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Vitamin B12 in Plant-Based Diets: Should You Take It?

by Jennifer Novakovich

b12_necessary_for_pregnant_mothers_picDue to a number of misconceptions, many vegetarians and vegans refuse to supplement with vitamin B12, often with the notion that it takes many years for a deficiency to develop. Unfortunately this is not the case, and deficiencies have been observed within two years on a plant-based diet. Long term vegetarians and vegans are at an especially high risk for a B12 deficiency. Among sampled vegetarian populations, deficiencies were present in 62% of pregnant women, between 25–86% in children, and anywhere between 11–90% in the elderly. Since many people chose plant-based diets for health reasons, it just doesn’t make sense to not to take a B12 supplement. Are you a vegetarian or vegan and haven’t picked up a B12 supplement yet? Or maybe you know someone who hasn’t been convinced? Read on for more!

Vitamin B12 (i.e. cobalamin) has the largest and most complex structure out of all of the vitamins. The name cobalamin was given because of the cobalt presence within the chemical structure. The cobalt-carbon bond reactivity is the key to B12’s bioactivity. Vitamin B12 is an essential nutrient and is required in a reaction which converts homocysteine to methionine, DNA synthesis, myelin production, axon maintenance, energy production by mitochondria, erythropoiesis in bone marrow, etc.

The biologically active forms of cobalamin include methylcobalamin and adenosylcobalamin. There are also two additional forms, hydroxycobalamin and cyanocobalamin (a synthetic form), which can be metabolized into either of the active forms. B12 is only synthesised by bacteria (usually in the form hydroxocobalamin) but is converted to different forms of the vitamin in other animal bodies and accumulates up the food chain. It is therefore not present in plants unless otherwise enriched (e.g. via organic fertilizer). Nutritional yeast may be B12 fortified, in which case about 2 tablespoons typically has 250mg (the RDA value). Blue-green algae (cyanobacteria) also has vitamin B12, but this B12 seems to be inactive in mammals. Nonetheless, vegetarians and vegans alike have limited to no natural sourced B12 in their diets and dietary B12 is typically only achieved through fortified foods (e.g. certain soy products, soy milk, many cereals), animal sourced foods (in vegetarians), or supplements. While theoretically B12 deficiencies can be prevented with fortified foods and nutritional yeast, supplements should be incorporated to ensure no deficiency.

How do we absorb vitamin B12? In human bodies, vitamin B12 in food is bound to protein and is released through stomach acid breakdown. Supplemental B12 is already in a free form and doesn’t need to be further broken down. Free B12 combines with intrinsic factor, which is secreted by the stomach’s parietal cells, and results in absorption of vitamin B12 within the ileum by receptor mediated endocytosis. People who are deficient in intrinsic factor (typically older people) will develop B12 deficiencies and require a B12 injection, which bypasses the intestinal absorption phase.

neural_degeneration_and_dementia_b12_deficiencySo why worry about a B12 deficiency? B12 deficiencies are associated with a number of health risks including a higher risk for heart disease, neural tube defects in newborns, low bone mineral density, neural degeneration, megaloblastic anemia, and dementia. While stored B12 depletion may take a relatively long time, once they are depleted, symptoms may occur more quickly, and some are irreversible. Neurologic degeneration can occur without anemia, so early management of a B12 deficiency is important to avoid irreversible damage.

Deficiency symptoms can be mild to severe and may include lethargy, tiredness, poor appetite, fatigue, numb hands, aging, depression, sore tongue, and forgetfulness. Symptoms are not always indicative of a B12 deficiency because they can be masked by either an iron deficiency or a high folate intake (in plants). Large amounts of folic acid can correct megaloblastic anemia, however it may actually worsen the cognitive symptoms associated with a B12 deficiency. Folic acid intake shouldn’t exceed 1000mcg from fortified foods and supplements. If you are on a diet with limited to no vitamin B12, it's a safer option to just supplement rather than wait for symptoms that may or may not occur because of either a high folate intake or low iron intake.

Supplements taken at an adequate dose can effectively treat and prevent a deficiency. They are also quite inexpensive. Most B12 supplements are in the form cyanocobalamin, which is a common synthetic form not found in nature (very cheap and stable, but fermented from cobalt and cyanide), but methylcobalamin, adenosylcobalamin, and hydroxycobalamin are also available in more expensive supplements. Doses of B12 can range from 100mg–5000mg. Since natural B12 is water soluble, it can be taken at very high doses without toxicity.

Only about 56% of a 1 mcg oral dose of vitamin B12 is absorbed, but absorption decreases even more when the capacity of intrinsic factor is exceeded (at high doses). Studies have indicated that the supplemental dose should be about 100 times higher than the RDA (250mg) and in deficiencies, 200 times higher. Large doses (between 1000–2000mcg) of B12 can effectively treat a deficiency and allow patients to avoid injections. This applies specifically to cyanocobalamin; natural cobalamin has a slightly higher absorption rate.

plant_based_diet_and_b12_deficiencies_picVitamin B12 supplements are an important addition to a plant-based diet. They are easy to take, cheap, safe, and typically will make you feel better and more energized. If you’re a vegetarian or vegan, for health reasons, or any reason for that matter, it just doesn’t make sense to not take B12 supplements.

Dali-Youcef N, Andrès E. (2009) An update on cobalamin deficiency in adults. QJM 102(1):17-28. Elzen W, Weele G, Gussekloo J, Westendorp R, Assendelft W. (2010) Subnormal vitamin B12 concentrations and anaemia in older people: a systematic review. BMC Geriatr. 10: 42.O'Leary F, Allman-Farinelli M, Samman S. (2012) Vitamin B12 status, cognitive decline and dementia: a systematic review of prospective cohort studies. British Journ Nut 108:1948–1961.O'Leary F, Samman S. (2010) Vitamin B12 in Health and Disease. Nutrients 2:299-316. Rawlak R, Parrott S, Raj S, Cullum-Dugan D, Lucas D. (2012) How prevalent is vitamin B12 deficiency among vegetarians? Nut Rev 71(2):110–117.Watanabe F. (2007) Vitamin B12 sources and bioavailability. Exp Biol Med. 232(10):1266-74.

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