162
Trace Elements in Abiotic and Biotic Environments
23.6 HUMANS
Total Fe in human body is 4 g, of which in the blood, the average is 415 mg/L (range
380–450); in the bone, it varies between 3 and 380 mg/kg; and in the tissue, the
average is 180 mg/kg (range 20–400) (Emsley 2011). The largest Fe fraction is present in the erythrocytes as hemoglobin, myoglobin, and heme-containing enzymes.
Significant Fe amounts are as metalloprotein ferritin and hemosiderin, mainly in the
spleen, liver, bone marrow, and striate muscle.
Iron has several vital functions in the body. It serves as a carrier of oxygen to the
tissues, from the lungs by red blood cell hemoglobin. It is also an integrated part
of important enzyme systems in various tissues. Fe-containing oxygen storage protein in the muscles, myoglobin, is similar in structure to hemoglobin, but has only
one heme unit and one globin chain. Several Fe-containing enzymes, cytochromes,
also have one heme group and one globin protein chain. Their role in the oxidative
metabolism is to transfer energy within the cell, and specifically in the mitochondria.
Other key functions for the Fe-containing enzymes (e.g., cytochrome P450) include
the synthesis of steroid hormones and bile acids; detoxification of foreign substances
in the liver; and signal controlling in some neurotransmitters, such as the dopamine
and serotonin systems in the brain. Iron is reversibly stored within the liver as ferritin
and hemosiderin, whereas it is transported between different compartments in the
body by the protein transferrin (Hallberg 1981).
There are two kinds of dietary Fe: heme Fe and nonheme Fe. Heme Fe 2+ present
in hemoglobin and myoglobin is well absorbed and is relatively unaffected by diet
composition. Nonheme Fe, the form of Fe 3+ present in vegetables and in human’s
staples, generally is poorly absorbed, and is greatly affected by enhancing or inhibiting substances in the diet. The primary sources of heme Fe 2+ are the hemoglobin and
myoglobin from consumption of meat, poultry, and fish, whereas nonheme Fe 3+ is
obtained from cereals, pulses, legumes, fruits, and vegetables. Average absorption of
heme Fe from meat-containing meals is about 25%. Absorption of heme Fe can vary
from about 40% during Fe deficiency to about 10% during Fe repletion (Hallberg
et al. 1997).
Heme Fe can be degraded and converted to nonheme Fe if foods are cooked at
a high temperature for too long. Calcium is the only dietary factor that negatively
influences the absorption of heme Fe and nonheme Fe.
Iron compounds used for the fortification of foods will only be partially available
for absorption. Reducing substances must be present for Fe to be absorbed. Presence
of meat, poultry, fish, and ascorbic acid (e.g., certain fruit juices, fruits, and certain
vegetables) in the diet enhance Fe absorption. Other foods contain chemical entities
(ligands) that strongly bind ferrous ions, and thus inhibit absorption. Examples are
phytates present in cereal grains, seeds, nuts, vegetables, roots, and fruits, as well
as certain Fe-binding polyphenols (e.g., tea, coffee, cocoa, certain spices, certain
vegetables, and most red wines) and Ca (e.g., from milk and cheese).
Iron deficiency is the most common nutritional problem leading to anemia. Its
deficiency ranges from depleted Fe stores without functional or health impairment to
Fe deficiency with anemia, which affects the functioning of several organ systems. Its
deficiency can delay normal infant motor function (normal activity and movement)
Trace Elements in Abiotic and Biotic Environments
23.6 HUMANS
Total Fe in human body is 4 g, of which in the blood, the average is 415 mg/L (range
380–450); in the bone, it varies between 3 and 380 mg/kg; and in the tissue, the
average is 180 mg/kg (range 20–400) (Emsley 2011). The largest Fe fraction is present in the erythrocytes as hemoglobin, myoglobin, and heme-containing enzymes.
Significant Fe amounts are as metalloprotein ferritin and hemosiderin, mainly in the
spleen, liver, bone marrow, and striate muscle.
Iron has several vital functions in the body. It serves as a carrier of oxygen to the
tissues, from the lungs by red blood cell hemoglobin. It is also an integrated part
of important enzyme systems in various tissues. Fe-containing oxygen storage protein in the muscles, myoglobin, is similar in structure to hemoglobin, but has only
one heme unit and one globin chain. Several Fe-containing enzymes, cytochromes,
also have one heme group and one globin protein chain. Their role in the oxidative
metabolism is to transfer energy within the cell, and specifically in the mitochondria.
Other key functions for the Fe-containing enzymes (e.g., cytochrome P450) include
the synthesis of steroid hormones and bile acids; detoxification of foreign substances
in the liver; and signal controlling in some neurotransmitters, such as the dopamine
and serotonin systems in the brain. Iron is reversibly stored within the liver as ferritin
and hemosiderin, whereas it is transported between different compartments in the
body by the protein transferrin (Hallberg 1981).
There are two kinds of dietary Fe: heme Fe and nonheme Fe. Heme Fe 2+ present
in hemoglobin and myoglobin is well absorbed and is relatively unaffected by diet
composition. Nonheme Fe, the form of Fe 3+ present in vegetables and in human’s
staples, generally is poorly absorbed, and is greatly affected by enhancing or inhibiting substances in the diet. The primary sources of heme Fe 2+ are the hemoglobin and
myoglobin from consumption of meat, poultry, and fish, whereas nonheme Fe 3+ is
obtained from cereals, pulses, legumes, fruits, and vegetables. Average absorption of
heme Fe from meat-containing meals is about 25%. Absorption of heme Fe can vary
from about 40% during Fe deficiency to about 10% during Fe repletion (Hallberg
et al. 1997).
Heme Fe can be degraded and converted to nonheme Fe if foods are cooked at
a high temperature for too long. Calcium is the only dietary factor that negatively
influences the absorption of heme Fe and nonheme Fe.
Iron compounds used for the fortification of foods will only be partially available
for absorption. Reducing substances must be present for Fe to be absorbed. Presence
of meat, poultry, fish, and ascorbic acid (e.g., certain fruit juices, fruits, and certain
vegetables) in the diet enhance Fe absorption. Other foods contain chemical entities
(ligands) that strongly bind ferrous ions, and thus inhibit absorption. Examples are
phytates present in cereal grains, seeds, nuts, vegetables, roots, and fruits, as well
as certain Fe-binding polyphenols (e.g., tea, coffee, cocoa, certain spices, certain
vegetables, and most red wines) and Ca (e.g., from milk and cheese).
Iron deficiency is the most common nutritional problem leading to anemia. Its
deficiency ranges from depleted Fe stores without functional or health impairment to
Fe deficiency with anemia, which affects the functioning of several organ systems. Its
deficiency can delay normal infant motor function (normal activity and movement)