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High vitamin B12 levels

by Georges Mouton MD

Physicians sometimes encounter cases of high vitamin B12 levels.These cases are often assumed to be due to an overdose resulting from excessive supplementation, consumption of cobalamin-fortified energy drinks or from intramuscular injectionsor oral supplements prescribed by a health care professional. In most cases, no action is taken upon discovering the anomaly.

Dr. Georges MoutonHowever, careful enquiry very frequently demonstrates that no external human intervention explains the finding of elevated vitamin B12 levels. Thus the answer to the puzzle must be found within the body’s internal metabolic processes. It is clear that the amount of vitamin B12 excreted in human faeces does not only correspond to what was not absorbed in the ileum (the last of the three sections of the small intestine), but also reflects the production of significant amounts of cobalamin by the colonic microflora [1].

Intestinal Vitamin B Synthesis
The fact that intestinal micro-organisms produce significant amounts of B vitamins is fully accepted and has been published in peer-reviewed international medical journals [2,3]. Intestinal bacterial B vitamin biosynthesis involves at least vitamin B1 (thiamine) [4], vitamin B2 (riboflavin) [5], vitamin B5 (pantothenic acid) [6], vitamin B8 (biotin) [6, 7], vitamin B9 (folic acid) [8,9],and vitamin B12 (cobalamin) [1]. As a matter of fact, bacteria obtained from dairy and belonging to the genus Propionibacterium (also abundant in the human intestinal microflora) are extensively used for the biological production of cobalamin [10].

Concerning vitamin B8, also called biotin, “it has long been recognized that the normal microflora of the large intestine synthesize considerable amounts of biotin” [6]. In fact, several studies have shown that the colon is capable of absorbing free biotin and HM Said has shown, for the first time in 1998, the functional existence of a specialized carrier-mediated system for biotin uptake in colonic epithelial cells [7]. “In addition, the uptake process is shared by another water-soluble vitamin, pantothenic acid, (…) which is also synthesized by the normal microflora of the large intestine”, as biotin inhibited the uptake of vitamin B5 and vice versa [6].

The specialized vitamin B transporter has been cloned in the rabbit intestine by another team in 1999 [11] and named the sodium-dependent multivitamin transporter (SMVT). This transporter is also highly expressed in human enterocytes (cells found in the internal lining of the intestines) [11,12], where it serves to take up not only pantothenate and biotin, but also lipoate (the ion from lipoic acid) [11].

Half a century ago, vitamin B2 (also called riboflavin) was known to be synthesized by intestinal bacteria and the amount provided by this source appears to become significantly higher when adhering to a vegetarian diet [13]. Interestingly, as he did for other water-soluble vitamins B, HM Said demonstrated in 2000 “for the first time, the existence of a specialized carrier-mediated mechanism for riboflavin uptake in an in vitro cellular model of human colonocytes” (cells found in the lining of the colon) [5]. Once again in 2001, HM Said showed that a model of human-derived colonic epithelial cells possesses a specific carrier-mediated system for thiamine (vitamin B1) uptake [4]. “It is suggested that bacterially synthesized thiamine in the large intestine may contribute to thiamine nutrition of the host, especially towards (…) the local colonocytes” [4].

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Certain bacterial species present in the rat colon are also capable of de novo synthesis of vitamin B9, better known as folic acid [8]. As clearly evidenced by the use of tritiated (marked with radioactive hydrogen) para-aminobenzoic acid (3H PABA), the experimental “data provide direct evidence that some of the folate synthesized by the microflora in the rat large intestine is incorporated into the tissue folate of the host” [8].

More recently, the same methodology has been utilized with humans in order to determine whether folate synthesized by bacteria in the small intestine rather than in the colon is assimilated by the human host [9]. Indeed, the perfusion of tritiated PABA, a classic precursor substrate for the bacterial folate synthesis, led to the identification of bacterially synthesized (as marked) folates aspirated from in the small intestine. Subsequently, tritiated 5-methyltetrahydrofolate, a major metabolite of folate, was isolated from the human host urine, demonstrating that the human host did absorb and consequently metabolized these bacterially synthesized folates [9].

Coming back to cobalamin, it has been shown, already in 1980, that “at least two groups of organisms in the small bowel, Pseudomonas and Klebsiella sp., may synthesize significant amounts of the vitamin [B12]” [1]. Obviously, the two accepted dogma of vitamin B metabolism in the digestive tract don’t seem to correspond to reality: several compounds (vitamins B1, B2, B5, B8 and B9) supposedly absorbed by the small intestine may be assimilated by the colonocytes, while several compounds (vitamins B9 and B12) supposedly synthesized by colonic bacteria may actually be generated in the small intestine! Unfortunately, if we wanted to explain the high vitamin B12 blood levels by some colonic absorption, we must underline that absolutely nothing has been published about this and what seems true for other vitamins B would not be so for cobalamin.

Consequently, we should rather focus on the possibility that bacterially-produced vitamin B12 is absorbed in the small intestine, where most of the assimilation process of other B vitamins takes place. Two different specific proteins ensure the uptake of thiamine (vitamin B1) in the enterocytes of the proximal small intestine and are structurally close to a specific folic acid carrier [14]. Indeed, the intestinal folate (vitamin B9) absorption process occurs via a specialized mechanism that involves the reduced folate carrier (RFC) in the jejunum (the middle part of the small intestine) [15, 16]. We have already mentioned earlier the existence, in the proximal small intestinal enterocytes, of a sodium-dependent multivitamin transporter (SMVT) taking care of biotin (vitamin B8) and of pantothenic acid (vitamin B5). The involvement of a specialized carrier-mediated mechanism for pyridoxine (vitamin B6) by the intestinal epithelial cells has been demonstrated for the first time in 2003 [17]. Finally, a specialized carrier for niacin (vitamin B3) has been uncovered very recently, the article only being published in July 2005 [18].

In contrast to all the other B vitamins, cobalamin is not absorbed in the jejunum or in the proximal (first part of) ileum as they are, but only in the terminal ileum from a quite complex absorption process. This makes absorption very sensitive to diseases affecting specifically, or more frequently, this portion of the digestive tract such as Crohn’s disease.

Vitamin B12 Absorption
The term vitamin B12 or cobalamin actually refers to four different forms found in the diet and mostly bound to proteins: methylcobalamin, hydroxocobalamin, cyanocobalamin, and deoxyadenosylcobalamin.

The absorption process involves five steps:

  1. The cobalamins are released from their protein complexes through the action of acid or pepsin in the stomach.
  2. They bind to R proteins – cobalamin-binding glycoproteins secreted in saliva and in gastric juice.
  3. The cobalamin-protein complexes must then be degraded by pancreatic proteases. This important step may be jeopardized in case of pancreatic insufficiency [19].
  4. The free cobalamin combines in the duodenum with another glycoprotein called intrinsic factor which is secreted by the stomach parietal (oxyntic) cells; this glycoprotein dimerises and each part of the dimer binds one molecule of cobalamin, making the complex resistant to digestion [20]. The formation of the cobalamin-intrinsic factor complex appears indispensable for the vitamin to be absorbed in the terminal ileum via an active transport system [19].
  5. The brush border membrane of the terminal ileum enterocytes contains a specific receptor for the dimeric complex and its importance in the process is shown by a congenital vitamin B12 malabsorption syndrome due to a defect in this receptor. The absorption is hampered by an abnormally low ileum pH, which may occur in some diseases such as the Zollinger-Ellison syndrome.

The problem with vitamin B12 absorption lies in the small safety margin between the dietary requirements for the vitamin and the maximal absorptive capacity of the five-step process outlined above. Cobalamins can also be absorbed passively, but the passive pathway only accounts for 1 or 2 % of the ingested vitamin, explaining the development of anemia when one of the five steps is not functioning properly [20]. The most frequent cause for vitamin B12 malabsorption is represented by the lack of intrinsic factor [19], which may be explained by a genetic defect, an auto-immune condition (auto-antibodies targeting either the parietal cells or the intrinsic factor itself), or a surgical gastrectomy (removal of part or all of the stomach). But further problems can occur at the level of the blood carriers, transcobalamin I and transcobalamin II, which may be impaired [21].

Now, supposing that all these steps leading to an effective absorption of vitamin B12 function adequately, then the presence in significant amounts of bacteria producing cobalamin in the terminal ileum would explain - at least theoretically - a sharp increase in absorption and lead to higher blood levels of this vitamin. If we consider some specific circumstances in the above mentioned study about folate absorption [9], we might discover the mechanism which could lead to an excessive absorption of cobalamin and to an elevation of blood levels.

Case Study Involving High Vitamin B-12 Levels
We present a second case study concerning a thirty-year old woman (in 1999) whose blood parameters were monitored for unrelated matters but strikingly presented repetitive high vitamin B12 levels without any related supplementation neither from the vitamin itself, nor through vitamin B complexes / multivitamin formulas, thus ruling out a toxic overdose.

The original data from our records follows [NOTE: All results for vitamin B12 are expressed in pg/ml and the normal range provided by the Belgian laboratory is 200pg/ml to 900pg/ml, even if the lower limit could be considered as too low to be compatible with optimal health].

The five first measurements, from February 1999 to April 2000, were quite consistently fluctuating around 2500pg/ml (respectively 2796pg/ml on 6/2/99, 2355pg/ml on 19/4/99,2572pg/ml on 30/7/99, 2697pg/ml on 7/3/2000 and 2325pg/ml on 17/4/00), which is much too high! At the time, the patient’s blood had to be monitored in relation to a drug-based anti-epileptic treatment. However, the young woman was not complaining about her digestive system, even if she occasionally mentioned some severe but transitory abdominal cramps.

Her digestive problems started during the summer season in 2000, with IBS (irritable bowel syndrome) like symptoms, bloating, diarrhoea and excruciating pain in the belly. She was examined thoroughly and the gastroenterologist initially suspected Crohn’s disease due to the presence of mucosal ulcerations in the proximal small intestine. During that period of major clinical deterioration, blood vitamin B12 level increased even further as seen from two measurements performed on 25/08/00 (3220pg/ml) and on 28/11/00 (3221pg/ml). Then, she refused to take the corticoids prescribed by the specialist and went on a natural treatment based on diet modifications (exclusion of high IgG foods, in her case: dairy products, beef, bananas and black pepper), supplements (according to her biological results in blood and in 24-hour urine), antimicrobial herbs (such as grapefruit seed extracts) and probiotics.

She didn’t improve dramatically, but slowly started to complain less within a few weeks, then was feeling slightly better in March 2001 and significantly better when she came back five months later, in August 2001. Very interestingly, vitamin B12 blood levels started to decrease to 2740pg/ml on 24/3/01 and then down to 2132pg/ml on 22/08/01. In fact, the last result provided her lowest blood value since the beginning of the study. In September 2001, we then asked the gastroenterologist to perform a new endoscopy, in order to dismiss the diagnosis of Crohn’s disease and make sure that we were not harming her by not giving the prescribed drugs. The digestive exploration was then considered as normal, besides some “non specific mucosal inflammation”.

So the case was much less worrying and it took about seven months before she consulted again, in March 2003. She was symptom-free, finally expressing a much better digestive capacity since she was on this diet, even though she hadn’t renewed her supplements for a while. The cramps had disappeared and her blood reading for the vitamin B12 was 1001pg/ml on 26/3/02, almost back to the normal range. She definitely reached and stayed within the normal range on further checks with 726pg/ml on 31/08/02, 677npg/ml on 21/5/03 and finally 516pg/ml on 15/5/04. The last time, she was still symptom-free, but also dairy-free. She might have to consider taking vitamin B12 supplements one day in the future, but that’s another story…

About the author: Georges Mouton MD is a medical doctor specializing in functional medicine with practices in Brussels, London and Madrid. His website can be found at

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