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A Functional Nutrition Guide to B12

Updated: Jun 23, 2022

I love the topic of vitamin B12, especially in the context of a predominantly plant-based diet. There’s no question about it, an exclusively plant-based diet will require the individual in case to supplement, yet there is still much controversy surrounding the topic. Please also don’t assume that those eating animal proteins are protected from B12 deficiency or the need for supplementation.

Vitamin B12 is an essential consideration to all human diets, not just those on a plant-based diet. Humans don’t make bioavailable sources of this water-soluble vitamin, which means we’re reliant on external sources including dietary sources (read meat, liver, white meat, fish, dairy, eggs and to a much lesser degree tempeh, chlorella and mushrooms), fortified foods (including Nutritional Yeast and other fortified plant-based products) and supplements.

B12 (also known as cobalamin) is not made in plants, instead it’s synthesised by bacteria and archaea. It is abundant in the meats and by-products of ruminant animals (with multiple stomachs) because it’s their gut bacteria that make the B12. In the case of humans, the bacteria in our large intestines do produce B12 however as B12 isn’t readily absorbed via the large intestine it’s really no help to us at this stage. It sounds like a flaw in the design of the human body, however I’m convinced we’ll one day discover the real reason for why.. watch this space!

B12 Requirements

Demands for B12 aren’t great and in an individual of good health, B12 bioavailability is adequate – approximately 56% of every 1mcg consumed via the diet is absorbed. The recommended daily intake (RDI) for adults is just 2.4 mcg (2.6 and 2.8 for pregnant and breastfeeding women) and if you consume animal products this could be met through having just a few eggs and a small piece of cheese. We also don’t lose much B12, a portion of B12 is supplied by recycled enterocytes (cells that line the large and small intestine) and recycled bile salts reabsorbed and delivered back to the liver and subsequently via the gall bladder back into the digestive tract.

In the case of B12 supplements, only about 1 – 2% of what’s had is absorbed.

The Signs of B12 Deficiency

There are multiple forms of B12. Methylcobalamin (found in dairy and human breast milk) and adenosylcobalamin (found in meat) are the two co-enzyme forms of B12. They’re crucial for supporting two key cycles: methylation and the krebs cycle. Which means they ultimately support DNA synthesis, red blood cell production, homocysteine clearance (elevated levels are a sign of inflammation and associated with cardiovascular disease), nervous system function and cellular energy production.

The other two forms of B12 are cyanocobalamin and hydroxocobalamin which are synthetic forms frequently used for supplementation. Each can be converted to the co-enzyme form to varying degrees.

If B12 availability is lacking it can cause reversible megaloblastic anaemia, demyelinating neurologic disease, or both. These could manifest in the form of numbness or tingling in the hands, legs or feet, low energy, cognitive difficulties, memory loss, weakness, yellowing skin (jaundice) and/or a swollen or inflamed tongue.

Those most at risk of deficiency

Although requirements are low and bioavailability of dietary B12 is good, the prevalence of B12 deficiency is alarmingly high. Some references cite as much as 15% of the general population could be deficient and a review of 40 studies on vegetarians showed that as much as 86% could be deficient, with the likeliness being higher for vegans. Lack of dietary intake is most certainly a risk factor, but what else causes low levels and who is at risk?

  • Those with hypochlorhydria (low stomach acid) as a result of gastrointestinal surgery, ageing, the use of medications including proton pump inhibitors and metformin and chronic stress are at increased risk of deficiency. Hydrochloric acid is required to free B12 from the protein it’s bound to in food.

  • Those with a lack of Intrinsic Factor (IF). IF is a glycoprotein secreted by the parietal cells of the stomach and it supports B12 absorption in the small intestine. This can also affect those that have had gastrointestinal surgery ans well as individuals with a condition known as Pernicious Anaemia who often fail to produce IF and left untreated, it can result in B12 deficiency.

  • Those with other disorders of the stomach and small intestine including coeliac disease and Crohn's disease.

  • B12 crosses the placenta during pregnancy and is present in breastmilk. Exclusively breastfed children of exclusively plant-based mothers (who may have limited B12 reserves) are therefore at risk of B12 deficiency within just months of birth. Plant-based women who are pregnant or breastfeeding must supplement in order to maintain adequate B12 levels.

  • Individuals on an exclusively plant based or largely plant-based diet, with little to no dietary B12 intake.

Assessing B12 status

The neurological symptoms of vitamin B12 deficiency can occur without anaemia, so early diagnosis and intervention is incredibly important to avoiding irreversible damage, not to mention maintaining quality of life.

For the plant-based community, I recommend assessing B12 status on commencing a plant-based diet and at 6 – 12 monthly intervals thereafter.

I recommend the below blood tests as part of a B12 status assessment with the addition of methylmalonic acid (MMA) where possible, though this isn’t always covered by Medicare (in Australia).

  • Total B12 (serum B12): On its own, Total B12 demonstrates poor sensitivity and as a result should always be accompanied by additional markers of B12 status, especially if the result is less than 400 pmol/L.

  • Active B12 (transcobalamin II): This is a much more sensitive and accurate marker as it reflects intake via the small intestine and cellular delivery. Greater than 100 pmol/L should be the goal.

  • Homocysteine: Consider this a functional marker of B12 status. Although other factors can impact homocysteine, higher levels (greater than 8 umol/L) may be reflective of B12 inadequacy. Aside from being a functional marker of B12 adequacy, homocysteine in high levels is associated with cardiovascular disease and needs to be addressed.

  • Full Blood Examination (FBE): Assessing Mean Corpuscular Volume (MCV) and Red cell Distribution Width (RDW) offers understanding of how advanced B12 deficiency is, with worse results reflecting later stages of deficiency.

Your Dr should support you in having the above tests done, especially if you follow a predominantly plant-based diet, show signs and symptoms of a deficiency or are at risk of deficiency. If you need extra support in obtaining any of these tests please consult with me or another Nutritionist, Naturopath or Dietitian.

Supplementing With B12 on a Plant-based Diet

It is true that some plant foods contain biologically active and available forms of B12 including tempeh, natto, chlorella, nori and certain mushrooms. As levels can differ depending on the source and preparation, for healthy adults looking to maintain B12 levels on a predominantly plant-based diet I recommend supplementing with 250 – 500mcg B12 per day. Supplementation trials confirm that methylcobalamin, hydroxocobalamin and cyanocobalamin are all capable of correcting B12 deficiency and reducing homocysteine, however I recommend the use of methylcobalamin and hydroxocobalamin. Please work with a practitioner if you have a history of anxiety, OCD, low mood or less than optimal B12 status, as above.

If you have a medical condition, high homocysteine, a history of anxiety, OCD or low mood, show signs of deficiency or have identified genetic mutations (such as MTHFR) your supplement requirements will vary so please work with myself or a healthcare provider with experience in B12 supplementation to determine what’s appropriate for you.

I do not recommend supplementing with Spirulina or fortified foods. Spirulina contains a ‘pseudo-B12’ (adeninyl cobamide) which is known to be biologically inactive in humans and can potentially negatively impact absorption of B12 from other sources. It acts as an anti-nutrient.

Please also don’t wait until overt deficiency occurs before supplementing. It is possible for suboptimal levels to go undetected for years all the while contributing to the subtle signs of low B12.

I hope this helps. If you require support in addressing low B12 or suspect less than optimal B12 please don’t hesitate to get in touch or book in for 15-minute Complimentary Consultation.


Pawlak R, et al. 2014. The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature. European Journal of Clinical Nutrition, 68, 5, 541-8.

Herrman W, Obeid R. 2008. Causes and early diagnosis of vitamin B12 deficiency. Dtsch. Arztebl. Int. 105, 40, 680Y5.

Sally P, Stabler M.D. 2013. Vitamin B12 Deficiency. New England Journal of Medicine 2013, 368, 149-160.

O’Leary F, Samman S. 2010. Vitamin B12 in health and disease. Nutrients. 2, 3, 299-316

Carmel R. 2008. How I treat cobalamin (vitamin B12) deficiency. Blood. 112, 2214-21.

Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes: Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: National Academy Press, 1998.

Harrington D. J. 2017. Laboratory assessment of vitamin B 12 status. Journal of Clinical Pathology. 70, 2, 168–173.

Herrmann W, et al. 2005. The usefulness of holotranscobalamin in predicting vitamin B12 status in different clinical settings. Current Drug Metabolism. 6,1, 47–53.

Spence, J. D. 2016. Metabolic vitamin B12 deficiency: A missed opportunity to prevent dementia and stroke. Nutrition Research. 36, 2, 109–116.

Biologically Active Vitamin B12 Compounds in Foods for Preventing Deficiency among Vegetarians and Elderly Subjects. Journal or Agriculture and Food Chemistry

Obeid R, et al. 2015. Cobalamin coenzyme forms are not likely to be superior to cyano- and hydroxyl-cobalamin in prevention or treatment of cobalamin deficiency. Molecular Nutrition & Food Research, 59, 7, 1364–1372.

Greibe E, et al. 2018. Increase in circulating holotranscobalamin after oral administration of cyanocobalamin or hydroxocobalamin in healthy adults with low and normal cobalamin status. European Journal of Nutrition. 57, 8, 2847–2855.

Herbert V. 1994. Staging vitamin B−12 (cobalamin) status in vegetarians. The American Journal of Clinical Nutrition. 59, 5, 1213S-1222S.

Paul C, Brady D. M. 2017. Comparative bioavailability and utilization of particular forms of B12 supplements with potential to mitigate B12-related genetic polymorphisms. Integrative Medicine (Encinitas, Calif.). 16, 1, 42–49.

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