iron
The essential micromineral required by our bodies
What does Iron do in our bodies?
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Iron is a significant blood constituent essential for hemoglobin in red blood cells.
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Iron has other vital roles in the body's growth and maintenance.
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Iron is also necessary for physical growth, neurological development, cellular functioning, and the synthesis of some hormones.
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Iron helps change beta-carotene to vitamin A.
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Iron helps make body proteins, collagen, and connective tissues.
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Iron also supports a healthy immune system.
Iron is a significant blood constituent in the hemoglobin of red blood cells.
Iron in hemoglobin:
Iron is a significant blood constituent essential for hemoglobin in red blood cells. The main job of mineral iron is to carry oxygen in the hemoglobin of red blood cells in our bloodstream. About two-thirds of our bodies' Iron is in hemoglobin. Hemoglobin takes oxygen to body cells; in body cells, oxygen is used to produce energy.
What is hemoglobin?
​Hemoglobin is an iron-containing protein in red blood cells that binds oxygen from the lungs and transports oxygen through the bloodstream to body cells. Hemoglobin is a hemoprotein composed of globin (a protein in globe fold) and heme that gives red blood cells their characteristic color. Hemoglobins function primarily to transport oxygen from the lungs to the body tissues. Heme is a complex red organic pigment in hemoglobin containing Iron and other atoms to which oxygen binds. The red color in the blood is due to the iron-containing protein hemoglobin, and iron deficiency decreases hemoglobin production.
Iron for myoglobin and collagen.
Iron is an essential nutrient for muscles.
The human body uses iron to make myoglobin. Myoglobin is a protein that provides oxygen to muscles. Iron, as part of myoglobin, supports muscle metabolism and healthy connective tissues.
Iron helps make body proteins and collagen. Collagen is a fibrous protein in bone, cartilage, tendon, and other connective tissue. Collagen is the main structural protein in the extracellular matrix in the body's various connective tissues.
The Food Sources of Iron:
There are primarily four dietary food sources of iron:
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Heme-iron from animal-based meat
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Nonheme iron from plant-based food
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Leached iron from cast-iron utensils
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Enriched and fortified iron added to foods
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Iron in the diet comes from both plant and animal sources. The iron from plant-based foods is nonheme. Much of the iron in animal products is heme iron. Meat, poultry, and fish are good sources of heme iron.
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Iron in plant-based foods is nonheme iron. For example, iron in leafy green vegetables is nonheme iron. In addition, when foods are cooked in cast iron utensils, leached iron from cast-iron utensils gets into the foods; Lastly, many foods on today’s supermarket shelves are enriched or fortified with iron, for example, iron-enriched flour and iron-fortified breakfast cereals.
Heme iron food sources:
1) Heme iron food sources
Heme iron is a chemical complex containing iron. A heme complex includes four 5-membered nitrogen-containing rings that form a cage around a central iron ion.
The iron in hemoglobin and myoglobin is heme iron, and the iron ion is in the ferrous state.
Heme iron is a complex red organic pigment containing iron and other atoms to which oxygen binds. The deep-red color of animal muscle comes from hemoglobin. The darker the color, the higher the heme iron content. For example, beef liver, redder than roast beef, has more iron. Likewise, dark turkey meat has more heme iron than light turkey meat.
Nonheme iron from our diet:
2) Nonheme iron from our diet
Some plant-based foods give the dieting iron in the form of nonheme iron. For example, leafy green vegetables, legumes, whole grains, and enriched wheat products are good sources of nonheme iron.
The leached nonheme iron from cast-iron utensils when cooking inside them:
The leached nonheme iron from cast-iron utensils when cooking inside them:
Cast iron utensils are a source of nonheme iron when dishes are cooked, especially simmered in them for a while. The iron in the cast-iron utensils can be leached from the surface of the utensils into the cooked foods.
Acids promote the dissolution of iron into foods that are cooked in iron utensils. When dishes are cooked in the presence of acids, such as tomato juice, citrus juice, and vinegar, more iron can be dissolved into the cooking liquid as acids can enhance ion leaching from the surface of the utensils into the cooked foods.
Iron-enriched foods sold in supermarkets:
Iron-enriched foods sold in supermarkets
Many foods on today’s supermarket shelves are enriched or fortified with iron, such as iron-enriched flour and iron-fortified breakfast cereals.
Iron absorption factors:
The iron in the food consumed isn’t absorbed into the bloodstream very efficiently.
Four parameters affect the absorption of iron:
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The iron form, whether the iron is heme iron or nonheme iron;
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The amount of iron the person consumes overall;
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Whether there are other nutrients in our meals and snacks that enhance or hinder iron absorption, the presence of heme iron enhances the absorption of nonheme iron; conversely, nonheme iron can not affect the absorption of heme iron.
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The quantity of iron the body has already stored in the human body.
Dietary foods have two types of iron: heme iron and nonheme iron.
Heme iron is the iron form contained in meat, poultry, and fish. Heme iron is the heme group in hemoglobin and myoglobin proteins in animal meats.
Nonheme iron is iron in plant-origin foods. Interestingly, egg yolks have mostly nonheme iron.
Heme iron is absorbed more readily than nonheme iron without the need for absorption-enhancing cofactors.
Depending on how much iron the body has stored, 15% to 35% of heme iron from food gets absorbed.
Even though foods with nonheme iron often contain more iron, only 2% to 20% of nonheme iron gets absorbed.
The bioavailability of iron in a mixed diet, animal- and plant-based foods, is about 18%; Iron from the diet is absorbed into the intestinal mucosal cells. Then, the iron is transported from the mucosal cells to the rest of the body, depending on needs.
Iron in the gastrointestinal tract and absorption:
Our bodies are highly adaptive, absorbing more iron when their iron stores are low and less when they are higher.
Iron from the diet is absorbed into the intestinal mucosal cells. The body's iron amount is controlled primarily by how much iron is absorbed. The amount of iron transported from the mucosal cells to the rest of the body depends on needs. If body stores of iron are high, less iron is transported from the mucosal cells. If iron stores are low, a more significant percentage of the iron absorbed into the mucosal cells is transported to other tissues.
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Iron in the gastrointestinal tract and absorption:
Several proteins regulate the transport and delivery of iron:
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Copper-containing protein ceruloplasmin
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The iron transport protein transferrin
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The iron storage protein ferritin
Transferrin is the main protein in blood that binds to iron and transports it throughout the body. Ceruloplasmin converts ferrous iron to ferric iron, which can bind to transferrin and ferritin. In ferrous iron, the iron is in a divalent ion state, whereas in ferric iron, the iron is in the trivalent ion state.
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The iron that has entered the mucosal cells of the small intestine can be bound to ferritin or picked up and transported in the blood to the liver, bones, and other body tissues by transferrin. Transferrin receptors on cell membranes bind to the transferrin-iron complex, allowing it to enter the cell where the iron is released for use.
Our bodies adjust iron absorption and homeostasis. When iron is in short supply, less iron storage protein ferritin is made. The number of transferrin receptors on cell membranes increases, allowing more iron to be transported into the cells.
When iron is plentiful, more ferritin is made to increase storage capacity. However, the number of transferrin receptors on cell membranes decreases, so the ability to pick up iron from the mucosal cells and transport it into body cells is reduced. The iron absorbed over immediate needs can be stored in the protein ferritin, primarily in the liver, spleen, and bone marrow. The iron not picked up from mucosal cells is excreted in the feces and mucosal cells when they die and are sloughed off into the intestinal lumen.
Levels of ferritin in the blood can be used to estimate iron stores. When ferritin concentrations in the liver become high, some are converted to an insoluble storage protein called hemosiderin. However, the bound iron can be mobilized from body stores as needed.
The absorption of nonheme iron:
When food containing nonheme iron is consumed, stomach acid helps convert the ferric form of iron to the ferrous format of iron. The ferrous form of iron remains more soluble when it enters the intestine and is absorbed into the mucosal cells more readily.
Two types of substances can enhance or inhibit the absorption of nonheme iron, referred to as iron absorption enhancers and inhibitors respectively.
Nonheme iron absorption enhancers:
The nonheme iron absorption enhancers give enhancement of iron absorption through food partnership. Pairing certain foods enhances nonheme iron absorption from eggs and plant-based foods.
Vitamin C and animal meat are enhancers of the absorption of nonheme iron.
Consuming vitamin C or iron from meat, poultry, and fish simultaneously with plant-based nonheme iron-rich foods helps maximize iron absorption.
Acidic substances are nonheme iron enhancers, including vitamin C, citric acid, or lactic acid.
When foods containing nonheme iron are consumed with these acids, iron absorption is enhanced because the acid helps to keep iron in the ferrous form.
Vitamin C is ascorbic acid, and it is a weak organic acid. It is a nonheme iron enhancer.
Vitamin C, the nonheme iron enhancer:
The studies show that vitamin C enhances nonheme iron absorption by keeping iron in its more absorbable ferrous form and forming a complex with iron; the developed iron complex remains soluble and more bio-available. As a result, vitamin C can enhance nonheme iron absorption up to six-fold.
For example, vitamin C-rich strawberries with oatmeal, papaya with whole-wheat toast, red bell pepper with whole-grain pasta, and orange juice with whole-wheat bread are all good examples of pairing vitamin C-rich foods to enhance nonheme iron absorption from plant-based foods.
Meat is non-heme iron enhancer:
​The second non-heme iron enhancer is meat.
Meat such as poultry, beef, and fish can increase the absorption of nonheme iron from plant-based foods.
For example, a small amount of group beef in a pot of chilli bean soup enhances the body's absorption of nonheme iron from the beans besides being a source of iron of its own.
For example, barbecued beef paired with roasted beans and tortillas, chicken with brown rice, and sirloin strips with a spinach salad are good examples of meat paired with plant-based foods to enhance nonheme iron absorption.
The nonheme iron absorption inhibitors:
The nonheme iron absorption inhibitor is the substance that inhibits the absorption of nonheme iron by the human body.
Fiber, phytates, tannins in tea, and oxalates in some leafy greens, such as spinach, are the dietary factors that inhibit the absorption of nonheme iron.
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The nonheme iron absorption inhibitors:
Some phytonutrients contained in plant-based foods are nonheme-iron inhibitors, including the following:
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Oxalic acid in spinach and chocolate.
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Phytic acid in some legumes and grains inhibits iron absorption.
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Polyphenols in coffee.
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Tannins in coffee and tea.
These phyto-substances in plant-based foods are large molecules that bind readily with nonheme iron and inhibit its absorption. The tip for better nonheme iron absorption is to drink coffee or tea between meals, not with meals.
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Now, let us discuss the three main functions of iron in the body in further detail.
The three primary functions of mineral iron in the body are the following:
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Iron in the body is essential for oxygen delivery to body cells.
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Iron is also a part of several proteins involved in the electron transport chain in the body, drug metabolism, and the immune system.
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Iron is also part of the enzyme catalase.
The enzyme catalase protects the cells from oxidative damage. Catalase is an enzyme that destroys hydrogen peroxide into oxygen and water before the peroxide can damage the cells.
The three main functions of iron in the body in further detail:
Iron is a component of two oxygen-carrying proteins in the body, such as hemoglobin and myoglobin.
Most of the iron in the body is part of hemoglobin. Hemoglobin in red blood cells transports oxygen to body cells and carries carbon dioxide away from cells for elimination by the lungs. Myoglobin is in the muscle. Iron in the myoglobin enhances the amount of oxygen available for muscle contraction.
Oxygen is essential for life, but excessive oxygen can form free radicals, peroxides, and other Reactive Oxygen Species (ROS), such as superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. These reactive oxygen species are highly reactive chemicals and can damage cells through oxidation. For example, peroxides are free radicals that can damage cells by oxidizing the cell components. Many trace elements interact with oxygen, some by assuring oxygen is delivered to cells and some by preventing oxidative damage. Several enzymes that contain trace minerals can destroy reactive oxygen molecules and protect cells from oxidative damage. Iron, copper, zinc, manganese, and selenium all serve as components of antioxidant enzymes.
How much iron do we need?
Now, let us discuss the recommended iron intake of iron.
The recommended dietary allowance, RDA, of iron is based on the amount needed to maintain normal function in the human body.
RDA means the Average Daily level of intake sufficient to meet the nutrient requirements of nearly all (97–98%) healthy individuals, often used to plan nutritionally adequate diets for individuals.
For males aged 14 to 18, the RDA is 11 milligrams daily.
The RDA is 8 milligrams per day for adult men aged 19 and older and women 51 years and older.
For females aged 14 through 18, the RDA is 15 milligrams daily.
The RDA for women aged from 19 to 50 years old is 18 milligrams daily.
During pregnancy, for females 14 to 50 years old, the RDA is 27 milligrams.
During breastfeeding for females 14 to 18 years old, the RDA is 10 milligrams; for women 19 to 50 years old in lactation, the RDA is 9 milligrams daily.
The above RDAs are for nonvegetarians.
The RDAs for vegetarians are 1.8 times higher than for people who eat meat. The nonheme iron in plant-based foods is less bioavailable than heme iron from meat. In addition, foods such as meat, poultry, and seafood that can increase the absorption of nonheme iron are not present in a vegetarian diet.
Recommended dietary allowances (RDAs) for iron are listed in the following table:
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Iron can be harmful if you get too much. High doses of iron can also decrease zinc absorption. The daily upper limits for iron include intakes from all sources—food, beverages, and supplements—and are listed below.
A doctor might prescribe more than the upper limit of iron to people who need higher doses for a while to treat iron deficiency.
Tolerable upper intake levels (ULs) for Iron are listed in the following table:
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How much iron do we need?
What if you don’t consume enough iron?
When iron stores in the body get too low, red blood cells can not carry sufficient oxygen, which likely makes the individual feel tired and weak, causing an inability to concentrate and poor memory, lowering the ability to fight infections.
Iron deficiency is more common among menstruating women.
Vegans are advised to consume more iron because the body does not absorb nonheme iron from plant-based foods to a large extent.
What if you consume excess amounts of iron?
Healthy adults have little risk of iron overload from iron-rich foods. However, iron can be harmful if you get too much. High doses of iron can also decrease zinc absorption. An intake of excessive amounts of iron supplements or medications may lead to abdominal pain, constipation, faintness, vomiting, nausea, and gastric upset. Taking adult iron supplements can be dangerous for children. Children should get immediate medical attention if they overdose on iron supplements.
UL: The Tolerable Upper Intake Level is the maximum daily intake unlikely to cause harmful effects on health. The UL for iron is 45 milligrams daily for all males and females 14 years and older. The daily upper limits (UL) for iron include intakes from all sources, food, beverages, and supplements. The ULs do not apply to individuals who are receiving supplemental iron under medical supervision. A doctor might prescribe more than the upper limit of iron to people who need higher doses for a while to treat iron deficiency.
The food sources of iron:
Which foods provide iron?
Iron is found naturally in many foods, including the following:
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Lean meat, poultry, and seafood
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Iron-fortified breakfast cereals and bread
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Lentils, spinach, beans and peas
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Nuts and some dried fruits, such as raisins
Iron in food comes in two forms: heme iron and nonheme iron. Plants and iron-fortified foods contain nonheme iron only, whereas meat, seafood, and poultry contain both heme and nonheme iron. Heme iron contributes 10% to 15% of total iron intake in Western populations. Heme iron has higher bio-availability than nonheme iron. The bioavailability of iron is approximately 14% to 18% from mixed diets that include substantial amounts of meat, poultry, seafood, and vitamin C. The bioavailability of iron is 5% to 12% in vegetarian diets.
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Top selected foods contain iron content:
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Breakfast cereals fortified with iron (you need to check the product label)
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Oysters
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White beans
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Beef liver
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Lentils
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Spinach
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Tofu
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Dark chocolate
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Kidney beans
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Sardines
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Chickpeas
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Tomatoes
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Beef
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Potatoes
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Cashew nuts
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Green peas
References:
National Institutes of Health Office of Dietary Supplements: Iron Fact Sheet for Health Professionals
https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/
Updated: June 15, 2023
Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc: a Report of the Panel on Micronutrientsexternal link disclaimer. Washington, DC: National Academy Press; 2001.
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