A unique feature of vitamin B12 is that, unlike other vitamins, animal sources are the only reliable sources for its intake. Vitamin B12 is found mainly in meat, eggs, dairy products, and fish.

A valuable source of vitamin B12 is calf liver (one pound per day), which contains such a high amount of vitamin B12 that even people lacking intrinsic factor can absorb it in sufficient amounts to prevent pernicious anemia (a disease caused by due to the deficiency of this vitamin).

Much controversy exists regarding the proposed plant sources of vitamin B12. Some findings suggest that fermented soy products, seaweed (called nori), and algae such as spirulina contain significant amounts of vitamin B12. However, the analysis reveals that due to the presence of compounds structurally similar to vitamin B12, called B12 analogues, it is difficult to distinguish between the two forms and cannot be used to meet dietary needs. The most prevalent opinion today is that plant sources of vitamin B12 are probably not available to humans and therefore cannot be labeled as safe sources of the vitamin.

There is some question as to whether vegetarians and vegans get enough vitamin B12. They are at risk of developing a deficiency syndrome because natural food sources of this vitamin are limited to foods of animal origin. Therefore, vegetarians are recommended to include a rich intake of vitamin B12-enriched foods in their diet program. Good sources of this vitamin are fortified cereals, dairy products such as soy milk, free-range eggs, and sunflower margarines. Strict vegetarians and vegans who do not even eat vitamin B12-fortified plant foods should consider taking a vitamin B12-containing supplement, either in oral or dietary form.

There are claims that vitamin B12 can be consistently obtained from nutritional yeast. However, one should be aware that there is no substantial evidence to validate such a claim.

Bacteria exclusively synthesize vitamin B12. Streptomyces griseus, a bacterium once thought to be a yeast, was once a source of vitamin B12. Now, the bacteria Propionibacterium shermanii and Pseudomonas denitrificans have replaced it as the last commercial sources.

Some foods, along with their vitamin B12 content in micrograms (mcg), include

Crab (steamed), 3 ounces – 8.8 mcg; Salmon (baked), 3 ounces – 2.4 mcg; Rockfish (baked), 3 ounces – 1.0 mcg; Beef (cooked), 3 ounces, 2.1 mcg; Chicken (roast), 3 ounces – 0.3 mcg; Turkey (roast), 3 ounces – 0.3 mcg; Egg (poached), 1 large: 0.4 mcg; Milk, 8 ounces–0.9 mcg; Brie (cheese), 1 ounce– 0.5 mcg.