Nutrition and Diet Therapy: Vitamins
Vitamins are organic (carbon-containing) compounds that are essential in small amounts for body processes. Vitamins themselves do not provide energy. They enable the body to use the energy provided by carbohydrates, fats, and proteins. The name vitamin implies their importance. Vita in Latin, means life. They do not, however, represent a panacea (universal remedy) for physi-cal or mental illness or a way to alleviate the stressors in life. They should not be overused—more is not necessarily better. In fact, megadoses can be toxic (poisonous). In the past it was believed that a healthy person eating a balanced diet would obtain all the nutrients—including vitamins—needed. That was in the past. Today’s reality is such that with after-school sports, dance lessons, music practice or lessons, both parents working, and more, people are in a time and energy crunch. So in many homes, home-cooked family meals have been replaced by fast food, home delivery, vending machines, and processed foods. Most of these choices are not found in the fruit and vegetable recommendation from MyPyramid.
The existence of vitamins has been known since early in the twentieth century. It was discovered that animals fed diets of pure proteins, carbohy-drates, fats, and minerals did not thrive as did those fed normal diets that included vitamins.
Vitamins were originally named by letter. Subsequent research has shown that many of the vitamins that were originally thought to be a single substance are actually groups of substances doing similar work in the body. Vitamin B proved to be more than one compound—B1, B6, B12, and so on—and consequently is now known as B complex. Many of the 13 known vitamins are currently named according to their chemical composition or function in the body (Table 7-1).
Vitamins are found in minute amounts in foods. The specific amounts and types of vitamins in foods vary.
Dietary Requirements of Vitamins
Dietary Requirements
Since 1997, the Food and Nutrition Board of the Institute of Medicine has been establishing Dietary Reference Intakes (DRIs) to replace the Recommended Di-etary Allowances (RDAs) as outlined in Table 7-2. Tolerable Upper Limits (ULs) have also been set for some vitamins and minerals. The UL is the maximum level of daily intake unlikely to cause adverse effects and is not a recommended level of intake. Vitamin allowances are given by weight—milligrams (mg) or micrograms (μg or mcg).
Vitamin deficiencies can occur and can result in disease. Persons inclined to vitamin deficiencies because they do not eat balanced diets include alcoholics, the poor and incapacitated elderly, patients with serious diseases that affect appetite, mentally retarded persons, and young children who receive inadequate care. Also, deficiencies of fat-soluble vitamins occur in patients with chronic malabsorption diseases such as cystic fibrosis, celiac disease, and Crohn’s disease.
The term avitaminosis means “without vitamins.” This word followed by the name of a specific vitamin is used to indicate a serious lack of that partic-ular vitamin. Hypervitaminosis is the excess of one or more vitamins. Either a lack or excess of vitamins can be detrimental to a person’s health.
Vitamins taken in addition to those received in the diet are called vitaminsupplements. These are available in concentrated forms in tablets, capsules,and drops. Vitamin concentrates are sometimes termed natural or synthetic (manufactured). Some people believe that a meaningful difference exists between the two types and that the natural are far superior in quality to the synthetic. However, according to the U.S. Food and Drug Administration (FDA), the body cannot distinguish between a vitamin of plant or animal origin and one manu-factured in a laboratory because once they have been dismantled by the digestive system, the two types of the same vitamin are chemically identical.
Synthetic vitamins are frequently added to foods during processing. When this is done, the foods are described as enriched or fortified. Examples of these foods are enriched breads and cereals to which thiamine, niacin, ribofla-vin, folate, and the mineral iron have been added. Vitamins A and D are added to milk and fortified margarine.
Preserving Vitamin Content in Food
Occasionally, vitamins are lost during food processing. In most cases, food producers can replace these vitamins with synthetic vitamins, making the processed food nutritionally equal to the unprocessed food. Foods in which vitamins have been replaced are called restored foods.
Because some vitamins are easily destroyed by light, air, heat, and water, it is important to know how to preserve the vitamin content of food during its preparation and cooking. Vitamin loss can be avoided by the following:
• Buying the freshest, unbruised vegetables and fruits locally andusing them raw whenever possible
• Preparing fresh vegetables and fruits just before serving
• Heating canned vegetables quickly and in their own liquid
• Following package directions when using frozen vegetables or fruit
• Using as little water as possible when cooking and having it boiling before adding vegetables, or, preferably, steaming them
• Covering the pan, cooking vegetables until bright in color andcrisp tender
• Saving any cooking liquid for later use in soups, stews, and gravies
• Storing fresh vegetables and most fruits in a cool, dark place
• Microwave fruits and vegetables in 1 to 2 tablespoons of water
Classification of Vitamins
Classification
Vitamins are commonly grouped according to solubility. A, D, E, and K are fat-soluble, and B complex and C are water-soluble (Table 7-3). In addi-tion, vitamin D is sometimes classified as a hormone, and the B-complex group may be classified as catalysts or coenzymes. When a vitamin has different chemical forms but serves the same purpose in the body, these forms are sometimes called vitamers. Vitamin E is an example. Sometimes a precursor, or provitamin, is found in foods. This is a substance from which the body can synthesize (manufacture) a specific vitamin. Carotenoids are examples of precursors of vitamin A and are referred to as provitamin A.
FAT-SOLUBLE VITAMINS
The fat-soluble vitamins A, D, E, and K are chemically similar. They are not lost easily in cooking but are lost when mineral oil is ingested. Mineral oil is not absorbed by humans. Consequently, it may be used in salad dressings to avoid the calories of vegetable oils. It is sometimes used as a laxative by the elderly. Its use should be discouraged because it picks up and carries with it fat-soluble vitamins that are then lost to the body. After absorption, fat-soluble vitamins are transported through the blood by lipoproteins because they are not soluble in water. Excess amounts can be stored in the liver. Therefore, deficiencies of fat-soluble vitamins are slower to appear than are those caused by a lack of water-soluble vitamins. Because of the body’s ability to store them, megadoses of fat-soluble vitamins should be avoided, as they can reach toxic levels.
Vitamin A
Vitamin A consists of two basic dietary forms: preformed vitamin A, also called retinol, which is the active form of vitamin A; and carotenoids, the inactiveform of vitamin A, which are found in plants.
Functions.Vitamin A is a family of fat-soluble compounds that play an impor-tant role in vision, bone growth, reproduction, and cell division. Vitamin A helps regulate the immune system, which helps fight infections. Vitamin A has been labeled as an antioxidant when, in fact, provitamin A (carotenoids) is the part of the family that functions as an antioxidant. Antioxidants protect cells from freeradicals. Free radicals are atoms or groups of atoms with an odd (unpaired) num-ber of electrons and can be formed when oxygen interacts with certain molecules. Once formed, these highly reactive radicals can start a chain reaction. When they react with important cellular components such as DNA or cell membranes, the most damage occurs. Antioxidants have the capability of safely interacting with free radicals and stopping the chain reaction before vital cells are damaged.
The first organic free radical was discovered in 1900 by Moses Gomberg. In the 1950s, Denman Harman, M.D., was the first to propose the free radical theory of aging.
Sources.There are two forms of vitamin A: preformed vitamin A and pro-vitamin A. Retinol is a preformed vitamin A and is one of the most active and usable forms of vitamin A. Retinol can be converted to retinal and retinoic acid, other active forms of vitamin A.
Provitamin A carotenoids can be converted to vitamin A from darkly colored pigments, both green and orange, in fruits and vegetables. Common car-otenoids are beta-carotene, lutein, lycopene, and zeaxanthin. Beta-carotene is most efficiently converted to retinol. Eating “five-a-day” of fruits and vegetables is highly recommended. The best sources of beta-carotene are carrots, sweet pota-toes, spinach, broccoli, pumpkin, squash (butternut), mango, and cantaloupe.
Research has shown that regular consumption of foods rich in carotenoids decreases the risk of some cancers because of its antioxidant effect. Taking a beta-carotene supplement has not shown the same results.
Preformed vitamin A (retinol) is found in fat-containing animal foods such as liver, butter, cream, whole milk, whole-milk cheeses, and egg yolk. It is also found in low-fat milk products and in cereals that have been fortified with vitamin A, but these are not the best sources.
Requirements.A well-balanced diet is the preferred way to obtain therequired amounts of vitamin A. Vitamin A values are commonly listed as a retinolequivalent (RE). A retinol equivalent is 1 μg retinol or 6μg beta carotene. Referto the inside back cover of this text for the dietary reference intakes of vitamin A as prescribed by the Food and Nutrition Board of the Institute of Medicine.
Hypervitaminosis.The use of a single vitamin supplement should be dis-couraged because an excess of vitamin A can have serious consequences. Signs of hypervitaminosis A may include birth defects, hair loss, dry skin, headaches, nausea, dryness of mucous membranes, liver damage, and bone and joint pain. In general, these symptoms tend to disappear when excessive intake is discontinued.
Deficiency.Signs of a deficiency of vitamin A include night blindness;dry, rough skin; and increased susceptibility to infections. Avitaminosis A can result in blindness or xerophthalmia, a condition characterized by dry, luster-less, mucous membranes of the eye. Lack of vitamin A is the leading cause of blindness in the world (discounting accidents).
Vitamin D
Vitamin D exists in two forms—D2 (ergocalciferol) and D3 (cholecalciferol). Each is formed from a provitamin when irradiated with (exposed to) ultraviolet light. They are equally effective in human nutrition, but D3 is the one that is formed in humans from cholesterol in the skin. D2 is formed in plants. Vitamin D is consid-ered a prohormone because it is converted to a hormone in the human body.
Vitamin D is heat-stable and not easily oxidized, so it is not harmed by storage, food processing, or cooking.
Functions.The major function of vitamin D is the promotion of calciumand phosphorus absorption in the body. By contributing to the absorption of these minerals, it helps to raise their concentration in the blood so that nor-mal bone and tooth mineralization can occur and tetany (involuntary muscle movement) can be prevented. (Tetany can occur when there is too little cal-cium in the blood. This condition is called hypocalcemia.)
Vitamin D is absorbed in the intestines and is chemically changed in the liver and kidneys. Excess amounts of vitamin D are stored in the liver and in adipose tissue.
Sources.The best source of vitamin D is sunlight, which changes a pro-vitamin to vitamin D3 in humans. It is sometimes referred to as the sunshine vitamin. The amount of vitamin D that is formed depends on the individual’s pigmentation (coloring matter in the skin) and the amount of sunlight avail-able. The best food sources of vitamin D are milk, fish liver oils, egg yolk, butter, and fortified margarine. Because of the rather limited number of food sources of vitamin D and the unpredictability of sunshine, health authorities decided that the vitamin should be added to a common food. Milk was selected.
Consequently, most milk available in the United States today has had 10μg of vitamin D concentrate added per quart.
Requirements.Under the DRIs, there are several reference values in-cluded. Vitamin D levels are given as Adequate Intake levels, or AI (Table 7-4).
People who are seldom outdoors, those who use sunscreens, and those who live in areas where there is little sunlight for 3 to 4 months a year should be especially careful that their diets provide their AI levels of vitamin D. Drink-ing 2 cups of vitamin D–fortified fat-free milk each day will provide sufficient vitamin D to those between birth and 50 years of age. Between the ages of 51 and 70, 1 quart of such milk will be needed each day to fulfill the AI. After 70, 11 ⁄2 quarts will be needed daily. In this last age-group, a vitamin D supplement may be needed.
Vitamin D or, specifically, cholecalciferol values are given in micrograms (μg or mcg.) or in international units; 5μg equals 200 international units.
Hypervitaminosis.Hypervitaminosis D must be avoided because it cancause deposits of calcium and phosphorus in soft tissues, kidney and heart damage, and bone fragility.
Deficiency.The deficiency of vitamin D inhibits the absorption of calciumand phosphorus in the small intestine and results in poor bone and tooth forma-tion. Young children suffering vitamin D deficiency may develop rickets, which causes malformed bones and pain, and their teeth may be poorly formed, late in appearing, and particularly subject to decay. Adults lacking sufficient vitamin D may develop osteomalacia, softening of bones. A deficiency of vitamin D con-tributes to osteoporosis (brittle, porous bones).
Vitamin E
Vitamin E consists of two groups of chemical compounds. They are the tocoph-erols and the tocotrienols. There are four types of tocopherols: alpha, beta,delta, and gamma. The most biologically active of these is alpha-tocopherol.
Functions.Vitamin E is an antioxidant. It is aided in this process by vitaminC and the mineral selenium. It is carried in the blood by lipoproteins. When the amount of vitamin E in the blood is low, the red blood cells become vulnerable to a higher-than-normal rate of hemolysis. Vitamin E has been found helpful in the prevention of hemolytic anemia among premature infants. It also may enhance the immune system. Because of its antioxidant properties, it is commonly used in commercial food products to retard spoilage.
Sources. Vegetable oils made from corn, soybean, safflower, and cottonseed and products made from them, such as margarine, are the best sources of vitamin E. Wheat germ, nuts, and green leafy vegetables also are good sources. Animal foods, fruits, and most vegetables are poor sources.
Requirements. Research indicates that the vitamin E requirement increases if the amount of polyunsaturated fatty acids in the diet increases. In general, however, the U.S. diet is thought to contain sufficient vitamin E.
Hypervitaminosis. Although vitamin E appears to be relatively nontoxic, it is a fat-soluble vitamin, and the excess is stored in adipose tissue. Consequently, it would seem advisable to avoid long-term megadoses of vitamin E.
Deficiency. A deficiency of vitamin E has been detected in premature, low-birthweight infants and in patients who are unable to absorb fat normally. Malabsorption can cause serious neurological defects in children, but in adults it takes 5 to 10 years before deficiency symptoms occur.
Vitamin K
Vitamin K is made up of several compounds that are essential to blood clot-ting. Vitamin K1, commonly called phylloquinone, is found in dietary sources, especially green leafy vegetables such as spinach and in animal tissue. Vitamin K2, called menaquinone, is synthesized in the intestine by bacteria and is also found in animal tissue. In addition, there is a synthetic vitamin K, called mena-dione. Vitamin K is destroyed by light and alkalies.
Vitamin K is absorbed like fats, mainly from the small intestine and slightly from the colon. Its absorption requires a normal flow of bile from theliver, and it is improved when there is fat in the diet.
Functions. Vitamin K is essential for the formation of prothrombin, which permits the proper clotting of the blood. It may be given to newborns immediately after birth because human milk contains little vitamin K and the intestines of newborns contain few bacteria. With insufficient vitamin K, newborns may be in danger of intracranial hemorrhage (bleeding within the head). Vitamin K may be given to patients who suffer from faulty fat absorption; to patients after extensive antibiotic therapy (ingestion of antibiotic drugs to combat infection) because these drugs destroy the bacteria in the intestines; as an antidote for an overdose of anticoagulant (blood thinner such as warfarin sometimes sold as Coumadin or Warnerin); or to treat cases of hemorrhage.
Sources. The best dietary sources of vitamin K are green leafy vegetables such as broccoli, cabbage, spinach, and kale. Dairy products, eggs, meats, fruits, and cereals also contain some vitamin K. Cow’s milk is a much better source of vitamin K than human milk. The synthesis of vitamin K by bacteriain the small intestine does not provide a sufficient supply by itself. It must be supplemented by dietary sources.
Requirements.Vitamin K is measured in micrograms. The AI for vita-min K is 120μg for men and 90μg for women. This is not increased during pregnancy or lactation. Infants up to 6 months should have 2.0μg a day. Those between 6 months and 1 year should receive 2.5μg a day. Vitamin K must be ingested daily. What is absorbed today will be utilized immediately with very little storage in the liver.
Hypervitaminosis.Ingestion of excessive amounts of synthetic vitaminK can be toxic and can cause a form of anemia.
Deficiency.The only major sign of a deficiency of vitamin K is defectiveblood coagulation. This increases clotting time, making the client more prone to hemorrhage. Human deficiency may be caused by faulty fat metabolism, antacids, antibiotic therapy, inadequate diet, or anticoagulants.
WATER-SOLUBLE VITAMINS
Water-soluble vitamins include B complex and C. These vitamins dissolve in water and are easily destroyed by air, light, and cooking. They are not stored in the body to the extent that fat-soluble vitamins are stored.
Vitamin B Complex
Beriberi is a disease that affects the nervous, cardiovascular, and gastroin-testinal systems. The legs feel heavy, the feet burn, and the muscles degener-ate. The patient is irritable and suffers from headaches, depression, anorexia, constipation, tachycardia (rapid heart rate), edema, and heart failure.
Toward the end of the nineteenth century, a doctor in Indonesia discovered that chickens that were fed table scraps of polished rice developed symptoms much like those of his patients suffering from beriberi. When these same chickens were later fed brown (unpolished) rice, they recovered.
Some years later, this mysterious component of unpolished rice was rec-ognized as an essential food substance and was named vitamin B. Subsequently, it was named vitamin B complex because the vitamin was found to be composed of several compounds. The B-complex vitamins are listed in Table 7-1.
Thiamine
Thiamine, a coenzyme, was originally named vitamin B1. It is partially de-stroyed by heat and alkalies, and it is lost in cooking water.
Functions.Thiamine is essential for the metabolism of carbohydrates andsome amino acids. It is also essential to nerve and muscle action. It is absorbed in the small intestine.
Sources.Thiamine is found in many foods, but generally in small quantities.(See Appendix D.) Some of the best natural food sources of thiamine are unrefined and enriched cereals, whole grains, lean pork, liver, seeds, nuts, and legumes.
Requirements.Thiamine is measured in milligrams. The daily thia-mine requirement for the average adult female is 1.1 mg a day, and for the average adult male it is 1.2 mg a day. The requirement is not thought to increase with age. In general, however, an increase in calories increases the need for thiamine.
Most breads and cereals in the United States are enriched with thiamine, so that the majority of people can and do easily fulfill their recommended intake.
Deficiency.Symptoms of thiamine deficiency include loss of appetite,fatigue, nervous irritability, and constipation. An extreme deficiency causes beriberi. Its deficiency is rare, however, occurring mainly among alcoholics whose diets include reduced amounts of thiamine while their requirements are increased and their absorption is decreased. Others at risk include renal clients undergoing long-term dialysis, clients undergoing bypass surgery for weight loss, and those who eat primarily rice.
Because some raw fish contain thiaminase, an enzyme that inhibits the normal action of thiamine, frequent consumption of large amounts of raw fish could cause thiamine deficiency. Eating raw fish is not recommended. Cooking inactivates this enzyme.
There are no known ill effects from excessive oral intake of thiamine, but it may be toxic if excessive amounts are given intravenously.
Riboflavin
Riboflavin is sometimes called B2. It is destroyed by light and irradiation andis unstable in alkalies.
Functions.Riboflavin is essential for carbohydrate, fat, and protein me-tabolism. It is also necessary for tissue maintenance, especially the skin around the mouth, and for healthy eyes. Riboflavin is absorbed in the small intestine.
Sources.Riboflavin is widely distributed in animal and plant foods but insmall amounts. Milk, meats, poultry, fish, and enriched breads and cereals are some of its richest sources. Some green vegetables such as broccoli, spinach, and asparagus are also good sources.
Requirement.Riboflavin is measured in milligrams. The average adultfemale daily requirement is thought to be 1.1 mg, and the adult male require-ment is 1.3 mg. The riboflavin requirement appears to increase with increased energy expenditure. The requirement does not diminish with age.
Deficiency.Because of the small quantities of riboflavin in foods and itslimited storage in the body, deficiencies of riboflavin can develop. The generous use of fat-free milk in the diet is a good way to prevent deficiency of this vitamin. It is important, however, that milk be stored in opaque containers because riboflavin can be destroyed by light. It appears that fiber laxatives can reduce riboflavin absorption, and their use over long periods should be discouraged.
A deficiency of riboflavin can result in cheilosis, a condition characterized by sores on the lips and cracks at the corners of the mouth (Figure 7-1), glossitis (inflammation of the tongue), dermatitis, and eye strain in the form of itching, burning, and eye fatigue. Its toxicity is unknown.
Thiamine
Thiamine, a coenzyme, was originally named vitamin B1. It is partially de-stroyed by heat and alkalies, and it is lost in cooking water.
Functions.Thiamine is essential for the metabolism of carbohydrates andsome amino acids. It is also essential to nerve and muscle action. It is absorbed in the small intestine.
Sources.Thiamine is found in many foods, but generally in small quantities.(See Appendix D.) Some of the best natural food sources of thiamine are unrefined and enriched cereals, whole grains, lean pork, liver, seeds, nuts, and legumes.
Requirements.Thiamine is measured in milligrams. The daily thia-mine requirement for the average adult female is 1.1 mg a day, and for the average adult male it is 1.2 mg a day. The requirement is not thought to increase with age. In general, however, an increase in calories increases the need for thiamine.
Most breads and cereals in the United States are enriched with thiamine, so that the majority of people can and do easily fulfill their recommended intake.
Deficiency.Symptoms of thiamine deficiency include loss of appetite,fatigue, nervous irritability, and constipation. An extreme deficiency causes beriberi. Its deficiency is rare, however, occurring mainly among alcoholics whose diets include reduced amounts of thiamine while their requirements are increased and their absorption is decreased. Others at risk include renal clients undergoing long-term dialysis, clients undergoing bypass surgery for weight loss, and those who eat primarily rice.
Because some raw fish contain thiaminase, an enzyme that inhibits the normal action of thiamine, frequent consumption of large amounts of raw fish could cause thiamine deficiency. Eating raw fish is not recommended. Cooking inactivates this enzyme.
There are no known ill effects from excessive oral intake of thiamine, but it may be toxic if excessive amounts are given intravenously.
Riboflavin
Riboflavin is sometimes called B2. It is destroyed by light and irradiation andis unstable in alkalies.
Functions.Riboflavin is essential for carbohydrate, fat, and protein me-tabolism. It is also necessary for tissue maintenance, especially the skin around the mouth, and for healthy eyes. Riboflavin is absorbed in the small intestine.
Sources.Riboflavin is widely distributed in animal and plant foods but insmall amounts. Milk, meats, poultry, fish, and enriched breads and cereals are some of its richest sources. Some green vegetables such as broccoli, spinach, and asparagus are also good sources.
Requirement.Riboflavin is measured in milligrams. The average adultfemale daily requirement is thought to be 1.1 mg, and the adult male require-ment is 1.3 mg. The riboflavin requirement appears to increase with increased energy expenditure. The requirement does not diminish with age.
Deficiency.Because of the small quantities of riboflavin in foods and itslimited storage in the body, deficiencies of riboflavin can develop. The generous use of fat-free milk in the diet is a good way to prevent deficiency of this vitamin. It is important, however, that milk be stored in opaque containers because riboflavin can be destroyed by light. It appears that fiber laxatives can reduce riboflavin absorption, and their use over long periods should be discouraged.
A deficiency of riboflavin can result in cheilosis, a condition characterized by sores on the lips and cracks at the corners of the mouth (Figure 7-1), glossitis (inflammation of the tongue), dermatitis, and eye strain in the form of itching, burning, and eye fatigue. Its toxicity is unknown.
Folate
Folate, folacin, and folic acid are chemically similar compounds. Their namesare often used interchangeably.
Functions.Folate is needed for DNA synthesis, protein metabolism, andthe formation of hemoglobin. Researchers have concluded that folic acid helps to prevent colon, cervical, esophageal, stomach, and pancreatic cancers. Folic acid also increases homocystein levels that help prevent strokes, blood vessel disease, macular degeneration, and Alzheimer’s disease.
Sources.Folate is found in many foods, but the best sources are cerealsfortified with folate, green leafy vegetables, legumes, sunflower seeds, and fruits such as orange juice and strawberries. Heat, oxidation, and ultraviolet light all destroy folate, and it is estimated that 50% to 90% of folate may be destroyed during food processing and preparation. Consequently, it is advisable that fruits and vegetables be eaten uncooked or lightly cooked whenever possible.
Requirements.Folate is measured in micrograms. The average dailyrequirement for the adult female is 400 g, and for the adult male it is also 400μg. There is an increased need for folate during pregnancy and periods of growth because of the increased rate of cell division and the DNA synthe-sis in the body of the mother and of the fetus. Consequently, it is extremely important that women of childbearing age maintain good folate intake. The recommended amount for a woman 1 month before conception and through the first 6 weeks of pregnancy is 600μg a day.
Deficiency.Folate deficiency has been linked toneural tube defects(NTDs) in the fetus, such as spina bifida (spinal cord or spinal fluid bulgethrough the back) and anencephaly (absence of a brain). Other signs of deficiency are inflammation of the mouth and tongue, poor growth, depression and mental confusion, problems with nerve functions, and megaloblasticanemia. Megaloblastic anemia is a condition wherein red blood cells are largeand immature and cannot carry oxygen properly.
Hypervitaminosis.The FDA limits the amount of folate in over-the-counter(OTC) supplements to 100μg for infants, 300μg for children, and 400μg for adults because consuming excessive amounts of folate can mask a vitamin B12 deficiency and inactivate phenytoin, an anticonvulsant drug used by epileptics.
Biotin
Function and Sources. Biotinparticipates as a coenzyme in the syn-thesis of fatty acids and amino acids. Some of its best dietary sources are egg yolks, milk, poultry, fish, broccoli, spinach, and cauliflower. Biotin is also synthesized in the intestine by microorganisms, but the amount that is available for absorption is unknown.
Requirements.Biotin is measured in micrograms. The Food and Nutri-tion Board of the Institute of Medicine has established an AI of 30μg for adults (see Table 7-2).
Deficiency.Deficiency symptoms include nausea, anorexia, depression,pallor (paleness of complexion), dermatitis (inflammation of skin), and an in-crease in serum cholesterol. Toxicity from excessive intake is unknown.
Pantothenic Acid
Pantothenic acid is appropriately named because the Greek wordpantothenmeans “from many places.” It is fairly stable, but it can be damaged by acids and alkalies.
Functions.Pantothenic acid is involved in metabolism of carbohydrates,fats, and proteins. It is also essential for the synthesis of the neurotransmitter acetylcholine and of steroid hormones.
Sources.Pantothenic acid is found extensively in foods, especially animalfoods such as meats, poultry, fish, and eggs. It is also found in whole-grain cere-als and legumes. In addition, it is thought to be synthesized by the body.
Requirements.There is no DRI for pantothenic acid, but the Food andNutrition Board has provided an estimated intake of 4 to 7 mg a day for normal adults (see Table 7-2).
Deficiency.Natural deficiencies are unknown. However, deficiencies havebeen produced experimentally. Signs include weakness, fatigue, and a burning sensation in the feet. Toxicity from excessive intake has not been confirmed.
Vitamin C
Vitamin C is also known as ascorbic acid. It has antioxidant properties and protects foods from oxidation, and it is required for all cell metabolism. It is read-ily destroyed by heat, air, and alkalies, and it is easily lost in cooking water.
Functions.Vitamin C is known to preventscurvy.This is a disease char-acterized by gingivitis (soft, bleeding gums and loose teeth); flesh that is easily bruised; tiny, pinpoint hemorrhages of the skin; poor wound healing; sore joints and muscles; and weight loss. In extreme cases, scurvy can result in death. Scurvy used to be common among sailors, who lived for months on bread, fish, and salted meat, with no fresh fruits or vegetables. During the middle of the eighteenth century, it was discovered that the addition of limes or lemons to their diets prevented this disease.
Vitamin C also has an important role in the formation of collagen, a pro-tein substance that holds body cells together, making it necessary for wound healing. Therefore, the requirement for vitamin C is increased during trauma, fever, and periods of growth. Tiny, pinpoint hemorrhages are symptoms of the breakdown of collagen.
Vitamin C aids in the absorption of nonheme iron (from plant and ani-mal sources and less easily absorbed than heme iron) in the small intestine when both nutrients are ingested at the same time. Because of this, it is called an iron enhancer.
Vitamin C also appears to have several other functions in the human body that are not well understood. For example, it may be involved with the forma-tion or functioning of norepinephrine (a neurotransmitter and vasoconstrictor that helps the body cope with stressful conditions), some amino acids, folate, leukocytes (white blood cells), the immune system, and allergic reactions.
It is believed to reduce the severity of colds because it is a natural antihista-mine, and it can reduce cancer risk in some cases by reducing nitrites in foods.
Vitamin C is absorbed in the small intestine.
Sources.The best sources of vitamin C are citrus fruits, melon, strawber-ries, tomatoes, potatoes, red and green peppers, cabbage, and broccoli.
Requirements.Vitamin C is measured in milligrams. Under normal cir-cumstances, an average female adult in the United States requires 75 mg a day and an average male 90 mg. In times of stress, the need is increased. Regular cigarette smokers are advised to ingest 125 mg or more a day.
It is generally considered nontoxic, but this has not been confirmed. An excess can cause diarrhea, nausea, cramps, an excessive absorption of food iron, rebound scurvy (when megadoses are stopped abruptly), and possibly oxalate kidney stones.
Deficiency.Deficiencies of vitamin C are indicated by bleeding gums, looseteeth, tendency to bruise easily, poor wound healing, and, ultimately, scurvy
Vitamin A
Vitamin A consists of two basic dietary forms: preformed vitamin A, also called retinol, which is the active form of vitamin A; and carotenoids, the inactiveform of vitamin A, which are found in plants.
Functions.Vitamin A is a family of fat-soluble compounds that play an impor-tant role in vision, bone growth, reproduction, and cell division. Vitamin A helps regulate the immune system, which helps fight infections. Vitamin A has been labeled as an antioxidant when, in fact, provitamin A (carotenoids) is the part of the family that functions as an antioxidant. Antioxidants protect cells from freeradicals. Free radicals are atoms or groups of atoms with an odd (unpaired) num-ber of electrons and can be formed when oxygen interacts with certain molecules. Once formed, these highly reactive radicals can start a chain reaction. When they react with important cellular components such as DNA or cell membranes, the most damage occurs. Antioxidants have the capability of safely interacting with free radicals and stopping the chain reaction before vital cells are damaged.
The first organic free radical was discovered in 1900 by Moses Gomberg. In the 1950s, Denman Harman, M.D., was the first to propose the free radical theory of aging.
Sources.There are two forms of vitamin A: preformed vitamin A and pro-vitamin A. Retinol is a preformed vitamin A and is one of the most active and usable forms of vitamin A. Retinol can be converted to retinal and retinoic acid, other active forms of vitamin A.
Provitamin A carotenoids can be converted to vitamin A from darkly colored pigments, both green and orange, in fruits and vegetables. Common car-otenoids are beta-carotene, lutein, lycopene, and zeaxanthin. Beta-carotene is most efficiently converted to retinol. Eating “five-a-day” of fruits and vegetables is highly recommended. The best sources of beta-carotene are carrots, sweet pota-toes, spinach, broccoli, pumpkin, squash (butternut), mango, and cantaloupe.
Research has shown that regular consumption of foods rich in carotenoids decreases the risk of some cancers because of its antioxidant effect. Taking a beta-carotene supplement has not shown the same results.
Preformed vitamin A (retinol) is found in fat-containing animal foods such as liver, butter, cream, whole milk, whole-milk cheeses, and egg yolk. It is also found in low-fat milk products and in cereals that have been fortified with vitamin A, but these are not the best sources.
Requirements.A well-balanced diet is the preferred way to obtain therequired amounts of vitamin A. Vitamin A values are commonly listed as a retinolequivalent (RE). A retinol equivalent is 1 μg retinol or 6μg beta carotene. Referto the inside back cover of this text for the dietary reference intakes of vitamin A as prescribed by the Food and Nutrition Board of the Institute of Medicine.
Hypervitaminosis.The use of a single vitamin supplement should be dis-couraged because an excess of vitamin A can have serious consequences. Signs of hypervitaminosis A may include birth defects, hair loss, dry skin, headaches, nausea, dryness of mucous membranes, liver damage, and bone and joint pain. In general, these symptoms tend to disappear when excessive intake is discontinued.
Deficiency.Signs of a deficiency of vitamin A include night blindness;dry, rough skin; and increased susceptibility to infections. Avitaminosis A can result in blindness or xerophthalmia, a condition characterized by dry, luster-less, mucous membranes of the eye. Lack of vitamin A is the leading cause of blindness in the world (discounting accidents).
Vitamin D
Vitamin D exists in two forms—D2 (ergocalciferol) and D3 (cholecalciferol). Each is formed from a provitamin when irradiated with (exposed to) ultraviolet light. They are equally effective in human nutrition, but D3 is the one that is formed in humans from cholesterol in the skin. D2 is formed in plants. Vitamin D is consid-ered a prohormone because it is converted to a hormone in the human body.
Vitamin D is heat-stable and not easily oxidized, so it is not harmed by storage, food processing, or cooking.
Functions.The major function of vitamin D is the promotion of calciumand phosphorus absorption in the body. By contributing to the absorption of these minerals, it helps to raise their concentration in the blood so that nor-mal bone and tooth mineralization can occur and tetany (involuntary muscle movement) can be prevented. (Tetany can occur when there is too little cal-cium in the blood. This condition is called hypocalcemia.)
Vitamin D is absorbed in the intestines and is chemically changed in the liver and kidneys. Excess amounts of vitamin D are stored in the liver and in adipose tissue.
Sources.The best source of vitamin D is sunlight, which changes a pro-vitamin to vitamin D3 in humans. It is sometimes referred to as the sunshine vitamin. The amount of vitamin D that is formed depends on the individual’s pigmentation (coloring matter in the skin) and the amount of sunlight avail-able. The best food sources of vitamin D are milk, fish liver oils, egg yolk, butter, and fortified margarine. Because of the rather limited number of food sources of vitamin D and the unpredictability of sunshine, health authorities decided that the vitamin should be added to a common food. Milk was selected.
Consequently, most milk available in the United States today has had 10μg of vitamin D concentrate added per quart.
Requirements.Under the DRIs, there are several reference values in-cluded. Vitamin D levels are given as Adequate Intake levels, or AI (Table 7-4).
People who are seldom outdoors, those who use sunscreens, and those who live in areas where there is little sunlight for 3 to 4 months a year should be especially careful that their diets provide their AI levels of vitamin D. Drink-ing 2 cups of vitamin D–fortified fat-free milk each day will provide sufficient vitamin D to those between bi
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