Micronutrients and Trace Elements

Question 1

Iron: physiologic functions

Answer 1

• Oxygen transport in blood (hemoglobin) and muscle (myoglobin)
• Electron transfer enzymes (cytochromes)
• Enzymes for activation of oxygen (oxidases and oxygenases)
• Enzymes for DA synthesis, myelination

Question 2

Iron: dietary sources

Answer 2

• Heme iron:
  
  • Cellular animal protein: meats, poultry, liver
  • Milk is poor source
• Non-heme iron:
  
  • Legumes, nuts, whole grains (especially enriched/fortified), green leafy vegetables
• Absorption of non-heme iron (< 10%) much lower compared to animal sources (> 40%)

Question 3

Iron: chemical factors affecting bioavailability

Answer 3

• Dietary factors that form insoluble complexes (phytate, tannins, phosphate, oxalate) decrease absorption
• Factors affecting oxidation state (absorption enhanced for reduced state of iron - Fe2+ over Fe3+)
  
  • Vitamin C helps with absorption
• Chemical form
  
  • Heme iron enhances absorption of non-heme iron
• Mineral-mineral interactions: excessive Zn or Cu decrease Fe absorption

Question 4

Iron: host factors affecting bioavailability

Answer 4

• Physiologic states
  
  • Pregnancy, growth, erythropoeisis increase absorption
• Iron deficiency increases absorption
• Inflammation –> increased hepcidin from liver –> decreased absorption at enterocyte
• Quantity present in meal/gut lumen (inverse relationship)

Question 5

Iron: key aspects of homeostasis

Answer 5

• Intestinal absorption is main point of regulation
  
  • Once in, hard to get out in conditions of excess
• Efficiently and effectively retained
  
  • e.g. recycling from RBC/Hb breakdown
• Bleeding: major route of iron loss
• Stores: liver, bone marrow, spleen

Question 6

Iron: etiology of deficiency

Answer 6

• Bleeding and cell sloughing –> iron deficiency
• No major regulated way to get rid of excess iron

Question 7

Iron: populations at risk for deficiency

Answer 7

• Infants > 6 mo old, premature infants, toddlers: low stores, high requirement
• Adolescents: relatively high requirement + poor intake
• Pregnant women: increased requirement
• Populations with chronic infestations (helminths, etc. causing intestinal blood loss)
• Bariatric surgery patients
• Hospitalized elderly or elderly in long-term care facilities
• Menstrual loss
• Sports anemia

Question 8

Iron: manifestations of deficiency

Answer 8

• Anemia (microcytic, hypochromic)
• Decreased exercise/work tolerance
• Fatigue
• Listlessness
• Impaired cognitive function
• Restless leg syndrome
• Impaired growth

Question 9

Iron: treatment of deficiency

Answer 9

• Oral iron supplements (ferrous sulfate)
• 30-60mg/day for 2-6 months
• Replenishment of iron stores
• For infants/children: 2-6 mg/kg/day

Question 10

Iron: prevalence of deficiency

Answer 10

• Most common nutritional deficiency in world, including USA

Question 11

Iron: toxicity potential

Answer 11

• Iron is potent pro-oxidant - avoid unnecessary supplementation
• Caution with IV iron and frequent blood transfusions (iron overload)
• Hemosiderin production generates excess iron
• Large supplementation can interfere with absorption of Zn, Cu, and other minerals
• Fe2+ overdose = hemorrhagic gastroenteritis, liver failure, shock
• In children: 1-2g can be fatal

Question 12

Zinc: physiologic functions

Answer 12

• Growth and tissue proliferation
• Sexual maturation
• Taste
• Immune function
• Wound healing
• Regulation of gene expression via zinc finger transcription proteins –> both DNA and RNA metabolism
• Structural roles in membrane stability
• Metalloenzymes (> 200!)

Question 13

Zinc: dietary sources

Answer 13

• Widely distributed animal products –> oysters extremely high
• Beef > poultry > fish, milk, eggs
• Relatively high in whole grains, legumes, seeds
• Lower absorption from plant foods

Question 14

Zinc: factors affecting bioavailability

Answer 14

• Absorption impaired by phytate (high in corn, legumes, nuts)
• Absorption NOT increased with deficiency (unlike iron)

Question 15

Zinc: key aspects of homeostasis

Answer 15

• Intestinal absorption and excretion of dietary zinc through intestine are important for body pool
• Zinc secreted into GI tract with biliary secretions, some reabsorption –> route for excessive loss

Question 16

Zinc: populations at risk for deficiency

Answer 16

• Low birthweight infants and youth: high growth rate and marginal intake
• Breastfed infants: human milk low in Zn
• Pregnant/lactating women: high demand for embryogenesis
• People on monotonous, plant-based diets (especially high in phytate)
• Bariatric surgery patients: up to 40% due to decreased protein intake
• Elderly: poor Zn intake associated with increased pneumonia incidence
  
  • Cu/Zn ratio in elderly associated with higher mortality and may be biomarker of aging

Question 17

Zinc: consequences of mild deficiency

Answer 17

• Growth delays/stunting
• Anorexia
• Impaired immune function
• Impaired neurocognitive development
• Much more common worldwide

Question 18

Zinc: consequences of moderate to severe deficiency

Answer 18

• Characteristic dermatitis (acro-orificial)
• Diarrhea
• Immune dysfunction
• Delayed wound healing
• Taste impairment
• Anorexia
• Personality changes

Question 19

Zinc deficiency: acrodermatitis enteropathica

Answer 19

• Inherited defect in enterocyte Zn transporter
• Fatal condition if not treated
• Responds to high doses of Zn supplements (lifetime)
• Presents with severe dermatitis, growth failure, diarrhea

Question 20

Zinc: prevalence of deficiency

Answer 20

• Worldwide, likely to be widespread
• Estimated to account for 0.4 million deaths/year in children < 5 years of age
• 2nd only to vitamin A deficiency (estimated to be responsible for 0.6 million deaths/year)
• One of the cheapest and most beneficial public health efforts is zinc supplementation

Question 21

Zinc: toxicity potential

Answer 21

• Relatively low
• > 50 mg/day can decrease HDL cholesterol
• Impair absorption of iron and copper
• Cause nausea
• Diarrhea