Nutrient Guide: Vitamin B12

Essential to mental and emotional well-being, Vitamin B12 helps metabolize every cell of the body. B12 maintains healthy nerve cells, and it aids in the production of DNA and RNA, the body’s genetic material9,11,53. In addition, B12 helps regulate the formation of red blood cells and helps iron function better. Vegetarians who eat no animal products need supplemental vitamin B12 to meet their requirements 7,9,11. Carbohydrate and fat processing requires B12 for its completion, insufficiency of the vitamin can also affect the movement of carbohydrates and fats through the body 11. Vitamin B12 acts differently than other vitamins because absorption of vitamin B12 from food requires normal function of the stomach, pancreas, and small intestine as it is dependent upon intrinsic factor (IF) (protein made in the stomach) to make its way from the GI tract (stomach and intestines) into the rest of the body. Lack of stomach acids also gets in the way of B12 absorption since stomach acids are necessary to release the B12 from proteins 54,59,60. Since intrinsic factor diminishes with age, older people are more prone to B12 deficiencies. Current Reference Daily Intake (RDI)* for Vitamin B12 is 6mcg for adults and children over the age of 4 and 3mcg for children under the age of 4.

Children from macrobiotic families in the Netherlands were reported to have impaired cognitive performance 62. In these children, B12 deficiency was associated with a suboptimal performance on Raven’s CPM, which measure intelligence. Deficient schoolchildren in Guatemala had slower reaction times on psychological tests, lower academic performance, attention problems, and delinquent behavior 63. Some studies in the Western world have shown that vitamin B12 deficient infants (born to vegetarian mothers) are anemic, irritable, and anorectic and thrive poorly. These infants also have marked developmental regression and poor brain growth 64,65. No plant or animal has been shown capable of producing B12, and the exclusive source of this vitamin appears to be tiny microorganisms like bacteria, yeasts, molds, and algae 7. During infancy, signs of a vitamin B12 deficiency include failure to thrive, movement disorders, developmental delays, and megaloblastic anemia 61. Deficiencies take a long time to develop, since the body stores a three- to five-year supply in the liver 4. Low levels of B12 can cause a range of symptoms including fatigue, shortness of breath, diarrhea, nervousness, numbness, or tingling sensation in the fingers and toes. Severe deficiency causes neurological damage 11,53.nd a deficiency in Iron may lead to pernicious anemia 4,11, an autoimmune disease that results in gastric atrophy resulting in vitamin B12 malabsorption 59,60 If pernicious anemia is left untreated, it causes vitamin B12 deficiency, leading to megaloblastic anemia and neurological disorders, even in the presence of adequate dietary intake of vitamin B12 11.

Vitamin B6 is required for proper absorption of vitamin B12, and deficiency of vitamin B6 has been shown to impair B12 absorption in animal studies. Conversion of vitamin B12 from its non-active into its biologically active form requires the presence of vitamin E. Individuals at risk for vitamin E deficiency may show signs of vitamin B12 deficiency as well 7. Vitamins B12B6, and B9 work together to control blood levels of homocysteine. Numerous studies indicate that even moderately elevated levels of homocysteine in the blood increase the risk of cardiovascular diseases 57 However, researchers aren’t sure whether homocysteine is a cause of heart disease or merely a marker that indicates someone may have heart disease 58. Studies have found that as many as 30% of patients hospitalized for depression are deficient in vitamin B12, but the reasons for the relationship between vitamin B12 deficiency and depression are not clear 55Vitamin B12 and folate work together to produce S-adenosylmethionine (SAMe), which is essential in immune function and mood. Several studies have shown supplementation with SAMe improves depressive symptoms 56

Sources of Vitamin B12 

Only bacteria can synthesize vitamin B12 and it cannot be made by animals or plants. Because of their greater ability to store vitamin B12, animals contain more of the vitamin than plants. The richest sources of vitamin B12 are snapper and calf’s liver, and to a lesser extent milk. When derived from animal foods, vitamin B12 is fairly well preserved under most cooking conditions. Other good sources of vitamin B12 include venison, shrimp, scallops, salmon, and beef. Within the plant world, sea plants (like kelp), algaes (like blue-green algae), yeasts (like brewer’s yeast), and fermented plant foods (like tempeh, miso, or tofu) are the most commonly consumed food sources of vitamin B127,9

For more information on the nutrient content of foods, search the USDA food composition database.

FoodServingVitamin B12 (mcg)
Clams (steamed)3 ounces84
Mussels (steamed)3 ounces20.4
Crab (steamed)3 ounces8.8
Salmon (baked)3 ounces2.4
Rockfish (baked)3 ounces1.0
Beef (cooked)3 ounces2.1
Chicken (roasted)3 ounces0.3
Turkey (roasted)3 ounces0.3
Egg (poached)1 large0.6
Milk (skim)8 ounces0.9
Brie (cheese)1 ounce0.5

* The Reference Daily Intake (RDI) is the value of established by the Food and Drug Administration (FDA) for use in nutrition labels. It was based initially on the highest 1968 Recommended Dietary Allowance (RDA) for each nutrient, to assure that needs were met for all age groups. To see more recent recommendations, please see the Dietary Reference Intakes (DRI). These are the most recent set of dietary recommendations established by the Food and Nutrition Board of the Institute of Medicine, 1997-2001. They replace previous RDAs and may be the basis for eventually updating the RDI’s. See here for a comparison of RDIs and DRIs.


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