8 Minerals Inorganic Nutrients

8.1 Overview

Minerals are inorganic substances in our foods. Major minerals are needed in amounts greater than one hundred milligrams daily, while trace minerals are needed in amounts less than one hundred milligrams.1 The availability of any mineral in a food is influenced by the body’s ability to absorb it. Absorption is determined by conditions in the gastrointestinal tract along with the presence of binding agents in foods.

Some minerals are electrolytes that dissociate into positively and negatively charged ions and can affect the hydrogen ion concentration in a solution. In the human body, a difference in ionic character exists between fluid in a cell, which is rich in potassium ions, and fluid outside a cell, which is rich in sodium ions. Cells use energy to maintain this difference, which is critical to metabolism. Minerals that act as electrolytes in the body include sodium, potassium, chloride, phosphate, and sulfur.2 These minerals are in the food and drink that we enjoy every day.

A host of other minerals are found in the body, including calcium, magnesium, iron, selenium, iodine, copper, fluoride, chromium, manganese, molybdenum, and zinc. A diverse food supply moderates concerns about mineral deficiency; however, calcium, iron, and potassium consumption among people living in the US may still be inadequate. It is possible to consume too much of a mineral; for example, many people living in the US consume too much sodium. There are health implications for both under- and overconsumption of minerals.

Most healthy Americans are not at risk of deficiencies in phosphorous, sulfur, selenium, iodine, copper, chromium, manganese, or molybdenum. Thanks to a diverse food supply and nutrition policies that include enrichment and fortification (adding back vitamins and minerals that have been removed during processing or adding ones previously not there), intakes of these minerals are generally adequate. Water supplies in most American cities now contain fluoride.3 As the population shifts to bottled water, fluoride adequacy may once again become a concern.

For most people living in the US, an adequate intake of minerals is not an issue. For certain subgroups of the population, deficiencies may exist. These groups include those who diet excessively, thereby radically reducing intake and food variety; those with certain conditions that limit absorption; and those with illnesses that deplete reserves. For many, the challenges are to eat a varied diet that promotes adequacy, avoid oversupplementation, and recognize the unique circumstances that may precipitate a deficiency. A multivitamin-multimineral tablet is more than adequate to meet the needs of most healthy adults.

Objectives

  • List minerals needed for health, their functions, and food sources
  • Recognize examples of mineral deficiencies and toxicity

8.2 Minerals as Inorganic Substances

Overview

Minerals are naturally occurring, inorganic substances that neither plants nor animals can make for themselves. Plants obtain minerals from surrounding rocks, soil, and water, while animals consume them in the foods they eat. Minerals are categorized as major minerals (needed in amounts greater than one hundred milligrams daily) or trace minerals (one hundred milligrams or fewer). Trace minerals are just as important to good health as major minerals but are needed in smaller amounts.

Key Concepts

  • How foods reflect mineral content of the soil where plants are grown
  • How the digestion and absorption of a mineral determines its availability
  • Health risks of excess sodium consumption

Minerals in Food

The mineral content of plant-based foods reflects the mineral content of the soils where the plants were grown. Early in the 1900s, goiter was prevalent in the central and northwest regions of the United States, in an area called the goiter belt, which included the Great Lakes and Appalachians. The soil in these parts of the country was deficient in iodine, and as a result, so were the local foods that people ate. To address this health problem, iodine was added to table salt, and the incidences of goiter dropped.4

Iodized salt is not universally used by everyone living in the US today, and processed foods typically are not manufactured using iodized salt. This has raised concerns about adequacy during pregnancy and breastfeeding. The American Thyroid Association and the Endocrine Society recommend that prior to and during pregnancy and breastfeeding, women take a supplement of 150 micrograms of iodine daily.5 Why? Fetuses and infants are especially susceptible to the effects of iodine deficiency.

Availability

The availability of a mineral is influenced by the amount in a food and your body’s ability to absorb it. For example, only about 10-30 percent of the calcium in the foods that you eat is effectively absorbed and enters the body.

The absorption of minerals is influenced by a variety of factors, from meal content to how food is prepared. Foods are complex mixtures of many different compounds, and for some foods, those compounds interfere with absorption. For example, spinach contains oxalic acid, a binding agent that reduces calcium availability from other foods consumed at the same time; however, the effect is minimal in a varied diet.6 Conversely, the presence of certain vitamins, such as vitamin D and C, increases absorption.

During digestion minerals of similar size and ionic charge often compete for absorption. Oversupplementing a diet with one mineral can have the unintended consequence of decreasing absorption of another. Variety in the foods that you eat is the best way to avoid these problems and ensure adequate adsorption of any mineral.

Minerals That Function as Electrolytes

What are electrolytes? Electrolytes are electrically charged minerals that help your body maintain acid-base balance and manage fluids.7,8 Sodium, chloride, potassium, and phosphorus (in the form of phosphate) are examples of electrolytes of dietary importance that must be consumed in the foods that you eat.

Sodium

Sodium is widely distributed throughout the modern food supply, and people living in the US have no problem obtaining enough. It is used in food manufacturing to create desirable taste and texture and to reduce food spoilage by inhibiting microbial growth.9 Hunter-gatherers of the past had diets that were rich in potassium and low in sodium. The ready availability of table salt and modern food-manufacturing practices has flipped this relationship resulting in a diet high in sodium and low in potassium.10

Sodium ions are found in fluids located outside of cells, or extracellular spaces. The body uses these ions to regulate the flow of water in and out of cells. The kidneys work to maintain desirable sodium levels by regulating the amount of sodium in urine; when intake is high, more sodium is expelled in urine. Prolonged vomiting or diarrhea can deplete the body’s sodium supply; however, in most cases, eating salty foods adequately replaces the loss.

Sodium is found throughout the food supply, even in tap water! While grazing animals such as cattle and deer can suffer from too little, omnivores such as humans seldom have this problem. In addition, our modern, highly processed foods are high in sodium, which is used in flavoring agents and preservatives. Sodium is also added to foods as part of salt during cooking or at the table.

Excess salt consumption is associated with hypertension for some but not all people.11 This phenomenon is called salt sensitivity of blood pressure, or salt sensitive. If you are salt sensitive, eating salty foods increases your risk of high blood pressure, known as hypertension. To minimize this effect, limit your sodium intake to fewer than 2,300 milligrams daily.12 Conscious effort is required to adhere to the Dietary Reference Intake recommendations and includes reading nutrition facts labels and minimizing salt use at the table.

Summary

Historically, people relied on local agriculture and developed deficiencies when the mineral content of the soil was depleted. Today our global food supply and modern agricultural practices bring us food from all over the world. Consequently, we develop mineral deficiencies and excesses as a result of our eating patterns rather than the soil where we live. Due to ready availability and widespread use in manufacturing, sodium is an example of an important electrolyte that we often consume in excess with subsequent risks to health.

8.3 Potassium, Chloride, Calcium, Iron, Magnesium, and Zinc

Overview

Potassium is found inside body cells, where it helps maintain fluid balance. Routinely eating foods high in potassium levels is associated with a decrease in blood pressure, which is why doctors recommend a banana a day. Nutrition facts labels list the potassium content of a serving to encourage the consumption of particular food sources. The major source of dietary chloride is salt. Chloride is an important electrolyte found in fluids outside of body cells.

Calcium is critical to bone health, and adequacy helps you manage the rate of bone loss throughout adult life. Adequate iron is necessary for the formation of red blood cells, energy production, and liver detoxification. Consuming too much in supplements is a concern, as the body has no ready mechanism for expelling excesses. Magnesium is part of coenzymes and is found in a variety of foods. Adequate zinc is needed for the senses of taste and smell to function.

Key Concepts

  • How potassium and chloride participate in maintaining fluid balance
  • Nutrition facts labels and how they list minerals to promote consumption
  • How calcium and iron are critical throughout life
  • How magnesium and zinc are found in a variety of foods

Potassium

Like sodium, potassium is used to maintain fluid balance; however, it is found inside body cells (intracellular), not outside. Most of the potassium in the foods that you eat is absorbed during digestion. Any excess is excreted by the kidneys once body stores are replete (filled). In contrast to sodium, dietary potassium is associated with a decrease in blood pressure, and as a result, eating a banana a day is a familiar recommendation.13 Individuals with kidney disorders, however, must monitor intake and avoid excesses that exceed the capacity of the kidney to excrete it.

Nutrition facts labels intentionally list the amount of potassium in a serving of food along with a percent daily value because of concerns about inadequate intake. People taking medications that deplete reserves, such as diuretics designed to flush excess fluid out of the body, should focus on foods that are good sources and discuss supplementation with their health care professionals.

Did you know that in the US, coffee, tea, and potatoes are leading sources of dietary potassium? While they might not be exceptionally rich in potassium, they are significant sources because of high levels of consumption across the population. National Health and Nutrient Assessment Survey (NHANES) data tell us this and other interesting facts about our eating habits. Using a nationally representative sample, NHANES collects demographic, socioeconomic, dietary, and health-related data that capture health and nutrition trends.14

Food sources of potassium include apricots, lentils, prunes, squash, kidney beans, bananas, milk, yogurt, chicken, and beef, among many others. A varied diet with contributions from whole grains, unprocessed fruits and vegetables, meats, milk, and vegetable oils is a good way to get your potassium.

Chloride

Chloride is an essential mineral found in extracellular (outside of cells) fluids. It is a necessary part of gastric (stomach) juices and helps maintain fluid and electrolyte balance.15 What is the primary source of dietary chloride? Salt! Sodium chloride (salt) is found in processed foods and the saltshaker on the table. While chloride is present in water, the amount is inconsequential when compared to that contributed by salting our foods. Human milk contains chloride at a slightly higher level than sodium. Heavy sweating, chronic diarrhea or vomiting, trauma, and renal (kidney) disease deplete both sodium and chloride levels.

Calcium

Most of the calcium in your body (99 percent) is in your bones and teeth. Bone remodeling, a metabolic process where bone tissue is reabsorbed (removed) and then reformed (rebuilt), continues throughout life and requires dietary calcium. Well known for its role in bone health, calcium also is important to blood clotting, muscle contraction, and nerve function. Levels in the blood, which represent less than 1 percent of the total calcium in the body, are held within a very narrow range through the action of the kidneys (excretion) and small intestine (absorption). Bones also help maintain blood levels by moving calcium in and out of bone tissue as needed.16

Nutrients in food influence how much calcium is retained from a meal. Vitamin D facilitates calcium absorption from the small intestine. Drinks containing vitamin C, such as orange juice, also promote absorption. Foods high in phytic and oxalic acids bind with calcium and reduce absorption. Foods in this category include spinach, sweet potatoes, and beans. Beverages high in caffeine and phosphate, such as carbonated drinks, increase calcium loss through secretion into the intestine and excretion in urine.

The criticality of maximizing bone mineralization during the first three decades of life cannot be overstated. Persistent, inadequate calcium intake results in osteopenia, a decrease in bone mineral density (BMD) that progresses over time to osteoporosis or fragile, porous bones. The bulk (95 percent) of adult bone mass is achieved during the first seventeen years of life for women and the first twenty-one years for men. Bone mineral accretion (accumulation) continues into the third decade, after which a gradual loss begins that continues throughout life.17 Rates of bone mass loss can be reduced by weight-bearing exercises and adequate calcium and vitamin D intake.

In the past, Americans relied on milk to meet calcium needs, but decreased consumption coupled with the widespread popularity of nondairy beverages has reduced overall intake. Green vegetables and legumes, calcium-fortified tofu, breads, cereals, and fortified orange juice are good sources, but few Americans eat enough of these foods to meet needs. A diet low in rich sources, such as milk and dairy products, calcium-fortified tofu, and enriched cereals, provides less than optimal amounts.

The maximum absorbable calcium through supplementation is 500 mg. Amounts beyond this are not absorbed when taken at the same time. Calcium carbonate and calcium citrate are two forms commonly found in supplements. Calcium carbonate requires stomach acids and is best absorbed when taken with a meal. Absorption of calcium citrate does not need stomach acids and can be taken with or without food.

Iron

Adequate iron is critical to muscular work and exercise performance. Iron-containing compounds participate in energy production, immune function, and drug detoxification in the liver. Iron is part of hemoglobin, a blood protein molecule that transports oxygen throughout the body; myoglobin, an oxygen-storage protein in muscle tissue; and ferritin, a storage form found in the liver, spleen, and bone marrow.18

A surprising number of people living in the US have inadequate iron reserves and are at risk of chronic fatigue, loss of appetite, and apathy. Left uncorrected, iron deficiency progresses from depleted stores to iron deficiency erythropoiesis (production of red blood cells) to iron-deficiency anemia, which is characterized by low hemoglobin levels in the blood and a decrease in hematocrit (proportion of red blood cells in the blood).

The bioavailability (the portion that is absorbed and utilized) of iron is greater from meats than from plant foods. A strict vegetarian diet has a significantly lower iron bioavailability than a mixed diet where the presence of heme-iron (animal source) enhances absorption of nonheme (plant source) iron. Vegetarians, then, should give particular attention to a varied diet that combines iron-rich foods with those high in vitamin C, which enhances absorption.19 Iron deficiency is common throughout the world where access to meat and seafood is limited and intestinal parasites cause blood losses that deplete stores.

Prolonged training, such as distance running, can increase iron loss for athletes. These individuals experience an increased rate of erythrocyte (blood cell) turnover and may have an inadequate diet. The EAR for athletes who intensely exercise on a regular basis is 30 percent greater than that for those who do not.20

The best sources of iron include meats, seafood, fortified breads, cereals, and breakfast bars. Iron absorption from foods varies from 5 percent to 10 percent, depending upon availability and need. To address public health needs, the FDA included iron content on nutrition facts labels to encourage the consumption of iron-rich foods.

The body does not have a ready mechanism for removing excess iron, so careless supplementation is dangerous. In the US, accidental overdose is a leading cause of death in children under the age of six, so iron supplements must be stored safely.

Hemochromatosis is a heredity disorder that disproportionately affects those of Northern European descent. It is characterized by excessive iron absorption despite a normal diet. Over time individuals with this disorder accumulate iron at a rate that damages organs and, if left untreated, results in cirrhosis of the liver, heart failure, and damaged endocrine organs.21 Regular blood donation or phlebotomy (removal of blood) is used to lower iron levels to a more normal range and avoid organ damage.

Magnesium

Magnesium is used by every cell in the body. It helps maintain normal nerve and muscle function along with optimal bone and cardiovascular health. Magnesium is a cofactor in multiple enzyme systems that regulate biochemical reactions.22 Only about 1 percent of the body’s magnesium supply is present in blood serum, making deficiencies hard to detect. NHANES data indicate that almost half of the people living in the US consume less than the recommended amount.

Certain medications, type 2 diabetes, and gastrointestinal diseases put people at risk of a magnesium deficiency, as does chronic alcoholism. Deficiency is connoted by poor appetite, nausea and vomiting, and fatigue and weakness. Severe deficiencies disrupt calcium and potassium homeostasis as well as nerve and heart function.

Magnesium is widely available in foods and beverages. Sources include unprocessed whole grains, legumes, and seeds. Soybeans, pumpkin seeds, cashew nuts, and baked beans are examples of foods that provide magnesium. Dark-green leafy vegetables, such as spinach, are good sources because magnesium is contained in chlorophyll. Milk and yogurt are sources as well as dried beans and legumes. While tap, mineral, and bottled waters can provide magnesium, the amount in each varies by source and brand. A dietary pattern high in refined foods, meats, and dairy products is low in magnesium.

Zinc

Zinc is a cofactor for enzymes, supports immune function, and aids in wound healing, protein synthesis, DNA synthesis, and cell division.23 Normal smell and taste require adequate zinc, which is not stored by the body and must be replenished on a regular basis. Zinc is a requirement for normal growth and development from prenatal through adolescence.

Foods rich in high-quality protein, such as red meat and poultry, are good sources of zinc. While whole grains, cereals, and legumes are sources of zinc, they also contain phytates (plant storage forms of phosphorous) that bind it and reduce absorption. Phytates are called antinutritional factors because of this property. Deficiencies are found in populations where grains (wheat, rice, maize) that contain phytates are the major source of zinc.24

Although zinc deficiencies are uncommon in the US, phytates in legumes and whole grains make it necessary for vegetarians to consume 50 percent more dietary zinc than their omnivore counterparts. Chronic alcoholism reduces absorption and increases excretion.

Zinc-containing throat lozenges and syrups reduce the effects of the common cold by inhibiting rhinovirus binding in the nose and sinuses.25 Zinc toxicity is possible, and supplementing the diet beyond the Tolerable Upper Limit risks adverse health effects, including heart disease (by depressing the formation of HDL) and compromising the immune function.25

Summary

The minerals potassium, chloride, iron, magnesium, and zinc are essential to good health. Concerns about adequacy are reflected in nutrition facts labels, where amounts per serving of potassium, calcium, and iron are listed so people can choose the best food sources. When supplementing a diet with any mineral, one should consider health status and possible interactions with medications. Overdosing with iron, a mineral the body can’t expel, can be dangerous.

References

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