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 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 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 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 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.