RD Resources for Professionals:
Vitamin B12 in Vegetarian Diets
What about Vitamin B12?
Vitamin B12, originally discovered as an anti-pernicious
factor, has been named cobalamin because it contains
the rare element cobalt in its chemical structure.1
Methylcobalamin and adenosylcobalamin are two
biologically active forms of B12. Other forms, such
as hydroxycobalamin (or aquacobalamin) and
cyanocobalamin, must be metabolized to either of the
two active forms in order to be utilized in human cells.2, 3
Function
Functions driven by Vitamin B12 include the activatation
of folate, which is essential in DNA synthesis and thus in
synthesis of all new cells in the body. B12 plays a critical
role in erythropoiesis. Since methionine is needed for
the synthesis of myelin, a coating of the nerve pathways,
B12 deficiency may result in inadequate myelin
synthesis, maintenance and repair and may impair nerve
transmission.4 B12 plays a critical role in erythropoiesis.
Digestion and Absorption
The process of digestion and absorption of B12 takes
place in a few stages and requires adequate synthesis
of hydrochloric acid, proteases and an intrinsic factor
(IF). 3-4 B12 is released from dietary proteins by pepsin,
an enzyme secreted in the stomach, and activated
by hydrochloric acid. Once released, B12 binds with
R-proteins secreted in the saliva. In the small intestine,
pancreatic proteases digest the R-proteins, making it
possible for the IF to bind to the newly freed B12, forming
an IF-B12 (or IF-Cbl) complex.3, 4 B12 can be absorbed
into the bloodstream either via receptor-mediated
endocytosis in the distal ileum or, in the absence of IF, by
passive diffusion.3
Recommendation
The RDA for B12 for an adult, as issued by the
Institute of Medicine, is 2.4μg/day.4 The daily amount
that is actually needed to maintain an adequate
serum level that promotes erythropoiesis and other
hematological functions is considerably smaller, but
this recommendation assumes a 50% absorption rate
of B12 from the amount ingested with foods. The RDA
issued by the Institute of Medicine is consistent with
recommendations of other organizations including
the World Health Organization (1.9μg/day).5 Intake
of relatively high doses of B12 does not appear to be
associated with any detrimental health problems and
thus no UL has been established.4
Deficiency
Although B12 deficiency was once believed to
be rare and unlikely to develop, except in strict
vegetarians, studies conducted in the last few decades
have shown that this view has been based on the
incorrect assumption that it takes many years for the
deficiency to develop.6 Although B12 deficiency was
once believed to be rare and unlikely to develop,
except in strict vegetarians, studies conducted in the
last few decades have shown that B12 deficiencies
“are common in wealthier countries, partularly
among the elderly, and are most prevalent in poorer
populations around the world.” It is now believed that
megaloblastic anemia, a common symptom of B12
deficiency, is common among people of European and
African descents, as well as those living on the Indian
subcontinent, and populations of Central and South
RD Resources for Professionals: Vitamin B12 in Vegetarian Diets
America. In Asia, B12 deficiency is less prevalent except
in vegetarians.6, 7
B12 deficiency may develop as a result of malabsorption
(due to gastrointestinal condition), inadequate intake,
lack of synthesis of holotranscobalamin II (TCII), genetic
defect of methylmalonyl co-enzyme A mutase and
nitrous oxide poisoning.3, 4, 8 Malabsorption, the main
cause of deficiency, may have two primary causes:
inadequate or complete inability of IF synthesis and/
or inadequate synthesis of, or synthesis of weak,
hydrochloric acid, which is essential in activating
pepsinogen to pepsin in order to digest dietary proteins.3,
4, 9 Although malabsorption due to the above listed
conditions is most prevalent among the elderly, it
may also occur at any age among people who have
undergone gastrectomy, such as in cases of ulcer disease
and gastric bypass for obesity.10,11 Deficiency due to
inadequate intake has been reported in strict
vegetarians, lacto-ovo vegetarians, people following
macrobiotic diets and alcoholics.3,4
B12 deficiency symptoms can be grouped into several
categories including hematological, neurological
and psychiatric. Specific symptoms of B12 deficiency
in each of the above categories are listed in Table 1.
Deficiency of B12 is very often misdiagnosed. This is
because symptoms can mimic those of other health
conditions such as Alzheimer’s disease, spinal cord
compression, amyotrophic lateral sclerosis, diabetic
peripheral neuropathy, alcoholic peripheral neuropathy
or congestive heart failure. The classic B12 deficiency
symptoms include the synthesis of large, immature,
oblong-shaped erythrocytes (megaloblasts) and myelin
deterioration in both central and peripheral nervous
systems. Erythrocytes may appear normal in case of
either high intake of folate or iron deficiency anemia.
Megaloblastic anemia leads to symptoms such as
weakness, fatigue, lightheadedness, tachycardia,
angina and pale skin. The deterioration of myelin may
progress to axonal degeneration and even axonal
death, which leads to neuropathy.3, 4, 10, 11
Food Sources and Bioavailability of B12
B12 is only synthesized by microorganisms.3 It is not found
in foods of plant origin. It is, however, found in meats
and foods of animal origin including dairy products and
eggs. Some foods believed to be made exclusively from
plant foods (e.g. cereal, breads, pies and even cookies)
do contain very small amounts of B12 due to either
contamination during processing, the addition of small
amounts of animal-derived ingredients such as milk
solids, or fortification.12 However, the amount of B12
found in these foods (see Table 2), except for foods that
have been fortified, is negligible.
Milk contains between 0.3–0.4 μg/100g of B12. Boiling
milk can destroy 30–50% of B12 depending on the
duration of cooking. Pasteurization destroys 5–10% of
B12. B12 content of dairy products, such as cheese or
cottage cheese, falls between 20–60% that of milk.
The B12 content of whole egg is between 0.9–1.4
μg/100g, mostly found in the yolk. Bioavailability of
B12 from various preparations of eggs (e.g., scrambled,
boiled) ranges from less than 4% to just over 9%.13
Fermented soy products, such as tempeh and other plant
foods, do not contain biologically active forms of B12.3
Supplements and fortified foods
B12 is widely available in supplement form, mostly
as cyanocobalamin, in pharmacies and health
food stores throughout the United States. Other
forms of this vitamin, such as methylcobalamin and
hydroxycobalamin, can also be found. “Living” vitamin
supplements, made from plants, do not contain
biologically active B12. Using probiotics is not a reliable
way to prevent B12 deficiency.14
In the U.S., many foods are fortified with B12. These
foods include breakfast cereals, soymeat analogues,
and soymilk. These foods contain from less than the
adult RDA to more than 200% of the adult RDA for
B12. Additionally, some brands of nutritional yeast
are fortified with vitamin B12. It is important to read
labels as not all cereals, meat analogues, soymilks, and
nutritional yeast are fortified with B12, and the amount
of fortification can change.12
Bacteria-synthesized B12 in the small intestine
Although some bacteria in the small intestine produce
B12, this amount does not appear to be substantial
enough to maintain adequate B12 status in humans.3, 15
Algae as a source of B12
It was previously thought that algae, such as spirulina,
nori and kombu, were believed to contain B12. However,
studies not only showed that they almost exclusively
contain inactive analogues of B12, but that they may
RD Resources for Professionals: Vitamin B12 in Vegetarian Diets
Table 1. Clinical Manifestations of B12 Deficiency
Types
Clinical manifestations
Hematological
Megaloblastic anemia4 (large, immature RBC), Leukopenia20 (lower than the normal amount of
white blood cells), Thrombocytopenia20 (abnormally low number of platelets in the bloodstream)
Neurological
Demyelination4,21 and Neuropathy21 (tingling and numbness in legs, paralysis), Dementia22 (memory
loss, brain atrophy, white matter damage, cognitive deficits)
Psychiatric
Psychosis,23 Irritability, Personality changes,24 Mild memory impairment,24 Hallucinations,24
Depression25
Other
Fatigue,24 Bone loss,26,27 Failure to thrive28,29 (developmental regression), Diabetic retinopathy22
(microvascular damage), Cardiovascular disease25 (vascular damage), Birth defects30
interfere with absorption and metabolism of the active
B12 forms and thus may contribute to the development
of B12 deficiency.3, 14
B12 Status of Vegetarians
The estimation of the prevalence of B12 deficiency
among vegetarians varies widely from 12% to as much as
94%, depending upon several factors including deficiency
definition, assessment method of B12 status and type
of vegetarian diet.3, 4, 16, 17, 18, 19 Although it is theoretically
possible to consume adequate B12 regardless of the type
of vegetarian diet (either by consuming enough dairy
and/or eggs or fortified foods), vegetarians are frequently
diagnosed with a deficiency regardless of the type of
diet to which they adhere. As a result, all vegetarians
should be screened for deficiency.3, 4
Assessment of B12 Status
There are several B12 assessment techniques, such
as serum or plasma B12, MCV, tHcy, TCII and serum or
urinary methylmalonic acid (MMA). Of these tests, TCII
and MMA are the most accurate, while serum B12 and
MCV are believed to be unreliable.3, 4
Recommendations for Vegetarians
In order to prevent deficiency, vegetarians should
include a reliable B12 source such as fortified foods
or supplements. Because food fortification levels and
practices change and the vitamin b12 content of eggs
and dairy products is variable, the most reliable way for
vegetarians to meet recommendations for B12 is to use a
vitamin B12 supplement.
The following supplementation levels are suggested:
• All vegetarians, regardless of type, should
periodically be screened for B12 deficiency, using
either MMA or TCII assessment.
• All women considering pregnancy, and those
already pregnant, should take 250 μg/d* of a B12
supplement.3
• All vegans should take 250 μg/d* of a B12
supplement.3
• All lacto-ovo vegetarians should consider taking 250
μg/d* of B12 supplement a few times per week.
*This amount is about 100 times higher than the RDA due to the fact
that only about 1% of ingested B12 from supplements is absorbed.
References
1. Smith E. Purification of anti-pernicious anemia factors from liver.
Nature, 1948 (April 24);161:638-639
2. Watanabe F. Vitamin B12 Sources and Bioavailability. Exp Biol Med,
2007;232:1266–1274.
3. Herbert V. Vitamin B-12: plant sources, requirements, and assay. Am J
Clin Nutr, l988;48:852-858.
4. Institute of Medicine of the National Academies. Dietary Reference
Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin
B12, Pantothenic Acid, Biotin, and Choline. http://www.iom.edu/
Reports/2000/Dietary-Reference-Intakes-for-Thiamin-RiboflavinNiacin-Vitamin-B6-Folate-Vitamin-B12-Pantothenic-Acid-Biotin-andCholine.aspx, Accessed 04/10/2011.
5. World Health Organization. Requirements of vitamin A, iron, folate
and vitamin B12. Report of a Joint FAO/WHO Expert Consultation.
Rome, Food and Agriculture Organization of the United Nations, 1988
(FAO Food and Nutrition Series, No. 23).
6. Allen LH. How common is vitamin B12 deficiency? Am J Clin Nutr,
2009;89(suppl):693S–696S.
RD Resources for Professionals: Vitamin B12 in Vegetarian Diets
Table 2. Content of B12 in selected plant products*
Description
Weight (g)
Common measure
B12 content (µg)
28.35
1 bar
0.05
Cereals ready-to-eat, Low Fat 100% Natural Granola with Raisins
50
1/2 cup
0.05
Mushrooms, white, raw
70
1 cup
0.03
Bread, mixed-grain, toasted (includes whole-grain, 7-grain)
24
1 slice
0.02
Rolls, dinner, plain, commercially prepared
28
1 roll
0.02
Cookies, fig bars
16
1 cookie
0.01
Snacks, granola bars, soft, uncoated, raisin
Margarine, regular, unspecified oils, with salt added
14.1
1 Tbsp
0.01
Pie, apple, commercially prepared, enriched flour
117
1 piece
0.01
Bread, cracked-wheat
25
1 slice
0.01
1 cup
< 1.0 to 6.2
Plant Products Fortified with B12
Cereal (ready-to-eat)
Meat analogues
1 serving according to pkg
1.2 – 4.2
B12 fortified nutritional yeast
1 Tbsp
4
Soymilk
8 fl oz
< 1.0 to 3.0
Rice drink
8 fl oz
1.5
* Source: https://www.ars.usda.gov/SP2UserFiles/Place/12354500/Data/SR24/nutrlist/sr24w418.pdf
7. Stabler SP and Allen RH. Vitamin B12 deficiency as a world-wide
problem. Annu Rev Nutr, 2004;24:299–326.
8. National Institutes of Health. Genetic home reference, http://ghr.nlm.
nih.gov/gene/MUT
9. Sumner AE, Chin MM, Abrahm JL, Berry JT, Gracely EG, Allen RH,
Stabler SP. Elevated Methylmalonic Acid and Total Homocysteine
Levels Show High Prevalence of Vitamin B12 Deficiency after Gastric
Surgery. Ann Intern Med, 1996;124:469-476.
10.Malizia RW, Baumann BM, Chansky ME, Kirchhoff MA. Ambulatory
dysfunction due to pernicious anemia. The Journal of Emergency
Medicine, 2010;38:302-307.
11.Stabler SP, Allen RH, Savage DG, Lindenbaum J. Clinical spectrumand
diagnosis of cobalamin deficiency. Blood, 1990(April 15);76:871–881.
12.USDA National Nutrient Database for Standard Reference. Vitamin B12
(μg) content of selected foods per common vitamin B-12 measure,
sorted by nutrient content. http://www.nal.usda.gov/fnic/foodcomp/
Data/SR18/nutrlist/sr18w418.pdf
13.Squires MW and Naber EC. Vitamin profiles of eggs as indicators
of nutritional status in the laying hen: vitamin B12 study. Poul Sci,
1992;71:275–282.
14.Donaldson M.S. Metabolic vitamin B12 status on a mostly raw vegan
diet with follow-up using tablets, nutritional yeast, or probiotic
supplements. Ann Nutr Metab, 2000;44:229–234.
15.Albert MJ, Mathan VI, Baker SJ. Vitamin B12 synthesis by human small
intestinal bacteria. Nature, 1980(Feb. 21);283:781-782.
16.Hokin BD and Butler T. Cyanocobalamin (vitamin B-12) status
in Seventh-day Adventist ministers in Australia. Am J Clin Nutr,
1999;70(suppl):576S–578S.
17.Herrmann W. and Geisel J. Vegetarian lifestyle and monitoring of
vitamin B12 status. Inter J Clin Chem, 2002;326:47-59.
18.Gilsing A., Crowe F., Lloyd-Wright Z., Sanders T., Appleby P., Allen N.,
and Key T. Serum concentrations of vitamin B12 and folate in British
male omnivores, vegetarians and vegans: results from a crosssectional analysis of the EPIC-Oxford cohort study. Eur J Clin Nutr,
2010;64:933-939.
19.Herrmann W., Schorr H., Obeid R., and Geisel J. Vitamin B-12
status, particularly holotranscolbalamin II and methylmalonic acid
concentrations, and hyperhomocysteinemia in vegetarians. Am J Clin
Nutr, 2003;78:131-136.
20.Oh RC, Brown DL. Vitamin B12 deficiency. Am Fam Physician.
2003;67:979-986.
21.Renault F, Verstichel P, Ploussard J, Costil J. Neuropathy in two
cobalamin-deficient breast-fed infants of vegetarian mothers. Muscle
Nerve. 1999;22:252-254.
More references for this resource are available at
http://vndpg.org/resources/B12
RD Resources are a member benefit of the Vegetarian Nutrition Dietetic Practice Group. They are available in the Members
Section of the website at www.VNDPG.org. © 2012 by Vegetarian Nutrition Dietetic Practice Group.
Written by: Roman Pawlak, Ph.D, RD
Expires August 2017.