Flashcards in Micronutrients: Trace Elements Deck (56)
Iron Function and Food Sources
i. Oxygen transport in blood (hemoglobin) and muscle (myoglobin)
ii. Electron transfer enzymes (cytochromes)
iii. Enzymes for activation of oxygen (oxidases and oxygenases)
b. Food Sources:
i. Heme iron: Cellular animal protein: meats, poultry, liver; (milk is poor source)
ii. Non-heme: legumes, nuts, whole grains (esp when enriched/fortified, green leafy vegetables;
Note: absorption of non-heme iron, much lower (<10%) compared to animal sources (≥ 20%)
Factors Affecting Iron Absorption
Factors affecting absorption:
a. Dietary factors that form insoluble complexes (phytate, tannins, phosphate, oxalate)
b. Factors affecting oxidation state (ascorbic acid: Fe3+--> Fe2+; absorption enhanced for reduced state)
c. Chemical form (non-heme/inorganic vs heme (heme iron enhances absorption of non-heme))
d. Mineral-mineral interactions: excessive Zn or Cu ---> decreased Fe absorption
e. Host factors: physiologic states (Pregnancy, growth, erythropoiesis); Fe deficiency ---> increased absorption;
i. inflammation: ↑ hepcidin from liver ↓ absorption at enterocyte
f. Quantity present in the meal/gut lumen (inverse relationship)
Homeostasis of Body Iron
a. Main site of regulation is intestinal absorption; once absorbed, very efficiently/ effectively retained (e.g. recycling from rbc/Hb breakdown); bleeding = major route of iron loss; stores: liver, bone marrow, spleen
b. Transport: Transferrin
Storage form: ferritin or hemosiderin (aggregated ferritin molecules)
c. Iron distribution:
i. Males: 2500 mg in circulating hemoglobin; 500-1000mg in stores
ii. Females: 1500 mg in circulation; stores 500 mg
Transport vs storage form of Fe
a. Transport: Transferrin
b. Storage form: ferritin or hemosiderin (aggregated ferritin molecules)
Deficiency of Iron
a. Most common nutritional deficiency in the world;
b. Populations “at risk”: infants > 6 mo old (low stores, high requirement); premature infants (very low stores, high requirement); adolescents (relatively high requirement + poor intake); pregnant women (increased requirement); populations with chronic infestations (e.g. helminths, causing intestinal blood loss), bariatric surgery patients, hospitalized elderly or elderly in long term care facilities.
c. Deficiency in men or in post-menopausal women merits investigation for source of bleeding.
d. Manifestations: Anemia (microcytic, hypochromic), exercise/work tolerance, fatigue, listlessness;
deficiency w/o anemia impaired cognitive function (permanent if onset in infancy?), impaired growth
e. Diagnosis: nutritional deficiency suggested by low Hb/Hct & microcytic/ hypochromic rbc (= severe deficiency); low ferritin (= mild, moderate or severe deficiency
i. Caveat: ferritin is an acute phase protein, and is elevated with inflammatory conditions; need to check inflammatory marker (ESR or CRP) coincidentally w/ ferritin for accurate interpretation); low serum Fe w/ high total Fe binding capacity (TIBC) low % saturation
f. Treatment: Oral iron supplements (ferrous sulfate) 30-60 mg/d x 2-6 mo for replenishment of iron stores (infants/children: 2-6 mg/kg/day)
Toxicity of high Fe
a. Iron is a potent pro-oxidant unnecessary iron supplementation to be avoided; normal individuals generally able to regulate absorption well enough to avoid iron overload syndrome; conditions requiring frequent blood transfusions can lead to iron overload (regular blood donation avoids excessive iron accumulation!)
b. Excess iron deposited mainly as hemosiderin in reticuloendothelial cells
c. Large doses of supplemental iron interfere with absorption of zinc, copper & possibly other minerals
d. Hereditary Hemochromatosis –relatively common inherited condition in which Fe absorption is excessive due to defect in hepcidin; individuals accumulate increased Fe stores that are damaging, esp to liver (Increased risk of hepatocellular carcinoma)
e. Medicinal Fe overdose is esp toxic; effects:
i. hemorrhagic gastroenteritis, shock & acidosis, coagulation defects, hepatic failure; in children, 1-2 grams of iron may be fatal.
Function of Zinc
a. Regulation of gene expression (zinc finger transcription proteins, both RNA & DNA metabolism)
b. Structural roles in membrane stability
c. Metalloenzymes (> 200 !)
d. Especially critical during periods of growth and cellular/tissue proliferation (immune system, wound healing, skin & gi tract integrity); physiologic functions for which zinc is essential include normal growth, sexual maturation, sense of taste, immune function, night vision (possibly mediated through Vit A & retinol binding protein)
Food Sources and Absorption of Zinc
Food Sources & Absorption:
a. Widely distributed in foods, but richest sources = animal products; (oysters extremely high); beef > poultry > fish, milk, eggs; relatively high in whole grains, legumes, seeds, etc but lower absorption from plant foods;
b. Absorption impaired by phytate (found only in plants; esp high in corn, legumes, nuts)
c. Absorption not increased w/ deficiency (unlike iron)
Homeostasis of Zinc
a. Absorption of dietary zinc and excretion of zinc from gi tract are important in regulating body zinc “pool”;
b. Zinc secreted into gi tract w/ digestion, as part of pancreatic-biliary secretions; some reabsorbed, some excreted, so route to excrete excess Zn exists (vs iron)
Populations at risk for zinc deficiency
Deficiency: Populations at risk:
a. Infants (esp premature) & young children (high growth rate +/- marginal intake); breastfed infants > 6 mo; human milk low [Zn] after 6 mo – need source from foods
b. Pregnant women (high demand; critical for normal embryogenesis)
c. Monotonous, plant based diets (esp if high in phytate);
d. Bariatric surgery patients (up to 40% may be deficient due to decreased protein intake and malabsorption)
e. Elderly: poor zinc status common and may be associated with higher incidence of pneumonia;
i. Copper to zinc ratio (CuZ) – increased ratio in elderly associated with higher mortality; may be biomarker of aging.
f. GI illness/injury: diarrhea associated w/ losses (World Health Organization: 20 mg/d x 10 days for acute diarrhea in young children)
g. Wounds, burns: increased requirement for synthesis of new tissue
Manifestations of Zinc deficiency:
a. Mild: growth delays/stunting, anorexia, impaired immune function; impaired neurocognitive development
b. Moderate – severe: severe, characteristic dermatitis (acro-orificial); diarrhea, immune dysfunction, delayed wound healing, taste impairment, anorexia, personality changes
c. Acrodermatitis Enteropathica: mutation in enterocyte Zn transporter (ZIP4); fatal condition if not treated; responds to high doses of Zn supplements (lifetime); presents w/ severe dermatitis, growth failure, diarrhea.
Toxicity and other uses for Zinc
a. Toxicity: relatively low; > 50 mg/d can HDL-cholesterol, impair absorption of Fe & Cu, cause nausea, diarrhea
b. Other uses for zinc: Lozenges within 24 hr of first symptoms of a cold, may decrease the duration of illness by one to four days & significantly reduce the severity of cold symptoms; more research needed re dose and formulation; postulate Zn may prevent viral replication or attachment to nasal membranes.
a. Function: Integral part of thyroid hormones: thyroxine (tetraiodothyronine) (T4) and triiodothyronine (T3); thyroid gland able to concentrate iodine; amount in the gland intake
Iodine food sources
a. Food Sources: Seafoods & seaweed (most iodine resides in the ocean); I content of crops grown and animal products (esp dairy & eggs) variable, dependent on composition of feeds and/or content of soil (“geochemical distribution”); iodized salt provides substantial source.
b. Iodine content of soil varies (geochemical), depending on glaciation, rainfall, runoff into rivers.
Iodine absorption and metabolism
Absorption, metabolism, and excretion:
a. Readily absorbed from food, reaches circulation as Iodide. In circulation, 95% as organic Iodine, 5% as Iodide; most T4 and T3 trannsported via carrier proteins (eg thyroxine binding globulin). Iodide uptake---> binding to T4 & T3---> circulation
Deficiency of Iodine
Deficiency: common worldwide endemic goiter & cretinism in children (5.7 million cretins exist; estimate 1 billion persons at risk for I deficiency disorders)
a. Cretin child: (“deaf mutism”) dwarfed, mentally retarded, typical “dull” facies, large tongue; results from I deficiency during pregnancy; fetus: abortions, stillbirths, congenital anomalies;
b. Goiter: enlarged thyroid gland as compensation for I for thyroid hormone synthesis
c. In populations w/ endemic iodine deficiency, estimated to be responsible for a mean IQ loss of 13.5 points in the population.
Copper Quick Stats
a. Functions: oxidative enzymes (cytochrome oxidase, ferroxidase, amine oxidase)
b. Food sources: shellfish, meats, nuts; low in milk
c. Absorption & Metabolism: 30-40% absorption from mixed diet; stored in liver; excreted in bile
i. Mild- Anemia
Neutropenia, osteoporosis, seborrheic skin lesions
ii. Severe- Mental retardation, seizures, connective tissue defects, fractures
Selenium Quick Stats
a. Functions: Glutathione peroxidase (GSHx); deiodinase; important anti-oxidant
b. Food sources: Present in foods associated w/ amino acids (e.g. selenomethionine); intake varies widely w/ soil content (“geochemical” distribution). E.g., in areas of China, New Zealand, Finland, Venezuela, soil levels low, populations w/ much lower blood levels - ? functional consequences.
i. Keshan disease, a cardiomyopathy in China which can be prevented w/ Se supplementation may represent interaction w/ other nutritional deficits (e.g. I) and/or viral infection (viral mutation to virulence in Se deficient host).
c. Absorption & Metabolism: 60-80% absorbed from diet; kidneys main site homeostasis & urine excretion
i. Mild- Macrocytosis, loss of hair pigment, hypothyroidism
ii. Severe- Cardiomyopathy
Definition of Trace elements
Major elements: 11 elements that accounts for 99.7% of the human body’s weight
*The remaining 0.3% consists of 25 trace elements.
Concepts of Bioavailability
Bioavailability: the extent to which other dietary constituents affect the absorption & retention of a nutrient;
Trace minerals especially susceptible to interference w/ absorption
IRON – What does it do?
a. Total body iron ~ 5 g
i. ~ 50% as hemoglobin iron, 10 % myoglobin and 5%in enzymes
ii. Storage Fe: adults 300-1500 mg; (Fe overload disorders: 40-50,000 mg)
1. O2 transport in blood & muscle (Hb & myoglobin)
2. Electron transport (cytochromes)/respiratory chain
3. Enzymes for activation of O2 ( oxidases, oxygenases)
CNS myelination: Dopamine synthesis, (< 1% of total body iron)
Where do we get Fe?
i. Plant sources: legumes, whole grains, nuts;
ii. Fe-fortified foods (infant formula, cereals/grains)
Fe Homeostasis – Factors Affecting Absorption/Bioavailability
a. Form: Heme Fe >> non-heme ( 30% vs < 10%)
b. Calcium: the only dietary factor that can decrease heme iron absorption
c. Positive Factors:
i. Ascorbic acid
ii. Meat or fish ( factors in meat other than heme iron enhance the absorption of non-heme iron)
d. Negative factors (decrease Fe absorb)
i. Phyate ( bran, oat, beans, rye..)
iii. Polyphenols ( tea, some vegetables)
iv. Dietary fiber
v. Soy protein
Phyate ( bran, oat, beans, rye..)
Polyphenols ( tea, some vegetables)
a. Binds cations – Zn, Fe, Ca – in gut lumen; humans w/o phytases
(Most common feed enzyme: added to ~90% poultry & ~70% pig diets; P pollution)
b. High in grains, legumes:
Maize/wheat > legumes > rice
Globally, likely = major cause of dietary deficiencies
Food Sources & Fe Absorption
a. Meat (3 oz)
3.2 mg x .30 = 0.96 mg
b. Kidney beans (1/2 c, )
2.6 mg x 0.05 = 0.13 mg
c. Fe-enriched bread (1 sl)
0.8 mg x 0.05 = 0.04 mg
d. Human milk vs formula:
0.5 mg/l x 0.2 = 0.1 mg
12 mg/l x 0.05= 0.6 mg
a. Occurs in the proximal duodenum
b. The first step is reduction: Fe3+ Fe2+ better absorbed
c. Absorption = main point of regulation.
i. Once absorbed beyond GI tract, very efficiently retained/recycled
d. Loss (excretion): bleeding, cell sloughing
e. Key factors: form of Fe/meal composition & host status.
f. Host factors:
i. Deficiency--> increased absorption
ii. Inflammation---> absorption
Overview of Iron Homeostasis
a. In the duodenal enterocyte, dietary iron is reduced to the ferrous state by the duodenal ferric reductase, then the divalent metal transporter (DMT1) shuttles Fe++ inside the enterocytes where it can be stored as ferritin or released by way of ferroportin into the circulation.
b. In the circulation iron is transferrin-bound and its uptake by the hepatocytes is facilitated by Transferrin receptor 1 and 2 .
c. This uptake by the hepatocytes influences the exression of the iron regulatory hormone hepcidin.
d. Another source of iron is the senescent erythrocytes that are taken up by macrophages.
i. Macrophages export the recovered iron via ferroportin.
Hepcidin: blocks transport of Fe:
↓ iron deficiency
a. When hepcidin levels are low, iron-exporting cells, including duodenal enterocytes, macrophages, and hepatocytes display abundant ferroportin and release iron intro plasma.
b. When hepcidin concentrations increase, hepcidin binds to ferroportin, ferroportin is degraded, and iron is retained within cells in cytoplasmic ferritin.
Iron Homeostasis/Correlation to Lab Values
a. Stores: ferritin - liver, bone marrow, spleen
b. Inflammation ↑Hepcidin ↓ uptake
i. However the ferritin level may be normal or high.
c. Always obtain inflammatory makers with Ferritin level.
d. Transferrin: transports Fe in body; ~ no “free Fe”