Causes of Vitamin B12 Deficiencies

In a previous post I talked about why Vitamin B12 was so important and how that directly related to the symptoms that one would experience with deficiencies.

But I think it is also just as important to touch on the causes for Vitamin B12 deficiencies.

So, basically there are 3 main reasons for Vitamin B12 deficiencies.

  • Inadequate dietary intake
  • Acquired disorders of Vitamin B12 absorption
  • Inherited disorders of Vitamin B12

As Vegans, the easiest cause to tackle and prevent deficiency is inadequate dietary intake.

To begin, Vitamin B12 is a vitamin that is synthesised by microorganisms (bacteria) in foods of animal origin such as meat, milk, cheese and eggs (pretty much everything that a Vegan actively avoids!) and some artificially fortified foods.

Whilst there are some plant-based sources of Vitamin B12, such as certain algae and plants exposed to bacterial action or contaminated by soil or insects, there is evidence that suggests that these particular class of Vitamin B12 can’t be used within the body.

The table below gives the Estimated Average Requirements and the Recommended Daily Intakes of Vitamin B12 needs throughout the different stages of life.

Note: The EAR is the median daily intake value that is estimated to meet the requirement of half the healthy individuals in a life-stage and gender group. At this level of intake, the other half of the individuals in the specified group would not have their needs met.

The RDI is the average daily dietary intake level that is sufficient to meet the nutrient requirement of nearly all (97 to 98 percent) healthy individuals in a particular life-stage and gender group.

The RDI is the goal for usual intake by an individual.

Life Stages EAR RDI
1-3 yrs. 0.7 ug/day 0.9 ug/day
4-8 yrs. 1.0 ug/day 1.2 ug/day
9-13 yrs. 1.5 ug/day 1.8 ug/day
14-18 yrs. 2.0 ug/day 2.4 ug/day
19-70 yrs. 2.0 ug/day 2.4 ug/day
Pregnancy 2.2 ug/day 2.6 ug/day
Lactation 2.4 ug/day 2.8 ug/day


Right now, you are probably looking at your Vitamin B12 supplement a little confused wondering why the heck it is giving you a dose of 500-1500mg per pill! That is over 600 times the Recommended Daily Intake!

Before you start dialling the ambulance for a potential overdose, let me explain the absorption of Vitamin B12 and why larger doses are a good thing for Vitamin B12.

Absorption of Vitamin B12 is massively complex. So complex in fact, that 12 Nobel Prizes have been awarded to dedicated scientists trying to figure out this process! And there are still many questions left to be answered.

In light of this complexity, here I am going to talk briefly about absorption for the purposes of covering dietary intake.

There are two possible routes for Vitamin B12 absorption.

  • Vitamin B12 binds to a highly specialised protein called Intrinsic Factor (IF)
  • Vitamin B12 can be taken up by the cells and enter the blood stream with no aid but rather by diffusion.

There are a number of advantages for taking a Vitamin B12 supplement as opposed to getting it from a food source.

Firstly, a supplement contains what is known as unbound Vitamin B12; it is free from a protein. This means that the many steps that happen in your stomach to prepare Vitamin B12 for absorption are bi-passed and it can directly link to IF for absorption.

The downside is though, the receptors in the GI tract that uptake Vitamin B12 bound to IF and transport it to the blood stream get saturated at 2ug per meal. It physically can’t transport any more than 2ug per meal.

Luckily, when Vitamin B12 is taken in large doses, ranging from 1000ug to 3000ug a day, this allows absorption to occur by diffusion. This a natural process by which higher concentrations of Vitamin B12 want to move to an area of lower concentrations, to create a balance. This allows Vitamin B12 to move from the stomach to the blood.

But, once again, this method of absorption by diffusion will only allow about 10% of what you consume in an oral dose at any one time to cross the barrier of the GI tract into the blood stream. But still, that can add up to 10ug from diffusion, and if your IF proteins are not defective, with an added 2ug, that’s 12ug a day of Vitamin B12.

And this is a good thing!

There is research that suggests 4-7ug of Vitamin B12 a day is associated with an adequate Vitamin B12 level within the body as opposed to the 2.4ug as suggested by the NRVs. This suggests that the current Recommended Daily Intake for Vitamin B12 might be inadequate for optimal status even in healthy populations.

Also, there is no need to be concerned about overdosing on Vitamin B12 supplements. There is no evidence that high doses represent a health risk. There is no recorded Upper Level of intake for Vitamin B12 and there is no evidence of toxicity.

To finish up on inadequate dietary intake as a cause of Vitamin B12 for Vegans, one should aim for a dietary supplement that provides a minimum of 1000ug per dose (1 tablet) and taken once or twice a day with a meal. This should protect you from inadequate intakes, but, if you are still experiencing problems with your Vitamin B12 levels there can be other reasons.

This is more the realm of a GP but acquired disorders relate to an inability to absorb the available Vitamin B12 within the GI tract, even in cases of adequate intake. Reasons for this can include:

  1. Pernicious anaemia – The body’s immune system attacks and destroys the cells that produce IF. If these cells are destroyed, the body can’t make IF and can’t absorb vitamin B-12.
  2. Ileal Resection – The Ileum (the major route of B12 absorption) is surgically removed.
  3. Gastrectomy – Removal or partial removal of the stomach. This will affect Vitamin B12 absorption.
  4. Crohn’s disease– Any inflammation of the absorption routes for Vitamin B12 will affect absorption. This is the same for Coeliac disease.
  5. Coeliac disease
  6. Chronic pancreatitis – Inflammation of the pancreas prevents the recycling of Vitamin B12 body stores.
  7. HIV infection 
  8. Parasite infestation

Once these issues are corrected (if they can be), or bypassed, absorption will generally improve.

Inherited disorders of Vitamin B12 include:

  1. Imerslund-Gräsbeck syndrome
  2. Hereditary IF deficiency
  3. Transcobalamin deficiency

Once again, if you believe that your dietary intake of Vitamin B12 is adequate but your levels are still low, work closely with your trusted GP to remove the possibility of these other issues which may be impacting optimum health.

If you have any more questions or want one of these issues explored or explained in more depth, please let me know and I am more than happy to do a post on it.

Anything to help the health of my fellow Vegans.

Until next time, and always, to your health!




Journal Articles.

Fenech, M. (2001) The role of folic acid and vitamin B12 in genomic stability of human cells. Mutation Research, 45(1-2). 57-67.

Moll, R., & Davis, B. (2017) Iron, Vitamin B12 and folate. Medicine, 45(4), 198-203. doi:10.1016/j.mpmed.2017.01.007

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: A Clinician’s Journal, 19(1), 42-49.


Essentials of Human Nutrition (4th ed.) (2012) by Jim Mann and Stewart Truswell.

Biochemsistry: A short Course (2nd ed.) (2013) by John Tymoczko, Jeremy Berg and Lubert Stryer



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