Vitamin B12 (Cobalamin): Reference and Dietary Sources

Abstract

In this article, we describe:

• the major purposes of this specific nutrient in the human body, 
• its experimentally confirmed health uses, 
• conventional ways to estimate nutrient status,
• nutrient’s toxicities and deficiencies,
• experimentally confirmed and approved levels of the nutrient intake for different demographics,
• dietary sources of the nutrient.

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Introduction

Vitamin B12 is a water-soluble vitamin that is naturally present in some foods. Vitamin B12 exists in several forms and contains the mineral cobalt so compounds with vitamin B12 activity are collectively called “cobalamins”. Methylcobalamin and 5-deoxyadenosylcobalamin are the forms of vitamin B12 that are active in human metabolism.

Vitamin B12 is required for proper red blood cell formation, neurological function, and DNA synthesis. Vitamin B12 functions as a cofactor for methionine synthase and L-methylmalonyl-CoA mutase. Methionine synthase catalyzes the conversion of homocysteine to methionine. Methionine is required for the formation of S-adenosylmethionine, a universal methyl donor for almost 100 different substrates, including DNA, RNA, hormones, proteins, and lipids. L-methylmalonyl-CoA mutase converts L-methylmalonyl-CoA to succinyl-CoA in the degradation of propionate, an essential biochemical reaction in fat and protein metabolism. Succinyl-CoA is also required for hemoglobin synthesis.

B Complex vitamins are all water soluble and are not stored very well in the body. Thus, they are needed daily through diet or supplement to support their many functions. Deficiencies of one or more of the B vitamins may occur fairly easily, especially during times of fasting or dieting for weight loss or with diets that include substantial amounts of refined and processed food, sugar, or alcohol.

B complex vitamins are fairly easily digested from food or supplements and then absorbed into the blood, mainly from the small intestine. When the amount of Bs taken exceeds the body’s needs, the excess is easily excreted in the urine, giving it a dark yellow color. 

Approximately 56% of a 1 mcg oral dose of vitamin B12 is absorbed, but absorption decreases drastically when the capacity of intrinsic factor is exceeded (at 1–2 mcg of vitamin B12).

Vitamin B12 status is typically assessed via serum or plasma vitamin B12 levels. Values below approximately 170–250 pg/mL (120–180 picomol/L) for adults indicate a vitamin B12 deficiency. An elevated serum homocysteine level (values >13 micromol/L) might suggest a vitamin B12 deficiency. However, this indicator has poor specificity because it is influenced by other factors, such as low vitamin B6 or folate levels. Elevated methylmalonic acid levels (values >0.4 micromol/L) might be a more reliable indicator of vitamin B12 status because they indicate a metabolic change that is highly specific to vitamin B12 deficiency.

Vitamin B12 deficiency can lead to neurological changes, such as numbness and tingling in the hands and feet, but the neurological symptoms of vitamin B12 deficiency can occur without anemia, so early diagnosis and intervention is important to avoid irreversible damage.

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Typically, vitamin B12 deficiency is treated with vitamin B12 injections, since this method bypasses potential barriers to absorption. However, high doses of oral vitamin B12 may also be effective. Overall, an individual patient’s ability to absorb vitamin B12 is the most important factor in determining whether vitamin B12 should be administered orally or via injection.

Large amounts of folic acid can mask the damaging effects of vitamin B12 deficiency by correcting the megaloblastic anemia caused by vitamin B12 deficiency without correcting the neurological damage that also occurs. Moreover, preliminary evidence suggests that high serum folate levels might not only mask vitamin B12 deficiency, but could also exacerbate the anemia and worsen the cognitive symptoms associated with vitamin B12 deficiency.

The main causes of vitamin B12 deficiency include vitamin B12 malabsorption from food, pernicious anemia, postsurgical malabsorption, and dietary deficiency. The following groups are among those most likely to be vitamin B12 deficient: older adults; individuals with pernicious anemia, gastrointestinal disorders; people who have had gastrointestinal surgery; vegetarians including pregnant and lactating women who follow strict vegetarian diets and their infants.

Considering the health benefits of vitamin B12, it is definitely worth mentioning that evidence supports a role for folic acid and vitamin B12 supplements in lowering homocysteine levels, but results from several large prospective studies have not shown that these supplements decrease the risk of cardiovascular disease. Researchers also have long been interested in the potential connection between vitamin B12 deficiency and dementia. Observational studies show positive associations between elevated homocysteine levels and the incidence of both Alzheimer’s disease and dementia. Low vitamin B12 status has also been positively associated with cognitive decline. Finally, due to its role in energy metabolism, vitamin B12 is frequently promoted as an energy enhancer and an athletic performance and endurance booster.

Vitamin B12 because has low potential for toxicity, no adverse effects were recorded.

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Recommended Daily Allowances (RDAs) for Vitamin B12

Study results confirm that vitamin B12 supplementation (in combination with folic acid and vitamin B6) did not cause any serious adverse events when administered at doses of 0.4 mg for 40 months.

Age	Male	Female	Pregnancy	Lactation
0–6 months*	0.4 mcg	0.4 mcg
7–12 months*	0.5 mcg	0.5 mcg
1–3 years	0.9 mcg	0.9 mcg
4–8 years	1.2 mcg	1.2 mcg
9–13 years	1.8 mcg	1.8 mcg
14+ years	2.4 mcg	2.4 mcg	2.6 mcg	2.8 mcg

* Adequate Intake (AI)

Food Sources of Vitamin B12

Vitamin B12 is naturally found in animal products, including fish, meat, poultry, eggs, milk, and milk products. Vitamin B12 is generally not present in plant foods, but fortified breakfast cereals are a readily available source of vitamin B12 with high bioavailability for vegetarians. Some nutritional yeast products also contain vitamin B12. Fortified foods vary in formulation, so it is important to read the Nutrition Facts labels on food products to determine the types and amounts of added nutrients they contain.

Food	Micrograms (mcg) per serving	Percent DV*
Cooked clams, 3 ounces	84.1	3,504
Cooked beef liver, 3 ounces	70.7	2,946
Cooked wild rainbow trout, 3 ounces	5.4	225
Cooked sockeye salmon, 3 ounces	4.8	200
Cooked farmed rainbow trout, 3 ounces	3.5	58
Canned in water tuna fish, 3 ounces	2.5	104
Fortified nutritional yeasts, 1 serving	2.4	100
Cheeseburger, double patty and bun, 1 sandwich	2.1	88
Cooked haddock, 3 ounces	1.8	75
Broiled beef top sirloin, 3 ounces	1.4	58
Milk, low-fat, 1 cup	1.2	50
Yogurt, fruit, low-fat, 8 ounces	1.1	46
Swiss cheese, 1 ounce	0.9	38
Beef taco, 1 soft taco	0.9	38
Roasted cured ham, 3 ounces	0.6	25
Fortified breakfast cereals, 1 serving	0.6	25
Hardboiled egg, 1 large	0.6	25
Roasted chicken breast, 3 ounces	0.3	13

Selected Food Sources of Vitamin B12

• *DV = Daily Value.
• The DV for is is 2.4 mcg for adults and children age 4 years and older.
• Foods providing 20% or more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet.

References

• Elson Haas. “Staying Healthy with Nutrition”
• U.S. Department of Health & Human Services: https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/