MICRONUTRIENTS

Anson LoweMedicine

October 06, 2015

The biology of iron metabolismVitamin B12

Calcium; vitamin b12; iron

IRON FUNCTONS

• component of oxygen carrying proteins• (hemoglobin and myoglobin)

• a co-factor in electron transport (cytochromes)• co-factor in other enzymatic reactions

IRON and Disease

• Excessive iron is a source of oxidative damage

N Engl J Med (1999), 341:1986-1995

MMWR vol. 47 / no. RR-3

Distribution of iron-containing compounds(mg Fe / kg body weight)

Compound Men Women

Storage complexes Ferritin 9 4 Hemosiderin 4 1Transport protein Transferrin <1 <1Functional compounds Hemoglobin 31 31 Myoglobin 4 4 Respiratory enzymes 2 2

Total 50 42

IRON BALANCE

• ~1mg of iron is lost/day in the feces and desquamated cells.• an additional 0.3-0.5 mg is lost by women in the

childbearing years due to menstrual losses

IRON DEFICIENCY

• developmental delays and behavioral disturbances• increase risk of lead poisoning• anemia (microcytic, hypochromic)

Iron Deficient Anemia

• Incidence in the United States– 3.3 million women of childbearing age– 240,000 children aged 1-2 years

MMWR vol. 47 / no. RR-3

Increased iron requirements Inadequate iron absorption

Blood loss Diet low in bioavailable ironMenstruation Impaired absorptionGastrointestinal tract Intestinal malabsorptionBlood donation Gastric surgeryHookworms HypochlorhydriaGenitourinary tractRespiratory tract

GrowthPregnancy

Causes of Iron Deficiency

MMWR vol. 47 / no. RR-3

National Health and Nutrition Examination Survey, 1988–1994

Sex and age (years) Iron deficiency Iron-deficiency anemiaBoth sexes1–2 9 3*3–5 3 <16–11 2 <1Nonpregnant females12–15 9 2*16–19 11* 3*20–49 11 5*50–69 5 2≥70 7* 2*Males12–15 1 <116–19 <1 <120–49 <1 <150–69 2 1≥70 4 2*Prevalence in nonblacks is 1 percentage point lower than prevalence in all races.

MMWR vol. 47 / no. RR-3

Iron absorption by infants fed formula or milk

Substance Iron content Bioavailable Absorbed (mg/L) iron (mg/L) iron (mg/L)

Nonfortified formula 1.5–4.8* ~10 0.15–0.48Iron-fortified formula† 10.0–12.8* ~ 4 0.40–0.51Whole cow’s milk 0.5 ~10 0.05Breast milk 0.5 ~50 0.25

*Values are given for commonly marketed infant formulas.† Iron-fortified formula contains ≥1.0 mg iron/100 kcal formula ( 8 ). Most iron-fortified formulas contain approximately 680 kcal/L, which is equivalent to ≥6.8 mg iron/L.

N Engl J Med (1999), 341:1986-1995

Iron Regulation

There is no mechanism that controls iron excretion◦ Sloughing of intestinal mucosal cells◦Menses

Iron Regulation

The absorption of heme iron (10-30%) is more efficient than inorganic iron (0-10%)

The absorption of inorganic iron can be greatly enhanced◦ Iron can only be absorbed in the reduced form, Fe+2

Nature 1997;388:482–488

Nature 1997;388:482–488

Nature 1997;388:482–488

DMT1=DCT1=NRAMP2

Nature 1997;388:482–488

Fe+2 uptake is coupled with protonsDMT1 can also transport other divalent cations

such as Zn+2, Mn+2, Cu+2, Co+2, Cd+2, and to a lesser extent Ni+2, and Pb+2.

Iron deficiency is associated with achlorhydria

Chronic Atrophic Gastritis◦atrophy of the gastric glands

association with anti-parietal cell antibodies is common (H:K-ATPase)

◦achlorhydria◦25% are iron deficient

Copyright ©2006 American Physiological Society

Donovan, A. et al. Physiology 21: 115-123 2006;doi:10.1152/physiol.00052.2005

FIGURE 1. Intestinal iron absorption An individual enterocyte is depicted

N Engl J Med (1999), 341:1986-1995

IRON OVERLOAD

• hemochromatosis is the most common genetic mutation observed

• the mutation results in excessive absorption of iron despite high total body stores

• one million persons in the United States may be affected• results in cirrhosis, hepatoma, heart failure, diabetes

Gastroenterology 1999;116:193–207

N Engl J Med (1999), 341:1986-1995

Increased Height in HFE Hemochromatosis

N Engl J Med. 2013 Aug 22;369(8):785-6. doi: 10.1056/NEJMc1303066.

Copyright ©2001 by the National Academy of Sciences

Nicolas, Ga&euml;l et al. (2001) Proc. Natl. Acad. Sci. USA 98, 8780-8785

Hepcidin

• Produced mainly by the liver• Mol. Wt. = 9,400• secreted

Copyright ©2001 by the National Academy of Sciences

Nicolas, Ga&euml;l et al. (2001) Proc. Natl. Acad. Sci. USA 98, 8780-8785

Copyright ©2006 American Physiological Society

Donovan, A. et al. Physiology 21: 115-123 2006;doi:10.1152/physiol.00052.2005

FIGURE 3. The hepcidin-ferroportin axis In hemochromatosis, hepcidin is deficient or absent, resulting in increased ferroportin on the cell surface and accelerated iron release

Ferroportin Distribution

Copyright ©2006 American Physiological Society

Donovan, A. et al. Physiology 21: 115-123 2006;doi:10.1152/physiol.00052.2005

FIGURE 2. Regulation of hepcidin expression Hepatic production of the peptide hormone hepcidin is influenced by iron needs and stores

Case

70year old man referred for iron deficiency anemia.◦Pan endoscopy is negative◦What do you do?

Case

70year old man referred for iron deficiency anemia.◦Gross endoscopy survey is negative◦Evaluate for iron malabsorption.

Evaluate for chronic atrophic gastritis (biopsies for pathology, screen for antiparietal cell antibodies).

Helicobactor pylori infection. Celiac disease

Celiac Sprue

Case

55 year old man with alcoholic cirrhosis presents with iron saturations of 80% and ferritin of 5,000, both of which are abnormally high.◦Why?

Copyright ©2006 American Physiological Society

Donovan, A. et al. Physiology 21: 115-123 2006;doi:10.1152/physiol.00052.2005

FIGURE 2. Regulation of hepcidin expression Hepatic production of the peptide hormone hepcidin is influenced by iron needs and stores

Vitamin B12

Methylation◦conversion of homocysteine to methionine

Intramolecular rearrangement◦ isomerization of methylmalonyl coenzyme A to

succinyl CoA

Vitamin B12 Source

Pernicious Anemia Loss of gastric parietal cells Often associated with anti-intrinsic factor and parietal cell antibodies.

◦ Antibodies specific for H:K-ATPase are believed to be the cause Achlorhydria - loss of acid secretion Loss of intrinsic factor secretion Most common cause of vitamin B12 deficiency

Schilling Test

Patient with low vitamin B12 levelsAbsorption of B12 is first tested with the

ingestion of radioactive vitamin B12◦Urinary excretion of B12 is examined as a measure of

absorptionIf abnormal, the test is repeated with the

addition of intrinsic factor

Megaloblastic Anemia-1

Autosomal recessive disorder Mutations in cubilin result in defective IF-B12

binding◦Normal intrinsic factor levels◦Megaloblastic anemia◦Neurologic abnormalities

Folate

An important cofactor for one-carbon transfersPresent in green leafy vegetables◦Storage for 4 months

Folate Absorption

• Two different receptors have been isolated– 12 transmembrane domains and is present in the intestine in

addition to a wide variety of other tissues.– A second receptor is linked to the membrane by a GPI-linkage

and is present in caveolae in all cells

Proton – Coupled Folate Transporter

Folate Deficiency

• Folate reserves are limited, ~ 4 months• Clinical expression of folate deficiency

• megaloblastic anemia• birth defects of the neural tube• cancer?