B12: the “Achilles Heel” of Vegetarians and Vegans

Article published at: Oct 26, 2023
B12: the “Achilles Heel” of Vegetarians and Vegans
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Sep 21

Vitamin B12 is responsible for a wide range of functions in our body, including the metabolism of proteins, lipids, and carbohydrates, as well as the proper functioning of the nervous system. It plays a role in the synthesis of hemoglobin and the production of red blood cells, iron absorption, the secretion of adrenal hormones such as cortisol, aldosterone, adrenaline, and noradrenaline, dehydroepiandrosterone sulfate, and the maintenance of vitamin A levels in tissues. It also contributes to the healthy functioning of the digestive system.

Vitamin B12 can be obtained from our diet or produced by the normal flora in the last part of the small intestine of a person with a well-functioning gut. After the age of 50, 10-30% of the population shows a decrease in B12 levels, regardless of their dietary habits, indicating a problem of incomplete absorption or continuous reduction in B12.

B12 is present in animal-based foods (meat, fish, dairy, eggs), while it is virtually absent in the plant world. One might wonder how herbivorous animals like cows, goats, and rabbits acquire it. The quantities of B12 in the tissues of these animals do not come from their plant-based diet, but rather from the bacteria present on the surface of the Earth, which colonize their bacterial flora, synthesizing this valuable vitamin.

Why does the bacterial flora of animals produce much more B12 than that of humans? One possible answer to this question is that herbivorous animals eat plants that have not been thoroughly washed and come directly from the ground or a feeder very close to the soil. In contrast, humans wash and clean their food meticulously for hygiene reasons or consume foods that have undergone industrial processing, which further reduces the already minimal B12 content in wild plants or organically grown produce.

Given that modern agriculture today makes food almost entirely devoid of bacteria, a vegetarian or vegan must be careful not to risk medium to long-term B12 deficiency.


Values above 200 pg/ml (or 150 pmol/l) are considered normal, while B12 levels below 80 pg/ml (or 60 pmol/l) are a sure sign of B12 deficiency. Reference values vary depending on the methodology of the clinical laboratory, the reference units, and also because some reference values have more statistical significance than physiological-pathological significance. To reach a very severe B12 deficiency, the path is somewhat long and always individual-dependent. Here, we must understand what a B12 deficiency does to our body.

The consequences primarily include hematological diseases (e.g., megaloblastic anemia) but also diseases of the nervous system, both central and peripheral. A deficiency in B12, along with a deficiency in folic or folinic acid, causes, among other things, an increase in homocysteine, a marker of cardiovascular function more significant than LDL or 'bad' cholesterol.

Scientific literature indicates that B12 deficiency is associated with various neurological and psychiatric disorders, complications during pregnancy, neonatal malformations, osteoporosis, and more.

The status of B12 in our body becomes apparent through specialized blood tests that include B12 levels, serum folate, red cell folate, homocysteine, and a blood count.


  • B12: 204 – 1130 pg/ml

  • Serum Folic Acid (Folate): 7 - 28 nmol/l

  • Folic Acid (Erythrocyte): 572 – 1988 pg/ml

  • Homocysteine: < 10.0 mcmol/l

A deficiency in B12 can also be detected through specialized blood or urine analysis by measuring the levels of MMA (methylmalonic acid), a substance that B12 rapidly metabolizes. Elevated MMA levels unequivocally indicate a B12 deficiency. Unfortunately, this analysis is not yet available in Greek laboratories. Therefore, this parameter is not applicable at the moment.

B12 is the weak point, perhaps the only one, in an exclusively plant-based diet. The discussion on this matter has ignited globally between proponents and opponents of a vegan diet. The ADA (American Dietetic Association) has taken a clear stance that vegetarians have no issues but should take supplements. They state, "No plant food has anything but minuscule amounts of active B12. Foods such as spirulina and various seaweeds are not serious B12 sources. Lacto-ovo vegetarians obtain substantial B12 from dairy products and eggs."

Given that the ADA is an organization that has demonstrated a serious approach to the choices of vegetarians and vegans, it should be taken into serious consideration. However, the issue still requires further investigation since there are positions with different views on what is rightfully called the "Achilles' heel" of vegans.

In general, researchers agree that the body needs a daily dose of 2-3 μg (micrograms) of B12. (1 μg = 1 millionth of a gram). Those who consume meat, fish, or dairy have a reserve, a storage of 2000 – 5000 μg, i.e., 2-5 mg, mainly in the liver, and it would take at least 3 years to deplete this reserve. To put it into perspective, if we had 1 gram of stored B12, it would last us for 600 years. There are cases of vegans who haven't experienced a B12 deficiency even after 30 years. Generally, after 3-5 years of a purely plant-based diet, B12 levels begin to drop. In some individuals, this may happen a bit earlier.


The perfect diet, if it ever existed, no longer exists today. We can talk about a better diet, one that gives us the best advantages and the fewest problems. This is certainly the case for a natural diet based on plant foods.


  • Meat – Fish – Seafood – Eggs (yolks) – Dairy (mainly cheeses and yogurt)

  • B12-enriched foods for vegetarians (mainly cereals)


  • Continuous fatigue, weakness, and a sensation like pins and needles

  • Pallor and yellowing of the skin

  • Constant dizziness

  • Loss of taste sensation on the tongue (inability to recognize tastes)

  • Feelings of sadness and anxiety (hormones responsible for happiness or neurotransmitters, serotonin and dopamine, become inactive)

  • Severe neurological disorders

  • Vision and hearing problems


  • Consumption of antioxidants

  • Helicobacter pylori in the pyloric region

  • Gastric interventions affecting the mechanisms of the digestive system

  • Increased coffee consumption (4 cups per day)

  • Endogenous Factor (Glycoprotein produced by the stomach's parietal cells responsible for B12 absorption in the small intestine).

How Much B12 Should We Take with Supplements

Most studies on this subject agree that B12 absorption is more effective when we take small doses at regular intervals. This can be achieved through the consumption of foods enriched with B12 or supplements that ideally contain only low-dose B12. In fact, when we take a dose of B12 less than 5 mcg, about 60% is absorbed, while taking a dose of 500 mcg or more results in an absorption rate of approximately 1%. Of course, the use of high-dose supplements is also satisfactory (e.g., 2000 mcg), as it provides an absorption of 20 mcg, which is 7-10 times higher than the recommended daily dose of 2-3 mcg. It's worth noting that vegetarians and vegans should regularly check their B12 levels because only testing can confirm that the product they are taking is capable of providing the required B12 amounts. Additionally, because some supplements contain folic acid, which can mask a B12 deficiency (overlap), the recommended panel of tests that should be conducted frequently includes B12, folic acid, homocysteine, and a complete blood count or general blood test. Elevated homocysteine levels, even with normal B12 levels, indicate a B12 deficiency and the need for supplementation. If everything goes well, the subsequent test will show that elevated homocysteine levels have returned to normal.

As mentioned earlier, B12 is found in meat, fish, dairy, and eggs. Some researchers believe that B12 is also found in seaweed and certain traditional Japanese macrobiotic foods like Tempeh and Miso, which are fermented with bacteria. In 2002, Watanabe et al. demonstrated that seaweed, spirulina, and these macrobiotic foods contain minimal amounts of B12, but in an inactive form, so they cannot be considered reliable sources of the vitamin. However, the same researcher in a 2007 study published that green algae and red seaweed (Nori) contain small amounts of active B12, possibly due to the action of certain bacteria that coexist with these algae.

This much-discussed topic appears to have no end in sight. This is quite reasonable since studies are conducted daily in scientific centers worldwide. My advice to vegan friends is simply to undergo periodic examinations to determine when they need to supplement their diet.

Postscript: Let's Not Unfairly Judge Vegans

Many times, negative articles are published about vegans and their B12 deficiency. One of these, in the "New England Journal of Medicine," carried the sensational title "BLINDNESS OF A VEGAN" (in reality, it was a letter and not a research-based article). This letter discussed the case of a 33-year-old who had been a vegan since the age of 13 and experienced vision problems (optic neuropathy). Various examinations revealed a long list of deficiencies in vitamins A, C, D, E, B1, B12, FOLIC ACID, and trace elements like selenium and zinc. All of these indicated poor nutrition. Injectable B12 was administered to address anemia, not the neuropathy in the eyes.

Because this individual had deficiencies not only in B12 but in many other vitamins and minerals, which a vegan diet typically provides in sufficient quantities, it was evident that this person had absorption issues or followed an extremely poor vegan diet with significant deficiencies (e.g., consuming only sweets, processed foods, or cooked foods). This was eventually confirmed. However, newspapers simply emphasized that the individual's problems were due to their vegan diet, which is highly misleading. In reality, most articles published regarding B12 deficiency, whether in infants, children, or adults, relate to cases of individuals with absorption problems or sometimes individuals living in extremely impoverished conditions or adopting an unconventional lifestyle. In other words, the health problems of these individuals are not the result of their chosen vegan diet but rather have other underlying causes.

It is well-known that vegetarians and vegans are generally healthier than the rest of the population. They feel better and contribute philosophically to making humanity and the planet better. At some point, we need to recognize that we can obtain protein from other sources and stop damaging the planet. It's not our sole property!"

Gerassimos Tsiolis, PhD in Biochemistry
University of Bologna, Italy