Trace Elements
Dr.Aws Hassan
MRCP UK
Trace Elements
-a naturally occurring, homogeneous,
inorganic substance required in humans
in amounts less than 100 mg/day
Essential nutrients in trace amounts
•
Assessment of trace mineral status is difficult and
requires specialized analytical instruments (atomic
absorption spectrometry
•
Serum measurements are complicated by
associated disease states that affect levels of
circulating binding proteins (e.g., albumin)
•
Diagnosis is dependent on high degree of suspicion,
careful inspection for signs and symptoms, thorough
understanding of predisposing causes and resolution
of symptoms with therapeutic trial.
Copper
Copper Metabolism
•
Intestinal absorption/membrane translocation
mediated by specific transporters
•
Copper circulates bound to ceruloplasmin
•
Relative tissue distribution of copper reflects levels
of cuproenzymes
•
Excretion occurs via transport of copper into bile
and elimination in feces
Copper: Biochemical Functions
Essential catalytic cofactor for many cuproenzymes
including:
•
Cu, Zn-superoxide dismutase (antioxidant)
•
Cytochrome C oxidase (ATP synthesis, neurologic function)
•
Ceruloplasmin (6 atoms per molecule)
Functions to oxidize Fe
+2
to Fe
+3
for binding to transferrin.
Congenital absence of this protein leads to tissue iron
accumulation and iron overload syndrome
(hemochromatosis)
Copper Physiology/Deficiency
•
Acquired deficiency is rare. Causes include:
High intake of Zinc
Renal dialysis patients
Use of copper chelating agents (penicillamine)
•
Manifestations:
Hypochromic microcytic anemia
Neutropenia
Hypopigmentation of hair and skin
Structural abnormalities in connective tissue (hair, teeth, bone
demineralization, vascular system with arterial aneurysms with risk
of hemorrhage and thrombosis)
Fetal and neonatal deprivation leads to neurologic dysfunction
Reduced levels of circulating copper and ceruloplasmin
Food Sources
-organ meats, seafood, nuts, seeds,
cereals, whole grains, cocoa
Iodine
Iodine
-body normally has 20-30 mg of iodine
and more than 75% is in the thyroid
gland
-the rest is in the mammary gland,
gastric mucosa, and blood
-
it’s only function is related to thyroid
hormone
Iodine
•
Required for synthesis of thyroid hormone
Thyroxine (T
4
)
– 4 atoms of iodine per molecule
Triiodothyronine (T
3
)
– 3 atoms of iodine per molecule
Thyroxine
Absorption and
Excretion
-iodine is absorbed in the form of iodide
-occurs both as free and protein-bound iodine
in circulation
-iodine is stored in the thyroid where it is used
for the synthesis of T3 and T4
-the hormone is degraded in target cells and
in the liver and the iodine is conserved if
needed
-excretion is primarily via urine
-small amts from bile are excreted in the
feces
Food Sources
-foods of marine origin (seaweed),
processed foods, iodized salt
Deficiency
-goiter
—enlargement of the thyroid
gland
-deficiency may be absolute
—in areas
of deficiency, or relative
—adolescence,
pregnancy, lactation
-goiters are more prevalent in women
and with increased age
-goitrogens occurring naturally in foods can
cause goiter by blocking absorption or
utilization of iodine (cabbage, peanuts,
soybeans)
-***severe deficiency during gestation and
early postnatal growth: cretinism
—mental
deficiency, quadriplegia, deaf mutism,
dysarthria, shuffling gait, short stature,
hypothyroidism.
Endemic Cretinism
Note normal man and
three adult women with
cretinism:
Short stature
Protuberant abdomen
Low IQ
(IDD)
Iodine deficiency is the most
common nutrient deficiency in the
world!
Iodine Excess and Toxicity
•
Humans are remarkably tolerant to high
iodine intakes
•
In iodine deficiency, repletion must be done
slowly to prevent hyperthyroidism
•
Paradoxical goiter (enlarged thyroid as a
result of very high intakes of iodine)
Occurs in Japan and China with high intake of
seaweed (50,000 - 80,000
mg/day)
Toxicity
-iodine has wide margin of safety
Goiter
Endemic to parts
of S. America and
India
Sporadic cases in
U.S.
Selenium
deficiency
(needed to
convert T4 to T3)
Goiter - Complications
Usually
asymptomatic
Acute pain from
thyroidal
hemorrhage
Dysphagia (trouble
swallowing)
Dyspnea (trouble
breathing)
Chromium
Chromium--Functions
-required for normal lipid and CHO
metabolism and for the fxn of insulin
-?can supplementation raise HDL
Absorption and
Excretion
-10-25% absorption in its trivalent form
-amount absorbed remains constant at
dietary intakes >40 ug (micrograms) at which
point excretion in urine is proportional to
intake
-increased intake of simple sugar, strenuous
exercise, or physical trauma also increase
urinary excretion
-both chromium and Fe are carried by Tf,
however albumin can also assume this role
Food Sources
-cereals, meats, poultry, fish, beer
Deficiency
-altered CHO metabolism, impaired glucose
tolerance, glycosuria, fasting hyperglycemia,
increased insulin levels and decreased insulin
binding
-impaired growth, peripheral neuropathy,
negative nitrogen balance
-increased chromium losses in stress
-hyperglycemia and wt loss reverse with IV
supplementation in TPN
Toxicity
-chronic renal failure
Cobalt
Cobalt
-most stored with vitamin B12
-component of B12
—cobalamin
-
essential for maturation of RBC’s and
normal function of all cells
Absorption and
Excretion
-shared with Fe
-absorption is increased in pts with
deficient Fe intake, portal cirrhosis with
Fe overload, and hemochromatosis
-excretion is mainly thru the urine
-small amts in feces, hair, sweat
Sources and Intakes
-microorganisms are able to synthesize
B12
-***humans must obtain B12 and cobalt
from animal foods such as organ and
muscle meat
-***takes a long time to become
deficient
—happens in vegetarians
Deficiency
-related to vit B12 deficiency
-**macrocytic anemia
-genetic defect: pernicious anemia
-tx: massive doses
-discussed in the vitamin lecture
Toxicity
-polycythemia
-hyperplasia of BM
-reticulocytosis
-increased blood volume
Selenium
Selenium
-glutathione peroxidase
-acts with other antioxidants and free
radical scavengers
-overlaps with vit E for antioxidant
effects
-fxn with vit E to protect cell and
organelle membranes from oxidative
damage
Selenium
– Biochemical Functions
•
Serves as a catalytic component in enzymes
and proteins
•
Iodothyronine 5
’- deiodinase
•
Thioredoxin reductase
•
Glutathione peroxidase (destroys hydrogen
peroxide)
Selenium
– Metabolism
•
Selenium is stored in the body as
selenocysteine in selenoproteins
•
Excreted in urine and in breath as dimethyl
selenide with a garlic-like odor
Relationship of glutathione peroxidase,
selenium, and vitamin E
GSH peroxidase
contains selenocysteine
Absorption and
Excretion
-upper segment of the small intestine
-increased absorption with deficiency
-status is measured by measuring
selenium or glutathione peroxidase in
plasma, platelets, and RBC
’s or
selenium levels in whole blood or urine
-RBC selenium is an indicator of long-
term status
Food Sources
Food content tends to follow Se content of soil
–
richest food sources are organ meats and sea
foods, followed by cereals and grains, dairy
products, fruits and vegetables
Se content of grains can vary by 10,000 fold
•
Requirements determined based on serum
glutathione peroxidase activity
Selenium Deficiency Diseases
•
Major problem in livestock
•
Human deficiency is rare except in areas with low
Se content in soil
Keshan disease occurs in Keshan China: endemic
cardiomyopathy and muscle weakness (due to oxidized
lipids)
Aggressive supplementation has eliminated disease
•
Iatrogenic deficiency
TPN without supplemental Se
Selenium Toxicity
Range of dietary Se intake without
toxicity is narrow
Acute selenium poisoning can result
in cardiorespiratory collapse (gram
amounts)
Chronic toxicity (selenosis) changes
in nail structure and loss of hair
(intakes ~6x UL)
Hair and nail brittleness
Selenium and Cancer Prevention
•
Epidemiologic evidence indicates low intakes of
Se are associated with higher risk of prostate
cancer
•
Prospective study of Se supplementation
demonstrated 42% reduction in cancer incidence
•
Small sample size and other confounding factors
have diminished enthusiasm for the results of
these studies
Molybendum
Molybendum
-relationship with copper and sulfate
-cofactor of many enzymes involved in
the catabolism of sulfur AA, purines and
pyridines
-Toxicity: gout-like syndrome,
reproductive SE
’s
-Deficiency: increased risk with co-
existing copper deficiency, TPN
Silicon, Vanadium,
Arsenic, Boron
-see handouts posted on the web
-will not be on the exam!!!