Trace Minerals in the Human Body PDF

C H A P T E R The Trace Minerals 13 CHAPTER OUTLINE...

C H A P T E R The Trace Minerals 13 CHAPTER OUTLINE The Trace Minerals— An Overview Iron Iron Roles in the Body Iron Absorption and Metabolism Iron Defi- ciency Iron Toxicity Iron Recommen- dations and Sources Iron Contamina- Figure 12-9 in the last chapter (p. 409) showed the tiny quantities of trace tion and Supplementation minerals in the human body. The trace minerals are so named because Zinc Zinc Roles in the Body Zinc Absorption and Metabolism Zinc they are present, and needed, in relatively small amounts in the body. All Deficiency Zinc Toxicity Zinc together, they would produce only a bit of dust, hardly enough to fill a tea- Recommendations and Sources Zinc spoon. Yet they are no less important than the major minerals or any of Supplementation the other nutrients. Each of the trace minerals performs a vital role. A de- Iodine ficiency of any of them may be fatal, and an excess of many is equally Selenium deadly. Remarkably, people’s diets normally supply just enough of these Copper minerals to maintain health. Manganese Fluoride The Trace Minerals—An Overview Chromium The body requires the trace minerals in minuscule quantities. They participate in di- Molybdenum verse tasks all over the body, each having special duties that only it can perform. Other Trace Minerals Food Sources The trace mineral contents of foods depend on soil and water com- Contaminant Minerals position and on how foods are processed. Furthermore, many factors in the diet and Closing Thoughts on the within the body affect the minerals’ bioavailability. ◆ Still, outstanding food sources Nutrients for each of the trace minerals, just like those for the other nutrients, include a wide HIGHLIGHT 13 Phytochemicals variety of foods, especially unprocessed, whole foods. and Functional Foods Deficiencies Severe deficiencies of the better-known minerals are easy to recog- nize. Deficiencies of the others may be harder to diagnose, and for all minerals, mild ◆ Reminder: Bioavailability refers to the rate at deficiencies are easy to overlook. Because the minerals are active in all the body sys- and the extent to which a nutrient is tems—the GI tract, cardiovascular system, blood, muscles, bones, and central ner- absorbed and used. vous system—deficiencies can have wide-reaching effects and can affect people of all ages. The most common result of a deficiency in children is failure to grow and thrive. Toxicities Some of the trace minerals are toxic at intakes not far above the esti- mated requirements. Thus it is important not to habitually exceed the Upper Level trace minerals: essential mineral nutrients of recommended intakes. Many vitamin-mineral supplements contain trace miner- found in the human body in amounts als, making it easy for users to exceed their needs. Highlight 10 discusses supple- smaller than 5 g; sometimes called ment use and some of the regulations included in the Dietary Supplement Health microminerals. 441 442 CHAPTER 13 and Education Act. As that discussion notes, the Food and Drug Administration (FDA) has no authority to limit the amounts of trace minerals in supplements; con- sumers have demanded the freedom to choose their own doses of nutrients.* Individ- uals who take supplements must therefore be aware of the possible dangers and select supplements that contain no more than 100 percent of the Daily Value. It would be easier and safer to meet nutrient needs by selecting a variety of foods than by combining an assortment of supplements (see Highlight 10). Interactions Interactions among the trace minerals are common and often well coordinated to meet the body’s needs. For example, several of the trace minerals support insulin’s work, influencing its synthesis, storage, release, and action. At other times, interactions lead to nutrient imbalances. An excess of one may cause a deficiency of another. (A slight manganese overload, for example, may ag- gravate an iron deficiency.) A deficiency of one may interfere with the work of an- other. (A selenium deficiency halts the activation of the iodine-containing thyroid hormones.) A deficiency of a trace mineral may even open the way for a contami- nant mineral to cause a toxic reaction. (Iron deficiency, for example, makes the body vulnerable to lead poisoning.) These examples reinforce the need to balance intakes and to use supplements wisely, if at all. A good food source of one nutrient may be a poor food source of another, and factors that enhance the action of some trace minerals may interfere with others. (Meats are a good source of iron but a poor source of calcium; vitamin C enhances the absorption of iron but hinders that of copper.) Research on the trace minerals is active, suggesting that we have much more to learn about them. IN SUMMARY Although the body uses only tiny amounts of the trace minerals, they are vi- tal to health. Because so little is required, the trace minerals can be toxic at levels not far above estimated requirements—a consideration for supplement users. Like the other nutrients, the trace minerals are best obtained by eating a variety of whole foods. Iron Iron is an essential nutrient, vital to many of the cells’ activities, but it poses a prob- lem for millions of people. Some people simply don’t eat enough iron-containing foods to support their health optimally, whereas others absorb so much iron that it threatens their health. Iron exemplifies the principle that both too little and too much of a nutrient in the body can be harmful. In its wisdom, the body has several ways to achieve iron homeostasis, protecting against both deficiency and overload.1 Iron Roles in the Body ◆ Iron’s two ionic states: Iron has the knack of switching back and forth between two ionic states. ◆ In the re- Ferrous iron (reduced): Fe++ duced state, iron has lost two electrons and therefore has a net positive charge of Ferric iron (oxidized): Fe+++ two; it is known as ferrous iron. In the oxidized state, iron has lost a third electron, has a net positive charge of three, and is known as ferric iron. Ferrous iron can be oxidized to ferric iron, and ferric iron can be reduced to ferrous iron. Thus iron can serve as a ◆ Reminder: A cofactor is a substance that cofactor ◆ to enzymes involved in oxidation-reduction reactions—reactions so wide- works with an enzyme to facilitate a chem- spread in metabolism that they occur in all cells. Enzymes involved in making ical reaction. amino acids, collagen, hormones, and neurotransmitters all require iron. (For de- tails about ions, oxidation, and reduction, see Appendix B.) * Canada regulates the amounts of trace minerals in supplements.  THE TRACE MINERALS 443 Iron forms a part of the electron carriers that participate in the electron trans- port chain (discussed in Chapter 7).* In this pathway, these carriers transfer hydro- gens and electrons to oxygen, forming water, and in the process, make ATP for the cells’ energy use. Most of the body’s iron is found in two proteins: hemoglobin ◆ in the red blood ◆ Reminder: Hemoglobin is the oxygen-carrying cells and myoglobin in the muscle cells. In both, iron helps accept, carry, and protein of the red blood cells that trans- then release oxygen. ports oxygen from the lungs to tissues throughout the body; hemoglobin accounts for 80% of the body’s iron. Iron Absorption and Metabolism The body conserves iron. Because it is difficult to excrete iron once it is in the body, balance is maintained primarily through absorption. More iron is absorbed when stores are empty and less is absorbed when stores are full.2 Iron Absorption Special proteins help the body absorb iron from food (see Figure 13-1). One protein, called mucosal ferritin, receives iron from food and stores it in the mucosal cells ◆ of the small intestine. When the body needs iron, mucosal fer- ◆ A mucous membrane such as the one that ritin releases some iron to another protein, called mucosal transferrin. Mucosal lines the GI tract is sometimes called the transferrin transfers the iron to another protein, blood transferrin, which transports mucosa (mu-KO-sa). The adjective of the iron to the rest of the body. If the body does not need iron, it is carried out when mucosa is mucosal (mu-KO-sal). the intestinal cells are shed and excreted in the feces; intestinal cells are replaced about every three to five days. By holding iron temporarily, these cells control iron absorption by either delivering iron when the day’s intake falls short or disposing of myoglobin: the oxygen-holding protein of the muscle cells. it when intakes exceed needs. myo = muscle Heme and Nonheme Iron Iron absorption depends in part on its dietary ferritin (FAIR-ih-tin): the iron storage protein. source.3 Iron occurs in two forms in foods: as heme iron, which is found only in transferrin (trans-FAIR-in): the iron transport foods derived from the flesh of animals, such as meats, poultry, and fish and as non- protein. heme iron, which is found in both plant-derived and animal-derived foods (see Fig- heme (HEEM): the iron-holding part of the ure 13-2, p. 444). On average, heme iron represents about 10 percent of the iron a hemoglobin and myoglobin proteins. About 40% of the iron in meat, fish, and poultry is * The iron-containing electron carriers of the electron transport chain are known as cytochromes. See bound into heme; the other 60% is Appendix C for details of this pathway. nonheme iron. FIGURE 13-1 Iron Absorption Iron in food If the body does not Mucosal cells in the need iron Iron is not absorbed and is excreted intestine store excess in shed intestinal cells instead. iron in mucosal ferritin Thus, iron absorption is reduced (a storage protein). when the body does not need iron. If the body needs iron Mucosal ferritin releases iron to mucosal transferrin (a transport protein), which hands off iron to another transferrin that travels through the blood to the rest of the body. 444 CHAPTER 13 FIGURE 13-2 Heme and Nonheme Iron in Foods Only foods derived from animal flesh provide heme, but they also contain nonheme iron. Heme accounts for about 10% of the average daily iron intake, but it is well Key: absorbed (about 25%). Heme Nonheme iron accounts Nonheme for the remaining 90%, but it is less well absorbed (about 17%). All of the iron in foods derived from plants is nonheme iron. person consumes in a day. Even though heme iron accounts for only a small propor- tion of the intake, it is so well absorbed that it contributes significant iron. About 25 percent of heme iron and 17 percent of nonheme iron is absorbed, depending on di- etary factors and the body’s iron stores.4 In iron deficiency, absorption increases. In iron overload, absorption declines.5 Researchers disagree as to whether heme iron © Benjamin F. Fink Jr./Brand X Pictures/Getty Images absorption responds to iron stores as sensitively as nonheme iron absorption does. Absorption-Enhancing Factors Meat, fish, and poultry contain not only the well-absorbed heme iron, but also a peptide (called the MFP factor) that promotes the absorption of nonheme iron ◆ from other foods eaten at the same meal.6 Vita- min C also enhances nonheme iron absorption from foods eaten in the same meal by capturing the iron and keeping it in the reduced ferrous form, ready for absorp- tion. Some acids and sugars also enhance nonheme iron absorption. Absorption-Inhibiting Factors Some dietary factors bind with nonheme iron, in- hibiting absorption. ◆ These factors include the phytates in legumes, whole grains, This chili dinner provides several factors that may enhance iron absorption: heme and non- and rice; the vegetable proteins in soybeans, other legumes, and nuts; the calcium in heme iron and MFP from meat, nonheme iron milk; and the polyphenols (such as tannic acid) in tea, coffee, grain products, from legumes, and vitamin C from tomatoes. oregano, and red wine. ◆ Factors that enhance nonheme iron Dietary Factors Combined The many dietary enhancers, inhibitors, and their absorption: combined effects make it difficult to estimate iron absorption. Most of these factors MFP factor exert a strong influence individually, but not when combined with the others in a Vitamin C (ascorbic acid) meal. Furthermore, the impact of the combined effects diminishes when a diet is evaluated over several days. When multiple meals are analyzed together, three fac- ◆ Factors that inhibit nonheme iron tors appear to be most relevant: MFP and vitamin C as enhancers and phytates as absorption: inhibitors. Phytates (legumes, grains, and rice) Vegetable proteins (soybeans, legumes, Individual Variation Overall, about 18 percent of dietary iron is absorbed from nuts) mixed diets and only about 10 percent from vegetarian diets.7 As you might expect, Calcium (milk) vegetarian diets do not have the benefit of easy-to-absorb heme iron or the help of Tannic acid (and other polyphenols in MFP in enhancing absorption. In addition to dietary influences, iron absorption also tea and coffee) depends on an individual’s health, stage in the life cycle, and iron status. Absorption can be as low as 2 percent in a person with GI disease or as high as 35 percent in a rapidly growing, healthy child. The body adapts to absorb more iron when a per- son’s iron stores fall short or when the need increases for any reason (such as preg- nancy). The body makes more mucosal transferrin to absorb more iron from the intestines and more blood transferrin to carry more iron around the body. Similarly, when iron stores are sufficient, the body adapts to absorb less iron. MFP factor: a peptide released during the digestion of meat, fish, and poultry that Iron Transport and Storage Blood transferrin delivers iron to the bone marrow enhances nonheme iron absorption. and other tissues. The bone marrow uses large quantities to make new red blood  THE TRACE MINERALS 445 cells, whereas other tissues use less. Surplus iron is stored in the protein ferritin, pri- marily in the liver, but also in the bone marrow and spleen. When dietary iron has been plentiful, ferritin is constantly and rapidly made and broken down, providing an ever-ready supply of iron. When iron concentrations become abnormally high, the liver converts some ferritin into another storage protein called hemosiderin. Hemosiderin releases iron more slowly than ferritin does. By storing excess iron, the body protects itself: free iron acts as a free radical, attacking cell lipids, DNA, and protein. (See Highlight 11 for more information on free radicals and the damage they can cause.) Iron Recycling The average red blood cell lives about four months; then the spleen and liver cells remove it from the blood, take it apart, and prepare the degra- dation products for excretion or recycling. The iron is salvaged: the liver attaches it to blood transferrin, which transports it back to the bone marrow to be reused in making new red blood cells. Thus, although red blood cells live for only about four months, the iron recycles through each new generation of cells (see Figure 13-3). The body loses some iron daily via the GI tract and, if bleeding occurs, in blood. Only tiny amounts of iron are lost in urine, sweat, and shed skin.* Iron Balance Maintaining iron balance depends on the careful regulation of iron absorption, transport, storage, recycling, and losses. The hormone hepcidin is cen- tral to the regulation of iron balance.8 Produced by the liver, hepcidin helps to main- tain blood iron within the normal range by inhibiting absorption from the intestines and transport out of storage as needed. Iron Deficiency hemosiderin (heem-oh-SID-er-in): an iron storage protein primarily made in times of iron overload. Worldwide, iron deficiency is the most common nutrient deficiency, affecting hepcidin: a hormone produced by the liver more than 1.2 billion people.9 In developing countries, almost half of preschool that regulates iron balance. * Adults lose about 1.0 milligram of iron per day. Women lose additional iron in menses. Menstrual iron deficiency: the state of having depleted losses vary considerably, but over a month, they average about 0.5 milligram per day. iron stores. FIGURE 13-3 Animated! Iron Recycled in the Body Once iron enters the body, most of it is recycled. Some is lost with body tissues and must be replaced by eating iron-containing food. Some losses via sweat, skin, Transferrin carries and urine iron in blood. Some iron delivered to myoglobin of muscle cells Liver (and spleen) dismantles red blood cells, packages Bone marrow incorporates iron into transferrin, and iron into hemoglobin of stores excess iron in red blood cells and stores ferritin (and hemosiderin). excess iron in ferritin (and hemosiderin). Iron-containing Some losses hemoglobin in red if bleeding blood cells occurs carries oxygen. To test your understanding of these concepts, log on to academic.cengage.com/login. 446 CHAPTER 13 children and pregnant women suffer from iron-deficiency anemia.10 In the United States, iron deficiency is less prevalent, but it still affects 10 percent of tod- dlers, adolescent girls, and women of childbearing age. Iron deficiency is also rel- atively common among overweight children and adolescents compared with those who are normal weight.11 Preventing and correcting iron deficiency are high priorities.12 ◆ High risk for iron deficiency: Vulnerable Stages of Life Some stages of life ◆ demand more iron but provide Women in their reproductive years less, making deficiency likely. Women in their reproductive years are especially Pregnant women prone to iron deficiency because of repeated blood losses during menstruation. Infants and young children Pregnancy demands additional iron to support the added blood volume, growth of Teenagers the fetus, and blood loss during childbirth. Infants and young children receive lit- tle iron from their high-milk diets, yet need extra iron to support their rapid growth. Iron deficiency among toddlers in the United States is common.13 The rapid growth of adolescence, especially for males, and the menstrual losses of females also demand extra iron that a typical teen diet may not provide. An adequate iron intake is especially important during these stages of life. ◆ The iron content of blood is about 0.5 Blood Losses Bleeding ◆ from any site incurs iron losses. In some cases, such as an mg/100 mL blood. A person donating a active ulcer, the bleeding may not be obvious, but even small chronic blood losses sig- pint of blood (approximately 500 mL) loses nificantly deplete iron reserves. In developing countries, blood loss is often brought about 2.5 mg of iron. on by malaria and parasitic infections of the GI tract. People who donate blood reg- ularly also incur losses and may benefit from iron supplements. As mentioned, men- strual losses can be considerable as they tap women’s iron stores regularly. ◆ Stages of iron deficiency: Assessment of Iron Deficiency Iron deficiency develops in stages. ◆ This section Iron stores diminish provides a brief overview of how to detect these stages, and Appendix E provides Transport iron decreases more details. In the first stage of iron deficiency, iron stores diminish. Measures of Hemoglobin production declines serum ferritin (in the blood) reflect iron stores and are most valuable in assessing iron status at this earliest stage. The second stage of iron deficiency is characterized by a decrease in transport iron: serum iron falls, and the iron-carrying protein transferrin increases (an adap- tation that enhances iron absorption). Together, measurements of serum iron and transferrin can determine the severity of the deficiency—the more transferrin and the less iron in the blood, the more advanced the deficiency is. Transferrin satura- tion—the percentage of transferrin that is saturated with iron—decreases as iron stores decline. The third stage of iron deficiency occurs when the lack of iron limits hemoglobin production. Now the hemoglobin precursor, erythrocyte protoporphyrin, be- gins to accumulate as hemoglobin and hematocrit values decline. ◆ Iron-deficiency anemia is a microcytic Hemoglobin and hematocrit tests are easy, quick, and inexpensive, so they are (my-cro-SIT-ic) hypochromic (high- the tests most commonly used in evaluating iron status. Their usefulness in detect- po-KROME-ic) anemia. ing iron deficiency is limited, however, because they are late indicators. Further- micro = small cytic = cell more, other nutrient deficiencies and medical conditions can influence their hypo = too little values. chrom = color Iron Deficiency and Anemia Iron deficiency and iron-deficiency anemia are not the same: people may be iron deficient without being anemic. The term iron defi- iron-deficiency anemia: severe depletion of ciency refers to depleted body iron stores without regard to the degree of depletion or iron stores that results in low hemoglobin to the presence of anemia. The term iron-deficiency anemia refers to the severe deple- and small, pale red blood cells. Anemias that tion of iron stores that results in a low hemoglobin concentration. In iron-deficiency impair hemoglobin synthesis are microcytic anemia, hemoglobin synthesis decreases, resulting in red blood cells that are pale (small cell). micro = small (hypochronic) and small (microcytic), ◆ as shown in Figure 13-4.14 These cells can’t cytic = cell carry enough oxygen from the lungs to the tissues. Without adequate iron, energy erythrocyte protoporphyrin (PRO-toe- metabolism in the cells falters. The result is fatigue, weakness, headaches, apathy, PORE-fe-rin): a precursor to hemoglobin. pallor, and poor resistance to cold temperatures. Because hemoglobin is the bright hematocrit (hee-MAT-oh-krit): measurement red pigment of the blood, the skin of a fair person who is anemic may become no- of the volume of the red blood cells packed ticeably pale. In a dark-skinned person, the tongue and eye lining, normally pink, by centrifuge in a given volume of blood. is very pale.  THE TRACE MINERALS 447 © Dr. Gladden Willis/Visuals Unlimited (both) FIGURE 13-4 Normal and Anemic Blood Cells Both size and color are normal in these blood cells. Blood cells in iron-deficiency anemia are small (microcytic) and pale (hypochromic) because they contain less hemoglobin. The fatigue that accompanies iron-deficiency anemia differs from the tiredness a person experiences from a simple lack of sleep. People with anemia feel fatigue only when they exert themselves. Iron supplementation can relieve the fatigue and improve the body’s response to physical activity.15 Iron Deficiency and Behavior Long before the red blood cells are affected and anemia is diagnosed, a developing iron deficiency affects behavior. Even at slightly lowered iron levels, energy metabolism is impaired and neurotransmitter synthesis is altered, reducing physical work capacity and mental productivity.16 Without the physical energy and mental alertness to work, plan, think, play, sing, or learn, peo- ple simply do these things less. They have no obvious deficiency symptoms; they just appear unmotivated, apathetic, and less physically fit. Work productivity and vol- untary activities decline. Many of the symptoms associated with iron deficiency are easily mistaken for behavioral or motivational problems. A restless child who fails to pay attention in class might be thought contrary. An apathetic homemaker who has let housework pile up might be thought lazy. No responsible dietitian would ever claim that all behavioral problems are caused by nutrient deficiencies, but poor nutrition is al- ways a possible contributor to problems like these. When investigating a behav- ioral problem, check the adequacy of the diet and seek a routine physical examination before undertaking more expensive, and possibly harmful, treatment options. (The effects of iron deficiency on children’s behavior are discussed further in Chapter 15.) Iron Deficiency and Pica A curious behavior seen in some iron-deficient people, especially in women and children of low-income groups, is pica—an appetite for ice, clay, paste, and other nonfood substances. These substances contain no iron and cannot remedy a deficiency; in fact, clay actually inhibits iron absorption, which may explain the iron deficiency that accompanies such behavior. Iron Toxicity pica (PIE-ka): a craving for nonfood In general, even a diet that includes fortified foods poses no special risk for iron substances. Also known as geophagia (gee- toxicity.17 The body normally absorbs less iron when its stores are full, but some oh-FAY-gee-uh) when referring to clay eating individuals are poorly defended against excess iron. Once considered rare, and pagophagia (pag-oh-FAY-gee-uh) iron overload has emerged as an important disorder of iron metabolism and when referring to ice craving. regulation. iron overload: toxicity from excess iron. 448 CHAPTER 13 Iron Overload The iron overload disorder known as hemochromatosis is usu- ally caused by a genetic failure to prevent unneeded iron in the diet from being absorbed.18 Recent research suggests that just as insulin supports normal glucose homeostasis and its absence or ineffectiveness causes diabetes, the hormone hep- cidin supports iron homeostasis and its absence or ineffectiveness causes he- mochromatosis. Hereditary hemochromatosis is the most common genetic disorder in the United States, affecting some 1.5 million people. Other causes of iron overload include re- peated blood transfusions (which bypass the intestinal defense), massive doses of supplementary iron (which overwhelm the intestinal defense), and other rare metabolic disorders. Excess iron may cause hemosiderosis, a condition character- ized by deposits of the iron storage protein hemosiderin in the liver, heart, joints, and other tissues. Some of the signs and symptoms of iron overload are similar to those of iron de- ficiency: apathy, lethargy, and fatigue. Therefore, taking iron supplements before assessing iron status is clearly unwise; hemoglobin tests alone would fail to make the distinction because excess iron accumulates in storage. Iron overload assess- ment tests measure transferrin saturation and serum ferritin. Iron overload is characterized by tissue damage, especially in iron-storing or- gans such as the liver. Infections are likely because bacteria thrive on iron-rich blood. Symptoms are most severe in alcohol abusers because alcohol damages the intestine, further impairing its defenses against absorbing excess iron. Untreated hemochromatosis increases the risks of diabetes, liver cancer, heart disease, and arthritis. Iron overload is more common in men than in women and is twice as prevalent among men as iron deficiency. The widespread fortification of foods with iron makes it difficult for people with hemochromatosis to follow a low-iron diet, and greater dangers lie in the indiscriminate use of iron and vitamin C supplements. Vi- tamin C not only enhances iron absorption, but also releases iron from ferritin, al- lowing free iron to wreak the damage typical of free radicals. Thus vitamin C acts as a prooxidant when taken in high doses. (See Highlight 11 for a discussion of free radicals and their effects on disease development.) Iron and Heart Disease Some research suggests a link between heart disease and iron, especially when accompanied by alcohol consumption.19 As mentioned, free radicals can attack ferritin, causing it to release iron from storage. Free iron, in turn, acts as an oxidant that can generate more free radicals. Whether iron’s role in oxidative stress contributes to the development of diseases is unclear.20 Iron and Cancer There may be an association between iron and some cancers.21 Explanations for how iron might be involved in causing cancer focus on its free- radical activity, which can damage DNA (see Highlight 11). One of the benefits of a high-fiber diet may be that the accompanying phytates bind iron, making it less available for such reactions. Iron Poisoning Large doses of iron supplements cause GI distress, including con- stipation, nausea, vomiting, and diarrhea. These effects may not be as serious as other consequences of iron toxicity, but they are consistent enough to establish an Upper Level of 45 milligrams per day for adults. Ingestion of iron-containing supplements remains a leading cause of accidental poisoning in small children. Symptoms of toxicity include nausea, vomiting, diar- rhea, a rapid heartbeat, a weak pulse, dizziness, shock, and confusion. As few as hemochromatosis (HE-moh-KRO-ma-toe- five iron tablets containing as little as 200 milligrams of iron have caused the sis): a genetically determined failure to deaths of dozens of young children. The exact cause of these deaths is uncertain, prevent absorption of unneeded dietary iron but excessive free-radical damage is thought to play a role in heart failure and res- that is characterized by iron overload and piratory distress. Autopsy reports reveal iron deposits and cell death in the stom- tissue damage. ach, small intestine, liver, and blood vessels (which can cause internal bleeding). hemosiderosis (HE-moh-sid-er-OH-sis): a Keep iron-containing tablets out of the reach of children. If you suspect iron poison- condition characterized by the deposition of hemosiderin in the liver and other tissues. ing, call the nearest poison control center or a physician immediately.  THE TRACE MINERALS 449 Iron Recommendations and Sources To obtain enough iron, people must first select iron-rich foods and then take advan- tage of factors that maximize iron absorption. This discussion begins by identifying iron-rich foods and then reviews the factors affecting absorption. Recommended Iron Intakes The usual diet in the United States provides about 6 to 7 milligrams of iron for every 1000 kcalories. The recommended daily intake for men is 8 milligrams, and because most men eat more than 2000 kcalories a day, they can meet their iron needs with little effort. Women in their reproductive years, however, need 18 milligrams a day. The accompanying “How to” explains how to calculate the recommended intake. Vegetarians need 1.8 times as much iron ◆ to make up for the low bioavailabil- ◆ To calculate the RDA for vegetarians, multi- ity typical of their diets.22 To maximize iron absorption, vegetarians should incor- ply by 1.8: porate iron-rich foods into a diet that is low in inhibitors (foods such as leavened 8 mg  1.8 = 14 mg/day (vegetarian breads and fermented soy products such as miso and tempeh) and high in en- men) 18 mg  1.8 = 32 mg/day (vegetarian hancers (foods rich in vitamin C and the organic acids found in fruits and vegeta- women, 19 to 50 yr) bles). Good vegetarian sources of iron include soy foods (such as soybeans and tofu), legumes (such as lentils and kidney beans), nuts (such as cashews and al- monds), seeds (such as pumpkin seeds and sunflower seeds), cereals (such as cream of wheat and oatmeal), dried fruit (such as apricots and raisins), vegetables (such as mushrooms and potatoes), and blackstrap molasses. Because women have higher iron needs and lower energy needs, they sometimes have trouble obtaining enough iron. On average, women receive only 12 to 13 mil- ligrams of iron per day, which is not enough iron for women until after menopause. To meet their iron needs from foods, premenopausal women need to select iron-rich foods at every meal. Dietary Guidelines for Americans 2005 Women of childbearing age who may become pregnant should eat foods high in heme-iron and/or consume iron-rich plant foods or iron-fortified foods with an enhancer of iron absorption, such as vitamin C–rich foods. Iron in Foods Figure 13-5 (p. 450) shows the amounts of iron in selected foods. © Craig M. Moore Meats, fish, and poultry contribute the most iron per serving; other protein-rich foods such as legumes and eggs are also good sources. Although an indispensable part of the diet, foods in the milk group are notoriously poor in iron. Grain products vary, with whole-grain, enriched, and fortified breads and cereals contributing sig- When the label on a grain product says nificantly to iron intakes. Finally, dark greens (such as broccoli) and dried fruits “enriched,” it means iron and several B vita- (such as raisins) contribute some iron. mins have been added. HOW TO Estimate the Recommended Daily Intake for Iron To calculate the recommended daily iron An estimated average requirement is intake, the DRI Committee considers a determined based on the daily need and number of factors. For example, for a the assumption that an average of 18 woman of childbearing age (19 to 50): percent of ingested iron is absorbed: Losses from feces, urine, sweat, and shed 1.5 mg iron (needed) skin: 1.0 milligram  0.18 (percent iron absorbed) Losses through menstruation: 0.5 mil-  8 mg iron (estimated average requirement) ligram (about 14 milligrams total aver- aged over 28 days) Then, a margin of safety is added to cover the needs of essentially all women of These losses reflect an average daily need childbearing age, and the RDA is set at 18 (total) of 1.5 milligrams of absorbed iron. milligrams. 450 CHAPTER 13 FIGURE 13-5 Iron in Selected Foods See the “How to” section on p. 329 for more information on using this figure. Milligrams Food Serving size (kcalories) 0 2 4 6 8 10 12 14 16 18 Bread, whole wheat 1 oz slice (70 kcal) Cornflakes, fortified 1 oz (110 kcal) 1 Spaghetti pasta ⁄2 c cooked (99 kcal) RDA for RDA for Tortilla, flour 1 10"-round (234 kcal) women women Broccoli 1 ⁄2 c cooked (22 kcal) 51+ 19–50 1 Carrots ⁄2 c shredded raw (24 kcal) Potato 1 medium baked w/skin (133 kcal) 1 Tomato juice ⁄2 c (31 kcal) RDA Banana 1 medium raw (109 kcal) for men Orange 1 medium raw (62 kcal) 1 Strawberries ⁄2 c fresh (22 kcal) IRON Watermelon 1 slice (92 kcal) Meats (red), legumes (brown), and some vegetables (green) Milk 1 c reduced-fat 2% (121 kcal) make the greatest contributions Yogurt, plain 1 c low-fat (155 kcal) of iron to the diet. Cheddar cheese 11⁄2 oz (171 kcal) 1 Cottage cheese ⁄2 c low-fat 2% (101 kcal) 1 Pinto beans ⁄2 c cooked (117 kcal) Key: Peanut butter 2 tbs (188 kcal) Breads and cereals Sunflower seeds 1 oz dry (165 kcal) 1 Vegetables Tofu (soybean curd) ⁄2 c (76 kcal) Fruits Ground beef, lean 3 oz broiled (244 kcal) Milk and milk products Chicken breast 3 oz roasted (140 kcal) Legumes, nuts, seeds Tuna, canned in water 3 oz (99 kcal) Meats Egg 1 hard cooked (78 kcal) Best sources per kcalorie Excellent, and sometimes unusual, sources: Clams, canned 3 oz (126 kcal) Beef liver 3 oz fried (184 kcal) Parsley 1 c raw (22 kcal) Iron-Enriched Foods Iron is one of the enrichment nutrients for grain products. One serving of enriched bread or cereal provides only a little iron, but because peo- ple eat many servings of these foods, the contribution can be significant. Iron added to foods is not absorbed as well as naturally occurring iron, but when eaten with ab- sorption-enhancing foods, enrichment iron can make a difference. In cases of iron overload, enrichment may exacerbate the problem.23 Maximizing Iron Absorption In general, the bioavailability of iron is high in meats, fish, and poultry, intermediate in grains and legumes, and low in most veg- etables, especially those containing oxalates such as spinach. As mentioned earlier, the amount of iron ultimately absorbed from a meal depends on the combined ef- fects of several enhancing and inhibiting factors. For maximum absorption of non- heme iron, eat meat for MFP and fruits or vegetables for vitamin C. The iron of baked beans, for example, will be enhanced by the MFP in a piece of ham served with them. The iron of bread will be enhanced by the vitamin C in a slice of tomato on a sandwich. Iron Contamination and Supplementation contamination iron: iron found in foods as the result of contamination by inorganic iron In addition to the iron from foods, contamination iron from nonfood sources of salts from iron cookware, iron-containing inorganic iron salts can contribute to the day’s intakes. People can also get iron from soils, and the like. supplements.  THE TRACE MINERALS 451 Contamination Iron Foods cooked in iron cookware take up iron salts. The more acidic the food and the longer it is cooked in iron cookware, the higher the iron content. The iron content of eggs can triple in the time it takes to scramble them in an iron pan. Admittedly, the absorption of this iron may be poor (perhaps only 1 to 2 percent), but every little bit helps a person who is trying to increase iron intake. Iron Supplements People who are iron deficient may need supplements as well as an iron-rich, absorption-enhancing diet. Many physicians routinely recommend iron supplements to pregnant women, infants, and young children. Iron from sup- plements is less well absorbed than that from food, so the doses must be high. The absorption of iron taken as ferrous sulfate or as an iron chelate is better than that from other iron supplements. Absorption also improves when supplements are © Polara Studios Inc. taken between meals, at bedtime on an empty stomach, and with liquids (other than milk, tea, or coffee, which inhibit absorption). Taking iron supplements in a single dose instead of several doses per day is equally effective and may improve a person’s willingness to take it regularly. There is no benefit to taking iron supplements with orange juice because vita- An old-fashioned iron skillet adds iron to foods. min C does not enhance absorption from supplements as it does from foods. (Vita- min C enhances iron absorption by converting insoluble ferric iron in foods to the more soluble ferrous iron, and supplemental iron is already in the ferrous form.) Constipation is a common side effect of iron supplementation; drinking plenty of water may help to relieve this problem. IN SUMMARY Most of the body’s iron is in hemoglobin and myoglobin where it carries oxy- gen for use in energy metabolism; some iron is also required for enzymes in- volved in a variety of reactions. Special proteins assist with iron absorption, transport, and storage—all helping to maintain an appropriate balance, be- cause both too little and too much iron can be damaging. Iron deficiency is most common among infants and young children, teenagers, women of child- bearing age, and pregnant women. Symptoms include fatigue and anemia. Iron overload is most common in men. Heme iron, which is found only in meat, fish, and poultry, is better absorbed than nonheme iron, which occurs in most foods. Nonheme iron absorption is improved by eating iron-containing foods with foods containing the MFP factor and vitamin C; absorption is lim- ited by phytates and oxalates. The summary table presents a few iron facts. Iron RDA Significant Sources Men: 8 mg/day Red meats, fish, poultry, shellfish, eggs, legumes, dried fruits Women: 18 mg/day (19–50 yr) 8 mg/day (51) Deficiency Symptoms Upper Level Anemia: weakness, fatigue, headaches; im- paired work performance and cognitive func- Adults: 45 mg/day tion; impaired immunity; pale skin, nailbeds, mucous membranes, and palm creases; con- Chief Functions in the Body cave nails; inability to regulate body tempera- ture; pica Part of the protein hemoglobin, which carries oxygen in the blood; part of the protein myo- Toxicity Symptoms globin in muscles, which makes oxygen avail- able for muscle contraction; necessary for the GI distress utilization of energy as part of the cells’ meta- Iron overload: infections, fatigue, joint pain, chelate (KEY-late): a substance that can grasp bolic machinery skin pigmentation, organ damage the positive ions of a mineral. chele = claw 452 CHAPTER 13 Zinc ◆ Reminder: A cofactor is a substance that Zinc is a versatile trace element required as a cofactor ◆ by more than 100 enzymes. works with an enzyme to facilitate a chem- Virtually all cells contain zinc, but the highest concentrations are found in muscle ical reaction. and bone.24 Zinc Roles in the Body Zinc supports the work of numerous proteins in the body, such as the metalloen- ◆ Metalloenzymes that require zinc: zymes, ◆ which are involved in a variety of metabolic processes, including the reg- Help make parts of the genetic materials ulation of gene expression.* In addition, zinc stabilizes cell membranes, helping to DNA and RNA strengthen their defense against free-radical attacks. Zinc also assists in immune Manufacture heme for hemoglobin function and in growth and development. Zinc participates in the synthesis, storage, Participate in essential fatty acid and release of the hormone insulin in the pancreas, although it does not appear to metabolism play a direct role in insulin’s action. Zinc interacts with platelets in blood clotting, Release vitamin A from liver stores affects thyroid hormone function, and influences behavior and learning perfor- Metabolize carbohydrates mance. It is needed to produce the active form of vitamin A (retinal) in visual pig- Synthesize proteins Metabolize alcohol in the liver ments and the retinol-binding protein that transports vitamin A. It is essential to Dispose of damaging free radicals normal taste perception, wound healing, the making of sperm, and fetal develop- ment. A zinc deficiency impairs all these and other functions, underlining the vast importance of zinc in supporting the body’s proteins. Zinc Absorption and Metabolism The body’s handling of zinc resembles that of iron in some ways and differs in oth- ers. A key difference is the circular passage of zinc from the intestine to the body and back again. Zinc Absorption The rate of zinc absorption varies from about 15 to 40 percent, depending on a person’s zinc status—if more is needed, more is absorbed. Also, di- etary factors influence zinc absorption. For example, phytates bind zinc, thus limit- ing its bioavailability.25 Upon absorption into an intestinal cell, zinc has two options. It may become in- volved in the metabolic functions of the cell itself. Alternatively, it may be retained within the cell by metallothionein, a special binding protein similar to the iron storage protein, mucosal ferritin. Metallothionein in the intestinal cells helps to regulate zinc absorption by hold- ing it in reserve until the body needs zinc. Then metallothionein releases zinc into the blood where it can be transported around the body. Metallothionein in the liver performs a similar role, binding zinc until other body tissues signal a need for it. Zinc Recycling Some zinc eventually reaches the pancreas, where it is incorpo- rated into many of the digestive enzymes that the pancreas releases into the intes- tine at mealtimes. The intestine thus receives two doses of zinc with each meal—one from foods and the other from the zinc-rich pancreatic secretions. The recycling of metalloenzymes (meh-TAL-oh-EN-zimes): zinc in the body from the pancreas to the intestine and back to the pancreas is re- enzymes that contain one or more minerals ferred to as the enteropancreatic circulation of zinc. As this zinc circulates as part of their structures. through the intestine, it may be excreted in shed intestinal cells or absorbed into the metallothionein (meh-TAL-oh-THIGH-oh- body on any of its times around (see Figure 13-6). The body loses zinc primarily neen): a sulfur-rich protein that avidly binds with and transports metals such as zinc. in feces. Smaller losses occur in urine, shed skin, hair, sweat, menstrual fluids, and metallo = containing a metal semen. thio = containing sulfur Zinc Transport Zinc’s main transport vehicle in the blood is the protein albumin. ein = a protein Some zinc also binds to transferrin—the same transferrin that carries iron in the enteropancreatic (EN-ter-oh-PAN-kree-AT- ik) circulation: the circulatory route from the pancreas to the intestine and back to the * Among the metalloenzymes requiring zinc are carbonic anhydrase, deoxythymidine kinase, DNA and pancreas. RNA polymerase, and alkaline phosphatase.  THE TRACE MINERALS 453 FIGURE 13-6 Animated! Enteropancreatic Circulation of Zinc Some zinc from food is absorbed by the small intestine and sent to the pancreas to be incorporated into digestive enzymes that return to the small intestine. This cycle is called the enteropancreatic circulation of zinc. To test your understanding of these concepts, log on to academic.cengage.com/login. Zinc in food If the body does not Zinc is not absorbed and is Mucosal cells in need zinc excreted in shed intestinal the intestine store cells instead. Thus, zinc excess zinc in absorption is reduced when metallothionein. the body does not need zinc. The pancreas uses zinc to make If the body digestive enzymes needs zinc and secretes them into the intestine. Metallothionein releases zinc to albumin and transferrin for transport to the rest of the body. blood. In healthy individuals, transferrin is usually less than 50 percent saturated FIGURE 13-7 Zinc-Deficiency Symp- with iron, but in iron overload, it is more saturated. Diets that deliver more than twice tom—The Stunted Growth of Dwarfism as much iron as zinc leave too few transferrin sites available for zinc. The result is poor The growth retardation, known as zinc absorption. The converse is also true: large doses of zinc inhibit iron absorption. dwarfism, is rightly ascribed to zinc defi- Large doses of zinc create a similar problem with another essential mineral, cop- ciency because it is partially reversible per. These nutrient interactions highlight one of the many reasons why people when zinc is restored to the diet. should use supplements conservatively, if at all: supplementation can easily create imbalances. Zinc Deficiency Severe zinc deficiencies are not widespread in developed countries, but they do occur in vulnerable groups—pregnant women, young children, the elderly, and the poor. Human zinc deficiency was first reported in the 1960s in children and adolescent boys in Egypt, Iran, and Turkey. Children have especially high zinc needs because they are growing rapidly and synthesizing many zinc-containing proteins, and the native diets among those populations were not meeting these needs. Middle Eastern © H. Sanstead, University of Texas at Galveston diets are typically low in the richest zinc source, meats, and the staple foods are legumes, unleavened breads, and other whole-grain foods—all high in fiber and phytates, which inhibit zinc absorption.* Figure 13-7 shows the severe growth retardation and mentions the immature sexual development characteristic of zinc deficiency. In addition, zinc deficiency hinders digestion and absorption, causing diarrhea, which worsens malnutrition not only for zinc, but for all nutrients. It also impairs the immune response, mak- ing infections likely—among them, GI tract infections, which worsen malnutrition, including zinc malnutrition (a classic downward spiral of events).26 Chronic zinc deficiency damages the central nervous system and brain and may lead to poor The Egyptian man on the right is an adult of motor development and cognitive performance. Because zinc deficiency directly average height. The Egyptian boy on the left is impairs vitamin A metabolism, vitamin A–deficiency symptoms often appear. Zinc 17 years old but is only 4 feet tall, like a 7-year-old in the United States. His genitalia are like those of a 6-year-old. * Unleavened bread contains no yeast, which normally breaks down phytates during fermentation. 454 CHAPTER 13 deficiency also disturbs thyroid function and the metabolic rate. It alters taste, causes loss of appetite, and slows wound healing—in fact, its symptoms are so per- vasive that generalized malnutrition and sickness are more likely to be the diagno- sis than simple zinc deficiency. Zinc Toxicity High doses (over 50 milligrams) of zinc may cause vomiting, diarrhea, headaches, exhaustion, and other symptoms. An Upper Level for adults was set at 40 mil- ligrams based on zinc’s interference in copper metabolism—an effect that, in ani- mals, leads to degeneration of the heart muscle. Zinc Recommendations and Sources Figure 13-8 shows zinc amounts in foods per serving. Zinc is highest in protein- © Polara Studios Inc. rich foods such as shellfish (especially oysters), meats, poultry, milk, and cheese. Legumes and whole-grain products are good sources of zinc if eaten in large quan- tities; in typical U.S. diets, the phytate content of grains is not high enough to im- pair zinc absorption. Vegetables vary in zinc content depending on the soil in Zinc is highest in protein-rich foods such as oysters, beef, poultry, legumes, and nuts. which they are grown. Average intakes in the United States are slightly higher than recommendations. FIGURE 13-8 Zinc in Selected Foods See the “How to” section on p. 329 for more information on using this figure. Milligrams Food Serving size (kcalories) 0 2 4 6 8 10 12 Bread, whole wheat 1 oz slice (70 kcal) Cornflakes, fortified 1 oz (110 kcal) 1 RDA Spaghetti pasta ⁄2 c cooked (99 kcal) for Tortilla, flour 1 10"-round (234 kcal) men 1 Broccoli ⁄2 c cooked (22 kcal) 1 Carrots ⁄2 c shredded raw (24 kcal) Potato 1 medium baked w/skin (133 kcal) RDA Tomato juice 3 ⁄4 c (31 kcal) for women Banana 1 medium raw (109 kcal) Orange 1 medium raw (62 kcal) 1 Strawberries ⁄2 c fresh (22 kcal) ZINC Watermelon 1 slice (92 kcal) Meat, fish, and poultry (red) are Milk 1 c reduced-fat 2% (121 kcal) concentrated sources of zinc. Yogurt, plain 1 c low-fat (155 kcal) Milk (white) and legumes (brown) contain some zinc. Cheddar cheese 11⁄2 oz (171 kcal) 1 Cottage cheese ⁄2 c low-fat 2% (101 kcal) Key: 1 Pinto beans ⁄2 c cooked (117 kcal) Breads and cereals Peanut butter 2 tbs (188 kcal) Vegetables Sunflower seeds 1 oz dry (165 kcal) Tofu (soybean curd) 1 ⁄2 c (76 kcal) Fruits Ground beef, lean 3 oz broiled (244 kcal) Milk and milk products Chicken breast 3 oz roasted (140 kcal) Legumes, nuts, seeds Tuna, canned in water 3 oz (99 kcal) Meats Egg 1 hard cooked (78 kcal) Best sources per kcalorie Excellent, and sometimes unusual, sources: Oysters 3 oz cooked (139 kcal) Sirloin steak, lean 3 oz broiled (172 kcal) Crab 3 oz cooked (94 kcal)  THE TRACE MINERALS 455 Zinc Supplementation In developed countries, most people obtain enough zinc from the diet without resort- ing to supplements. In developing countries, zinc supplements play a major role in the treatment of childhood infectious diseases. Zinc supplements effectively reduce the incidence of disease and death associated with diarrhea.27 The use of zinc lozenges to treat the common cold has been controversial and in- conclusive, with some studies finding them effective and others not.28 The different study results may reflect the effectiveness of various zinc compounds. Some studies using zinc gluconate report shorter duration of cold symptoms, whereas most stud- ies using other combinations of zinc report no effect. Common side effects of zinc lozenges include nausea and bad taste reactions. IN SUMMARY Zinc-requiring enzymes participate in a multitude of reactions affecting growth, vitamin A activity, and pancreatic digestive enzyme synthesis, among others. Both dietary zinc and zinc-rich pancreatic secretions (via enteropancre- atic circulation) are available for absorption. Absorption is monitored by a special binding protein (metallothionein) in the intestine. Protein-rich foods derived from animals are the best sources of bioavailable zinc. Fiber and phy- tates in cereals bind zinc, limiting absorption. Growth retardation and sexual immaturity are hallmark symptoms of zinc deficiency. These facts and others are included in the following table. Zinc RDA Significant Sources Men: 11 mg/day Protein-containing foods: red meats, shellfish, whole grains; some fortified cereals Women: 8 mg/day Upper Level Deficiency Symptomsa Growth retardation, delayed sexual maturation, Adults: 40 mg/day impaired immune function, hair loss, eye and skin lesions, loss of appetite Chief Functions in the Body Part of many enzymes; associated with the Toxicity Symptoms hormone insulin; involved in making genetic Loss of appetite, impaired immunity, low HDL, material and proteins, immune reactions, copper and iron deficiencies transport of vitamin A, taste perception, wound healing, the making of sperm, and the normal development of the fetus aA rare inherited disease of zinc malabsorption, acrodermatitis (AK-roh-der-ma-TIE-tis) enteropathica (EN-ter-oh-PATH-ick- ah), causes additional and more severe symptoms. Iodine Traces of the iodine ion (called iodide) ◆ are indispensable to life. In the GI tract, io- ◆ The ion form of iodine is called iodide. dine from foods becomes iodide. This chapter uses the term iodine when referring to the nutrient in foods and iodide when referring to it in the body. Iodide occurs in the body in minuscule amounts, but its principal role in the body and its requirement are well established. ◆ The thyroid gland releases tetraiodothyro- Iodide Roles in the Body Iodide is an integral part of the thyroid hormones ◆ nine (T4), commonly known as thyroxine that regulate body temperature, metabolic rate, reproduction, growth, blood cell (thigh-ROCKS-in), to its target tissues. Upon production, nerve and muscle function, and more. By controlling the rate at which reaching the cells, T4 is deiodinated to tri- the cells use oxygen, these hormones influence the amount of energy released dur- iodothyronine (T3), which is the active form ing basal metabolism. of the hormone. 456 CHAPTER 13 FIGURE 13-9 Iodine-Deficiency Symp- Iodine Deficiency The hypothalamus regulates thyroid hormone production by controlling the release of the pituitary’s thyroid-stimulating hormone (TSH). ◆ With tom—The Enlarged Thyroid of Goiter iodine deficiency, thyroid hormone production declines, and the body responds by secreting more TSH in a futile attempt to accelerate iodide uptake by the thyroid gland. If a deficiency persists, the cells of the thyroid gland enlarge to trap as much iodide as possible. Sometimes the gland enlarges until it makes a visible lump in the neck, a simple goiter (shown in Figure 13-9). Goiter afflicts about 200 million people the world over, many of them in South America, Asia, and Africa. In all but 4 percent of these cases, the cause is iodine de- ficiency. As for the 4 percent (8 million), most have goiter because they regularly eat excessive amounts of foods ◆ that contain an antithyroid substance (goitro- gen) whose effect is not counteracted by dietary iodine. The goitrogens present in plants remind us that even natural components of foods can cause harm when eaten in excess. Goiter may be the earliest and most obvious sign of iodine deficiency, but the © Bob Daemmrich/The Image Works most tragic and prevalent damage occurs in the brain. Children with even a mild iodine deficiency typically have goiters and perform poorly in school. With sus- tained treatment, however, mental performance in the classroom as well as thyroid function improves.29 A severe iodine deficiency during pregnancy causes the extreme and irreversible mental and physical retardation known as cretinism. ◆ Cretinism affects approx- imately 6 million people worldwide and can be averted by the early diagnosis and In iodine deficiency, the thyroid gland treatment of maternal iodine deficiency. A worldwide effort to provide iodized salt enlarges—a condition known as simple to people living in iodine-deficient areas has been dramatically successful. Because goiter. iron deficiency is common among people with iodine deficiency and because iron deficiency reduces the effectiveness of iodized salt, dual fortification with both iron ◆ Thyroid-stimulating hormone is also called and iodine may be most beneficial.30 thyrotropin. ◆ Examples of goitrogen-containing foods: Iodine Toxicity Excessive intakes of iodine can interfere with thyroid function Cabbage, spinach, radishes, rutabagas and enlarge the gland, just as deficiency can.31 During pregnancy, exposure to ex- Soybeans, peanuts cessive iodine from foods, prenatal supplements, or medications is especially dam- Peaches, strawberries aging to the developing infant. An infant exposed to toxic amounts of iodine during gestation may develop a goiter so severe as to block the airways and cause ◆ The underactivity of the thyroid gland is suffocation. The Upper Level is 1100 micrograms per day for an adult—several known as hypothyroidism and may be caused by iodine deficiency or any number times higher than average intakes. of other causes. Without treatment, an Iodine Recommendations and Sources The ocean is the world’s major source infant with congenital hypothyroidism will of iodine. In coastal areas, seafood, water, and even iodine-containing sea mist are develop the physical and mental retarda- dependable iodine sources. Further inland, the amount of iodine in foods is variable tion of cretinism. and generally reflects the amount present in the soil in which plants are grown or ◆ Iodized salt contains about 60 μg iodine on which animals graze. Landmasses that were once under the ocean have soils rich per gram salt. in iodine; those in flood-prone areas where water leaches iodine from the soil are ◆ On average, 1/2 tsp iodized salt provides the poor in iodine. In the United States and Canada, the iodization of salt ◆ has elimi- RDA for iodine. nated the widespread misery caused by iodine deficiency during the 1930s, but iodized salt is not available in many parts of the world. Some countries add iodine goiter (GOY-ter): an enlargement of the to bread, fish paste, or drinking water instead. thyroid gland due to an iodine deficiency, Although average consumption of iodine in the United States exceeds recom- malfunction of the gland, or mendations, it falls below toxic levels. Some of the excess iodine in the U.S. diet overconsumption of a goitrogen. Goiter caused by iodine deficiency is simple stems from fast foods, which use iodized salt liberally. Some iodine comes from bak- goiter. ery products and from milk. The baking industry uses iodates (iodine salts) as goitrogen (GOY-troh-jen): a substance that dough conditioners, and most dairies feed cows iodine-containing medications and enlarges the thyroid gland and causes toxic use iodine to disinfect milking equipment. Now that these sources have been iden- goiter. Goitrogens occur naturally in such tified, food industries have reduced their use of these compounds, but the sudden foods as cabbage, kale, brussels sprouts, emergence of this problem points to a need for continued surveillance of the food cauliflower, broccoli, and kohlrabi. supply. Processed foods in the United States use regular salt, not iodized salt. cretinism (CREE-tin-ism): a congenital The recommended intake of iodine for adults is a minuscule amount. The need disease characterized by mental and physical retardation and commonly caused by for iodine is easily met by consuming seafood, vegetables grown in iodine-rich soil, maternal iodine deficiency during and iodized salt. ◆ In the United States, labels indicate whether salt is iodized; in pregnancy. Canada, all table salt is iodized.  THE TRACE MINERALS 457 IN SUMMARY Iodide, the ion of the mineral iodine, is an essential component of the thyroid hormone. An iodine deficiency can lead to simple goiter (enlargement of the thyroid gland) and can impair fetal development, causing cretinism. Iodiza- tion of salt has largely eliminated iodine deficiency in the United States and Canada. The table provides a summary of iodine. Iodine