The B12 Conundrum

The following article probably contains WAY more information than you might want to know about any single vitamin but vitamin B12 is an important vitamin for the consideration of those who are not eating meat. The main point is that it is important to be sure that you are taking in a reliable source of vitamin B12.

Where is our B12?

Vitamin B12 (cobalamin) is the only nutrient that is not available in sufficient amounts in a plant-based diet. This is not the fault of plants. B12 is actually only produced by microorganisms. Neither plants nor animals can produce it on their own. Tiny microbes that live in the soil, in natural water sources and in the intestines of humans and animals are the world’s producers of Vitamin B12. Yes, in our own intestines! Sadly however, those B12-producing microbes living within the microbiomes of our guts reside well past our stomach and small intestines where B12 is actually absorbed. Because our intestinal contents do not usually travel backward along our digestive tracts (fortunately!) we are unable to take advantage of the source of B12 within our own intestines (1,7,8). Most meat-eaters acquire enough B12 when they eat meat that contains B12 producing bacteria although studies have shown that eating meat does not always translate into sufficient B12 blood levels (20,21). Animals themselves derive much of their Vitamin B12 from dirt on the plants that they eat and from the untreated water that they drink. In the past, humans enjoyed the same easy sources. Traces of dirt and small insects on inefficiently cleaned vegetables provided a constant source of B12. However, in today’s modern society with its emphasis on cleanliness, we do not eat dirty vegetables or drink water out of a natural stream. We thoroughly wash our food before preparing it to eat and we treat our water with chlorine and other chemicals which kill the very bacteria that produce Vitamin B12. The bottom line is that plant-based eaters need a reliable external source of this important vitamin (5).

Why is B12 important?

Vitamin B12 plays an essential role in human beings. It is necessary for DNA synthesis and for the maturation of red blood cells in the bone marrow. It supports the formation and function of nerve fibers in the brain, spinal cord and peripheral nerves. B12 is also needed to convert the amino acid, homocysteine, into methionine. A B12 deficiency will always lead to high homocysteine levels which cause artery damage and atherosclerosis (26).

How do we lose B12?

B12 is stored in the blood, liver and muscles and is lost at the rate of 0.2% every day (9) Once losing a stable source of B12, it is usually 2 to 3 years before the body stores are depleted although, in some people, this can happen more quickly. Breast fed babies born to B12 deficient mothers can develop deficiency signs within weeks of birth. All pregnant women should be sure to have a reliable B12 source (9,10,11,12). A deficiency of B12 can be very serious if it is not taken care of. The first signs are unusual fatigue, irritability, nausea and numbness and tingling in the extremities (fingers and toes). As the deficiency advances, symptoms such as difficulty with balance and walking, memory loss, depression, confusion and macrocytic anemia appear and further progression can produce permanent nerve damage and psychosis (1,6). Don’t let this frighten you however. Fortunately, vitamin B12 supplements are easily absorbed orally and most people have no problem maintaining a healthy level of B12 in their body.

Where can I get my B12?

The easiest method of ensuring that you are getting adequate B12 is by taking a weekly or daily supplement. The actual amount of the vitamin needed to maintain a healthy blood level is only 4 mcg per day. However, the complicated absorption of B12 in our bodies requires much larger doses (16). The absorption of B12 is aided by intrinsic factor, a protein secreted by the stomach lining. Intrinsic factor allows only about 2 mcg to be absorbed at one time. This is fine when B12 is ingested several times a day but, when taking a once a day supplement, does not allow enough to be absorbed. A second absorption method called passive diffusion also occurs in the stomach and allows the absorption of about 2% of the total dose taken (14,15). The upshot of this is that a daily B12 supplement at a dose of 200 to 250 mcg should be taken. If you prefer to just take one dose a week the dose should be about 1500 mcg. Any extra B12 that is not absorbed is not harmful and is easily excreted from the body. All people over the age of 50 are advised to take B12 supplements even if they do eat meat because aging reduces both proteases (enzymes that break down protein) and gastric acid in the stomach and make it more difficult for the body to separate the B12 from the meat protein (4,18).  In people over 65 years of age, doses of around 1000 mcg per day or 2500 mcg twice a week are a well-researched amount to aim for (27).  Although B12 was once administered by intramuscular injection for better availability, it has been shown more recently that high dose oral B12 is just as effective and a cheaper and less invasive method of administration. (17,19)

It is possible to obtain some vitamin B12 from fortified foods. Non-dairy milks such as soy milk and rice milk usually contain about 2 mcg per cup so it is conceivable that, if you consume 2 cups of soy or rice milk daily, you could obtain enough B12 for health. However, this amount would have to be consumed consistently every day or you would soon fall below the amount needed for health. Fermented foods once supplied a good source of vitamin B12 but this was when the fermentation took place in wooden barrels that could provide an excellent environment for bacteria that produce B12. Modern stainless steel barrels no longer perform this function. Mushrooms produce a tiny amount of B12 but not enough to be of any practical use (24,25). Seaweed is reputed to contain B12 but it is mostly in the form of an inactive B12 analog that will attach to a B12 receptor on a cell but has no activity. In fact, these inactive forms can exacerbate a B12 deficiency by filling up the receptor sites so that active B12 cannot do its job. Fairly new on the market, some types of insect flour may prove to be a good source of B12 although many species have very low levels. More research is needed (22).

Cyanocobalamin, methylcobalamin, adenosylcobalamin, hydroxycobalamin

There seems to be some controversy about the best form of vitamin B12 to take. The most common and least expensive form is cyanocobalamin but B12 is also available as methylcobalamin, adenosylcobalamin and hydroxycobalamin. All four of these forms have been shown in clinical studies to improve the vitamin B12 status of individuals. Although cyanocobalamin is a synthetic form of the vitamin, there are no conclusive trials showing that cyanocobalamin is more difficult for the human body to absorb or that it has any negative side effects. Until more research offers more input, most practitioners consider cyanocobalamin a safe source of vitamin B12 (23).

Testing for B12

A serum blood test result for vitamin B12 should be at least 148 to 443 pmol/l . At these levels homocysteine levels will remain low. (High homocysteine can increase the risk of heart disease (1)). As with other aspects of vitamin B12, serum testing is problematic. The test cannot differentiate between true B12 and inactive forms of the vitamin so you can end up with falsely high serum levels (1). Also, if you are taking supplements your B12 serum level might be high even though tissue levels are low. More accurate tests include measuring homocysteine blood levels and MMA (methylmalonic acid) blood levels. If either of these levels are elevated you are presumed to have a B12 deficiency which should be immediately treated (13).


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My name is Debra Harley (BScPhm) and I welcome you to my retirement project, this website. Over the course of a life many lessons are learned, altering deeply-rooted ideas and creating new passions.


  1. glenda johnson on July 10, 2017 at 8:30 am

    Thanks for the B12 article….it answered a lot of my questions

    • Deb on July 10, 2017 at 9:05 am

      Glad to be of help, Glenda. It is a complicated topic!

  2. Carolyn Harley on July 10, 2017 at 6:18 am

    Very clear and interesting discussion

    • Deb on July 10, 2017 at 9:05 am

      Thanks Carolyn.

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