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8.4: Trace Minerals

Trace minerals are needed in amounts less than 100 mg per day and they are present in the body in amounts less than 5 grams. Important trace minerals include: iron, cobalt, copper, zinc, manganese, molybdenum, iodine, bromine, and selenium. These are also called minor minerals, with "minor" referring to their amount, as opposed to their importance. 


Bromine has sometimes been considered to be possibly essential in humans, but with the support of only limited circumstantial evidence, and no clear biological role. As a pharmaceutical, the simple bromide ion (Br) has inhibitory effects on the central nervous system, and bromide salts were once a major medical sedative, before replacement by shorter-acting drugs. They retain niche uses as antiepileptics.

Unlike its neighbors in the halogen column (chlorine and iodine), bromine is not known to have a confirmed biological role in humans. A single 2014 study suggests that it may be a necessary catalyst to make collagen IV, making it essential to basement membrane architecture and tissue development, but these findings have not been confirmed and the precise role posited for bromine is not certain. Aside from this, only circumstantial evidence in the form of hypothesized deprivation symptoms exists for a trace role for bromine. 


Cobalt is the active center of coenzymes called cobalamins, the most common example of which is vitamin B12. As such, it is an essential trace dietary mineral for all animals. Cobalt is essential to the metabolism of all animals. In the early 20th century during the development for farming of the North Island Volcanic Plateau of New Zealand, cattle suffered from what was termed "bush sickness". It was discovered that the volcanic soils lacked the cobalt salts essential for the cattle food chain.

The "coast disease" of sheep in the Ninety Mile Desert of the Southeast of South Australia in the 1930s was found to originate in nutritional deficiencies of the trace elements, cobalt and copper. The cobalt deficiency was overcome by the development of "cobalt bullets", dense pellets of cobalt oxide mixed with clay given orally for lodging in the animal's rumen.


Copper is essential to all living organisms as a trace dietary mineral because it is a key constituent of the respiratory enzyme complex cytochrome c oxidase. In humans, copper is found mainly in the liver, muscle, and bone. The adult body contains between 1.4 and 2.1 mg of copper per kilogram of body weight. Rich sources of copper include oysters, beef and lamb liver, Brazil nuts, blackstrap molasses, cocoa, and black pepper. Good sources include lobster, nuts and sunflower seeds, green olives, avocados, and wheat bran.

The human body contains copper at a level of about 1.4 to 2.1 mg per kg of body mass. Copper is absorbed in the gut, then transported to the liver bound to albumin. After processing in the liver, copper is distributed to other tissues in a second phase, which involves the protein ceruloplasmin, carrying the majority of copper in blood. Ceruloplasmin also carries the copper that is excreted in milk, and is particularly well-absorbed as a copper source. Copper in the body normally undergoes enterohepatic circulation (about 5 mg a day, vs. about 1 mg per day absorbed in the diet and excreted from the body), and the body is able to excrete some excess copper, if needed, via bile, which carries some copper out of the liver that is not then reabsorbed by the intestine.

The Food and Nutrition Board of the U.S. Institute of Medicine updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for copper in 2001. The current EAR for copper for people ages 14 and up is 0.7 mg/day. The RDA is 0.9 mg/day. RDAs are higher than EARs so as to identify amounts that will cover people with higher than average requirements. RDA for pregnancy equals 1.0 mg/day. RDA for lactation equals 1.3 mg/day. For infants up to 12 months the AI is 0.22 mg/day and for children ages 1–13 years the RDA increases with age from 0.34 to 0.7 mg/day. 


Iodine is found in the thyroid hormones. Iodine deficiency affects about two billion people and is the leading preventable cause of intellectual disabilities. Iodine is an essential element for life and is the heaviest element commonly needed by living organisms. It is required for the synthesis of the growth-regulating thyroid hormones thyroxine and triiodothyronine (T4 and T3 respectively, named after their number of iodine atoms). A deficiency of iodine leads to decreased production of T3 and T4 and a concomitant enlargement of the thyroid tissue in an attempt to obtain more iodine, causing the disease known as simple goitre. The major form of thyroid hormone in the blood is thyroxine (T4), which has a longer half-life than T3.

The daily Dietary Reference Intake recommended by the United States Institute of Medicine is between 110 and 130 µg for infants up to 12 months, 90 µg for children up to eight years, 130 µg for children up to 13 years, 150 µg for adults, 220 µg for pregnant women and 290 µg for lactating mothers. The Tolerable Upper Intake Level (UL) for adults is 1,100 μg/day. The tolerable upper limit was assessed by analyzing the effect of supplementation on thyroid-stimulating hormone. The thyroid gland needs no more than 70 μg/day to synthesize the requisite daily amounts of T4 and T3. The higher recommended daily allowance levels of iodine seem necessary for optimal function of a number of body systems, including lactating breast, gastric mucosa, salivary glands, brain cells, choroid plexus, oral mucosa, and arterial walls.

Natural sources of dietary iodine include seafood, such as fish, seaweeds (such as kelp) and shellfish, dairy products and eggs so long as the animals received enough iodine, and plants grown on iodine-rich soil. Iodized salt is fortified with iodine in the form of sodium iodide. The general US population has adequate iodine nutrition, with women of childbearing age and pregnant women having a possible mild risk of deficiency. 

Salt iodization

After iodine fortification programs such as iodization of salt have been implemented, some cases of iodine-induced hyperthyroidism have been observed. The condition seems to occur mainly in people over forty, and the risk appears higher when iodine deficiency is severe and the initial rise in iodine intake is high. In areas where there is little iodine in the diet, typically remote inland areas and semi-arid equatorial climates where no marine foods are eaten, iodine deficiency gives rise to hypothyroidism, symptoms of which are extreme fatigue, goitre, mental slowing, depression, weight gain, and low basal body temperatures. Iodine deficiency is the leading cause of preventable intellectual disability, a result that occurs primarily when babies or small children are rendered hypothyroidic by a lack of the element. The addition of iodine to table salt has largely eliminated this problem in the wealthier nations, but iodine deficiency remains a serious public health problem today in the developing world. Iodine deficiency is also a problem in certain areas of Europe. Information processing, fine motor skills, and visual problem solving are improved by iodine repletion in moderately iodine-deficient children.


Iron plays an important role in biology, forming complexes with molecular oxygen in hemoglobin and myoglobin; these two compounds are common oxygen transport proteins in vertebrates. Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals. A human male of average height has about 4 grams of iron in his body, a female about 3.5 grams. This iron is distributed throughout the body in hemoglobin, tissues, muscles, bone marrow, blood proteins, enzymes, ferritin, hemosiderin, and transport in plasma.

Iron is pervasive, but particularly rich sources of dietary iron include red meat, lentils, beans, poultry, fish, leaf vegetables, watercress, tofu, chickpeas, black-eyed peas, and blackstrap molasses. Bread and breakfast cereals are sometimes specifically fortified with iron. Iron in low amounts is found in molasses, teff, and farina. The Recommended Dietary Allowance (RDA) for iron varies considerably depending on age, sex, and source of dietary iron: for example, heme-based iron has higher bioavailability.

The Food and Nutrition Board of the U.S. Institute of Medicine updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for iron in 2001. The current EAR for iron for women ages 14-18 is 7.9 mg/day, 8.1 for ages 19-50 and 5.0 thereafter (post menopause). For men the EAR is 6.0 mg/day for ages 19 and up. The RDA is 15.0 mg/day for women ages 15-18, 18.0 for 19-50 and 8.0 thereafter. For men, 8.0 mg/day for ages 19 and up. RDAs are higher than EARs so as to identify amounts that will cover people with higher than average requirements. RDA for pregnancy equals 27 mg/day. RDA for lactation equals 9 mg/day. For children ages 1–3 years 7 mg/day, 10 for ages 4-8 and 8 for ages 9-13. 

For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For iron labeling purposes 100% of the Daily Value was 18.0 mg, and as of May 2016 remained unchanged at 18.0 mg. Food and supplement companies have until July 28, 2018 to comply with the change. A table of the pre-change adult Daily Values is provided at Reference Daily Intake.

Iron deficiency is the most common nutritional deficiency in the world. When loss of iron is not adequately compensated by adequate dietary iron intake, a state of latent iron deficiency occurs, which over time leads to iron-deficiency anemia if left untreated, which is characterized by an insufficient number of red blood cells and an insufficient amount of hemoglobin. Children, pre-menopausal women (women of child-bearing age), and people with poor diet are most susceptible to the disease. Most cases of iron-deficiency anemia are mild, but if not treated can cause problems like fast or irregular heartbeat, complications during pregnancy, and delayed growth in infants and children.


In biology, manganese(II) ions function as cofactors for a large variety of enzymes with many functions. Manganese enzymes are particularly essential in detoxification of superoxide free radicals in organisms that must deal with elemental oxygen. Manganese also functions in the oxygen-evolving complex of photosynthetic plants. The element is a required trace mineral for all known living organisms, but is a neurotoxin. In larger amounts, and apparently with far greater effectiveness through inhalation, it can cause a poisoning in mammals with neurological damage that is sometimes irreversible.

Manganese is an important element for human health, essential for development, metabolism, and the antioxidant system. Nevertheless, excessive exposure or intake may lead to a condition known as manganism, a neurodegenerative disorder that causes dopaminergic neuronal death and symptoms similar to Parkinson's disease. The manganese Adequate Intake for a 44 year old human male is 2.3 mg per day, for a woman of the same age 1.8 mg, with 11 mg estimated as the tolerable upper limit for daily intake to avoid toxicity. AIs for children are less. The essential minimum intake is unknown since manganese deficiency is so rare. The human body contains about 12 mg of manganese, mostly in the bones. The soft tissue remainder is concentrated in the liver and kidneys. In the human brain, the manganese is bound to manganese metalloproteins, most notably glutamine synthetase in astrocytes.


The most important role of molybdenum in living organisms is as a metal heteroatom at the active site in certain enzymes. Molybdenum-bearing enzymes are by far the most common bacterial catalysts for breaking the chemical bond in atmospheric molecular nitrogen in the process of biological nitrogen fixation. At least 50 molybdenum enzymes are now known in bacteria and animals, although only bacterial and cyanobacterial enzymes are involved in nitrogen fixation. These nitrogenases contain molybdenum in a form different from other molybdenum enzymes, which all contain fully oxidized molybdenum in a molybdenum cofactor. These various molybdenum cofactor enzymes are vital to the organisms, and molybdenum is an essential element for life in all higher eukaryote organisms, though not in all bacteria.

Molybdenum is a trace dietary element necessary for the survival of humans and the few mammals that have been studied. The human body contains about 0.07 mg of molybdenum per kilogram of body weight, with higher concentrations in the liver and kidneys and in lower in the vertebrae. Molybdenum is also present within human tooth enamel and may help prevent its decay. The average daily intake of molybdenum varies between 0.12 and 0.24 mg, depending on the molybdenum content of the food. Pork, lamb, and beef liver each have approximately 1.5 parts per million of molybdenum. Other significant dietary sources include green beans, eggs, sunflower seeds, wheat flour, lentils, cucumbers and cereal grain.

Acute toxicity has not been seen in humans, and the toxicity depends strongly on the chemical state. Studies on rats show a median lethal dose (LD50) as low as 180 mg/kg for some Mo compounds. Although human toxicity data is unavailable, animal studies have shown that chronic ingestion of more than 10 mg/day of molybdenum can cause diarrhea, growth retardation, infertility, low birth weight, and gout; it can also affect the lungs, kidneys, and liver. Low soil concentration of molybdenum in a geographical band from northern China to Iran results in a general dietary molybdenum deficiency, and is associated with increased rates of esophageal cancer. Compared to the United States, which has a greater supply of molybdenum in the soil, people living in those areas have about 16 times greater risk for esophageal squamous cell carcinoma.


Selenium salts are toxic in large amounts, but trace amounts are necessary for cellular function in many organisms, including all animals. Selenium is an ingredient in many multivitamins and other dietary supplements, including infant formula. It is a component of the antioxidant enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants). It is also found in three deiodinase enzymes, which convert one thyroid hormone to another. Selenium requirements in plants differ by species, with some plants requiring relatively large amounts and others apparently requiring none.

Dietary selenium comes from nuts, cereals and mushrooms. Brazil nuts are the richest dietary source (though this is soil-dependent, since the Brazil nut does not require high levels of the element for its own needs). Recommended Dietary Allowance ~ 55 µg/day. Selenium as a dietary supplement is available in many forms, including multi-vitamins/mineral supplements - typically 20 µg/day. Selenium-specific supplements may have -200 µg/day.

Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis. This 400 µg Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow up study on the same five people in 1992. The 1992 study actually found the maximum safe dietary Se intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to avoid creating an imbalance of nutrients in the diet and to accord with data from other countries. In China, people who ingested corn grown in extremely selenium-rich stony coal (carbonaceous shale) have suffered from selenium toxicity. This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded.


Zinc is an essential mineral perceived by the public today as being of "exceptional biologic and public health importance", especially regarding prenatal and postnatal development. Zinc deficiency affects about two billion people in the developing world and is associated with many diseases. In children, deficiency causes growth retardation, delayed sexual maturation, infection susceptibility, and diarrhea. Enzymes with a zinc atom in the reactive center are widespread in biochemistry, such as alcohol dehydrogenase in humans. Consumption of excess zinc can cause ataxia, lethargy and copper deficiency. In humans, the biological roles of zinc are ubiquitous. It interacts with "a wide range of organic ligands", and has roles in the metabolism of RNA and DNA, signal transduction, and gene expression. It is estimated that about 10% of human proteins (2800) potentially bind zinc, in addition to hundreds more that transport and traffic zinc.

The Food and Nutrition Board of the U.S. Institute of Medicine updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for zinc in 2001. The current EARs for zinc for women and men ages 14 and up are 6.8 mg/day and 9.4 mg/day, respectively. The RDAs are 8 and 11 mg/day. RDAs are higher than EARs so as to identify amounts that will cover people with higher than average requirements. RDA for pregnancy equals 11 mg/day. RDA for lactation equals 12 mg/day. For infants up to 12 months the RDA is 3 mg/day and for children ages 1–13 years the RDA increases with age from 3 to 8 mg/day.

For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For zinc labeling purposes 100% of the Daily Value was 15 mg, but as of May 2016 it has been revised to 11 mg. A table of the pre-change adult Daily Values is provided at Reference Daily Intake. Food and supplement companies have until July 28, 2018 to comply with the change. Animal-sourced foods (meat, fish, shellfish, fowl, eggs, dairy) provide zinc. The concentration of zinc in plants varies with the level in the soil. With adequate zinc in the soil, the food plants that contain the most zinc are wheat (germ and bran) and various seeds (sesame, poppy, alfalfa, celery, mustard). Zinc is also found in beans, nuts, almonds, whole grains, pumpkin seeds, sunflower seeds and blackcurrant.[196]


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