Sources of VitaminsVegetables, fruits, grains, and nuts constitute a rich source of vitamins. Most vitamins come from plant foods, but a few are found only in animal products.Charles D. Winters/Photo Researchers, Inc.
Vitamin, any of the organic (carbon-containing) compounds that the body requires in small amounts to maintain health and function properly. Children additionally need vitamins to grow. The body gets most of its vitamins from the foods we eat. A healthy diet with plenty of fruits and vegetables should provide nearly all of the vitamins a person needs. See also Human Nutrition.
Scientists have classified 13 compounds as vitamins. They have given most of these vitamins letter or letter plus number names, such as A, B12, and D. Most vitamins are produced by plants. Some, such as vitamin D, are produced only by animals. A few vitamins are made by the body itself. For example, bacteria in the digestive tract help produce vitamin K, and the skin uses sunlight to produce vitamin D.
Vitamins are also manufactured for sale as supplements for people who need additional vitamins to meet their body’s requirements. For example, doctors often prescribe vitamin supplements for pregnant and nursing women to provide the additional nutrients needed by a rapidly growing fetus or infant. Older adults may not meet their vitamin requirements through food because the body’s ability to absorb vitamins is impaired with age.
In the United States, since 1940, the Food and Nutrition Board of the National Research Council has published recommended dietary allowances (RDA) for vitamins, minerals, and other nutrients. Expressed in milligrams (mg) or international units (IU) for adults and children of normal health, these recommendations provide useful guidelines for daily intake. Such guidelines are useful not only for professionals in nutrition but also for the growing number of families and individuals who eat irregular meals and rely on prepared foods, most of which are now required to carry nutritional labeling.
II VITAMINS ARE VITAL
Christiaan EijkmanDutch physician Christiaan Eijkman won the 1929 Nobel Prize in physiology or medicine. Eijkman discovered the substance in food, later identified as a B vitamin, whose deficiency caused beriberi.© The Nobel Foundation
The word vitamine, later shortened to vitamin, was coined by Polish-American chemist Casimir Funk in the early 20th century. Funk was searching for the then-unknown substance in foods that prevents such diseases as beriberi, rickets, and scurvy. After experiments on pigeons, Funk guessed that an amine—a compound containing nitrogen and hydrogen—in foods was responsible. He called this substance a vital (necessary for life) amine, or vitamine. Scientists later discovered that not all vitamins were amines, however.
Vitamins help the body carry out essential biochemical processes. Vitamins generally combine with proteins to create enzymes that promote chemical reactions. These enzymes play an important role in metabolism—the reactions that break down the fats, carbohydrates, and proteins in food so that the body can use them for energy and cell repair. The enzymes also promote reactions involved in the formation of bone, hormones, blood cells, nervous-system chemicals, and genetic material. Without vitamins, many of these reactions would slow down or cease. The different vitamins are not chemically related, and most differ in their actions in the body.
Deficiency of particular vitamins can lead to various diseases. Too little vitamin C, for example, can cause rickets, a disease in which the bones fail to develop properly. Bowlegs or “knock knees” are signs of rickets. Rickets and the other vitamin-deficiency diseases that interested Casimir Funk have largely disappeared in the developed world as the result of fortified foods and improved nutrition. But these diseases still occur in the developing world, especially where malnutrition is common.
Interest in vitamins grew during the 1990s, especially in those vitamins that act as antioxidants—vitamins A, C, and E. Antioxidants neutralize molecules known free radicals that cause cell damage. Free radicals have been linked to a number of disorders, including Alzheimer’s disease, arteriosclerosis, cancer, diabetes, and Parkinson disease. Researchers theorized that increased doses of antioxidants might also prevent and even cure such diseases. These claims have so far not panned out, however, as scientific studies have failed to demonstrate the preventive abilities of antioxidants. Research on antioxidants and other vitamins continues.
III KINDS OF VITAMINS
Scientists classify the 13 well-identified vitamins as fat-soluble or water-soluble. The fat-soluble vitamins—A, D, E, and K—dissolve in fat and are generally consumed in foods that contain fat. Because these vitamins also can be stored in the body’s fat, we do not have to consume them every day. The water-soluble vitamins—the eight B vitamins and vitamin C—do not dissolve in fat and cannot be stored. They pass from the body in urine and must be consumed frequently, preferably daily.
Vitamins are unstable and can be destroyed during the cooking or processing of foods. Heat in combination with water can remove water-soluble vitamins from food. Because many vegetables are rich in these vitamins, nutritionists advise people to cook vegetables by steaming, roasting, or microwaving rather than by boiling in water. The longer vegetables cook, the more vitamins they lose. In addition, some vitamins can be destroyed by exposure to sunlight or air. Frozen vegetables, which generally are frozen while still fresh, are often preferable to vegetables that have been shipped long distances. Fresh vegetables should be cooked soon after purchase.
A Vitamin A
CarrotsAn important source of vitamin A is carotene, an orange pigment found in such vegetables and fruits as carrots, squash, sweet potatoes, tomatoes, and cantaloupes.Dorling Kindersley
Vitamin A is a pale yellow, fat-soluble substance. It is formed from an orange pigment in plants called carotene, which animals convert into vitamin A. Vitamin A plays an important role in cell growth, vision, and the immune system. It helps skin develop and stay healthy and promotes the growth of bones and teeth. It is present in the retina (the light-sensitive membrane at the back of the eye) and aids vision in low light. It also helps maintain the mucous membranes that trap microbes and fight infection.
Vitamin A deficiency is rare in the developed world but more common in the developing world. An early symptom is night blindness (difficulty in adapting to darkness). The skin, eyes, and mucous membranes may also become extremely dry. The long-term risks of excess vitamin A are not yet certain. Some studies indicate that too much vitamin A—more than 1,500 micrograms per day—over a prolonged period may reduce bone density and increase the likelihood of fractures. In the short term too much vitamin A can cause headaches, nausea, blurred vision, and dizziness.
The body obtains vitamin A in two ways. One is by manufacturing it from beta-carotene, a vitamin precursor found in orange vegetables, such as carrots, squash, and sweet potatoes, as well as in broccoli, spinach, and other dark green, leafy vegetables. The other way of obtaining vitamin A is by absorbing it from plant-eating animals. In animal form, vitamin A is found in milk, butter, cheese, eggs, liver, and fish.
B The B Vitamins
Known also as vitamin B complex, the eight B vitamins are water-soluble substances. A number of enzymes involved with converting food into useful energy contain one of the B vitamins. The B vitamins are chemically dissimilar but are found in many of the same foods. Several of the B vitamins are easily destroyed by heat.
B1 B1 (Thiamine)
GrainsGrains, the seeds produced from a number of cereal grasses, are among the most important food crops in the world. Whole grains are an important source of many B vitamins, including thiamine, riboflavin, and niacin.Dorling Kindersley
Vitamin B1, or thiamine, promotes the metabolism of carbohydrates, enabling these nutrients to release their energy. Thiamine also plays a role in the functioning of the nervous system, muscles, and heart. The body does not store thiamine and people who are malnourished may develop thiamine deficiency. Mild thiamine deficiency can cause fatigue, muscle weakness, and loss of appetite. Severe thiamine deficiency causes beriberi, a disease characterized by muscle weakness, swelling of the heart, and leg cramps. Beriberi may, in severe cases, lead to heart failure and death.
Many foods contain thiamine, but few supply it in concentrated amounts. Foods richest in thiamine are pork, liver, yeast, whole or enriched grains and cereals, nuts, seeds, and legumes (dried peas and beans). Milling of cereal removes those portions of the grain richest in thiamine; consequently, white flour and polished white rice may be lacking in the vitamin. Enrichment of flour and cereal products has largely eliminated the risk of thiamine deficiency in developed countries, although it still occurs in malnourished alcoholics, older adults, and people with certain chronic illnesses.
B2 B2 (Riboflavin)
Vitamin B2, or riboflavin, like thiamine, is involved in metabolism. In addition to metabolizing carbohydrates, fats, and protein, it helps maintain the mucous membranes of the respiratory system. Riboflavin is more stable in heat than thiamine, but it can be destroyed by light.
Signs of riboflavin deficiency generally appear in the skin and eyes. Cracks or open sores may develop on the skin, especially around the lips or nostrils. The eyes may become red, itchy, and sensitive to light. The best sources of riboflavin are liver, milk and dairy products, fish, dark green leafy vegetables, and whole or enriched grains and cereals.
B3 B3 (Niacin)
Atherosclerotic AortaResearchers have found that the B vitamin niacin helps lower blood levels of so-called bad cholesterol and raise levels of good cholesterol. A buildup of cholesterol-containing deposits in the arteries, as in the aorta shown here, slows blood flow and can lead to heart attacks.Science Pictures Limited/Corbis
Vitamin B3, also known as niacin and nicotinic acid, like other B vitamins helps release energy from nutrients. In addition it helps the proper functioning of the digestive system, nervous system, and skin. Researchers also have found that niacin acts on cholesterol levels in the blood, raising levels of high-density lipoproteins (HDLs, or the so-called good cholesterol) and lowering levels of low-density lipoproteins (LDLs, or “bad” cholesterol).
Mild niacin deficiency can cause loss of appetite, indigestion, nausea and vomiting, and weakness. Severe niacin deficiency causes pellagra, the first symptom of which is skin inflammation. Diarrhea, confusion, and irritability may follow. However, pellagra develops only when the amino acid tryptophan, found in eggs and milk, also is missing from the diet. The best sources of niacin are protein-rich foods such as liver, poultry, meat, and fish and seafood. It is also found in whole or enriched grains and cereals, legumes, and nuts. The body also makes niacin from tryptophan. Too much niacin can cause skin flushing. Large doses of niacin over long periods can lead to liver damage.
BananasBananas are an excellent source of vitamin B6. So are avocados and baked potatoes.Corbis
Vitamin B6 is necessary for the absorption and metabolism of carbohydrates and protein. It also plays roles in the use of fats in the body; in the formation of red blood cells; and in the functioning of nerve and muscle cells and the immune system. It is needed for the production of myelin (the material that surrounds the nerves) and of certain neurotransmitters—chemicals that carry messages between nerve cells. Vitamin B6 comes in three forms known as pyridoxine, pyridoxal, and pyridoxamine.
Vitamin B6 deficiency affects the skin and mucous membranes, causing itching, inflammation, and sores (resembling the symptoms of riboflavin and niacin deficiencies). The effect of a B6 deficiency on the nervous system includes confusion, depression, and insomnia. Vitamin B6 deficiency also can cause anemia. The best sources of vitamin B6 are whole grains and cereals, such as brown rice and oatmeal; pork; chicken (white meat); avocadoes; spinach; beans; potatoes; peanut butter; and bananas.
SushiFish and other seafood are good sources of vitamin B12. The Japanese diet includes lots of raw fish on top of small cakes of cooked rice, called sushi. Esbin-Anderson/The Image Works
Vitamin B12 is necessary for the formation of red blood cells and deoxyribonucleic acid (DNA), the genetic material found in all cells. It also is needed for the functioning of the nervous system. Vitamin B12 is also called cobalamin because it contains the metal cobalt.
Vitamin B12 is bound to protein in food. During digestion hydrochloric acid in the stomach frees the vitamin, which combines with another substance so it can be absorbed by the intestines. Vitamin B12 deficiency generally results from a breakdown in this process, so that the body absorbs too little of this vitamin. People with stomach or intestinal disorders are at particular risk of developing B12 deficiency. Older adults, too, often lose some ability to absorb the vitamin. Certain drugs also can interfere with the absorption of B12, particularly drugs that treat heartburn and acid reflux.
Early signs of B12 deficiency include memory problems, confusion, and other difficulties related to thought. Numbness and tingling in the arms and legs may follow. People who are unable to absorb vitamin B12 can develop pernicious anemia, a chronic condition in which the red blood cells are unable to deliver sufficient oxygen to body tissues. Pernicious anemia is treated by vitamin injections. Vitamin B12 is obtained only from animal sources, such as meat, fish, poultry, eggs, cheese, and milk. Vegans—strict vegetarians who eat no animal products—can obtain B12 from soy milk, cereals, and other products that have been fortified with the vitamin.
B6 Folate, or Folic Acid
AsparagusAsparagus is a good source of the B vitamin folate, or folic acid. Lentils, lima beans, black beans, and chickpeas also provide folate.Garry D. McMichael/Photo Researchers, Inc.
Folic acid, also known as folate or folacin, is a B vitamin needed for forming new body cells. Folate occurs naturally in foods; folic acid is the form produced synthetically for supplements. Folate is especially important during pregnancy and in infancy, when new cells are forming rapidly. Researchers have found that folate deficiency may contribute to certain birth defects, including spina bifida and anencephaly (see Neural Tube Defects). Pregnant women who do not receive enough folic acid may also give birth prematurely or to low-birthweight infants. For this reason the U.S. Department of Agriculture recommends that all women of child-bearing age take at least 0.4 mg of folic acid daily, because they may not know they are pregnant during the first weeks. Women should take 0.6 mg of folic acid during pregnancy and 0.5 mg while breastfeeding.
Enriched BreadManufacturers have been adding the B vitamin folic acid to bread, flour, and other products since 1998. The U.S. Food and Drug Administration instituted this practice to make sure that Americans get enough folic acid in their diets. Many other countries also enrich these foods with folic acid.Tom Tracy/FPG International, LLC
Folic acid deficiency can also lead to a form of anemia. In addition, low levels of folic acid may lead to elevated levels of homocysteine, an amino acid associated with blood flow through the arteries and heart disease. Some studies have found that folic acid supplements can lower homocysteine levels and may thereby lower the risk of heart attacks and strokes. Some evidence has linked folic acid deficiency with certain cancers and with depression. This evidence does not mean that folic acid supplements can prevent cancer, however. The rapid cell division that characterizes cancer may simply use up the body’s folate.
Dietary sources of folate include leafy green vegetables, asparagus, legumes, oranges and orange juice, liver, and whole grains. Under a fortification program instituted by the U.S. Food and Drug Administration in 1998, manufacturers now add folic acid to certain foods, including enriched bread, flour, pasta, and rice. Many other countries also fortify these foods with folate. Folic acid is destroyed in foods during processing and cooking, so it is important to avoid overcooking vegetables.
B7 Other B Vitamins
Pantothenic acid, or vitamin B5, plays a role in the metabolism of proteins, carbohydrates, and fats. It is found in most foods, and people rarely develop dietary deficiencies in it.
Biotin, also a B vitamin, plays a role in the formation of fatty acids and the release of energy from carbohydrates. It is produced by bacteria in the intestine and is also found in many foods. Biotin’s deficiency in humans is rare.
C Vitamin C
OrangesOranges and other citrus fruits, along with their juices, provide lots of vitamin C. This vitamin plays many useful roles in the body.Milt and Pattty Putnam/The Stock Market
Vitamin C is a water-soluble vitamin also known as ascorbic acid. It is important in the formation and repair of bones, teeth, and collagen—a substance in the skin, muscles, blood vessels, and other tissues. Vitamin C helps heal wounds and also helps the body absorb iron from plant foods.
During the 1970s Nobel Prize-winning chemist Linus Pauling became an advocate of the health benefits of vitamin C, claiming it could prevent a variety of ailments, especially colds. Many people began taking vitamin C supplements as a result. However, scientific studies have failed to find evidence to support Pauling’s claim, although there is some indication that vitamin C may reduce the time a cold lasts. In the 1990s much excitement surrounded findings that vitamin C and other antioxidants block some of the damage to the body caused by molecules called free radicals. These molecules are involved in aging and diseases such as cancer. Antioxidants do seem to slow the damage caused by cigarette smoke and other pollutants. However, research using vitamin C and other antioxidants to treat or prevent such diseases and disorders as asthma, cataracts, and cancer has produced mixed or inconclusive results. Researchers are designing improved studies.
Vitamin C deficiency can lead to weakness and fatigue, inflamed or bleeding gums, greater likelihood of infection, and poorer ability to heal. Scurvy is the classic manifestation of severe vitamin C deficiency. Its symptoms are due to loss of the cementing action of collagen and include hemorrhages, loosening of teeth, and cellular changes in the long bones of children. Although unused vitamin C is quickly excreted in the urine, physicians recommend that people not take more than 2,000 mg per day. Larger doses can result in upset stomachs and diarrhea. Sources of vitamin C include citrus fruits, fresh strawberries, cantaloupe, pineapple, and guava. Good vegetable sources are broccoli, brussels sprouts, tomatoes, spinach, kale, green peppers, cabbage, and turnips.
D Vitamin D
The Sunshine VitaminVitamin D is sometimes called the sunshine vitamin because it is manufactured by the skin after exposure to the ultraviolet radiation in sunlight. Ultraviolet radiation also causes freckles.Louis Goldman/Photo Researchers, Inc.
Vitamin D is necessary for keeping the bones and teeth strong and healthy. It performs this function by helping the intestines absorb calcium and by regulating levels of the minerals calcium and phosphorus in the blood. These minerals play a vital role in building bones and teeth. Calcium taken without sufficient vitamin D has little effect on maintaining bones. Vitamin D deficiency can lead to a softening of the bones known as rickets in children or contribute to osteoporosis (loss of bone mass). Recent research suggests that vitamin D may also bolster the immune system and help protect against some cancers. Vitamin D deficiency has also been associated with Alzheimer’s disease. Further research should clarify these potential risks and benefits.
Bone DeformationRickets can result from insufficient vitamin D in the diet or from insufficient amounts of ultraviolet radiation from the sun. Rickets can lead to skeletal deformation, such as vertebral or leg curvature. This X ray reveals bone deformation due to rickets.Biophoto Associates/Science Source/Photo Researchers, Inc.
Also called the sunshine vitamin, vitamin D is manufactured by the skin after exposure to the ultraviolet radiation of the Sun. It is also obtained from eggs; liver; salmon, tuna, sardines, and other fatty fish; and fortified milk. Rickets is a disease once common among children in northern cities who had little exposure to sunlight in winter. The introduction of milk fortified with vitamin D largely banished the disorder among children. However, infants fed only with breast milk may receive too little vitamin D; physicians generally recommend supplements to prevent rickets. Increased use of sunscreens to prevent skin cancer lowers the body’s exposure to ultraviolet radiation and increases the likelihood of vitamin D deficiency. In addition, African Americans and other people with dark skin pigmentation have less ability to produce vitamin D from ultraviolet radiation. Vitamin D supplements are recommended for people with dark skin who receive little exposure to sunlight.
Because vitamin D is fat-soluble and stored in the body, too much vitamin D taken regularly can cause health problems. Over time, excessive vitamin D consumption—more than 50 mg (2,000 IU) per day—can lead to vitamin D poisoning, causing nausea, vomiting, constipation, and weight loss. By raising the level of calcium in the blood, excess vitamin D also can lead to confusion and abnormal heartbeats. In addition, it can interfere with kidney function and result in deposits of calcium throughout the body, especially in the kidneys.
E Vitamin E
AlmondsAlmonds and other nuts are excellent sources of vitamin E. So are margarine and vegetable oils.Harry Taylor/Oxford Scientific Films
Vitamin E is a fat-soluble vitamin that acts in the body as an antioxidant. Vitamin E plays a role in forming red blood cells. The vitamin comes in eight different forms, but one form—alpha-tocopherol—has the greatest impact on the human body. This is the form found in most vitamin supplements.
Vitamin E is found in vegetable oils, margarine, wheat germ, whole grains, nuts, and leafy green vegetables. Although vitamin E deficiency is rare, people who eat extremely low-fat diets or who are unable to absorb dietary fats may be at risk and should consider supplements. Research on the benefits of vitamin E for preventing cataracts, heart disease, and certain cancers has so far provided inconsistent and inconclusive results.
Although vitamin E is stored in the body, overdoses appear to have lower toxic effects than do overdoses of other fat-soluble vitamins. Because vitamin E can prevent blood clots, high doses—more than 1,000 mg per day—over a prolonged period may lead to bleeding problems. People who take anticoagulants (blood-thinning drugs) such as warfarin should check with their physicians before taking vitamin E supplements.
F Vitamin K
SpinachSpinach is a good source of vitamin K. It also is rich in vitamin A, riboflavin, and iron.Deni Bown/Oxford Scientific Films
Vitamin K is a fat-soluble vitamin necessary mainly for the formation of blood clots. Without this vitamin, bleeding would not stop. Recent research indicates that vitamin K also plays a role in building bone. Results from the ongoing Nurses’ Health Study showed that women who got at least 110 mg of vitamin K a day had a lower-than-average risk of fracturing a hip.
Dietary sources of vitamin K include all leafy green vegetables, eggs, dairy products, soybeans, and liver. Vitamin K is also produced by bacteria in the intestine. People who eat a healthy diet rarely develop vitamin K deficiencies. Anticoagulant drugs work by interfering with the action of vitamin K. Physicians advise people taking an anticoagulant to keep their intake of vitamin K stable.