cells involves dividing large, inactive cells into smaller, active cells. Because of the methyl-folate trap, people with a B12 deficiency can sometimes have large red blood cells. People with a folate deficiency can also have large red blood cells. This problem is known as megaloblastic anemia or macrocytic anemia (depending on how the large red blood cells are measured). Megaloblastic anemia is measured by determining the quantity of large red blood cells. Macrocytic anemia is determined by the average volume of the red blood cells, known as mean corpuscular volume (MCV). In B12 deficiency, there may be high enough folate levels to allow red blood cell division to continue, preventing macrocytic anemia.
To add insult to injury, an iron deficiency (which results in small red blood cells) can counteract the large red blood cells making it appear as though the blood cells are normal in the face of multiple nutritional deficiencies.46
B12 helps transport and store folate in the cells. When serum B12 is low, folate is unable to be stored in the cells and it starts to accumulate in the serum, increasing serum folate levels.39
Intestinal cells are also rapidly dying and being replaced. Ironically, a B12 deficiency can make itself worse because it can prevent the production of the intestinal cells needed to absorb B12.
Methylmalonic Acid (MMA)
There is one metabolic pathway in which B12 is the only co-enzyme: the conversion of one molecule, methylmalonyl-CoA, to another molecule, Succinyl-CoA. When B12 is not available, methylmalonyl-CoA levels increase. Because it is toxic, methylmalonyl-CoA is converted to methylmalonic acid (MMA) which then accumulates in the blood and urine. Since this reaction only requires B12 as a co-enzyme, MMA levels are excellent indicators of B12 status. Rare genetic defects can also cause high MMA levels.
Measuring B12 in the Body and in Food
Traditionally, B12 has been measured by "feeding" B12 to certain bacteria and seeing if those bacteria are able to thrive on it. This is known as microbiological assay. However, many bacteria used in these assays also thrive on noncobalamin corrinoids.48 B12 has also been measured through radioassay (which has other names, such as competitive binding assay) by seeing whether it binds to R protein. It is now known that R protein can bind to noncobalamin corrinoids. So, these methods that were once thought to measure only B12 are now known to measure B12 analogues also. Despite this, some laboratories continue to rely on these methods to measure the B12 content of foods and the body (although it is becoming less common). Radioassays using IF as the binding agent have proven more accurate because IF does not measure noncobalamin corrinoids. However, problems have also been found even when using IF radioassays (as will be discussed in more detail later). IF from pigs is normally used for these radioassays, and there could be a slight possibility that pig IF is selective for B12 analogues that human IF would not select.
Determining B12 vs. Folate Deficiency: dU Suppression Test
To determine whether someone is deficient in B12, folate, or both, a deoxyuridine (dU) suppression test can be performed. To make DNA, cells require a molecule called deoxythymidine. If cells are given dU, they must convert dU into deoxythymidine before they can use it to make DNA. Cells with slowed DNA synthesis can be fed dU in combination with B12 or folate, or both. The deficient vitamin will produce the most noticeable increase in DNA synthesis.46
Stages of B12 Deficiency
von Schenck et al.103 (1997) reported that the average adult stores 3000 µg B12, while losing only about 3 µg/day. Herbert48 (1994) reports that when one stops eating B12, they pass through 4 stages of B12 deficiency as follows:
Depletion Stages:
Stage 1 Serum depletion: Shown by low amounts of B12 on TC2; also can be measured by lower than normal B12 in red blood cells since only young red blood cells contain B12.
Stage 2 Cell depletion: Shown by low TC1+TC3 and low B12 in red blood cells; low total serum B12 levels (i.e., TC1 + TC2 + TC3) are a relatively late indicator.
Deficiency Stages:
Stage 3 Biochemical deficiency: Shown by slowed DNA synthesis, elevated serum Hcy and MMA.
Stage 4 Clinical deficiency: Megaloblastic anemia, nerve damage, etc.
Because liver cells store more B12 than bone marrow or nervous tissue, liver cells can be in Stage 2 while bone marrow (which makes red blood cells) and nervous tissue is already in Stage 3 and possibly even 4.48
Herbert says that vegans can stabilize in Stage 2 for years because depleted stores trigger increased absorption, making more efficient the absorption of the trace amount of B12 from bacterial contamination in the small intestine and B12 secreted in the bile. Stage 2 will move on to Stage 3 much sooner in people with B12 absorption problems. The slight negative balance of vegans will eventually deplete stores and Stage 3 will be entered.
Symptoms
Neurological Symptoms
Neurological symptoms are the biggest worry in B12 deficiency because they can be irreversible. However, if caught early enough, they can be reversed in many cases.
One theory of why nerve problems occur in B12 deficiency is from a lack of methionine (from B12 not converting Hcy back into methionine) which creates a lack of S-adenosylmethionine (SAM).35 SAM is required for the production of phosphatidylcholine36 which is part of the myelin (the fatty material that insulates many nerves). Phosphatidylcholine improves nerve transmission.36
Another theory is that the altered nerve function of B12 deficiency is possibly due to the body's inability to convert methylmalonyl-CoA (a 3 carbon molecule) to succinyl-CoA (a 4 carbon molecule). This inability results in a build up of propionyl-CoA (a 3 carbon molecule). Fatty acids are normally made by adding 2 carbons at a time to an even numbered carbon molecule. In an overabundance of 3 carbon molecules, large amounts of unusual 15-carbon and 17-carbon fatty acids could be produced and incorporated into nerve sheets, causing altered nerve function.103
Early Signs of B12 Deficiency
According to Crane et al.18 (1994, USA), the usual vegan patient has no clinical symptoms or signs of inadequate B12. Early manifestations are unusual fatigue, faulty digestion (no appetite or nausea) and loss of menstruation. Other symptoms are nervousness, numbness and tingling of the hands and feet, mild depression, striking behavioral changes, paranoia, hyperactive reflexes, fever of unknown origin,18 frequent upper respiratory infections,19 impotence, impaired memory,49 infertility,55 sore tongue, and diarrhea.60
6 Ways to Get B12 Deficiency
Herbert48 (1994) reports the ways to get B12 deficiency as follows:
Inadequate dietary intake.
Inadequate absorption:
Loss of IF: This is genetically predetermined and age-dependent (sometimes as early as 45 yrs). It is the most common cause of B12 deficiency in nonvegetarians. Because of their lower stores, vegans will more rapidly express a genetically predisposed B12 deficiency.
Loss of gastric acid and/or protein digesting enzymes which break the protein-B12 bonds in food. According to Ho49 (1999), this can be caused by stomach surgery, atrophy or inflammation of the stomach, medications that suppress acid secretion, or a stomach infection by H. pylori or anaerobic bacteria.
Pancreatic disease that reduces free calcium in the ileum. This can be improved by giving calcium and/or bicarbonate.
Unhealthy ileum.
The oral, diabetic drug metformin ties up free calcium in the intestines producing B12 malabsorption.
Autoimmunity to IF: Circulating antibodies to IF indicate eventual PA if not treated. A chronic B12 deficiency damages immune function and the antibodies may disappear as B12 deficiency progresses.
Inadequate utilization: Defects in B12 enzymes, transport proteins, or storage proteins.
Increased requirement during pregnancy or hyperthyroidism.
Increased excretion caused by alcoholism.
Increased destruction: Megadoses of vitamin C can create free radicals which can damage B12 and IF. Nitrous oxide anesthesia in people who are low in B12 (nitrous oxide can change the cobalt atom of B1235).
Others causes of B12 deficiency are tapeworms,35 hypothyroidism (this relationship is possibly autoimmune),53 Giardia lamblia infection, chronic use of gastric acid secretion suppressors, or drugs affecting absorption (including cimetidine, metformin, potassium chloride, and cholestyramine).98
Alcoholism & AIDS
B12 deficiency can be masked by alcoholism because excess B12 is released into the blood from the damaged liver.48 AIDS can cause B12 deficiency as shown through macrocytic anemia and neurological problems, but without elevated Hcy levels.
Correcting & Preventing Deficiency
According to Herbert, measuring MMA or Hcy levels will not prevent deficiency. Instead, depletion must be measured by way of low B12-TC2 levels. Depletion precedes deficiency by months to years in over 95% of the cases.48 It only takes .1 µg of B12 to start reversing deficiency symptoms, though the response can be improved with more.47
Small Amounts of Animal Products Not Enough to Restore Optimal B12 Status
van Dusseldorp et al.102 (1999, Netherlands) investigated whether moderate consumption of animal products is sufficient for achieving normal B12 function in 73 adolescents (in good health) who had received a macrobiotic diet until 6 years of age and had then switched to a LOV or omnivorous diet. 94 nonvegetarian (NV) adolescents from birth were used as a reference. In macrobiotics, dairy products supplied on average 0.95 µg B12/day. Additionally, they consumed fish, meat, or chicken 2-3 times/week. Serum B12 was significantly lower and MMA, folate, and MCV were significantly higher in macrobiotics. Of macrobiotics, 21% had abnormal MMA levels, 10% had abnormal total homocysteine, and 15% had abnormal MCV (> 89 fl). Authors concluded that a substantial number of the formerly strict macrobiotic adolescents still had impaired B12 function. Thus, moderate consumption of animal products is not sufficient for restoring normal B12 status in adolescents with inadequate B12 intake during the early years of life. They might need B12 intakes higher than recommended to obtain normal B12 status.
Supplements, Fortified Foods, & Animal Products
Tucker et al.98 (2000, USA) examined the B12 status of 2999 subjects in the Framingham Offspring Study. Average sB12 for the entire group was 473 pg/ml. 39% of subjects had sB12 < 348, 17% < 250, and 9% < 200 pg/ml. There was a significant trend towards lower B12 levels with increased age. In contrast with previous reports, sB12 was associated with B12 intake. Supplement users were significantly less likely than non-supplement users to have B12 < 250 pg/ml (8% vs. 20%). Among non-supplement users, those who consumed fortified cereal over 4 times/week were significantly less likely to have B12 < 250 pg/ml. Those in the highest one-third vs. lowest one-third of dairy intake were significantly less likely to have B12 < 250 pg/ml (13% vs. 24%). There was no difference between meat intake groups. Tucker et al. concluded that the use of supplements, fortified cereal, and milk appears to protect against lower SB12. This could be because additional sources may have increased total intake and/or some sources may be more bioavailable. Meat, eggs, and seafood may not be as protective as dairy because cooking may destroy B12. There was a clear and strong increase in B12 levels with greater B12 intake up to about 10 µg/day.
Supplements
In 1988, Herbert cautioned that large amounts of B12 may eventually be found to be harmful.47 However, Hathcock & Troendle40 (1991) point out that there appears to be little or no question that B12 intakes of 500-1000 µg/day are safe. The cobalt and the cyanide contribution of 1000 µg/day of cyanocobalamin are toxicologically insignificant.40
Crane et al.18 (1994, USA) noted that tablets of one vitamin company dissolved slowly in water and acid. They then conducted a study to see if vegan patients who had not responded to oral B12 tablets could improve their B12 response by chewing the tablets. 7 participants chewed the tablets of 100 µg (once a week for 6 weeks) and their average levels went from 116 to 291 pg/ml. Of the 9 who didn't chew, levels increased from 123 to 139 pg/ml. These 9 then chewed 500 µg/day for 10 days and their levels rose to normal with a final average of 524 ± 235 pg/ml. 3 participants could not raise their levels orally and required injections to maintain sB12 above 300 pg/ml. Crane et al. recommend that vegans chew, or let dissolve in the mouth, a 100-500 µg B12 tablet once a week or more often.
Supplements should not be left in the light as prolonged light causes irreversible destruction of cyanocobalamin.89
Oral B12 Even for People with Malabsorption
Intramuscular injections (IMI) of B12 are the typical way to treat B12 deficiency. The injections can be painful and expensive. Kuzminski et al.56 (1998, USA) studied 33 newly diagnosed B12 deficient patients (almost all had malabsorption) to receive cyanocobalamin as either 1 mg intramuscularly on days 1, 3, 7, 10, 14, 21, 30, 60, and 90, or 2000 µg orally on a daily basis for 120 days. Results were:
oral
injection
Pretreatment:
serum B12 (pg/ml)
93
95
serum MMA (nmol/l)
3850
3630
serum homocysteine (µmol/l)
37.2
40.0
After 4 months:
serum B12
1005
325
serum MMA
169
265
serum homocysteine
10.6
12.2
The higher serum B12 and lower serum MMA levels at 4 months in the oral group were significantly higher than the injected group. Kuzminski et al. concluded that 2000 µg/day of oral cyanocobalamin was as effective as 1000 µg injected intramuscularly each month, and may be superior.
Delpre & Stark28 (1999, Israel) studied patients with B12 deficiency to see if B12 can be absorbed by holding a tablet under the tongue, known as sublingual. 5 patients had PA, 7 were vegetarians, and 2 had Crohn's disease (which can prevent the absorption of B12 in the ileum). The patients held two 1000 µg B12 tablets (equaling 2000 µg/day) under their tongues for 30 minutes until completely dissolved. This was done for 7 to 12 days. Average sB12 levels went from 127.9 ± 42.6 to 515.7 ± 235 pg/ml. All the patients' sB12 normalized. There were no side effects and all patients preferred this to IMI. Bodamer & Scaglia10 (1999) pointed out that in addition to the B12 absorption problems listed in Delpre & Stark's28 study, some people have inborn errors in B12 metabolism and people such as this might not benefit from oral cyanocobalamin therapy. Such people will need more careful medical attention.
Norberg75 (1999, Sweden) points out that investigations in the 1950s and 60s showed that oral B12 is absorbed by an alternative pathway not dependent on IF or an intact distal ileum. Approximately 1% of an oral dose in the range of 200-2000 µg/day was absorbed by the alternative pathway. Based on this research, oral treatment, rather than IMI, has been in use for the majority of B12 deficiency cases in Sweden since the early 70s.
In a literature review encouraging the use of oral cobalamin therapy over injections for patients with PA, Lederle58 (1991) reported that Swedish investigators recommend 2000 µg B12 twice a day or injections to replenish B12 stores of patients with PA. After initial therapy, doses of 1000 µg/day appear to be enough.58
Please note that the above doses are for people with B12 malabsorption. People without malabsorption problems or current B12 deficiency should only need doses much closer to the RDA.
