Micronutrients and trace minerals Flashcards
Physiologic functions of iron
Oxygen transport in blood (hemoglobin) and muscle (myoglobin)
Electron transfer enzymes (cytochromes)
Enzymes for activation of oxygen (oxidases and oxygenases)
Physiologic functions of zinc
Regulation of gene expression (zinc finger transcription proteins, both RNA & DNA metabolism)
Structural roles in membrane stability
Metalloenzymes (> 200 !)
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)
Dietary sources of iron
Heme iron: Cellular animal protein: meats, poultry, liver; (milk is poor source)
Non-heme: legumes, nuts, whole grains (esp when enriched/fortified, green leafy vegetables; Note: absorption of non-heme iron, much lower ( less than 10%) compared to animal sources (≥ 20%)
Dietary sources of zinc
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;
Absorption impaired by phytate (found only in plants; esp high in corn, legumes, nuts)
Absorption not increased w/ deficiency (unlike iron)
Factors that affect bioavailability of iron
Dietary factors that form insoluble complexes (phytate, tannins, phosphate, oxalate)
Factors affecting oxidation state (ascorbic acid: Fe3+ → Fe2+; absorption enhanced for reduced state)
Chemical form (non-heme/inorganic vs heme (heme iron enhances absorption of non-heme))
Mineral-mineral interactions: excessive Zn or Cu decreases Fe absorption
Host factors: physiologic states (Pregnancy, growth, erythropoiesis); Fe deficiency → increased absorption; inflammation: →increased hepcidin from liver → decreased absorption at enterocyte
Quantity present in the meal/gut lumen (inverse relationship)
Factors that affect bioavailability of zinc
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Iron homeostasis
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
Transport: Transferrin
Storage form: ferritin or hemosiderin (aggregated ferritin molecules)
Iron distribution:
Males: 2500 mg in circulating hemoglobin; 500-1000mg in stores
Females: 1500 mg in circulation; stores ≤ 500 mg
Iron deficiency (prevalence and consequences)
Most common nutritional deficiency in the world;
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 (↑ requirement); populations with chronic infestations (e.g. helminths, causing intestinal blood loss), bariatric surgery patients, hospitalized elderly or elderly in long term care facilities.
Deficiency in men or in post-menopausal women merits investigation for source of bleeding.
Manifestations: Anemia (microcytic, hypochromic), decreased exercise/work tolerance, fatigue, listlessness;
deficiency w/o anemia → impaired cognitive function (permanent if onset in infancy?), impaired growth
Diagnosis: nutritional deficiency suggested by low Hb/Hct & microcytic/hypochromic rbc (= severe deficiency); low ferritin (= mild, moderate or severe deficiency; 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
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)
Who is at risk for zinc deficiency
Populations at risk:
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
Pregnant women (high demand; critical for normal embryogenesis)
Monotonous, plant based diets (esp if high in phytate);
Bariatric surgery patients (up to 40% may be deficient due to decreased protein intake and malabsorption)
Elderly: poor zinc status common and may be associated with higher incidence of pneumonia; Copper to zinc ratio (CuZ) – increased ratio in elderly associated with higher mortality; may be biomarker of aging.
GI illness/injury: diarrhea associated w/ ↑↑ losses
Wounds, burns: ↑ requirement for synthesis of new tissue
Worldwide, prevalence of zinc deficiency likely to be widespread, especially in populations on primarily plant based diet (high phytate); Zn deficiency estimated to account for 0.4 million deaths/yr in children
Definition of trace minerals
less than 100 mg needed daily
Iron toxicity
Iron is a potent pro-oxidant so avoid unnecessary iron supplementation
Normal individuals generally able to regulate absorption well enough to avoid iron overload syndrome; conditions requiring frequent blood transfusions can lead to iron overload
Excess iron deposited mainly as hemosiderin in reticuloendothelial cells
Large doses of supplemental iron interfere with absorption of zinc, copper & possibly other minerals
Hereditary Hemochromatosis
Medicinal Fe overdose is esp toxic; effects: hemorrhagic gastroenteritis, shock & acidosis, coagulation defects, hepatic failure; in children, 1-2 grams of iron may be fatal.
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 ( ↑↑ risk of hepatocellular carcinoma)
Symptoms of zinc deficiency
Mild: growth delays/stunting, anorexia, impaired immune function; impaired neurocognitive development
Moderate – severe: severe, characteristic dermatitis (acro-orificial); diarrhea, immune dysfunction, delayed wound healing, taste impairment, anorexia, personality changes
Acrodermatitis Enteropathica
Disease of zinc deficiency
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.
Zinc toxicity
Relatively low; > 50 mg/d can ↓ HDL-cholesterol, impair absorption of Fe & Cu, cause nausea, diarrhea
