Trace elements

Question 1

1. List the dietary sources, factors that affect bioavailabilty and physiological functions of iron

Answer 1

Function: Oxygen transport in blood (hemoglobin) and muscle (myoglobin)
Electron transfer enzymes (cytochromes)
Enzymes for activation of oxygen (oxidases and oxygenases)

Food Sources:
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 ( 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 ↓’s Fe absorption
Host factors: physiologic states (Pregnancy, growth, erythropoiesis); Fe deficiency ↑ absorption; inflammation: →↑ hepcidin from liver → ↓ absorption at enterocyte
Quantity present in the meal/gut lumen (inverse relationship)

Question 2

3. Describe the causes, consequences and clinical presentation of deficiencies of iron

Answer 2

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), ↓ 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)

Question 3

4. Describe the potential for toxicity for iron

Answer 3

Toxicity:
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!)
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 –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); public health concern re common Fe fortification, supplementation programs being harmful to these individuals;
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.

Question 4

1. List the dietary sources, factors that affect bioavailabilty and physiological functions of zinc

Answer 4

Functions: 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)

Food Sources & Absorption:
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)

Question 5

3. Describe the causes, consequences and clinical presentation of deficiencies of zinc

Answer 5

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; (Nutr. Rev 2010 Jan; 68(1):30-7). Copper to zinc ratio (CuZ) – increased ratio in elderly associated with higher mortality; may be biomarker of aging. (Age 2011, May 5).
GI illness/injury: diarrhea associated w/ ↑↑ losses (World Health Organization: 20 mg/d x 10 days for acute diarrhea in young children)
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

Question 6

4. Describe the potential for toxicity for zinc

Answer 6

Toxicity: relatively low; > 50 mg/d can ↓ HDL-cholesterol, impair absorption of Fe & Cu, cause nausea, diarrhea

Question 7

major stores of iron, 
site of regulation, 
how we lose it,
transport form
storage form

Answer 7

once absorbed, very efficiently/effectively retained bleeding = major route of iron loss;
stores: liver, bone marrow, spleen
regulation site-> intestine
Transport: Transferrin
Storage form: ferritin or hemosiderin (aggragated ferritin)

Question 8

Absorption and excretion of zinc

Answer 8

Absorption of dietary zinc and excretion of zinc from gi tract are important in regulating body zinc “pool”;
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)