(a) Heme group
(a) A hemoglobin molecule contains four heme groups, each of which houses a single iron atom (Fe) that can combine with oxygen. Iron deficiency anemia can result from a diet poor in iron-containing foods. The red blood cells in (b) are normal (400x), but many of those in (c) are small and pale (280x). They contain too little hemoglobin, because iron is lacking in the diet. Vegetarians must be especially careful to consume sufficient iron.
A compulsive disorder that may result from mineral deficiency is pica, in which people consume huge amounts of nondietary substances such as ice chips, soil, sand, laundry starch, clay and plaster, and even such strange things as hair, toilet paper, matchheads, inner tubes, mothballs, and charcoal. The condition is named for the magpie bird, Pica pica, which eats a range of odd things.
Pica affects people of all cultures and was noted as early as 40 B.C. The connection to dietary deficiency stems from the observation that slaves suffering from pica in colonial America recovered when their diets improved, particularly when they were given iron supplements. Another clue comes from a variation on pica called geophagy — "eating dirt"—that affects many types of animals, including humans. Researchers discovered that when parrots eat a certain claylike soil in their native Peru, soil particles bind alkaloid toxins in their seed food and carry the toxins out of the body. Perhaps pica in humans is protective in some way, too.
U What is the primary function of iron?
^9 Why does the usual diet provide only a narrow margin of safety in supplying iron?
^9 How is manganese utilized? Q Which foods are good sources of manganese?
Copper (Cu) is found in all body tissues but is most highly concentrated in the liver, heart, and brain. It is essential for hemoglobin synthesis, bone development, melanin production, and formation of myelin within the nervous system.
A daily intake of 2 mg of copper is sufficient to supply cells. Because a typical adult diet has about 2-5 mg of this mineral, adults seldom develop copper deficiencies. Foods rich in copper include liver, oysters, crab-meat, nuts, whole-grain cereals, and legumes.
Iodine (I) is found in minute quantities in all tissues but is highly concentrated within the thyroid gland. Its only known function is as an essential component of thyroid hormones. (The molecular structures of two of these hormones, thyroxine and triiodothyronine, are shown in fig. 13.18.)
A daily intake of 1 microgram (0.001 mg) of iodine per kilogram of body weight is adequate for most adults. Since the iodine content of foods varies with the iodine content of soils in different geographic regions, many people use iodized table salt to season foods to prevent deficiencies.
Cobalt (Co) is widely distributed in the body because it is an essential part of cyanocobalamin (vitamin
B12). It is also necessary for the synthesis oi several important enzymes.
The amount of cobalt required in the daily diet is unknown. This mineral is found in a great variety of foods, and the quantity in the average diet is apparently sufficient. Good sources of cobalt include liver, lean meats, and milk.
Zinc (Zn) is most concentrated in the liver, kidneys, and brain. It is part of many enzymes involved in digestion, respiration, and bone and liver metabolism. It is also necessary for normal wound healing and for maintaining the integrity of the skin.
The daily requirement for zinc is about 15 mg, and most diets provide 10-15 mg. Since only a portion of this amount may be absorbed, zinc deficiencies may occur. The richest sources of zinc are meats; cereals, legumes, nuts, and vegetables provide lesser amounts.
Fluorine (F), as part of the compound fluoroapatite, replaces hydroxyapatite in teeth, strengthening the enamel and preventing dental caries. Selenium (Se) is stored in the liver and kidneys. It is a constituent of certain enzymes and participates in heart function. This mineral is found in lean meats, whole-grain cereals, and onions. Chromium (Cr) is widely distributed throughout the body and regulates glucose utilization. It is found in liver, lean meats, yeast, and pork kidneys. Table 18.9 summarizes the characteristics of trace elements.
The term "dietary supplement" is used to refer to minerals, vitamins, carbohydrates, proteins, and fats—the micronutrients and macronutrients. Clinical Application 18.3 discusses the current broadened meaning of "dietary supplement."
99 How is copper used? Q What is the function of iodine? B Why might zinc deficiencies be common?
An adequate diet provides sufficient energy (calories), essential fatty acids, essential amino acids, vitamins, and minerals to support optimal growth and to maintain and repair body tissues. However, because individual nutrient requirements vary greatly with age, sex, growth rate, amount of physical activity, and level of stress, as well as with genetic and environmental factors, it is not possible to design a diet that is adequate for everyone. However, nutrients are so widely distributed in foods that satisfactory amounts and combinations can usually be obtained in spite of individual food preferences.
Shier-Butler-Lewis: I V. Absorption and I 18. Nutrition and I I © The McGraw-Hill
Human Anatomy and Excretion Metabolism Companies, 2001
Physiology, Ninth Edition
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