Humans cannot produce vitamin B12 naturally, making it one of the eight essential B vitamins.
Bacteria are required to produce vitamin B12 naturally, but the majority of the bacteria in our GI tract lives at the end of our digestive tract, after the point at which B12 gets absorbed.
Therefore, in order to satisfy our need for B12, we have to get it from our diet.
Plants do not contain any vitamin B12, so individuals that don’t eat many, or any, animal products should pay special attention to their intake. Vitamin B12 comprises many forms, including cyano-, methyl-, deoxyadenosyl-, and hydroxy-cobalamin.
Cyanocobalamin is the synthetic form of vitamin B12 and can be found in supplements and fortified foods.
The biggest dietary sources of vitamin B12 are viscera, such as liver (26–58 μg), meat (3–10 μg), dairy foods (0.3–2.4 μg), eggs (1–2.5 μg), poultry (trace amounts to 1 μg) in 100 g wet weight.
Bonito fish and clam extracts contain considerable amounts of free vitamin B12, 41 μg and 132 μg/100 g wet weight, respectively.
Gastrointestinal fermentation supports the growth of this vitamin B12–synthesising microorganisms, and this vitamin is subsequently absorbed and incorporated into animal tissues, such as those of ruminants.
Vitamin B12 is not synthesised by plants; therefore, low serum B12 levels may be more prevalent among vegetarians, and especially vegans.
Vegans and even lacto-ovo-vegetarians with only a small intake of eggs and dairy foods may require supplemental vitamin B12 from fortified foods or supplements.
The US Institute of Medicine has recommended that adults older than 51 years consume most of their vitamin B12 from fortified foods or supplements, bearing in mind that older adults are at higher risk of B12 deficiency due to the physiological reduction in intrinsic factor secretion necessary for absorbing this vitamin, as well as due to the use of drugs that can reduce the bioavailability of cobalamin.
Vitamin B12 has also been reported to be present in lower levels in non-animal foods, including edible algae, some mushrooms, and fermented foods such as tempeh, kimchi, miso, and tea.
For example, Chlorella, Spirulina and Porphyra yezoensis, commonly known as purple laver or nori, can produce a cobalamin-like compound, also called pseudo-cobalamin.
Vitamin B12 is essential for DNA synthesis and regulation. It is involved in many important metabolic pathways, especially in the metabolism of lipids, carbohydrates, and proteins, and plays a central role in hemopoiesis.
Treatments such as antibiotics, proton pump inhibitor medications, antihyperglycemic medicines (i.e. metformin), nitrous oxide anaesthesia, a non-steroidal anti-inflammatory drug, some anticonvulsant, and colchicine interfere with B12 absorption and metabolism.
Deficiency can also result in a form of anaemia called megaloblastic macrocytic anaemia. This anaemia causes large and abnormal red blood cells. Both a B12 and a folate deficiency can lead to megaloblastic anaemia. In B12 deficiency, folate metabolism is halted which disrupts DNA synthesis required for red blood cell production. Too much folate in the diet can force folate metabolism to appear normal even though B12 is deficient, which is why it is critical to test B12 levels.
Finally, another integral role of B12 is its role in the conversion of homocysteine to methionine. Methionine is essential for the methylation of literally hundreds of reactions in our body, which is one reason a B12 deficiency can affect us so severely. Also, without adequate B12, homocysteine accumulates since it is not converted to methionine. High levels of homocysteine, called hyperhomocysteinemia, may be a risk factor for developing coronary artery disease because it makes blood vessels more prone to injury and subsequent inflammation.
References:
Gille D, Schmid A. Vitamin B12 in meat and dairy products. Nutr Rev. 2015;73:106–11; Wolffenbuttel BHR, Wouters HJCM, Heiner-Fokkema MR, et al. The many faces of cobalamin (vitamin B12) deficiency. Mayo Clin Proc Innov Qual Outcomes. 2019;3:200–214.