Vitamins

Definition

Vitamins are organic components in food that are needed for growth and for maintaining good health. They include the fat-soluble vitamins, such as vitamin D, vitamin E, vitamin A, and vitamin K; and the watersoluble vitamins, such as folate (folic acid), vitamin B₁₂, biotin, vitamin B₆, niacin, thiamine, riboflavin, pantothenic acid, and vitamin C (ascorbic acid). Vitamins are required in the diet in only tiny amounts, in contrast to the energy components (sugars, starches, fats, and oils).

Purpose

All of the vitamins serve several important functions in the body and provide many health benefits. Therefore, a lack of a particular vitamin in the diet can cause a corresponding vitamin-deficiency disease.

Vitamin D, which helps to fight infection, is available from butter, cream, salmon, egg yolks, and adequate sun exposure. Vitamin D deficiency in children is called rickets, which is a disease of the bones. Symptoms include knocked knees, bowed legs, and protruding chests. Osteomalacia is the adult form of vitamin D deficiency; it is a result of low calcium intakes or lack of sun exposure during childhood and the adult years. Vitamin E acts as an antioxidant in the body for cells that are highly exposed to oxygen, and it resists hemolysis of red blood cells. Vitamin E is very widespread in food, so a deficiency is rare. However, when vitamin E deficiency does occur, it can cause serious nerve damage. Hemolytic anemia results if there is a vitamin E deficiency. A deficiency in vitamin E usually occurs only in premature babies due to the fact that they are born before the vitamin can be transferred to the developing baby during the last few weeks of pregnancy.

Vitamin A functions in maintaining vision, immune defense, bone development, cell growth, and reproduction. Food sources of vitamin A include fortified milk, spinach, carrots, and sweet potatoes. Vitamin A deficiency is common throughout the poorer parts of the world, and causes night blindness (nyctalopia). Severe vitamin A deficiency can result in xerophthalamia, a disease that, if left untreated, results in total blindness. Dry, scaly skin (hyperkeratosis) also results from vitamin A deficiency. Vitamin K is a nutrient that is essential for blood clotting; it can be obtained from intestinal bacteria. It is also present in dark-green leafy vegetables, cabbage, liver, eggs, cereals, and fruit. Vitamin K deficiency results in spontaneous bleeding. It can be affected by mineral oil, antibiotics, and anticoagulants.

Folate, also known as folic acid, is required for the synthesis of new cells in the body. Mild or moderate folate deficiency is common throughout the world, and can result from the failure to eat green leafy vegetables or fruits and fruit juices. Folate deficiency causes megaloblastic anemia, which is characterized by the presence of large abnormal cells, called megaloblasts, in the circulating blood. The symptoms of megaloblastic anemia are tiredness and weakness. Folic-acid deficiency is also associated with neuraltube birth defects such as spina bifida and anencephaly. These serious congenital malformations are the result of inadequate folate intake during pregnancy. Neural-tube defects occur early in pregnancy before most women even know they are pregnant, so it is essential that women of childbearing age receive adequate amounts of folate before they become pregnant, as well as during pregnancy. For this reason, folate fortification of enriched flour, breads, rice, and other grain products was approved by the U.S. Food and Drug Administration (FDA) in 1996. Vitamin B₁₂ helps make red blood cells in the body and protect nerve fibers. A deficiency occurs with the failure to consume sufficient meat or milk or other dairy products. Vitamin B₁₂ deficiency causes pernicious anemia, which is the result of a lack of the intrinsic factor needed for the absorption of vitamin B₁₂. A deficiency of vitamin B₁₂ can be masked by folate deficiency.

The B vitamins niacin, thiamine, and riboflavin each play a role in the energy metabolism of cells. Niacin can be found in such foods as tuna, chicken, mushrooms, and baked potatoes. A deficiency of niacin results in the disease known as pellagra, which involves skin rashes, scabs, diarrhea, and mental depression. Thiamine is found in pork, ham, green leafy vegetables, legumes, and whole-grain cereals. Thiamine deficiency results in beriberi, a disease resulting in atrophy, weakness of the legs, nerve damage, and heart failure. Vitamin C helps protect against infection and enhances the absorption of iron. Orange juice, grapefruit, broccoli, green peppers, and brussels sprouts are significant sources of vitamin C. A deficiency results in scurvy, a disease that contributes to the breakdown of collagen, which causes loose teeth, bleeding gums, and swollen wrists and ankles. Specific diseases uniquely associated with deficiencies in vitamin B₆, riboflavin, or pantothenic acidhave not been found in humans, though people who have been starving, or consuming poor diets for several months, might be expected to be deficient in most of the nutrients, including vitamin B₆, riboflavin, and pantothenic acid. Homocystinurias, a group of autosomal recessive disorders, are associated with low levels of folate and vitamins B₆ and B₁₂.

Some of the vitamins serve only one function in the body, while other vitamins serve a variety of unrelated functions. Hence, some vitamin deficiencies tend to result in one type of defect, while other deficiencies result in a variety of problems.

Purpose

People are treated with vitamins for three reasons. The primary reason is to relieve a vitamin deficiency when one has been detected. Chemical tests suitable for the detection of all vitamin deficiencies are available. The diagnosis of vitamin deficiency is often aided by visual tests, such as the examination of blood cells with a microscope, the x-ray examination of bones, or a visual examination of the eyes or skin.

A second reason for vitamin treatment is to prevent the development of an expected deficiency. In this case, vitamins are administered even with no test for possible deficiency. One example is vitamin K treatment of newborn infants to prevent bleeding. Food supplementation is another form of vitamin treatment. The vitamin D added to foods serves the purpose of preventing the deficiency from occurring in people who may not be exposed much to sunlight and who fail to consume foods that are fortified with vitamin D, such as milk. Niacin supplementation prevents pellagra among people who rely on corn as the main source of food and who do not eat much meat or milk. In general, the U.S. food supply is fortified with niacin.

A third reason for vitamin treatment is to reduce the risk for diseases that may occur even when vitamin deficiency cannot be detected by chemical tests. One example is folate deficiency. The risk for cardiovascular disease can be slightly reduced for a large fraction of the population by folic-acid supplements. These supplements can also sharply reduce the risk for certain birth defects.

Vitamin treatment is important during specific diseases where the body's normal processing of a vitamin is impaired. In these cases, high doses of the needed vitamin can force the body to process or utilize it in the normal manner. One example is pernicious anemia, a disease that tends to occur in middle age or old age; it impairs the absorption of vitamin B₁₂. Surveys have revealed that about 0.1% of the general population, and 2% to 3% of the elderly, may have the disease. If left untreated, pernicious anemia leads to nervous-system damage. The disease can easily be treated with large daily oral doses of vitamin B₁₂ (hydroxocobalamin) or with monthly injections of the vitamin.

Vitamin supplements are widely available as over-the-counter products. But whether they work to prevent or curtail certain illnesses, particularly in people with a balanced diet, is a matter of debate and ongoing research. For example, vitamin C is not proven to prevent the common cold, yet millions of people take it for that reason.

Precautions

Vitamin A and vitamin D can be toxic in high doses; side effects range from dizziness to kidney failure. High doses of niacin can be toxic to the liver, while excessive intake may occur with vitamin C, especially among the elderly. Doses of vitamin K can have toxic effects in infants. A physician or pharmacist should be consulted about the correct use of a multivitamin supplement that contains these vitamins.

Description

Vitamin treatment is usually done in three ways: by replacing a poor diet with one that supplies the recommended dietary allowance (RDA), by consuming oral supplements, or by injections. Injections are useful for persons with diseases that prevent absorption of fat-soluble vitamins. Oral vitamin supplements are especially useful for people who otherwise cannot or will not consume food that is a good vitamin source, such as meat and dairy products. For example, a vegetarian who will not consume meat may be encouraged to consume oral supplements of vitamin B₁₂.

Treatment of genetic diseases that impair the absorption or utilization of specific vitamins may require megadoses of the vitamin throughout one's lifetime. Megadose means a level of about 10 to 1,000 times greater than the RDA for a particular vitamin. Pernicious anemia, homocystinuria, and biotinidase deficiency are three examples of genetic diseases that are treated with megadoses of vitamins.

Preparation

The diagnosis of a vitamin deficiency usually involves a blood test. An overnight fast is usually recommended as preparation prior to the blood test so that vitamin-fortified foods do not affect the test results.

Aftercare

The response to vitamin treatment can be monitored by chemical tests, by an examination of red or white blood cells, or by physiological tests, depending on the exact vitamin deficiency.

KEY TERMS

Anencephaly— A neural-tube defect that causes lack of brain formation and results in death shortly after birth.

Antioxidant— A compound that prevents other compounds from being damaged by oxygen by reacting with oxygen itself.

Genetic disease— A disease that is passed from one generation to the next but does not necessarily appear in each generation. An example of genetic disease is Down syndrome.

Neural tube defects— A group of birth defects that affect the brain and spinal cord.

Recommended dietary allowance (RDA)— The recommended dietary allowances (RDAs) are quantities of nutrients of the diet that are required to maintain human health. RDAs are established by the Food and Nutrition Board of the National Academy of Sciences and may be revised every few years. A separate RDA value exists for each nutrient. The RDA values refer to the amount of nutrient expected to maintain health in the greatest number of people.

Vitamin status— The state of vitamin sufficiency or deficiency of any person. For example, a test may reveal that a patient's folate status is sufficient, borderline, or severely inadequate.

Complications

Although there are few complications associated with vitamin treatment, possible risks depend on the vitamin and the reason why it was prescribed. In general, the higher the dose that is taken, the higher the risk of toxicity. It is also important to remember that vitamins are better absorbed from food rather than in concentrated pill form. Physicians or pharmacists should be consulted about how and when to take vitamin supplements, particularly those that have not been prescribed by a physician.

Health care team roles

Dietitians can provide a wide range of information concerning the well-balanced diet that is necessary to receive adequate amounts of all the vitamins. Dietitians also play an important role in educating people about the dangers of consuming too much or too little of a particular vitamin. When a particular vitamin deficiency is present, consulting a dietitian, pharmacist, or physician about how and when to take vitamin supplements is advised.

Resources

BOOKS

Brody, T. Nutritional Biochemistry. San Diego: Academic Press, 1998.

Food and Nutrition Board. Recommended Dietary Allowances, 10th Edition. Washington, DC: National Academy Press, 1989.

Sizer, F., and E. Whitney. Nutrition: Concepts and Controversies, 7th Edition. Belmont, CA: Wadsworth, 1997.

Worthington-Roberts, B. S., and S. Rodwell Williams. Nutrition Throughout the Life Cycle, 4th Edition. Boston: McGraw-Hill, 2000.