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by Jack Norris, RD &
Ginny Messina, MPH, RD
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Coenzyme Functions


B12 Functions

In the cells of mammals (1), there are two different co-enzyme forms of vitamin B12 (2):
  • Methylcobalamin
    • Used by the enzyme methionine synthase to turn homocysteine (HCY) into methionine. Methionine is further converted to the important methyl donor, S-adenosylmethionine (SAM, aka SAMe) (more info at S-adenosylmethionine (SAMe)).
  • 5'-deoxyadenosylcobalamin
    • Used by the enzyme methylmalonyl-CoA mutase to convert methylmalonyl-CoA to succinyl-CoA.
    • Used by the enzyme leucine aminomutase to convert B-leucine into L-leucine and vice-versa.

Homocysteine Clearance

Methionine is an essential amino acid provided by the diet. Some methionine is turned into homocysteine. Homocysteine appears to be a nerve and vessel toxin, promoting cardiovascular disease (CVD) at elevated levels. HCY is thought to cause CVD by way of oxidative and vessel wall damage (3). The body normally turns HCY into other molecules, one of which is back into methionine. If this pathway is blocked, HCY levels increase. Methylcobalamin (B12) is needed by methionine synthase to convert HCY into methionine. Thus, if someone is B12 deficient, HCY levels will increase.

Anemia, DNA, and Folate

Traditionally, B12 deficiency, normally resulting from the inablity to absorb B12, was diagnosed by finding abnormally large red blood cells. This sort of anemia has two names:

  • Macrocytic anemia - when the average volume of the red blood cells, known as the Mean Corpuscular Volume (MCV), is larger than normal
  • Megaloblastic anemia - when abnormally large red blood cells are observed under a microscope

The vitamin folate (aka folic acid) affects the anemia symptoms of B12 deficiency. Folate is needed to turn uracil into thymidine, an essential building block of DNA (4). DNA is needed for new red blood cell production and division. B12 is involved in this process because in creating methylcobalamin (used in the HCY to methionine reaction), B12 produces a form of folate needed to make DNA. If there is no B12 available, this form of folate can become depleted (known as the methyl-folate trap) and DNA production slows (5). See Methionine-Homocysteine-Folate-B12 Cycle for an illustration of this pathway.

Only RNA is needed to produce the hemoglobin found in the red blood cells. Unlike DNA, RNA does not require thymidine. Therefore, if there is not adequate folate, the new red blood cells (which start out as large cells called reticulocytes) divide slowly, as they are dependent on DNA for division. At the same time, their hemoglobin is only dependent on RNA and it is produced at a normal rate. This causes large red blood cells known as macrocytes (4, 6). If enough of these macrocytes accumulate, the result is macrocytic anemia.

If there are large amounts of incoming folate from the diet, the body does not need to rely on regeneration of folate from the B12 cycle. Instead, it can use the extra dietary folate to produce DNA, thus preventing macrocytic anemia (see Methionine-Homocysteine-Folate-B12 Cycle, bottom right-hand portion). This is why high intakes of folate are said to "mask" a B12 deficiency.

To add insult to injury, an iron deficiency (which results in small red blood cells from inadequate hemoglobin synthesis) can counteract the macrocytic cells, making it appear as though the blood cells are normal in the face of multiple nutritional deficiencies (7).

Intestinal cells are also rapidly dying and being replaced using DNA. A B12 deficiency can make itself worse because it can prevent the production of the intestinal cells needed to absorb B12.

Lack of Anemia Does Not Mean B12 Status Is Healthy

Table 1. B12 Levels in Neurological Patients Without Macrocytic Anemia (pg/ml)
Number of Patients Serum B12
2 > 200
16 100-200
22 < 100

Traditionally, the existence of macrocytic anemia was relied on to indicate a B12 deficiency. However, neurological disorders due to B12 deficiency commonly occur in the absence of a macrocytic anemia.

Lindenbaum et al. (8) (1988, USA) examined 141 cases of neurological problems due to B12 deficiency. 40 (28%) had no macrocytic anemia (iron deficiency may have contributed to a lack in 6 patients, and folate therapy could account for 2 others). These 40 had very high serum MMA levels (range: .76-187 µmol/l, 78% > 2 µmol/l) and homocysteine levels (23-289 µmol/l, 45% > 100 µmol/l). Characteristic features of patients with B12 deficiency but without macrocytic anemia included: sensory loss, inability to move muscles smoothly (ataxia), dementia, and psychiatric disorders. They also had borderline (and sometimes normal) B12 levels (see Table 1). One patient died during the first week of treatment, but the other 39 benefited from B12 therapy. Some patients had residual abnormalities after years of treatment.

In a 2011 study from Korea, among 35 patients with vitamin B12 deficiency, most of whom had neurological symptoms, none had anemia (12).

Methylmalonic Acid (MMA)

The second coenzyme form of B12, adenosylcobalamin, takes part in the conversion of methylmalonyl-CoA to succinyl-CoA. When B12 is not available, methylmalonyl-CoA levels increase. Methylmalonyl-CoA is then converted to methylmalonic acid (MMA) which then accumulates in the blood and urine. Since B12 is the only coenzyme required in this pathway, MMA levels are the best indicators of a B12 deficiency.

High MMA levels can also (but rarely) be caused by genetic defects, kidney failure, low blood volume, gut bacteria changes, pregnancy, and thyroid disease (9, 10).

A normal serum MMA level is .07 to .27 µmol/l. Above, under Lack of Anemia Does Not Mean B12 Status is Healthy, we saw that patients with serum MMA levels in the range of .76 to 187 µmol/l had neurological problems. What about the range between .27 and .76 µmol/l?

In a study of non-vegetarian, older adults with slightly elevated methylmalonic acid (MMA) levels (.29-3.6 µmol/l), higher sMMA levels did not predict neurological problems (10). However, these individuals were not compared to people with normal sMMA levels. Because there was no control group, we cannot say that people with slightly elevated sMMA are not at risk for neurological problems. We can only suggest that increasing sMMA from .29 to 3.6 may not do any further, measurable neurological harm.

In another study (11), older adults with slightly elevated MMA levels (.27 - 2.00 µmol/l) were treated with cyanocobalamin injections: MMA levels decreased 66% and homocysteine levels decreased 23%. Patients with MMA in the range of .60-2.00 µmol/l had neurological improvements after B12 therapy.

These studies indicate:

  • Slightly increasing sMMA levels from .29 to .60 µmol/l may not increase one's risk for neurological problems.
  • People with MMA levels above .27 µmol/l may have elevated homocysteine which can benefit from B12 therapy.
  • People with sMMA levels above .60 µmol/l may have neurological problems that can benefit from B12 therapy.


1. Scalabrino G. Subacute combined degeneration one century later. The neurotrophic action of cobalamin (vitamin B12) revisited. J Neuropathol Exp Neurol. 2001 Feb;60(2):109-20.

2. Seetharam B, Li N. Transcobalamin II and its cell surface receptor. Vitam Horm. 2000;59:337-66.

3. Hackam DG, Peterson JC, Spence JD. What level of plasma homocyst(e)ine should be treated? Effects of vitamin therapy on progression of carotid atherosclerosis in patients with homocyst(e)ine levels above and below 14 micromol/L. Am J Hypertens. 2000 Jan;13(1 Pt 1):105-10.

4. Guyton AC, Hall JE. Textbook of Medical Physiology, 9th ed. Philadelphia, PA: W.B. Saunders, Co: 1996. p. 845-7.

5. Murray RK, Granner DK, Mayes PA, Rodwell VW. Harper's Biochemistry, 24th ed. Appleton & Lange, 1996.

6. Groff J, Gropper S. Advanced Nutrition and Human Metabolism, 3rd ed. Wadsworth: 2000.

7. Herbert V. The 1986 Herman Award Lecture. Nutrition science as a continually unfolding story: the folate and vitamin B-12 paradigm. Am J Clin Nutr. 1987;46:387-402.

8. Lindenbaum J, Healton EB, Savage DG, Brust JC, Garrett TJ, Podell ER, Marcell PD, Stabler SP, Allen RH. Neuropsychiatric disorders caused by cobalamin deficiency in the absence of anemia or macrocytosis. N Engl J Med. 1988 Jun 30;318(26):1720-8.   |   Link

9. Minet JC, Bisse E, Aebischer CP, Beil A, Wieland H, Lutschg J. Assessment of vitamin B-12, folate, and vitamin B-6 status and relation to sulfur amino acid metabolism in neonates. Am J Clin Nutr. 2000 Sep;72(3):751-7.

10. Hvas AM, Ellegaard J, Nexo E. Increased plasma methylmalonic acid level does not predict clinical manifestations of vitamin B12 deficiency. Arch Intern Med. 2001 Jun 25;161(12):1534-41.

11. Hvas AM, Ellegaard J, Nexo E. Vitamin B12 treatment normalizes metabolic markers but has limited clinical effect: a randomized placebo-controlled study. Clin Chem. 2001 Aug;47(8):1396-404.

12. Kim HI, Hyung WJ, Song KJ, Choi SH, Kim CB, Noh SH. Oral vitamin B12 replacement: an effective treatment for vitamin B12 deficiency after total gastrectomy in gastric cancer patients. Ann Surg Oncol. 2011 Dec;18(13):3711-7. | link