Trace Elements in Human Nutrition: Functions, Sources, and Health Implications

Trace Elements: Overview

Definition and General Characteristics

Trace elements, also known as trace minerals, are essential nutrients required by the human body in amounts less than 100 mg per day. They play critical roles in various physiological processes, despite their small required quantities.

• Examples: Iron, zinc, copper, manganese, selenium, iodine, fluoride, chromium, and molybdenum.

• Trace elements are difficult to study due to their low concentrations and challenges in removing them from the diet.

• Bioavailability (the rate and extent to which a nutrient is absorbed and used) is a significant concern for trace elements.

Iron

Biological Roles and Forms

Iron is the most common nutritional deficiency in Canada and is vital for oxygen transport and cellular metabolism.

• Heme iron: Found in animal foods; more efficiently absorbed.

• Nonheme iron: Found in plant foods and iron cookware; less efficiently absorbed.

• Ferritin: Major iron storage protein in the body.

• Transferrin: Main iron transport protein in the blood.

Functions of Iron

• Hemoglobin: Located in red blood cells; transports oxygen ($O_2$) to tissues.

• Myoglobin: Located in muscle; increases available $O_2$ for muscle contraction.

• Other functions: Drug metabolism, immune function, antioxidant enzyme activity.

Absorption and Bioavailability

• Heme iron is absorbed about twice as efficiently as nonheme iron.

• Absorption occurs in the intestinal mucosal cells and depends on iron intake and body stores.

• Vitamin C enhances nonheme iron absorption.

• Absorption is inhibited by dietary fiber, phytates, oxalates, tannins, and calcium—posing challenges for vegans and vegetarians.

Iron Deficiency: Stages and Symptoms

• Three stages of deficiency:

  1. Depletion of iron stores

  2. Reduced iron transport

  3. Decreased hemoglobin production

• Results in microcytic, hypochromic anemia (small, pale red blood cells).

Iron Toxicity and Overload

• UL (Upper Limit): 45 mg/day from all sources.

• Excess iron accumulates in tissues such as the heart and liver.

• Hemochromatosis: Genetic disorder causing increased iron absorption and tissue deposition; symptoms include weight loss, fatigue, abdominal pain, and organ damage.

• Treatment: Regular blood donation.

Dietary Recommendations and Sources

• RDA:

  • 8 mg/day: Adult males and postmenopausal females

  • 15 mg/day: Females 14–18 years

  • 18 mg/day: Females 19–50 years

• Sources: Red and organ meats, legumes, leafy greens, whole and enriched grains.

Iron in the Diet: Food Sources

Food	Iron Content (mg)
Beef, cooked (75 g)	~2.0
Spinach, cooked (125 mL)	~2.0
Chickpeas, cooked (175 mL)	~2.4
Oatmeal, cooked (175 mL)	~1.5
Chicken, cooked (75 g)	~0.7
Additional info: See Figure 12.1 for more details.

Zinc

Functions and Importance

Zinc is essential for growth, tissue repair, immune system activity, and the development of sex organs and bone. It is the most abundant intracellular trace element and is involved in over 300 enzymatic reactions.

• Better absorbed from animal sources; plant sources contain phytates that reduce bioavailability.

• Key roles include:

  • Scavenging free radicals (antioxidant defense)

  • DNA and RNA synthesis

  • Carbohydrate metabolism

  • Acid-base balance

  • Folate absorption

  • Insulin storage and release

  • Mobilization of vitamin A from the liver

  • Stabilization of cell membranes

  • Hormonal regulation of cell division

Zinc in the Canadian Diet

• Many Canadians do not meet zinc requirements.

• Richest sources: Animal products (oysters, beef, lamb, crab, lobster).

Food	Zinc (mg per 75 g)
Oysters	59.0
Beef, sirloin steak	8.5
Lamb, fore shank	6.5
Crab, Alaska	5.7
Beef, brisket	5.2
Lobster, raw	4.1
Beef, ground, lean/raw (90 g)	4.0

Vulnerable Populations

• Vegetarians and especially vegans are more vulnerable to inadequate zinc intake due to lower bioavailability from plant sources.

• Diets high in phytate, fiber, tannins, and oxalates further reduce zinc absorption.

• High nut intake may lower zinc status; excess copper can also upset zinc balance.

Dietary Recommendations and Sources

• RDA: Males = 11 mg/day; Females = 8 mg/day

• Sources: Red meat, liver, eggs, dairy products, seafood

Population	RDA (mg) – Mixed Diets	RDA (mg) – Vegan
Male (+19 years)	11	Up to 17
Female (+19 years)	8	Up to 12

Deficiency and Excess

• Deficiency: Decreased growth, development, immunity; skin rashes; diarrhea

• Excess: Gastrointestinal irritation, vomiting, appetite loss, diarrhea, abdominal cramps, headaches; decreased immunity, HDL, copper and iron absorption

Zinc Fingers and Gene Expression

Zinc "fingers" are structural motifs in proteins that bind to DNA, influencing gene expression. They are essential for the action of certain hormones (e.g., vitamin A, vitamin D) on DNA regulatory regions.

Copper

Functions and Interactions

Copper is necessary for preventing certain types of anemia and is a component of proteins and enzymes involved in connective tissue synthesis, lipid metabolism, heart muscle maintenance, and immune and nervous system function.

• Organ meats are a rich dietary source; also found in seafood, chocolate, nuts, seeds, and whole grains.

• Zinc and copper are antagonistic; high zinc intake can decrease copper bioavailability.

• Excess vitamin C can decrease copper absorption.

Dietary Recommendations and Sources

• RDA: Adults = 900 micrograms/day

• UL: 10 mg/day

• Sources: Organ meats, seafood, nuts, seeds, whole-grain breads and cereals, chocolate

Deficiency and Excess

• Deficiency: Iron-deficiency anemia, decreased collagen, high blood cholesterol, impaired growth, heart and nervous system degeneration, hair color and structure changes, increased infections, decreased antioxidants

• Excess: From supplements, copper containers, contaminated water; symptoms include anxiety, abdominal pain, vomiting, diarrhea

Manganese

Functions and Sources

Manganese is a constituent and activator of enzymes involved in carbohydrate and cholesterol metabolism, bone formation, urea synthesis, and oxidative damage prevention (e.g., superoxide dismutase).

• RDA: Male = 2.3 mg/day; Female = 1.8 mg/day

• Sources: Whole grains, nuts, legumes, leafy green vegetables

Selenium

Functions and Health Implications

Selenium content in food depends on soil concentration. It is incorporated into proteins such as glutathione peroxidase, which decreases oxidative damage, and is needed for thyroid hormone synthesis.

• Deficiency can lead to Keshan disease (heart muscle disorder), increased cancer risk, and thyroid imbalances.

• Symptoms: Muscular discomfort and weakness.

• RDA: Adults = 55 micrograms/day

• UL: 400 micrograms/day

• Sources: Seafood, kidney, liver, eggs, grains, nuts, seeds

Iodine

Functions and Sources

Iodine is essential for the synthesis of thyroid hormones, which regulate metabolic rate, growth, and development. Most dietary iodine comes from iodized salt; ocean and sea sources are higher in iodine.

• More than half of body iodine is stored in the thyroid gland.

• Low iodine intake leads to thyroid problems (e.g., goiter).

• Goitrogens (found in raw cabbage, cassava, millet) can limit iodine bioavailability.

• Toxicity is possible; self-medication is not recommended.

• RDA: Adults = 150 micrograms/day

• UL: 1100 micrograms/day

• Sources: Seafood, iodized salt, food contaminants and additives

• Deficiency: Decreased thyroid hormones, decreased metabolic rate, fatigue, weight gain, goiter, pregnancy complications (spontaneous abortions, stillbirths, cretinism)

• Excess: Goiter

Chromium

Functions and Sources

Chromium is involved in carbohydrate and lipid metabolism and is required to maintain normal blood glucose levels.

• Dietary sources: Brewer's yeast, liver, nuts, whole grains

• Cooking in stainless steel can increase food's chromium content.

• RDA: Ages 19–50: Male = 35 micrograms/day; Female = 25 micrograms/day

• Deficiency: Rare

• Excess: Insufficient evidence

Fluoride

Functions and Sources

Fluoride is present in small amounts in almost all soil, water, plants, and animals. It is incorporated into crystals in bone and teeth, strengthening them and reducing the risk of dental caries.

• Most common sources: Fluoridated water, tea, marine fish eaten with bones, topical toothpaste

• Calcium-rich foods reduce fluoride bioavailability.

• In saliva, fluoride decreases bacterial acid production, inhibits dissolution of tooth enamel by acid, and increases enamel re-mineralization after acid exposure.

Fluoride Toxicity

• Excess fluoride can cause fluorosis, resulting in black and brown stains, cracking, and pitting of the teeth.

• RDA: 0.05 mg/kg/day

• UL: 0.1 mg/kg/day for infants and children <9 years; 10 mg/day for ages 9–70 years

• Deficiency: Tooth decay

Molybdenum (Mo)

Functions and Sources

Molybdenum is a cofactor for enzymes involved in the metabolism of sulfur-containing amino acids and nitrogen-containing compounds (DNA, RNA), production of uric acid, and oxidation/reduction reactions.

• RDA: 45 micrograms/day (adults)

• UL: 2,000 micrograms/day

Summary of Trace Elements

Mineral	Sources	Recommended Intake for Adults	Major Functions	Deficiency Diseases and Symptoms	Groups at Risk of Deficiency	Toxicity
Iron	Red meats, fish, poultry, legumes, whole and enriched grains	8–18 mg/d	Part of hemoglobin, which delivers oxygen to cells; immune function; cognitive development	Anemia, weakness, fatigue, impaired immunity, cognitive impairment	Infants, children, women of childbearing age, pregnant women	45 mg/d; GI upset, heart, liver damage
Zinc	Meat, seafood, whole grains, eggs	8–11 mg/d	Regulates protein synthesis, immune function, growth, development	Poor growth, delayed sexual maturation, decreased immunity	Vegetarians, elderly, alcoholics	40 mg/d; decreased copper absorption
Copper	Organ meats, nuts, seeds, whole grains, seafood, chocolate	900 mcg/d	Part of proteins needed for iron metabolism, connective tissue synthesis	Anemia, poor growth, bone abnormalities	Preterm infants, those with genetic disorders	10 mg/d; vomiting, liver damage
Manganese	Nuts, legumes, whole grains, leafy vegetables	1.8–2.3 mg/d	Cofactor for enzymes in carbohydrate and cholesterol metabolism	Growth retardation	None identified	11 mg/d; nerve damage
Selenium	Seafood, eggs, grains, nuts, seeds	55 mcg/d	Antioxidant, synthesis of thyroid hormones	Keshan disease, increased cancer risk, thyroid imbalances	People in low-selenium regions	400 mcg/d; brittle hair/nails, GI upset
Iodine	Iodized salt, seafood, dairy, plants grown in iodine-rich soil	150 mcg/d	Synthesis of thyroid hormones	Goiter, hypothyroidism, cretinism	People in low-iodine regions, pregnant women	1100 mcg/d; enlarged thyroid
Chromium	Brewer's yeast, liver, nuts, whole grains	25–35 mcg/d	Enhances insulin action	Impaired glucose tolerance	Malnourished, elderly	ND
Fluoride	Fluoridated water, tea, marine fish with bones, toothpaste	3–4 mg/d*	Strengthens teeth and bones	Increased risk of dental caries	People in non-fluoridated areas	10 mg/d; mottled teeth, bone abnormalities
Molybdenum	Milk, grains, legumes	45 mcg/d	Cofactor for enzymes	Unknown in humans	Rare genetic disorders	2 mg/d; arthritis, joint pain

*AI = Adequate Intake; UL = Tolerable Upper Intake Level; ND = Insufficient data to determine a UL.

Benefits and Risks of Trace Element Supplements

Supplement	Main Claims	Actual Benefits or Risks
Iron	Increases energy	Needed to make hemoglobin to deliver oxygen to tissues. Supplements are beneficial if an iron deficiency exists. High doses cause constipation, liver damage, and heart disease risk.
Zinc	Treats colds, promotes aging, improves immune function, enhances fertility	Needed for immune function, protein synthesis, and cell division. Supplements may reduce cold duration but can impair copper and iron absorption and lower HDL cholesterol.
Copper	Prevents heart disease and osteoporosis, alleviates arthritis symptoms, maintains healthy skin and hair color	Supplements may help with treating iron-deficiency anemia but can cause toxicity if taken in excess. Not recommended for the general population.
Selenium	Protects against cancer, promotes heart health and longevity	Antioxidant; evidence that it might protect against some cancers is mixed. High doses are toxic.
Chromium	Treats diabetes, lowers cholesterol, increases muscle mass	May improve blood glucose regulation in those with impaired glucose regulation but not in healthy individuals. No evidence for muscle mass or fat loss benefits.
Vanadium	Builds muscle, enhances rapid and intense muscle response for body builders	No evidence to support a benefit for body builders. Supplements can reduce blood glucose but may be toxic at high intakes.