VITAMIN B-12
What foods provide vitamin B12?
What is the Recommended Dietary Allowance for vitamin B12 for adults?
When is a deficiency of vitamin B12 likely to occur?
Who may need a vitamin B12 supplement to prevent a deficiency?
Individuals with pernicious anemia
Individuals with gastrointestinal disordersCaution: Folic acid may mask signs of vitamin B12 deficiency
What is the relationship between vitamin B12, homocysteine, and heart disease?
What is the health risk of too much vitamin B12?
Selected Food Sources of Vitamin B12
Vitamin B12 : What is it?
Vitamin B12, also called cobalamin,
is important to good health. It helps maintain healthy nerve cells and red blood
cells, and is also needed to make DNA, the genetic material in all cells (1-4).
Vitamin B12 is bound to the protein in food. Hydrochloric acid in the stomach
releases B12 from protein during digestion. Once released, B12 combines with a
substance called intrinsic factor (IF) before it is absorbed into the
bloodstream.
What foods provide vitamin B12?
Vitamin B12 is naturally
found in animal foods including fish, milk and milk products, eggs, meat, and
poultry. Fortified breakfast cereals are an excellent source of vitamin B12 and
a particularly valuable source for vegetarians (5, 6, 7). The table of selected
food sources of vitamin B12 suggests dietary sources of vitamin B12.
What is the Recommended Dietary Allowance for
vitamin B12 for adults?
The Recommended Dietary
Allowance (RDA) is the average daily dietary intake level that is sufficient to
meet the nutrient requirements of nearly all (97 to 98 percent) healthy
individuals in each life-stage and gender group (7). The 1998 RDAs for vitamin
B12 (in micrograms) for adults (7) are:
| Life-Stage | Men | Women | Pregnancy | Lactation |
| Ages 19+ | 2.4 mcg | 2.4 mcg | ||
| All ages | 2.6 mcg | 2.8 mcg | ||
| Results of two national surveys, the National Health and Nutrition Examination Survey (NHANES III-1988-91) (8) and the Continuing Survey of Food Intakes by Individuals (CSFII 1994-96) (7) found that most adult men and women consume recommended amounts of vitamin B12 (6-8). | ||||
When is a deficiency of vitamin B12 likely to
occur?
Diets of most adult Americans provide recommended intakes of vitamin B12, but
deficiency may still occur as a result of an inability to absorb B12 from food.
It can also occur in individuals with dietary patterns that exclude animal or
fortified foods (9). As a general rule, most individuals who develop a vitamin
B12 deficiency have an underlying stomach or intestinal disorder that limits the
absorption of vitamin B12 (10). Sometimes the only symptom of these intestinal
disorders is anemia resulting from B12 deficiency.
Characteristic signs of B12 deficiency include fatigue, weakness, nausea, constipation, flatulence (gas), loss of appetite, and weight loss (1, 3, 11). Deficiency also can lead to neurological changes such as numbness and tingling in the hands and feet (7, 12). Additional symptoms of B12 deficiency are difficulty in maintaining balance, depression, confusion, poor memory, and soreness of the mouth or tongue (13). Some of these symptoms can also result from a variety of medical conditions other than vitamin B12 deficiency. It is important to have a physician evaluate these symptoms so that appropriate medical care can be given.
Who may need a vitamin B12 supplement to prevent a
deficiency?
Individuals with pernicious
anemia
Pernicious anemia is a form of anemia that occurs when there is an
absence of intrinsic factor, a substance normally present in the stomach.
Vitamin B12 binds with intrinsic factor before it is absorbed and used by your
body (7,14,15). An absence of intrinsic factor prevents normal absorption of B12
and results in pernicious anemia.
Anyone with pernicious anemia usually needs intramuscular (IM) injections (shots) of vitamin B12. It is very important to remember that pernicious anemia is a chronic condition that should be monitored by a physician. Anyone with pernicious anemia has to take lifelong supplemental vitamin B12.
Individuals with gastrointestinal disorders
Individuals with stomach and small intestinal disorders may not
absorb enough vitamin B12 from food to maintain healthy body stores (16). Sprue
and celiac disease are intestinal disorders caused by intolerance to protein in
wheat and wheat products. Regional enteritis, localized inflammation of the
stomach or small intestine, also results in generalized malabsorption of vitamin
B12 (7). Excess bacteria in the stomach and small intestine also can decrease
vitamin B12 absorption.
Surgical procedures of the gastrointestinal tract such as surgery to remove all or part of the stomach often result in a loss of cells that secrete stomach acid and intrinsic factor (7, 17, 18). Surgical removal of the distal ileum, a section of the intestines, also can result in the inability to absorb B12. Anyone who has had either of these surgeries usually requires lifelong supplemental B12 to prevent a deficiency.
Older Adults
Vitamin B12 must be separated from protein in food before it can bind with
intrinsic factor and be absorbed by your body. Bacterial overgrowth in the
stomach and/or atrophic gastritis, an inflammation of the stomach, contribute to
vitamin B12 deficiency in adults by limiting secretions of stomach acid needed
to separate vitamin B12 from protein in food (10, 20-24). Adults 50 years of age
and older with these conditions are able to absorb the B12 in fortified foods
and dietary supplements. Health care professionals may advise adults over the
age of 50 to get their vitamin B12 from a dietary supplement or from foods
fortified with vitamin B12 because 10 to 30 percent of older people may be
unable to absorb vitamin B12 in food (7, 19).
Vegetarians
Vegetarians who do not eat meats, fish, eggs, milk or milk products,
or B12 fortified foods consume no vitamin B12 and are at high risk of developing
a deficiency of vitamin B12 (9, 25). When adults adopt a vegetarian diet,
deficiency symptoms can be slow to appear because it usually takes years to
deplete normal body stores of B12. However, severe symptoms of B12 deficiency,
most often featuring poor neurological development, can show up quickly in
children and breast-fed infants of women who follow a strict vegetarian diet
(26).
Fortified cereals are one of the few plant food sources of vitamin B12, and are an important dietary source of B12 for vegetarians who consume no eggs, milk or milk products. Vegetarian adults who do not consume plant foods fortified with vitamin B12 need to consider taking a B12-containing supplement. Vegetarian mothers should consult with a pediatrician regarding appropriate vitamin B12 supplementation for their infants and children.
Caution: Folic acid may mask signs of vitamin B12
deficiency
Folic acid can correct the anemia that is caused by vitamin B12 deficiency.
Unfortunately, folic acid will not correct the underlying B12 deficiency (1, 27,
28). Permanent nerve damage can occur if vitamin B12 deficiency is not treated.
Folic acid intake from food and supplements should not exceed 1,000 micrograms
(mcg) daily because large amounts of folic acid can hide the damaging effects of
vitamin B12 deficiency (7). Adults older than 50 years are advised to consult
with their physician about the advisability of taking folic acid without also
taking a vitamin B12 supplement.
What is the relationship between vitamin B12,
homocysteine, and heart disease?
A deficiency of vitamin B12, folate, or vitamin B6 may increase your blood level
of homocysteine, an amino acid normally found in your blood. There is evidence
that an elevated blood level of homocysteine is an independent risk factor for
heart disease and stroke (29-38). The evidence suggests that high levels of
homocysteine may damage coronary arteries (34) or make it easier for blood
clotting cells called platelets to clump together and form a clot. However,
there is currently no evidence available to suggest that lowering homocysteine
level with vitamins will actually reduce your risk of heart disease. Clinical
intervention trials are needed to determine whether supplementation with vitamin
B12, folic acid, or vitamin B6 can help protect you against developing coronary
heart disease.
What is the health risk of too much vitamin B12?
Vitamin B12 has a very low potential for toxicity. The Institute of Medicine
states that "no adverse effects have been associated with excess vitamin B12
intake from food and supplements in healthy individuals (7)." The Institute
recommends that adults over 50 years of age get most of their vitamin B12 from
supplements or fortified food because of the high incidence of impaired
absorption of B12 from unfortified foods in this population (7).
Selected
Food Sources of Vitamin B12
As the 2000 Dietary Guidelines for Americans state, "Different foods contain
different nutrients and other healthful substances. No single food can supply
all the nutrients in the amounts you need" (39). As the following table
indicates, vitamin B12 is found naturally in animal foods. It is also found in
fortified foods such as fortified breakfast cereals. If you want more
information about building a healthful diet, refer to
Dietary Guidelines for Americans and the
Food
Guide Pyramid.
| Food | Micrograms
|
%DV*
|
| Beef liver, cooked, 3 oz |
60.0
|
1000
|
|
Fortified
breakfast cereals, (100%) fortified), 3/4 c |
6.0
|
100
|
| Trout, rainbow, cooked, 3 oz |
5.3
|
90
|
| Salmon, sockeye, cooked, 3 oz |
4.9
|
80
|
| Beef, cooked, 3 oz |
2.1
|
35
|
|
Fortified
breakfast cereals (25% fortified), 3/4 c |
1.5
|
25
|
| Haddock, cooked, 3 oz |
1.2
|
20
|
| Clams, breaded and fried, 3/4 c |
1.1
|
20
|
| Oysters, breaded and fried, 6 pieces |
1.0
|
15
|
| Tuna, white, canned in water, 3 oz |
0.9
|
15
|
| Milk, 1 cup |
0.9
|
15
|
| Yogurt, 8 oz |
0.9
|
15
|
| Pork, cooked, 3 oz |
0.6
|
10
|
| Egg, 1 large |
0.5
|
8
|
| American Cheese, 1 oz |
0.4
|
6
|
| Chicken, cooked, 3 oz |
0.3
|
6
|
| Cheddar cheese, 1 oz |
0.2
|
4
|
| Mozzarella cheese, 1 oz |
0.2
|
4
|
| * DV = Daily Value. DVs are reference numbers based on the Recommended Dietary Allowance (RDA). They were developed to help consumers determine if a food contains a lot or a little of a specific nutrient. The DV for vitamin B12 is 6.0 micrograms (mcg). The percent DV (%DV) listed on the nutrition facts panel of food labels tells adults what percentage of the DV is provided by one serving. Percent DVs are based on a 2,000 calorie diet. Your Daily Values may be higher or lower depending on your calorie needs. Foods that provide lower percentages of the DV also contribute to a healthful diet. | ||
This fact sheet was developed by the Clinical Nutrition Service, Warren Grant Magnuson Clinical Center, National Institutes of Health (NIH), Bethesda, MD, in conjunction with the Office of Dietary Supplements (ODS) in the Office of the Director of NIH. The mission of ODS is to strengthen knowledge and understanding of dietary supplements by evaluating scientific information, stimulating and supporting research, disseminating research results, and educating the public to foster an enhanced quality of life and health for the U.S. population. The Clinical Nutrition Service and the ODS would like to thank the expert scientific reviewers for their role in ensuring the scientific accuracy of the information discussed in this fact sheet
| Vitamin B12
Introduction -- Functions -- Dietary Sources -- Required Intakes IntroductionVitamin B12 is a member of the vitamin B complex. It contains cobalt, and so is also known as cobalamin. It is exclusively synthesised by bacteria and is found primarily in meat, eggs and dairy products. There has been considerable research into proposed plant sources of vitamin B12. Fermented soya products, seaweeds, and algae such as spirulina have all been suggested as containing significant B12. However, the present consensus is that any B12 present in plant foods is likely to be unavailable to humans and so these foods should not be relied upon as safe sources. Many vegan foods are supplemented with B12. Vitamin B12 is necessary for the synthesis of red blood cells, the maintenance of the nervous system, and growth and development in children. Deficiency can cause anaemia. Vitamin B12 neuropathy, involving the degeneration of nerve fibres and irreversible neurological damage, can also occur. FunctionsVitamin B12's primary functions are in the formation of red blood cells and the maintenence of a healthy nervous system. B12 is necessary for the rapid synthesis of DNA during cell division. This is especially important in tissues where cells are dividing rapidly, particularly the bone marrow tissues responsible for red blood cell formation. If B12 deficiency occurs, DNA production is disrupted and abnormal cells called megaloblasts occur. This results in anaemia. Symptoms include excessive tiredness, breathlessness, listlessness, pallor, and poor resistance to infection. Other symptoms can include a smooth, sore tongue and menstrual disorders. Anaemia may also be due to folic acid deficiency, folic acid also being necessary for DNA synthesis. B12 is also important in maintaining the nervous system. Nerves are surrounded by an insulating fatty sheath comprised of a complex protein called myelin. B12 plays a vital role in the metabolism of fatty acids essential for the maintainence of myelin. Prolonged B12 deficiency can lead to nerve degeneration and irreversible neurological damage. When deficiency occurs, it is more commonly linked to a failure to effectively absorb B12 from the intestine rather than a dietary deficiency. Absorption of B12 requires the secretion from the cells lining the stomach of a glycoprotein, known as intrinsic factor. The B12-intrinsic factor complex is then absorbed in the ileum (part of the small intestine) in the presence of calcium. Certain people are unable to produce intrinsic factor and the subsequent pernicious anaemia is treated with injections of B12. Vitamin B12 can be stored in small amounts by the body. Total body store is 2-5mg in adults. Around 80% of this is stored in the liver. Vitamin B12 is excreted in the bile and is effectively reabsorbed. This is known as enterohepatic circulation. The amount of B12 excreted in the bile can vary from 1 to 10ug (micrograms) a day. People on diets low in B12, including vegans and some vegetarians, may be obtaining more B12 from reabsorption than from dietary sources. Reabsorption is the reason it can take over 20 years for deficiency disease to develop in people changing to diets absent in B12. In comparison, if B12 deficiency is due to a failure in absorption it can take only 3 years for deficiency disease to occur. Dietary SourcesThe only reliable unfortified sources of vitamin B12 are meat, dairy products and eggs. There has been considerable research into possible plant food sources of B12. Fermented soya products, seaweeds and algae have all been proposed as possible sources of B12. However, analysis of fermented soya products, including tempeh, miso, shoyu and tamari, found no significant B12. Spirulina, an algae available as a dietary supplement in tablet form, and nori, a seaweed, have both appeared to contain significant amounts of B12 after analysis. However, it is thought that this is due to the presence of compounds structurally similar to B12, known as B12 analogues. These cannot be utilised to satisfy dietary needs. Assay methods used to detect B12 are unable to differentiate between B12 and it's analogues, Analysis of possible B12 sources may give false positive results due to the presence of these analogues. Researchers have suggested that supposed B12 supplements such as spirulina may in fact increase the risk of B12 deficiency disease, as the B12 analogues can compete with B12 and inhibit metabolism. The current nutritional consensus is that no plant foods can be relied on as a safe source of vitamin B12. Bacteria present in the large intestine are able to synthesise B12. In the past, it has been thought that the B12 produced by these colonic bacteria could be absorbed and utilised by humans. However, the bacteria produce B12 too far down the intestine for absorption to occur, B12 not being absorbed through the colon lining. Human faeces can contain significant B12. A study has shown that a group of Iranian vegans obtained adequate B12 from unwashed vegetables which had been fertilised with human manure. Faecal contamination of vegetables and other plant foods can make a significant contribution to dietary needs, particularly in areas where hygiene standards may be low. This may be responsible for the lack of aneamia due to B12 deficiency in vegan communities in developing countries. Good sources of vitamin B12 for vegetarians are dairy products or free-range eggs. ½ pint of milk (full fat or semi skimmed) contains 1.2 µg. A slice of vegetarian cheddar cheese (40g) contains 0.5 µg. A boiled egg contains 0.7 µg. Fermentation in the manufacture of yoghurt destroys much of the B12 present. Boiling milk can also destroy much of the B12. Vegans are recommended to ensure their diet includes foods fortified with vitamin B12. A range of B12 fortified foods are available. These include yeast extracts, Vecon vegetable stock, veggieburger mixes, textured vegetable protein, soya milks, vegetable and sunflower margarines, and breakfast cereals. Required IntakesThe old Recommended Daily Amounts (RDA's) have now been replaced by the term Reference Nutrient intake (RNI). The RNI is the amount of nutrient which is enough for at least 97% of the population.
Reference Nutrient Intakes for Vitamin B12, µg/day. (1000 µg = 1mg)
Pregnant women are not thought to require any extra B12, though little is known about this. Lactating women need extra B12 to ensure an adequate supply in breast milk. B12 has very low toxicity and high intakes are not thought to be dangerous |
