Physiological Functions

Vitamin B12 is required for folate absorption, storage and activation to its coenzyme forms. Thus vitamin B12 works in tandem with folate to support cellular replication and a deficiency in either vitamin can adversely affect the functions of both. The only unique role identified for the vitamin is the synthesis of myelin a constituent of the sheaths that protect nerve fibers. Neurological damage associated with a vitamin B12 deficiency can occur in spite of adequate folate intake.

Factors Affecting Availability

Vitamin B12 is generally stable and is minimally destroyed with normal methods of cooking and processing. However, up to a third of this vitamin can be destroyed if cooking time is prolonged.

Vitamin B12 is absorbed by both passive and facilitated diffusion. Only small amounts of the vitamin are passively absorbed so for optimal absorption, facilitated diffusion involving binding to intrinsic factor is required. Intrinsic factor, produced by gastric mucosa, promotes vitamin B12 absorption in the ileum. Gastric acidity also promotes availability of vitamin B12 by releasing it from the polypeptides to which it is bound in food sources. Decreased gastric acidity due to medications or age can significantly compromise vitamin B12 status.

Deficiency

Vitamin B12 deficiency is most commonly the result of inadequate absorption. Low intake can also contribute since vitamin B12 is exclusively found in foods of animal origin.

Megloblastic anemia is a clinical manifestation of vitamin B12 deficiency which reflects the dependence between folic acid and vitamin B12. Megaloblastic anemia caused by inadequate production of intrinsic factor is designated pernicious anemia. Neurological symptoms associated with vitamin B12 deficiency include progressive neuropathy with demyelination of the peripheral nerves advancing to the central nervous system. Numbness and tingling in the hands and feet, stiffness and weakness in the legs, and an unsteady gait are often observed. In the elderly, vitamin B12 deficiency is characterized by mental disorientation, memory loss, and a yellowish tint to the skin due to concurrent anemia and jaundice.

The dependence of folate metabolism on vitamin B12 can be bypassed by high dose folate supplements which will reverse the hematological changes, but not the neurological damage. To prevent the progressive irreversible neurological damage of undetected vitamin B12 deficiency, nonprescription folate supplements cannot exceed 400 mcg.

Toxicity

Vitamin B12 is not toxic in oral doses up to 100 mcg daily because absorptive efficiency decreases with increased dose.

The upper limit of safety for vitamin B12 has not been determined due to lack of data substantiating adverse effects at high intakes. The Food and Nutrition Board of the Institute of Medicine recommends that vitamin B12 should be consumed only from food sources to prevent intake of potentially toxic levels.

Requirements

The Daily Reference Intakes (DRI) for vitamin B12 are shown in the table below.

Life cycle	Vitamin B12 (mcg)
Infants
     0-6 months	.4
     7-12 months	.5
Children
     1-3 years	.9
     4-8 years	1.2
Males
     9-13 years	1.8
     14-18 years	2.4
     19-30 years	2.4
     31-50 years	2.4
     51-70 years	2.4
     >70 years	2.4
Females
     9-13 years	1.8
     14-18 years 	2.4
     19-30 years	2.4
     31-50 years	2.4
     51-70 years	2.4
     >70 	2.4
Pregnancy
     18 years	2.6
     19-30 years	2.6
     31-50 years	2.6
Lactation
     < 18 years	2.8
     19-30 years	2.8
     31-50 years	2.8

Dietary Sources

Vitamin B12 is obtained mainly from foods of animal origin. Some grain products and nondairy products intended to replace dairy foods may be fortified with vitamin B12 to provide dietary sources of this nutrient for vegetarians. See the table below for dietary sources of vitamin B12.

Vitamin B12  Content of Food

FOOD	               Vitamin B12 (mcg)
Liver, beef, 3.5 oz	112
Clams, steamed, 3.5 oz	99
Oysters, raw, 3 oz	         16
Crab, cooked, 3 oz	         9
Trout, cooked, 3.5 oz	5
Tuna, light, canned, 3.5 oz	1.8
Low fat yogurt, 1 cup	1.4
Milk, nonfat, 1 cup	.93
Frankfurters, beef, 1	.88
Pork chop, broiled, 3.5 oz	.6
Egg, cooked	         .50
Cheese, provolone, 1 oz	.41
Chicken, white meat, 3.5 oz .36
Chicken, dark meat, 3.5 oz 	.32
Cheddar cheese, 1 oz	.24