VITAMIN B-12 IN FOOD
Vitamin B-12, also knows as cobalamin, is unique among the vitamins on 2 accounts. First, food of animal origin, such as meat, poultry, fish, and dairy products, are the only reliable sources of vitamin B-12. Second, it is the only vitamin that contains a mineral (cobalt) as part of its structure.
Plants do not synthesize vitamin B-12. In fact, all vitamin B-12 compounds are synthesized exclusively by microorganisms, mainly bacteria. Animals acquire vitamin B-12 from soil ingested while eating and grazing. Ruminant animals, such as cows and sheep, also synthesize vitamin B-12 from bacteria in the multiple compartments of their stomach.
For humans, the sources of vitamin B-12 are foods of animal origin, such as meat, poultry, seafood, eggs, and dairy products. Especially rich sources of vitamin B-12 are organ meats, such as liver, kidneys, and heart, and fortified foods, such as ready to eat cereals. Although algae and fermented soy products, such as tempeh and miso, are sometimes advertised as being good plant sources of vitamin B-12, vegans should not rely on them to meet vitamin D-12 requirements. These foods often contain vitamin B-12 analogs (compounds similar to vitamin B-12) that do not function as vitamin B-12 in the body.
VITAMIN B-12 NEEDS AND UPPER LIMIT
The RDA for vitamin B-12 for adults is 2.4 μg/day. The Daily Value on food and supplement labels is 6 μg. On average, adult men consume 3 times the RDA and women consume 2 times the RDA. This high intake provides the average meat-eating person with a 2 to 3 years’ storage of vitamin B-12 in the liver. No adverse effects have been observed with excess vitamin B-12 intake from food or from supplements, so there is no Upper Level for this vitamin found so far.
FUNCTIONS OF VITAMIN B-12
Vitamin B-12 is required for 2 enzymatic reactions. First, the formation of the amino acid methionine from the amino acid homocysteine ia catalyzed by the enzyme methionine synthase, which requires the vitamin B-12 coenzyme methylcobalamin. Homocysteine accepts a methyl group from methylcobalmin, which forms methionine.
Methionine, in turn, is the source of S-adenosyl methionine (SAM). In many reactions, SAM served as a methyl donor. Methylation reactions are important for DNA and RNA regulation, myelin regulation, the the synthesis of many biological compounds. The methionine synthesis reaction also explains the close link between vitamin B-12 and folate: methylcobalamin obtains its methyl group from the folate coenzyme 5-methyl-tetrahydrofolate.
When the methyl group is donated to vitamin B-12, the folate coenzyme THFA is reformed. When vitamin B-12 is lacking, THFA declines and the symptoms of folate deficiency can ensue. When either folate or vitamin B-12 is lacking, methionine and SAM synthesis decline and the amount of homocysteine in the body increases.
VITAMIN B-12 DEFICIENCY
Researchers in mid-19th century England noted a form of anemia that causes death within 2 to 5 years if initial diagnosis. They called the disease pernicious anemia. It is now known that this disease can result from the inadequate production of the intrinsic factor required for vitamin B-12 absorption. Poor vitamin B-12 status is still fairly common today, and impaired absorption of the vitamin B-12 found in foods is most often to blame.
When Vitamin B-12 deficiency is severe enough that body stores are gone or almost gone, a megaloblastic, macrocytic anemia results. The anemia produced in vitamin B-12 deficiency is identical to that produced in a folate deficiency. Because a lack of vitamin B-12 impairs folate metabolism, normal DNA and red blood cells synthesis is disrupted, resulting in macrocytic anemia.
A vitamin B-12 deficiency produces severe nerve degeneration, which can be fatal. The neurological complications produce sensory disturbances in the legs, such as burning, tingling, prickling, and numbness. Walking is difficult and balance is seriously affected. Many mental problems exist as well, such as loss of concentration and memory, disorientation, and dementia. As the condition worsens, bowel and bladder control is lost. Visual disturbances are common, too. There are also numerous GI tract problems, ranging from a sore tongue to constipation. The neurological complications often precede the development of anemia.
ELEVATED PLASMA HOMOCYSTEINE CONCENTRATIONS
Poor vitamin B-12, folate, and vitamin B-6 status van each result in high circulating levels if the amino acid homocysteine. Many studies have shown that high homocysteine levels in the blood are a risk factor for heart attacks and strokes. Other studies have shown that high levels of plasma homocysteine also are associated with cognitive dysfunction and osteoporotic fractures.
Supplementing the diet with folate, vitamin B-12 and vitamin B-6 can reduce blood levels of homocysteine. However, the evidence that supplementing with these vitamins can reduce the diseases associated with high blood levels of homocysteine is not strong. Several studies that compared vitamin supplements with placebo treatments found that the supplements did not prevent heart disease, even though homocysteine levels declined. In addition, it appears that supplements of vitamin B-12, vitamin B06 and folate do not improve cognitive function. Even though B-vitamin supplements have not been shown to decrease the risk of heart disease or improve cognition, ample B-vitamin intakes are vital for normal physiological functions and good health.
PERSONS AT RISK OF VITAMIN B-12 DEFICIENCY
Poor vitamin B-12 status affects about 20% of older Americans. In the elderly, most cases are due to impaired absorption of vitamin B-12 in foods due to atrophic gastritis. This deficiency is not usually severe enough to produce anemia, but it can cause neurological problems and elevated blood homocysteine. The consumption of crystalline vitamin B-12, either as a dietary supplement, or in fortified foods, can improve vitamin B-12 status in older persons with or without gastric atrophy.
Those with malabsorption syndrome of any kind have an increased need for vitamin B-12. for those diagnosed with vitamin B-12 deficiency due to impaired absorption, 3 options are available: 1- monthly injections of vitamin B-12 to bypass the GI tract, 2-the use of a vitamin B-12 nasal gel, which also bypasses the GI tract, and 3- very high oral doses (1 to 2 mg) daily of vitamin B-12.
Vegetarians can also become vitamin B-12 deficient. However, if a person becomes a vegetarian in adulthood, vitamin B-12 stored in the liver delay the severe deficiency for a long time. Infants born to or breastfed by vegetarian or vegan mothers also can develop vitamin B-12 deficiency, accompanied with anemia and long-tern neurological problems, such as diminished brain growth, degeneration of the spinal cord, and poor intellectual development. Vegetarians have several options for obtaining vitamin B-12. If they are not vegans, they can obtain vitamin B-12 from dairy products and eggs. In addition, vitamin B-12 supplements and food products fortified with vitamin B-12 are available.