April 2018 Issue

Vitamin B12 and the Vegan Diet
By Sharon Palmer, RDN
Today's Dietitian
Vol. 20, No. 4, P. 38

Experts weigh in on nutrition recommendations for this essential micronutrient that has been a source of confusion for vegan consumers.

"Vegans can get vitamin B12 through natural bacteria in their mouths."

"It's a myth that vegans need to take vitamin B12 supplements."

"You can get enough B12 from the soil on your vegetables if you don't wash them."

These are just three of the common myths floating around the plant-based community surrounding vitamin B12 intake and vegan diets. In fact, some popular vegan websites and publications hold the philosophy that vegans can meet all of their essential nutrient needs without the use of any supplements.

To be sure, this viewpoint isn't prevalent among all plant-based experts and websites. Most respected sources place great emphasis on supplementing the vegan diet with adequate B12 sources. The Vegetarian Resource Group suggests that vegans need to have reliable sources of vitamin B12 in their diets.1 The Vegetarian Nutrition Dietetic Practice Group (VNDPG) of the Academy of Nutrition and Dietetics (the Academy) says that all vegetarians (including vegans) should include a reliable B12 source in their diets, such as fortified foods or supplements.2 And The Vegan Society goes so far as to state, "What every vegan should know about B12: the only reliable sources of B12 are foods fortified with B12 and supplements."3

It's extremely important for health care professionals—as well as clients—to understand the significance and intricacies of sufficient vitamin B12 intake for vegan diets, because deficiency of this essential micronutrient is serious business.

What Is Vitamin B12 and What Does It Do?
Vitamin B12 is a water-soluble vitamin that contains the mineral cobalt and exists in several forms. Compounds with vitamin B12 activity are called cobalamins. Methylcobalamin and adenosylcobalamin are two forms that are active in human metabolism. The other forms are hydroxocobalamin and cyanocobalamin that must be metabolized into the two active forms to be used in human cells. B12 analogues—inactive forms—may be found in algae and some plant foods.2,4,5

Vitamin B12 is needed for proper red blood cell formation, neurological function, and DNA synthesis. The vitamin is a cofactor for methionine synthase, which catalyzes the conversion of homocysteine to methionine—required for the formation of S-adenosylmethionine, a universal methyl donor for almost 100 substrates, including DNA, RNA, hormones, proteins, and lipids. Methionine also is needed for the synthesis of myelin, a coating of the nerve pathways. Vitamin B12 is a cofactor for L-methylmalonyl-CoA mutase, which converts L-methylmalonyl-CoA to succinyl-CoA in the degradation of propionate, an essential biochemical reaction in fat and protein metabolism. Succinyl-CoA also is required for hemoglobin synthesis.2,4

B12 digestion and absorption requires the adequate synthesis of hydrochloric acid, proteases (enzymes that break down proteins and peptides), and intrinsic factor (a glycoprotein secreted in the stomach). The vitamin is released from dietary proteins by pepsin (an enzyme secreted in the stomach) and activated by hydrochloric acid. B12 binds with proteins secreted in the saliva, and in the small intestine pancreatic proteases digest the proteins, liberating the B12 to form a complex with intrinsic factor. Vitamin B12 can then be absorbed into the bloodstream via endocytosis in the distal ileum, or by passive diffusion in the absence of intrinsic factor.2

Pernicious anemia (an autoimmune disease) affects the gastric mucosa, leading to destruction of parietal cells and failure to produce intrinsic factor, resulting in vitamin B12 malabsorption and eventually deficiency. Older adults, who often suffer from decreased hydrochloric acid in the stomach, and individuals with gastrointestinal (GI) disorders may have lower absorption of B12.2,4

Vitamin B12 deficiency may be characterized by megaloblastic anemia, fatigue, weakness, constipation, loss of appetite, soreness of the mouth or tongue, and weight loss; and neurological conditions, such as numbness and tingling of the hands and feet, difficulty with balance, depression, confusion, dementia, and poor memory. The neurological symptoms can occur without the presence of anemia, thus early detection is important to avoid irreversible damage. Deficiency is also commonly misdiagnosed, because symptoms can mimic other conditions.2,4

Large intake of folic acid—common in vegan diets—can mask the damaging effects of B12 deficiency by correcting anemia without addressing the neurological damage that occurs. High folic acid may even exacerbate anemia and cognitive symptoms. It isn't recommended to exceed 1,000 mcg/day of folic acid in healthy adults.4

Sources of Vitamin B12
Vitamin B12 is only synthesized by microorganisms. Thus, it isn't found in foods of plant origin. Natural animal food sources of vitamin B12 (which come from the animal's intestines or from their diet) include fish, meat, poultry, eggs, milk, and milk products. Though some bacteria in the small intestines produce B12, it isn't enough to maintain adequate status in humans.2,4

According to Jack Norris, RD, a vegan expert and coauthor of Vegan for Life: Everything You Need to Know to Be Healthy and Fit on a Plant-Based Diet, there are no reliable sources of B12 in plants, contrary to many rumors about sources such as tempeh, seaweeds, and organic produce. Plants have no B12 requirement; therefore, they don't have any active mechanisms to make or store it. When you find B12 in plants, it's due to contamination and thus isn't a reliable source. Many seaweeds have B12 analogues through their symbiotic relationship with cobalamin-producing bacteria; however, the evidence isn't clear that this form is active B12 in humans. And fermented foods, such as tempeh, aren't fermented through B12-producing bacteria, thus they aren't a source of B12. Rumors about bacteria on the surface of organic produce producing B12 haven't been verified. "Chlorella may improve B12 status, but it's by such a small amount that I wouldn't rely on chlorella for B12," Norris adds. He stresses that unless a food obtained from multiple regions consistently improves B12 status, it shouldn't be relied upon as a source of B12.5

Status in Vegans
Naturally, vegetarians—especially vegans—are at risk of vitamin B12 deficiency. In fact, there's a high prevalence of low serum vitamin B12 among vegetarians, in particular vegans, where prevalence ranges at levels between 43% and 88%.6 Going back as far as 1955, studies have shown that vegans have experienced vitamin B12 deficiency.7 In addition, B12 crosses the placenta during pregnancy and is present in breast milk—thus, if the mother doesn't receive adequate B12 intake, infants may suffer from B12 deficiency, which can lead to severe, permanent neurological damage.4

Norris says the story doesn't end there. He reports that most vegans show adequate B12 levels to make clinical deficiency unlikely, but they may show restricted activity of B12-related enzymes, leading to elevated homocysteine levels, which has been linked to increased risk of heart disease and stroke. Thus, he warns that repeated observations of elevated homocysteine levels in vegans show that B12 intake must be addressed.

The RDA for B12 in adults is 2.4 mcg per day (see Table 1), which assumes a 50% absorption rate from the amount ingested in foods. However, higher levels may be recommended to prevent potential deficiency symptoms in vegans. Intake of higher doses doesn't appear to be associated with detrimental health problems.2

The VNDPG recommends the following B12 guidance2:

  • All vegetarians, regardless of type, should periodically be screened for B12 deficiency, using either methylmalonic acid (MMA) or TCII (transcobalamin) assessment.
  • All women considering pregnancy and those already pregnant should take 250 mcg per day of a B12 supplement. (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.)
  • All vegans should take 250 mcg per day of a B12 supplement.
  • All lacto-ovo vegetarians should consider taking 250 mcg per day of a B12 supplement a few times per week.

Norris recommends vegan adults 18 to 65 years old should take one of the following regimens: two doses per day of 2 to 3.5 mcg per serving, one dose per day of 25 to 100 mcg per serving, or two doses per week of 1,000 mcg per serving. To read his full recommendations, visit http://veganhealth.org/b12/rec.8

In supplements, B12 is usually present as cyanocobalamin—a form the body readily converts into the active forms methylcobalamin and adenosylcobalamin. Other forms may be present in supplements, such as methylcobalamin and hydroxocobalamin. The vitamin B12 forms cyanocobalamin and occasionally hydroxocobalamin can be administered by intramuscular injection to treat deficiency or cases of malabsorption. While absorption may not differ among forms of B12, the body's ability to absorb the vitamin is largely limited by the capacity of intrinsic factor.2,4

Many foods are fortified with B12, including breakfast cereals, meat analogues, soymilk, and nutritional yeast (see Table 2). They may be fortified from small amounts to more than 200% of the RDA. It's important to read labels, as not all of these products are fortified, and fortification can change in products over time.2 Norris reports that amounts listed on a nutrition label are based on 6 mcg per day. For example, a food that provides 25% DV for B12 would provide 1.5 mcg.8

B12 Testing
The status of B12 typically is assessed through serum or plasma B12 levels. Values below 170 to 250 pg/mL for adults indicate a deficiency. However, this indicator may be influenced by other factors, such as low vitamin B6 or folate levels. Elevated MMA levels (greater or equal to 0.4 mcmol/L) may be a more reliable indicator of B12 status, because it indicates metabolic changes more specific to B12 deficiency.4

Norris recommends MMA and serum vitamin B12 tests for vegans, adding, "You don't need to get tested just because you're vegan; rather, you should make sure you follow the recommendations for intake."

Reed Mangels, PhD, RD, LDN, a vegan nutrition expert and associate professor of nutrition at the University of Massachusetts Amherst, also looks at MMA, as well as homocysteine levels, to see if B12 status is adequate. "B12 works with other B vitamins in many biochemical cycles, so it's important to understand the whole picture and look at a variety of nutrients," she says.

Clearing Up Confusions
So, what can dietitians do to clear up confusions on B12? The first step may be identifying some of the common myths. Norris says, "There's less confusion than during the 1990s and early 2000s, but there's still a lot. Many vegans don't think [they] need to worry about B12, or they think that meat-eaters get B12 deficiency just as often as vegans do, while others think you have to supplement with methylcobalamin rather than the less expensive, more reliable cyanocobalamin."

Mangels notes, "Some people think they have enough vitamin B12 stored and they don't need to worry about [it]. There's some confusion about reliable sources also. I wouldn't count on sources like seaweed, spirulina, or soil."

"I hear people that likely do need to supplement with B12 say that they're OK because they eat foods that may include B12, but they can be unreliable sources, such as sea vegetables or nutritional yeast," says Ginger Hultin, MS, RDN, CSO, a lifelong vegetarian, chair of the VNDPG, and spokesperson for the Academy.

After setting clients straight on vegan vitamin B12 myths, it's important to educate them on meeting their specific needs through supplements and fortified foods, Hultin says. "Each person should be treated as an individual, with their diet and health history in mind."

— Sharon Palmer, RDN, is the nutrition editor for Today's Dietitian and is an MS student studying sustainable food systems at Green Mountain College in Vermont.


1. Mangels R. Nutrition hotline. Vegetarian J. 2015;34(4):2

2. Pawlak R; Vegetarian Nutrition Dietetic Practice Group. RD resources for professionals: vitamin B12 in vegetarian diets. https://vndpg.org/docs/rd-resources/B12-RD.pdf. Published 2012. Accessed February 2, 2018.

3. Walsh S. What every vegan should know about vitamin B12. The Vegan Society website. https://www.vegansociety.com/resources/nutrition-and-health/nutrients/vitamin-b12/what-every-vegan-should-know-about-vitamin-b12. Published October 2001. Accessed February 2, 2018.

4. Vitamin B12: fact sheet for health professionals. National Institutes of Health, Office of Dietary Supplements website. https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/. Updated March 2, 2018. Accessed February 2, 2018.

5. Norris J. B12 in plant foods. VeganHealth.org website. http://www.veganhealth.org/vitamin-b12-plant-foods/. Updated October 2015. Accessed February 5, 2018.

6. Melina V, Craig W, Levin S. Position of the Academy of Nutrition and Dietetics: vegetarian diets. J Acad Nutr Diet. 2016;116(12):1970-1980.

7. Wokes F, Badenoch J, Sinclair HM. Human dietary deficiency of vitamin B12. Am J Clin Nutr. 1955;3(5):375-382.

8. Norris J. Vitamin B12. Vegan Health website. https://veganhealth.org/vitamin-b12/. Accessed February 5, 2018.