Lately I have been really tired. About two weeks ago I started taking Vitamin B-12. Vitamin B-12 is not found in vegetables and I had been avoiding meat.
Ultimately, animals must obtain vitamin B12 directly or indirectly from bacteria, and these bacteria may inhabit a section of the gut which is posterior to the section where B12 is absorbed. Thus, herbivorous animals must either obtain B12 from bacteria in their rumens, or (if fermenting plant material in the hindgut) by reingestion of cecotrope fæces.
Vitamin B12 is found in foods that come from animals, including fish and shellfish, meat (especially liver), poultry, eggs, milk, and milk products. One half chicken breast provides some 0.3 µg per serving or 6.0% of one’s daily value (DV); 3 ounces of beef, 2.4 µg, or 40% of one’s DV; one slice of liver 47.9 µg or 780% of DV; and 3 ounces of molluscs 84.1 µg, or 1,400 % of DV.
Eggs are often mentioned as a good B12 source, but they also contain a factor (avidin) that blocks absorption. Certain insects such as termites contain B12 produced by their gut bacteria, in a way analogous to ruminant animals. An NIH Fact Sheet lists a variety of food sources of vitamin B12.
While lacto-ovo vegetarians usually get enough B12 through consuming dairy products, vegans will lack B12 unless they consume multivitamin supplements or B12-fortified foods. Examples of fortified foods include fortified breakfast cereals, fortified soy products, fortified energy bars, and fortified nutritional yeast. According to the UK Vegan Society, the present consensus is that any B12 present in plant foods is likely to be unavailable to humans because B12 analogues can compete with B12 and inhibit metabolism.
Claimed sources of B12 that have been shown to be inadequate or unreliable through direct studies of vegans include laver (a seaweed), barley grass, and human gut bacteria.
Vitamin B12 is provided as a supplement in many processed foods, and is also available in vitamin pill form, including multi-vitamins. Vitamin B12 can be supplemented in healthy subjects also by liquid, transdermal patch, nasal spray, or injection and is available singly or in combination with other supplements.