Minerals

Minerals are inorganic elements that originate in the earth and cannot be made in the body. They play important roles in various bodily functions and are necessary to sustain life and maintain optimal health, and thus are essential nutrients. Most of the minerals in the human diet come directly from plants and water, or indirectly from animal foods. However, the mineral content of water and plant foods varies geographically because of variations in the mineral content of soil from region to region.

Minerals provide structure to bones and teeth and participate in energy production, the building of protein, blood formation, and several other metabolic processes.

SOURCE: U.S. Department of Agriculture, National Agricultural Library, Food and Nutrition Information Center and USDA Agricultural Research Service.

Recommended dietary allowances (RDAs) for select minerals

The major minerals present in the body include sodium, potassium, chloride, calcium, magnesium, phosphorus, and sulfur.

The fluid balance in the body, vital for all life processes, is maintained largely by sodium, potassium, and chloride. Fluid balance is regulated by charged sodium and chloride ions in the extracellular fluid (outside the cell) and potassium in the intracellular fluid (inside the cell), and by some other electrolytes across cell membranes. Tight control is critical for normal muscle contraction, nerve impulse transmission, heart function, and blood pressure. Sodium plays an important role in the absorption of other nutrients, such as glucose, amino acids, and water. Chloride is a component of hydrochloric acid, an important part of gastric juice (an acidic liquid secreted by glands in the stomach lining) and aids in food digestion. Potassium and sodium act as cofactors for certain enzymes.

Calcium, magnesium, and phosphorus are known for their structural roles, as they are essential for the development and maintenance of bones and teeth. They are also needed for maintaining cell membranes and connective tissue. Several enzymes, hormones, and proteins that regulate energy and fat metabolism require calcium, magnesium and/or phosphorus to become active. Calcium also aids in blood clotting. Sulfur is a key component of various proteins and vitamins and participates in drug-detoxifying pathways in the body.

Trace minerals are present (and required) in very small amounts in the body. An understanding of the important roles and requirements of trace minerals in the human body is fairly recent, and research is still ongoing. The most important trace minerals are iron, zinc, copper, chromium, fluoride, iodine, selenium, manganese, and molybdenum. Some others, such as arsenic, boron, cobalt, nickel, silicon, and vanadium, are recognized as essential for some animals, while others, such as barium, bromine, cadmium, gold, silver, and aluminum, are found in the body, though little is known about their role in health.

Trace minerals have specific biological functions. They are essential in the absorption and utilization of many nutrients and aid enzymes and hormones in activities that are vital to life. Iron plays a major role in oxygen transport and storage and is a component of hemoglobin in red blood cells and myoglobin in muscle cells. Cellular energy production requires many trace minerals, including iron, copper, and zinc, which act as enzyme cofactors in the synthesis of many proteins, hormones, neurotransmitters, and genetic material.

Iron and zinc support immune function, while chromium and zinc aid insulin action. Zinc is also essential for many other bodily functions, such as growth, development of sexual organs, and reproduction. Zinc, copper and selenium prevent oxidative damage to cells. Fluoride stabilizes bone mineral and hardens tooth enamel, thus increasing resistance to tooth decay. Iodine is essential for normal thyroid function, which is critical for many aspects of growth and development, particularly brain development. Thus, trace minerals contribute to physical growth and mental development.

The bioavailability of a nutrient refers to how effectively the nutrient can be absorbed and used by the body. Mineral bioavailability depends on several factors. Higher absorption occurs among individuals who are deficient in a mineral, while some elements in the diet (e.g., oxalic acid or oxalate in spinach) can decrease mineral availability by chemically binding to the mineral. In addition, excess intake of one mineral can influence the absorption and metabolism of other minerals. For example, the presence of a large amount of zinc in the diet decreases the absorption of iron and copper. On the other hand, the presence of vitamins in a meal enhances the absorption of minerals in the meal. For example, vitamin C improves iron absorption, and vitamin D aids in the absorption of calcium, phosphorous, and magnesium.

In general, minerals from animal sources are absorbed better than those from plant sources as minerals are present in forms that are readily absorbed and binders that inhibit absorption, such as phytates, are absent. Vegans (those who restrict their diets to plant foods) need to be aware of the factors affecting mineral bioavailability. Careful meal planning is necessary to include foods rich in minerals and absorption-enhancing factors.

The Food and Nutrition Board currently recommends that supplements or fortified foods be used to obtain desirable amounts of some nutrients, such as calcium and iron. The recommendations for calcium are higher than the average intake in the United States. Women, who generally consume lower energy diets than men, and individuals who do not consume dairy products can particularly benefit from calcium supplements. Because of the increased need for iron in women of childbearing age, as well as the many negative consequences of iron-deficiency anemia, iron supplementation is recommended for vulnerable groups in the United States, as well as in developing countries.

Although severe deficiencies of better-understood trace minerals are easy to recognize, diagnosis is difficult for less-understood minerals and for mild deficiencies. Even mild deficiencies of trace minerals however, can result in poor growth and development in children.

Iron deficiency is the most common nutrient deficiency worldwide, including in the United States. Iron-deficiency anemia affects hundreds of millions of people, with highest prevalence in developing countries. Infants, young children, adolescents, and pregnant and lactating women are especially vulnerable due to their high demand for iron. Menstruating women are also vulnerable due to blood loss. Vegetarians are another vulnerable group, as iron from plant foods is less bioavailable than that from animal sources.

Zinc deficiency, marked by severe growth retardation and arrested sexual development, was first reported in children and adolescent boys in Egypt, Iran, and Turkey. Diets in Middle Eastern countries are typically high in fiber and phytates, which inhibit zinc absorption. Mild zinc deficiency has been found in vulnerable groups in the United States. Copper deficiency is rare, but can be caused by excess zinc from supplementation.

Deficiencies of fluoride, iodine, and selenium mainly occur due to a low mineral content in either the water or soil in some areas of the world. Fluoride deficiency is marked by a high prevalence of dental caries and is common in geographic regions with low water-fluoride concentration, which has led to the fluoridation of water in the United States and many other parts of the world. Goiter and cretinism (a condition in which body growth and mental development are stunted) have been eliminated by iodization of salt in the United States, but still occur in parts of the world where salt manufacture and distribution are not regulated. Selenium deficiency due to low levels of the mineral in soil is found in northeast China, and it has been associated with Keshan disease, a heart disorder prevalent among people of that area.

Dietary deficiency is unlikely for most major minerals, except in starving people or those with proteinenergy malnutrition in developing countries, or people on poor diets for an extended period, such as those suffering from alcoholism, anorexia, or bulimia. Most people in the world consume a lot of salt, and it is recommended that they moderate their intake to prevent chronic diseases (high salt intake has been associated with an increased risk of death from stroke and cardiovascular disease). However, certain conditions, such as severe or prolonged vomiting or diarrhea, the use of diuretics, and some forms of kidney disease, lead to an increased loss of minerals, particularly sodium, chloride, potassium, and magnesium. Calcium intakes tend to be lower in women and vegans who do not consume dairy products. Elderly people with suboptimal diets are also at risk of mineral deficiencies because of decreased absorption and increased excretion of minerals in the urine.

Toxicity from excessive dietary intake of major minerals rarely occurs in healthy individuals. Kidneys that are functioning normally can regulate mineral concentrations in the body by excreting the excess amounts in urine. Toxicity symptoms from excess intakes are more likely to appear with acute or chronic kidney failure.

Trace minerals can be toxic at higher intakes, especially for those minerals whose absorption is not regulated in the body (e.g., selenium and iodine). Thus, it is important not to habitually exceed the recommended intake levels. Although toxicity from dietary sources is unlikely, certain genetic disorders can make people vulnerable to overloads from food or supplements. One such disorder, hereditary hemochromatosis, is characterized by iron deposition in the liver and other tissues due to increased intestinal iron absorption over many years.

In addition to clinical deficiency diseases such as anemia and goiter, research indicates that trace minerals play a role in the development, prevention, and treatment of chronic diseases. A marginal status of several trace minerals has been found to be associated with infectious diseases, disorders of the stomach, intestine, bone, heart, and liver, and cancer, although further research is necessary in many cases to understand the effect of supplementation. Iron, zinc, copper, and selenium have been associated with immune response conditions. Copper, chromium and selenium have been linked to the prevention of cardiovascular disease. Excess iron in the body, on the other hand, can increase the risk of cardiovascular disease, liver and colorectal cancer, and neurodegenerative diseases such as Alzheimer's disease. Chromium supplementation has been found to be beneficial in many studies of impaired glucose tolerance, a metabolic state between normal glucose regulation and diabetes. Fluoride has been known to prevent dental caries and osteoporosis, while potassium iodide supplements taken immediately before or after exposure to radiation can decrease the risk of radiation-induced thyroid cancer.

Sodium, chloride, and potassium are linked to high blood pressure (hypertension) due to their role in the body's fluid balance. High salt or sodium chloride intake has been linked to cardiovascular disease as well. High potassium intakes, on the other hand, have been associated with a lower risk of stroke, particularly in people with hypertension. Research also suggests a preventive role for magnesium in hypertension and cardiovascular disease, as well as a beneficial effect in the treatment of diabetes, osteoporosis, and migraine headaches.

Sodium and chloride toxicity can develop due to low intake or excess loss of water. Accumulation of excess potassium in plasma may result from the use of potassium-sparing diuretics (medications used to treat high blood pressure, which increase urine production, excreting sodium but not potassium), insufficient aldosterone secretion (a hormone that acts on the kidney to decrease sodium secretion and increase potassium secretion), or tissue damage (e.g., from severe burns). Magnesium intake from foods has no adverse effects, but a high intake from supplements when kidney function is limited increases the risk of toxicity. The most serious complication of potassium or magnesium toxicity is cardiac arrest. Adverse effects from excess calcium have been reported only with consumption of large quantities of supplements. Phosphate toxicity can occur due to absorption from phosphate salts taken by mouth or in enemas.

Osteoporosis is a bone disorder in which bone strength is compromised, leading to an increased risk of fracture. Along with other lifestyle factors, intake of calcium and vitamin D plays an important role in the maintenance of bone health and the prevention and treatment of osteoporosis. Good calcium nutrition, along with low salt and high potassium intake, has been linked to prevention of hypertension and kidney stones.

Some body processes require several minerals to work together. For example, calcium, magnesium, and phosphorus are all important for the formation and maintenance of healthy bones. Some minerals compete with each other for absorption, and they interact with other nutrients as well, which can affect their bioavailability.

With the exception of iron, dietary deficiencies are rare in the United States and other developed nations. However, malnutrition in developing countries increases the risk for trace-mineral deficiencies among children and other vulnerable groups. In overzealous supplement users, interactions among nutrients can inhibit absorption of some minerals leading to deficiencies. Patients on intravenous feedings without mineral supplements are at risk of developing deficiencies as well.

Mineral supplementation may be appropriate for people with prolonged illnesses or extensive injuries, for those undergoing surgery, or for those being treated for alcoholism. However, extra caution must be taken to avoid intakes greater than the RDA or AI for specific nutrients because of problems related to nutrient excesses, imbalances, or adverse interactions with medical treatments. Although toxic symptoms or adverse effects from excess supplementation have been reported for various minerals (e.g., calcium, magnesium, iron, zinc, copper, and selenium) and tolerable upper limits set, the amounts of nutrients in supplements are not regulated by the Food and Drug Administration (FDA). Therefore, supplement users must be aware of the potential adverse effects and choose supplements with moderate amounts of nutrients.

See also Açaí berry ; Adolescent nutrition ; Adult nutrition ; Dietary supplements ; Infant nutrition .