Trace Minerals

Chapter 13: Trace Minerals

Introduction to Trace Minerals

  • Definition: Trace minerals, also referred to as microminerals, are essential mineral nutrients that the human body requires in relatively small amounts, specifically less than 100 mg per day.

  • Functions: They play numerous crucial roles in maintaining health and performing essential functions in the body.

  • Sources: Generally, humans can obtain all necessary trace minerals through a varied diet that includes fruits, vegetables, legumes, and nuts.

  • Toxicity: It is important to note that toxic levels of trace minerals can easily be reached, particularly through supplement use, as the FDA does not regulate the amounts found in dietary supplements.

  • Examples: Trace minerals that have a Recommended Dietary Allowance (RDA) include iron, zinc, iodide, selenium, copper, and molybdenum. Those with an Adequate Intake (AI) include manganese, fluoride, and chromium.

Iron (Fe)

Forms of Iron

  • Iron exists in two primary ionic forms:

    • Ferrous iron (Fe²⁺): The reduced form.

    • Ferric iron (Fe³⁺): The oxidized form.

  • Distribution: Approximately 80% of the body’s iron is found in hemoglobin, which is critical for oxygen transport in the blood. Iron is also present in myoglobin in muscles, and stored in the liver, spleen, and bone marrow.

Functions of Iron in the Body

  1. Oxygen Transport: Iron is a key component of hemoglobin, with the structure defined as:
    Heme (iron-containing pigment) + globin (protein) = hemoglobin.

  2. Cofactor in REDOX Reactions: Iron’s ability to exist in two ionic states allows it to serve as a cofactor in various oxidation-reduction reactions that are pivotal in energy metabolism.

  3. Synthesis of Biomolecules: It is necessary for synthesizing amino acids, hormones, and neurotransmitters by acting as a cofactor for necessary enzymes.

Iron Absorption

  • The body efficiently conserves iron due to the difficulties associated with its excretion. Balance is maintained primarily through absorption regulation based on the body's current iron stores.

  • Iron Absorption Process:

    • Iron from food is absorbed and stored in the mucosal cells of the intestine in a storage protein known as mucosal ferritin.

    • If the body needs iron, mucosal ferritin releases iron to mucosal transferrin, a transport protein that carries iron through the bloodstream to various body tissues.

    • If the body does not require additional iron, it is not absorbed and instead is excreted through shed intestinal cells.

    • Mechanism:

    1. Iron in food → Mucosal ferritin stores excess iron.

    2. If needed, iron is released to mucosal transferrin → Transferrin delivers iron to cells (bone marrow, spleen, liver).

    3. Iron is not absorbed when unnecessary, leading to excretion in intestinal cells.

Iron Utilization

  1. Storage: Ferritin in mucosal cells serves as a storage site for iron. When the body needs iron, ferritin releases it to transferrin for distribution.

  2. Transport: Transferrin transports iron to target cells including bone marrow and spleen.

  3. Excretion and Replacement: Iron levels are adjusted according to physiological needs, factoring in losses during bleeding, sweat, skin cell shedding, and urine.

Food Sources of Iron

  • The iron present in foods is categorized into two types: heme and nonheme iron.

Heme Iron

  • Characteristics: It is better absorbed and less regulated by the body, leading to a higher risk of toxicity.

  • Health Implications: Linked with increased risk of metabolic syndrome, coronary heart disease (CHD), and colon cancer. Research suggests that each additional gram of heme iron consumed daily may increase CHD risk by 27%. An example given is that 3.5 oz of ground beef contains approximately 1.2 mg of heme iron.

Nonheme Iron

  • Enhancers: The absorption of nonheme iron is improved by factors such as:

    • MFP factor (meat, fish, poultry) enhances absorption of nonheme iron when consumed in the same meal.

    • Vitamin C consumption alongside nonheme iron enhances absorption significantly.

    • Organics acids (citric and lactic) also facilitate absorption, as does hydrochloric acid (HCl) in stomach acid and fructose.

Inhibitors

  • Certain dietary components can hinder iron absorption:

    • Calcium: Present in fortified orange juice and other dairy products.

    • Phytates and fibers: Found in grains and legumes.

    • Vegetable proteins: Present in soybeans and various nuts.

    • Tannins: Located in tea, coffee, and specific grains and wines.

Iron Deficiency

  • Prevalence: It is the most common nutrient deficiency, impacting about 2 billion people worldwide.

  • Causes: Includes chronic bleeding from ulcers, blood donations, menstruation, increased demands during pregnancy, and rapid growth in infants, children, and teenagers.

  • Symptoms: Symptoms of iron deficiency can include impaired coordination, diminished attention span, behavioral changes, and decreased physical capabilities.

Iron Deficiency Anemia

  • In cases where anemia occurs alongside deficiency, red blood cells (RBCs) are produced in smaller, paler forms and cannot efficiently transport oxygen, leading to fatigue, weakness, pallor, headaches, apathy, and cold intolerance.

Comparison of Normal Blood Cells with Those in Iron Deficiency Anemia

  • In a healthy state, DNA synthesis and cell division are regular:

    • With iron deficiency, synthesis is impaired, leading to smaller and paler cells (microcytic and hypochromic, respectively).

Iron Toxicity/Overload

  1. Hemochromatosis: A genetic disorder leading to excessive iron absorption, resulting in fatigue, tissue damage, infections, and liver enlargement or disease. Treatment includes chelation therapy or phlebotomy.

  2. Iron overload: Often caused by high doses of iron supplements or accidental ingestions; symptoms can mirror those of hemochromatosis, along with gastrointestinal distress including nausea and vomiting.

Health Risks and Recommendations Related to Iron

  • Potential Risks: High iron levels have been associated with heart disease and cancer due to increased free radical activity damaging cells.

  • Tolerable Upper Intake Levels: For adults, the upper limit is set at 45 mg/day.

  • Recommended Dietary Allowances:

    • Men: 8 mg/day for ages 19-50.

    • Women: 18 mg/day for ages 19-50.

    • Women: 8 mg/day for ages 51 and over.

Iron Sources in Foods

  • Rich dietary sources of iron include:

    • Red meats, fish, poultry, shellfish, eggs.

    • Legumes and grain products, particularly whole-grain, enriched, and fortified breads and cereals.

    • Dark leafy greens and dried fruits such as raisins.

    • Cooking with cast iron pans also contributes to dietary iron intake.

Iron Supplements

  • The most bioavailable form is ferrous sulfate (FeSO₄).

  • Recommended to be taken on an empty stomach for best absorption, ensuring to avoid milk or caffeinated beverages as they inhibit absorption.

  • A potential side effect noted is constipation.

Zinc (Zn)

Importance of Zinc

  • Functions: Zinc is required by over 100 enzyme systems, is integral to all cellular functions, and constitutes roughly 60% of stored zinc within muscle tissues besides being stored in bone, prostate gland, and eyes.

Roles of Zinc in the Body

  • It functions as a part of metalloenzymes essential for the synthesis of proteins and heme, as well as the metabolism of fatty acids, carbohydrates, and alcohol.

  • It is involved in growth and development, immune function, vitamin A production in its active form, thyroid hormone activity, taste perception, wound healing, and sperm and fetal development.

Zinc Absorption Mechanism

  • Absorption rates are contingent on current zinc status. More zinc is absorbed when the body requires it.

  1. Intestinal cells absorb zinc and store it in metallothionein.

  2. If zinc is needed, metallothionein releases it to proteins (albumin and transferrin) for systemic distribution.

  3. Enteropancreatic Circulation: Transport of zinc back to the pancreas from the bloodstream is crucial, where it is used in the production of digestive enzymes.

  4. Losses of zinc occur through feces, urine, and skin shedding, among other biological processes.

Zinc Deficiency

  • While zinc deficiency is not widespread in developed countries, it is more prevalent among pregnant women, young children, the elderly, and individuals with limited economic means.

  • Symptoms include growth impairment, delayed sexual maturation, impaired immune function, hair loss, skin lesions, altered taste preferences, loss of appetite, and delayed wound healing.

Zinc Toxicity

  • Zinc toxicity is characterized by symptoms resulting from intakes exceeding 50 mg, including:

    • Loss of appetite, impaired immune function, decreased HDL cholesterol levels, and potential deficiencies in copper and iron.

    • Adverse gastrointestinal effects include vomiting and diarrhea, along with extreme fatigue, headaches, and confusion.

  • Tolerable Upper Intake Level (UL): Adults should not exceed 40 mg/day.

Zinc Recommendations and Food Sources

  • Recommended Dietary Allowances:

    • Men: 11 mg/day.

    • Women: 8 mg/day.

  • Food Sources: High sources of zinc include shellfish, meats, poultry, milk, cheese, whole grains, and legumes.