Figure 18.9

A molecule of beta-carotene can be converted into two molecules of retinal, which in turn can be changed into retinol.

Do Vitamins Protect Against Heart Disease and Cancer?

Vitamins A and E are popularly believed to protect against cardiovascular disease and cancer. Although studies have shown that people who eat diets rich in fruits and vegetables containing these vitamins have slightly lower incidences of these disorders, these investigations do not prove that the vitamins, and not something else in these foods, provide protection. A more meaningful study would carefully follow the result of taking a specific vitamin supplement. The Finnish Alpha Tocopherol, Beta Carotene Cancer Prevention Study did just that — with surprising results.

Researchers monitored the health of 29,000 adult male smokers over a six-year period. Each man received vitamin A or E, both vitamins, or neither. The "endpoint" of the study was to determine which men developed lung cancer. Not only did these vitamins fail to protect against this specific cancer, but the men who received beta carotene actually had an increased incidence of lung cancer! Neither vitamin lowered incidences of death from stroke or coronary heart disease.

The Finnish study has several limitations. Some critics claim that it was not conducted over a long-enough time period. Also, the study considers only one type of person who engages in a dangerous activity— smoking—and may not account for all possible contributing factors to disease development. Unraveling the connections between specific nutrients and health is extremely complex, because of the many variables. Overall, a diet rich in fruits and vegetables is healthful. But the Finnish study shows that claims that certain vitamins protect against cancer or heart disease may be premature. ■

Vitamin A is relatively stable to the effects of heat, acids, and bases. However, it is readily destroyed by oxidation and is unstable in light.

Vitamin A is important in vision. Retinal is used to synthesize rhodopsin (visual purple) in the rods of the retina and may be required for production of light-sensitive pigments in the cones as well. The vitamin also functions in the synthesis of mucoproteins and mucopolysaccha-rides, in development of normal bones and teeth, and in maintenance of epithelial cells in skin and mucous membranes. Vitamin A and beta carotenes also act as antioxidants (an"te-ok'si-dantz) because they readily combine with oxygen and prevent oxidation of other compounds. Clinical Application 18.2 examines the ongoing controversy concerning the value of vitamins A and E in preventing illness.

Only foods of animal origin such as liver, fish, whole milk, butter, and eggs are sources of vitamin A. However, the vitamin's precursor, carotene, is widespread in leafy green vegetables and in yellow or orange vegetables and fruits.

Excess vitamin A produces peeling skin, hair loss, nausea, headache, and dizziness, a condition called hy-pervitaminosis A. Chronic overdoses of the vitamin may inhibit growth and cause the bones and joints to degenerate. "Megadosing" on fat-soluble vitamins is particularly dangerous during pregnancy. Excess vitamin A, for example, can cause birth defects.

"Yellow rice" is a genetically modified variety that, thanks to genes from a bacterium and petunia, manufactures beta carotene, a vitamin A precursor. In addition, alteration of a rice gene enables the plant to double its iron content. Once these traits are bred into a commercial strain of rice, the new crop may help prevent vitamin A and iron deficiencies in malnourished people living in developing nations. It will be available to them free of charge.

A deficiency of vitamin A causes night blindness, in which a person cannot see normally in dim light. Xe-ropthalmia, a dryness of the conjunctiva and cornea, is due to vitamin A deficiency. Vitamin A deficiency also causes degenerative changes in certain epithelial tissues, and the body becomes more susceptible to infection.

99 What biochemical do body cells use to synthesize vitamin A?

^9 What conditions destroy vitamin A?

^9 Which foods are good sources of vitamin A?

Vitamin D is a group of steroids that have similar properties. One of these substances, vitamin D3 (chole-calciferol), is found in foods such as milk, egg yolk, and fish liver oils. Vitamin D2 (ergocalciferol) is produced commercially by exposing a steroid obtained from yeasts (ergosterol) to ultraviolet light. Vitamin D can also be synthesized from dietary cholesterol that has been converted to provitamin D by intestinal enzymes, then stored in the skin and exposed to ultraviolet light (see chapter 13, p. 505).

Like other fat-soluble vitamins, vitamin D resists the effects of heat, oxidation, acids, and bases. It is primarily stored in the liver and is less abundant in the skin, brain, spleen, and bones.

As it is needed, vitamin D stored in the form of hy-droxycholecalciferol is released into the blood. When parathyroid hormone is present, this form of vitamin D is converted in the kidneys into an active form of the vitamin (dihydroxycholecalciferol). This substance, in turn, is carried as a hormone in the blood to the intestines where it stimulates production of calcium-binding protein. Here, it promotes absorption of calcium and phosphorus, ensuring that adequate amounts of these minerals are available in the blood for tooth and bone formation and metabolic processes.

Because natural foods are often poor sources of vitamin D, it is often added to food during processing. For example, homogenized, nonfat, and evaporated milk are typically fortified with vitamin D. Fortified means essential nutrients have been added to a food where they originally were absent or scarce. Enriched means essential nutrients have been partially replaced in a food that has lost nutrients during processing.

Excess vitamin D, or hypervitaminosis D, produces diarrhea, nausea, and weight loss. Over time, it may also cause calcification of certain soft tissues and irreversible kidney damage.

In children, vitamin D deficiency results in rickets, in which the bones and teeth fail to develop normally (fig. 18.10). In adults or in the elderly who have little exposure to sunlight, such a deficiency may lead to osteomalacia, in which the bones decalcify and weaken due to disturbances in calcium and phosphorus metabolism.

99 Where is vitamin D stored?

^9 What are the functions of vitamin D?

^9 Which foods are good sources of vitamin D?

□ What are symptoms of vitamin D excess and deficiency?

Vitamin E includes a group of compounds, the most active of which is alpha tocopherol. This vitamin is resistant to the effects of heat, acids, and visible light but is unstable in bases and in the presence of ultraviolet light or oxygen. In fact, it combines so readily with oxygen that vitamin E is called an antioxidant.

Vitamin E is found in all tissues but is primarily stored in the muscles and adipose tissue. It is also highly concentrated in the pituitary and adrenal glands.

Figure 18.10

Vitamin D deficiency causes rickets, in which the bones and teeth do not develop normally.

Figure 18.10

Vitamin D deficiency causes rickets, in which the bones and teeth do not develop normally.

The precise functions of vitamin E are unknown, but it is thought to act as an antioxidant by inhibiting the breakdown of polyunsaturated fatty acids in the tissues. It may also help maintain the stability of cell membranes. Antioxidants prevent formation of free radicals, which build up in certain diseases and aging.

Although vitamin E is widely distributed among foods, its richest sources are oils from cereal seeds such as wheat germ. Other good sources are salad oils, margarine, shortenings, fruits, nuts, and vegetables. Since this vitamin is so easily obtained, deficiency conditions are rare.

99 Where is vitamin E stored? Q What are the functions of vitamin E? Q Which foods are good sources of vitamin E?

Vitamin K, like the other fat-soluble vitamins, occurs in several chemical forms. One of these, vitamin K1 (phylloquinone), is found in foods, whereas another, vitamin K2, is produced by bacteria (Escherichia coli) that normally inhabit the human intestinal tract. These vitamins resist the effects of heat but are destroyed by oxidation or by exposure to acids, bases, or light. The liver stores them to a limited degree.

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