The Blood Maker
Vitamin B12 is unusual for a vitamin in that it contains cobalt. The name, cobalamin, derives from “cobal” from the word cobalt plus “amin” from the word vitamin (cobal-amin). During the 1930s, researchers were trying to identify an “antipernicious anemia factor.” Vitamin B12 was discovered simultaneously by two research teams, one in the United States and one in England. It was not until 1956 that the chemical structure of this complex vitamin was revealed. Compounds having B12 activity have cobalamin as part of their name. In the human body, the active coenzyme forms of vitamin B12 are methylcobalamin and deoxyadenosyl cobalamin. Vitamin B12 is the most complex of the vitamins. The crystals are a striking dark red in color.
Cobalamin Coenzyme forms are: Methylcobalamin Deoxyadenosyl cobalamin
Vitamin B12 supplements are usually in the form of cyanocobalamin, which is easily converted into the two active forms used in the body.
ROLES OF VITAMIN B12
One function of vitamin B12 is to convert homocysteine to methionine. One of the active forms of cobalamin, methylcobalamin, converts homocysteine using an enzyme that also requires folate. When folate gives up a methyl group, the methylcobalamin coenzyme becomes reactivated. This is important because, as mentioned,
homocysteine is a risk factor for cardiovascular disease. The methionine that is converted can go on to become part of SAMe. SAMe is a methyl group donor that can protect DNA and may be important in cancer prevention.
The other active form of vitamin B12, deoxyadenosyl cobalamin, has an important role in the production of energy from proteins and fats as you can see from Figure -21. Vitamin B12 is essential for the maintenance of the nervous system and for the synthesis of molecules involved in fatty acid biosynthesis. Vitamin B12 is needed to maintain the myelin sheath that surrounds nerve cells.
Vitamin B12 is needed to make red blood cells
Figure -21 Vitamin B12 in energy metabolism.
Vitamin B12 is one of the nutrients required for the synthesis of hemoglobin, the oxygen-carrying pigment in blood. Vitamin B12 is needed for DNA synthesis in the rapidly dividing cells of the bone marrow. Lack of vitamin B12 or folate can lead to the production of large, immature, hemoglobin-poor red blood cells. This
can result in pernicious anemia.
Circulating vitamin B12 is recovered by the liver and returned to the intestines in bile. Most of this vitamin B12 is reabsorbed from the intestines. Because of this efficient recycling, most people do not develop a deficiency, even with minimal intake.
Figure -22 Assimilation of cobalamin.
Pernicious anemia results from inadequate absorption of vitamin B12, which can be caused by damaged stomach cells. It is most common in those over 60 years of age. The damaged stomach cells cannot make enough hydrochloric acid and/or enough intrinsic factor. Even a more than adequate dietary intake will not help because the vitamin B12 is not absorbed through the intestines. With pernicious anemia, Vitamin B12 must be injected or absorbed through membranes directly into the bloodstream via a nasal spray or absorbed from under the tongue. It may take several years to develop deficiency symptoms as vitamin B12 is efficiently recycled. Whether the deficiency is from poor absorption or from a diet low in animal products, onset of symptoms is typically slow.
Folate annot be properly utilized if vitamin B12 is low or absent. This is because vitamin B12 is needed to convert methyl folate to its active form. If folate or vitamin B12 is absent, symptoms of folate anemia can be present. Folate anemia causes slowed DNA synthesis, which shows up first as defective red blood cells. Extra folate will clear up the anemia. However, if the anemia is due to lack of vitamin B12, then the nerves will continue to suffer damage from lack of vitamin B12. In this way folate can be dangerous because it can mask a vitamin B12 deficiency.
Deficiency of vitamin B12 first shows as a paralysis that begins in the extremities and moves inward. Correct identification of vitamin B12 deficiency can prevent permanent paralysis and nerve damage. Remember that there are no symptoms of anemia when folate levels are high and only the vitamin B12 levels are low.
VITAMIN B12 IN FOOD
Adults need only 2.4 micrograms (2.4 millionths of a gram) of vitamin B12 each day. This tiny amount, still billions of molecules, provides enough for all of our needs. Vitamin B12 is found only in foods made from animal products, such as meat, dairy products, and eggs; please refer to Graph 7. It is also available from nutritionally fortified yeast, although it is not found in brewer’s yeast.
Vitamin B12 is also found in many fortified vegetarian foods, such as soy milk. Vitamin B12 is made by bacteria in the colon. More research is needed to determine if enough vitamin B12 can be absorbed from the colon to be of significant value. Vitamin B12 is easily lost through leaching into water while cooking. One unusual thing about vitamin B12 is that microwave cooking inactivates it, as shown in Figure -23. Cooking in the oven or in soups and stews preserves the most vitamin B12.
Figure -23 Some cooking methods reduce vitamin B12 content.
Graph 7 Vitamin B12 amounts in some common foods.
Summary for Cobalamin—Vitamin B12
Main function: cell synthesis and red blood cells.
RDA: men and women, 2.4 mcg.
No toxicity reported from food or supplements.
Deficiency disease: pernicious anemia.
Healthy food sources: fortified cereals, nutritional yeast.
Degradation: easily destroyed by microwave cooking.
Coenzyme forms: methylcobalamin and deoxyadenosyl cobalamin.
Vitamin B12 is unusual for a vitamin in that it contains cobalt. The name, cobalamin, derives from “cobal” from the word cobalt plus “amin” from the word vitamin (cobal-amin). During the 1930s, researchers were trying to identify an “antipernicious anemia factor.” Vitamin B12 was discovered simultaneously by two research teams, one in the United States and one in England. It was not until 1956 that the chemical structure of this complex vitamin was revealed. Compounds having B12 activity have cobalamin as part of their name. In the human body, the active coenzyme forms of vitamin B12 are methylcobalamin and deoxyadenosyl cobalamin. Vitamin B12 is the most complex of the vitamins. The crystals are a striking dark red in color.
Cobalamin Coenzyme forms are: Methylcobalamin Deoxyadenosyl cobalamin
Vitamin B12 supplements are usually in the form of cyanocobalamin, which is easily converted into the two active forms used in the body.
ROLES OF VITAMIN B12
One function of vitamin B12 is to convert homocysteine to methionine. One of the active forms of cobalamin, methylcobalamin, converts homocysteine using an enzyme that also requires folate. When folate gives up a methyl group, the methylcobalamin coenzyme becomes reactivated. This is important because, as mentioned,
homocysteine is a risk factor for cardiovascular disease. The methionine that is converted can go on to become part of SAMe. SAMe is a methyl group donor that can protect DNA and may be important in cancer prevention.
The other active form of vitamin B12, deoxyadenosyl cobalamin, has an important role in the production of energy from proteins and fats as you can see from Figure -21. Vitamin B12 is essential for the maintenance of the nervous system and for the synthesis of molecules involved in fatty acid biosynthesis. Vitamin B12 is needed to maintain the myelin sheath that surrounds nerve cells.
Vitamin B12 is needed to make red blood cells
Figure -21 Vitamin B12 in energy metabolism.
Vitamin B12 is one of the nutrients required for the synthesis of hemoglobin, the oxygen-carrying pigment in blood. Vitamin B12 is needed for DNA synthesis in the rapidly dividing cells of the bone marrow. Lack of vitamin B12 or folate can lead to the production of large, immature, hemoglobin-poor red blood cells. This
can result in pernicious anemia.
ASSIMILATION OF VITAMIN B12
Vitamin B12 is normally bound to protein in food. Vitamin B12 is released from a protein by the digestive action of hydrochloric acid and pepsin as seen in Figure -22. The stomach then makes an intrinsic factor that binds to the vitamin B12. In the intestines, the complex of intrinsic factor and vitamin B12 is slowly absorbed into the bloodstream. Once in the blood, transport of vitamin B12 is dependent on specific binding proteins.Circulating vitamin B12 is recovered by the liver and returned to the intestines in bile. Most of this vitamin B12 is reabsorbed from the intestines. Because of this efficient recycling, most people do not develop a deficiency, even with minimal intake.
Figure -22 Assimilation of cobalamin.
DEFICIENCY OF VITAMIN B12
Deficiency of vitamin B12 is not usually from lack of intake, but rather from lack of absorption. Inadequate hydrochloric acid in the stomach will prevent vitamin B12 from being released from dietary proteins so it cannot be utilized. Lack of intrinsic factor can also prevent absorption.Pernicious anemia results from inadequate absorption of vitamin B12, which can be caused by damaged stomach cells. It is most common in those over 60 years of age. The damaged stomach cells cannot make enough hydrochloric acid and/or enough intrinsic factor. Even a more than adequate dietary intake will not help because the vitamin B12 is not absorbed through the intestines. With pernicious anemia, Vitamin B12 must be injected or absorbed through membranes directly into the bloodstream via a nasal spray or absorbed from under the tongue. It may take several years to develop deficiency symptoms as vitamin B12 is efficiently recycled. Whether the deficiency is from poor absorption or from a diet low in animal products, onset of symptoms is typically slow.
Folate annot be properly utilized if vitamin B12 is low or absent. This is because vitamin B12 is needed to convert methyl folate to its active form. If folate or vitamin B12 is absent, symptoms of folate anemia can be present. Folate anemia causes slowed DNA synthesis, which shows up first as defective red blood cells. Extra folate will clear up the anemia. However, if the anemia is due to lack of vitamin B12, then the nerves will continue to suffer damage from lack of vitamin B12. In this way folate can be dangerous because it can mask a vitamin B12 deficiency.
Deficiency of vitamin B12 first shows as a paralysis that begins in the extremities and moves inward. Correct identification of vitamin B12 deficiency can prevent permanent paralysis and nerve damage. Remember that there are no symptoms of anemia when folate levels are high and only the vitamin B12 levels are low.
VITAMIN B12 IN FOOD
Adults need only 2.4 micrograms (2.4 millionths of a gram) of vitamin B12 each day. This tiny amount, still billions of molecules, provides enough for all of our needs. Vitamin B12 is found only in foods made from animal products, such as meat, dairy products, and eggs; please refer to Graph 7. It is also available from nutritionally fortified yeast, although it is not found in brewer’s yeast.
Vitamin B12 is also found in many fortified vegetarian foods, such as soy milk. Vitamin B12 is made by bacteria in the colon. More research is needed to determine if enough vitamin B12 can be absorbed from the colon to be of significant value. Vitamin B12 is easily lost through leaching into water while cooking. One unusual thing about vitamin B12 is that microwave cooking inactivates it, as shown in Figure -23. Cooking in the oven or in soups and stews preserves the most vitamin B12.
Figure -23 Some cooking methods reduce vitamin B12 content.
Graph 7 Vitamin B12 amounts in some common foods.
Summary for Cobalamin—Vitamin B12
Main function: cell synthesis and red blood cells.
RDA: men and women, 2.4 mcg.
No toxicity reported from food or supplements.
Deficiency disease: pernicious anemia.
Healthy food sources: fortified cereals, nutritional yeast.
Degradation: easily destroyed by microwave cooking.
Coenzyme forms: methylcobalamin and deoxyadenosyl cobalamin.
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