Enhancing Iron Absorption


Douglas Lobay, BSc, ND

“When you hear hoof beats in the Pacific Northwest think horses not zebras,” a wise old professor said when discussing diagnostics during a third-year naturopathic course at Bastyr University (nee College).  Looking back, I think what he was saying was that common things commonly occur, while rare and unusual things occur rarely and not that commonly. 

Iron deficiency is one of the most common nutrient deficiencies in the world.  An estimated 20% of the entire world’s population has iron deficiency and iron-deficiency anemia.  Forty percent of people in developing countries and 10% of people in developed countries have iron deficiency. Iron deficiency occurs so commonly because of poor dietary intake, poor absorption, or blood loss, or a combination of these factors. Iron deficiency and iron-deficiency anemia present with a whole host of symptoms that include fatigue and lethargy.  As a clinician it is important to diagnose the condition correctly with proper lab tests. Secondly, it is important to identify the cause(s) of the condition and thirdly, to implement therapy that corrects the deficiency.1,2 

Most often diagnostics for iron deficiency and iron deficiency anemia are fairly straight forward.  Common lab tests include a complete blood count or CBC, ferritin, iron, total iron binding capacity (TIBC) and % saturation.  Identifying an underlying cause(s) can be more challenging.  Suffice it to say that poor nutrient intake, impaired absorption, and blood loss through gastro-intestinal bleeding and menstrual losses are the most common causes.  Recommending iron-rich foods, supplements to improve digestion and absorption, and iron supplements are an appropriate standard of care. Lists of iron-rich foods are easily available that include animal products and darker vegetables and fruits.  There are many iron supplements available in health foods stores and pharmacies. I will not delve in the merits of one commercial iron supplement over another.  Rather I will focus on iron absorption and bioavailability and specifically on ways to enhance absorption.1,2

Iron supplementation can improve iron levels in individuals low in iron, most of the time.  However, there are patients who still have low iron levels despite diagnosing the condition, identifying the cause(s) and recommending dietary changes and nutritional supplementation.  These challenging patients usually present with a plethora of symptoms and lab tests that reveal low iron and ferritin levels with or without iron deficiency anemia.   They often don’t respond to basic iron supplementation therapy. It can be disconcerting to have a patient with low iron and ferritin that doesn’t respond to iron supplementation and still remain fatigued and tired.  It can be difficult to avoid the quandary of treating the patient or the lab test.  A different iron supplementation is recommended and then iron levels are measured at some time in the future. It can be a veritable merry-go-round of different iron supplements to find the one that works. This paper includes some practical strategies that can improve iron absorption and increase serum iron and ferritin levels.1,2

Red Blood Cells, Hemoglobin, Oxygen, Ferritin, and Transferrin

Red blood cells (RBCs) are the most common cell type in the human body. The main function of red blood cells is to transport oxygen.  RBCs account for between 70 to 84% of all cells in the human body.  Their numbers are estimated to be between 20 to 30 trillion cells.  Red blood cells are made in the bone marrow.  During their synthesis, they lose their cell nucleus.  The anucleated RBC appears as biconcave disc, which is red in color because of the numerous iron containing hemoglobin molecules.  The average red blood cell lives approximately 100 to 120 days. After which senescent, RBCs are broken down by macrophages in the liver and spleen.  Iron is generally conserved and recycled to make new hemoglobin.3

Hemoglobin is a globular protein made of four subunits that contains four iron molecules.  The main function of hemoglobin molecules is to bind oxygen from the lungs and then deliver oxygen to cells throughout the body.  There are an estimated 270 million hemoglobin molecules per red blood cell.  Each hemoglobin molecule can bind four oxygen molecules.4  

Oxygen is a vital nutrient in the generation of ATP (adenosine triphosphate) from ingested food glucose.  According to basic biochemistry, there are three main steps in the breakdown of glucose to the generation of ATP.  The first step is glycolysis whereby glucose is broken down to three 2 carbon fragments.  The next step is Kreb’s cycle or the tricarboxylic acid cycle whereby the two carbon fragments generated in glycolysis go through a series of reactions to generate energy-rich molecules that can then generate ATP in the final step of the electron transport chain. The electron transport chain is the final step of ATP generation whereby electrons are transferred from one high energy molecule to another. Oxygen is the final electron acceptor in the electron transport chain in mitochondria.  Without oxygen the electron transport chain fails to work and will not then generate ATP.4

Ferritin is the main iron storage protein in the body found primarily in the cellular cytoplasm but also in the serum.  Serum ferritin levels are a reliable indicator of iron storage levels in the human body.  Ferritin is a globular protein made up of 24 subunits.  Ferritin can occur in every human cell but tends to be more concentrated in the liver, spleen, kidneys, and bone marrow. Each ferritin molecule can bind up to 4500 iron molecules.5 

Transferrin is the main iron transport molecule in the serum.  Transferrin has a high affinity for oxidized ferric +3 iron. Each transferrin molecule binds two iron atoms. Transferrin iron accounts for only 1/10 of 1% of total body iron stores.6

Iron

The average amount of iron in the human body is calculated to be 3.8 grams for males and 2.3 grams in females.  Iron is vital for human life and is involved in oxygen transfer, mitochondrial energy generation, DNA synthesis and as a cofactor in various enzyme reactions. The average human diet supplies 10 to 20 milligrams of iron per day. One to 2 milligrams of iron is absorbed. Epithelial desquamation accounts for a daily iron loss of approximately 1 to 2 milligrams. Seventy-five percent of total body iron is utilized in hemoglobin and to a lesser extent in myoglobin in muscles. Ten to 20% of iron is stored in ferritin. And 5 to 15% is utilized in other biochemical processes throughout the body.7,8

Ferrous fumarate is an ionic iron compound that contains approximately 33% elemental iron.  Three hundred milligrams of ferrous fumarate supplies about 100 milligrams of elemental iron. Ferrous gluconate is an ionic iron compound that contains approximately 12% elemental iron.  Three hundred milligrams of ferrous gluconate supplies about 42 milligrams of elemental iron.  Ferrous sulphate is an ionic iron compound that contains approximately 20% elemental iron. Three hundred milligrams of ferrous sulphate contains about 60 milligrams of elemental iron. Ferrous bisglycinate is a covalent iron compound that contains approximately 27% elemental iron.9,10

Iron is mainly absorbed in the duodenum and proximal jejunum. The iron absorption rate varies with the type of iron, when it is taken and what it is taken with.  Heme iron has an estimated absorption rate between 15 to 35% with an average absorption rate calculated to be about 25%.  Non-heme iron has an estimated absorption rate between 2 to 20% with an average absorption rate calculated to be about 11%.  Some studies show that ferrous fumarate taken with vitamin C has an absorption rate between 6.3 to 10.4%.  Other studies show that ferrous bisglycinate has an absorption rate between 2.6 to 13.6% with an average absorption rate of about 6.0%.10,11

Ferroportin and Hepcidin

The ferroportin-hepcidin mechanism of iron absorption is a fairly recent discovery.  The exact biochemical pathway of iron absorption was elucidated and confirmed in the year 2000 by various scientific groups.7,8

Ferroportin is a transmembrane protein that spans the apical side of intestinal luminal cells.  It is the only known transporter of iron in humans.  It allows efflux of iron from intestinal cells into capillaries of the bloodstream.  The ability of ferroportin to transfer iron is highly controlled by hepcidin. When hepcidin levels in the serum are low, iron transfer through ferroportin is high. Conversely, when hepcidin levels are high, iron transfer through ferroportin is low.7,8

Hepcidin is a 25 amino acid length hormone produced in the liver that selectively binds to ferroportin. Hepcidin is essentially a gatekeeper that controls the efflux of iron across ferroportin.  Hepcidin binds to ferroportin and causes the degradation of ferroportin by lysosomes and promotes its destruction inside the cell. Hepcidin is produced in liver cells in response to high iron levels and acute inflammation.  When iron levels are level are low, oxygen levels are low, inflammation is increased, and the production of red blood cells is stimulated, hepcidin is suppressed in the liver.7,8,12

Heme and Non-Heme Iron

Iron is present in the diet as two distinct forms known as heme and non-heme iron.  Heme iron is iron that is complexed with animal protein to form a metalloprotein.  Heme iron is found only in animal products such as red meat, fish, and poultry.  Non-heme iron is iron that is found in plants, fruits, vegetables, nuts, seeds and grains.  Non-heme iron is typically available is ionic or covalent bonded molecules.  Heme and non-heme iron have two distinctive absorption pathways through human intestinal cells.  Heme iron usually accounts for about 10% of total iron intake.  Non-heme iron accounts for up to 90% of total iron intake.  Obviously, these numbers can be manipulated by adjusting the diet in relationship to heme and non-heme iron food sources.7,8,12

Iron Absorption

Iron absorption is dependent on intestinal pH and the type of iron that is available.  Most intestinal iron received in the duodenum at an average pH of 6 is mainly in an oxidized ferric +3 state or bound in heme molecules.  Ferric iron is relatively non-soluble and must be converted to a more soluble reduced ferrous +2 state. A ferric reductase enzyme located on the luminal brush border side of intestinal cells called duodenal cytochrome B (DcytB) converts ferric +3 iron to ferrous +2 iron. Once the iron is reduced to ferrous iron it can be absorbed by an intestinal luminal transporter protein called divalent metal transporter 1 (DMT1). Once inside the cell, ferrous iron is converted back to ferric iron and either stored in ferritin molecules or transported through the basolateral side into the bloodstream through ferroportin. 

The activity of ferroportin appears to exclusively under control of liver hormone hepcidin. High levels of hepcidin, cause ferroportin degradation and inhibit iron efflux to the bloodstream while low levels of hepcidin prevent ferroportin destruction and allow iron transfer across the basolateral membrane.  Once transferred through ferroportin the ferrous iron is converted back to ferric iron by two copper-containing enzymes located on the apical outer cell membrane.  The ferric iron is then bound to transferrin in capillaries and transported in the bloodstream. The activity of DcytB and DMT1 can be upregulated by low serum iron levels and hypoxia.  Also the activity of DcyB and DMT1 can be downregulated by a variety of digestive conditions that include the use of proton pump inhibitor drugs, celiac disease, and other conditions that promote damage to the brush border and malabsorption.12,13,14

Heme iron has a separate and distinctive absorption pathway across duodenal intestinal cells that involves fusion of heme and incorporation into small vesicles in a process called pinocytosis.  Heme iron is released from animal protein that has been ingested by stomach acid.  Heme iron then binds to heme carrier proteins in the luminal brush border of the duodenum.  Then via endocytosis the heme molecule is absorbed across the cell membrane.  Once inside the cell a large portion of ingested heme is degraded by an enzyme called heme oxygenase (HO).  Ferrous iron is released and then quickly converted to ferric iron. The ferric iron then bounds ferritin inside the duodenal cells for storage. The degraded heme iron thus contributes to overall level of free intracellular iron. The absorption of heme iron is also sensitive to iron levels, hypoxia, and a variety of intestinal conditions that cause malabsorption.15,16

The human body is highly proficient at conserving and recycling iron.  The average total body stores of iron are about 3.8 grams for males and 2.3 grams for females.  In western industrial countries total iron levels are slightly higher somewhere between 4 to 5 grams.  The average human diet supplies about 10 to 20 milligrams of elemental iron per day. Only 1 to 2 milligrams of this iron is absorbed into the body.  1 to 2 milligrams of iron are lost per day through the intestines and skin desquamation.  1800 milligrams of iron are used in hemoglobin in red blood cells.  300 milligrams of iron are sequestered in bone marrow to be incorporated in red blood cells. 1000 milligrams of iron are stored in the liver primarily as ferritin.  300 milligrams of iron are in heart and muscle tissue primarily as myoglobin. 600 milligrams of iron are located in macrophages form senescent red blood cells for breakdown and recycling.  The human body requires between 20 and 25 milligrams of iron per day for the production of new hemoglobin molecules.16,17 

Vitamin C

Vitamin C enhances iron absorption. This has been demonstrated in both giving vitamin C with a meal to enhance iron extraction from food and giving ascorbic acid together with an iron supplement.  The mechanism by which vitamin C enhances iron absorption appears to be two-fold.  One, ascorbic acid forms a complex with non-heme iron molecules that prevent other dietary factors such as phytates and polyphenols from binding to the iron.  And secondly, vitamin C helps to reduce ferric iron to more soluble and preferentially absorbed ferrous iron that is delivered to intestinal cell surface for more efficient absorption.  

Vitamin C in the form of one glass of orange juice added to a meal enhances iron absorption in heme iron by 2.5 times.  Meals that contain low to moderate doses of iron inhibitors appear to require a molar ratio of vitamin C to iron about 2:1, which equals 20 milligrams vitamin C to 3 milligrams elemental iron.  Meals that contain a high amount of iron inhibitors require a molar ratio of 4:1 equivalent to 40 milligrams of vitamin C per 3 milligrams of elemental iron.  This ratio can be extrapolated to iron supplementation to give an approximate dose of vitamin C when consuming non-heme iron.18,19

Other Food Acids

Other acids found in different foods may help to increase iron absorption, including citric acid, lactic acid, malic acid and tartaric acids.  Citric, malic, and tartaric acids are found in many fruit juices while lactic acid is found in fermented like pickles sauerkraut, kefir, and yogurt.  Low stomach acid or hypochlorhydria is known to be associated with poor iron absorption.11,19

Alcohol

Alcohol has been shown to increase iron absorption, especially heme iron.  Even mild to moderate alcohol consumption has been shown to enhance iron absorption.  Alcohol has been shown to suppress hepcidin expression in liver cells, thereby allowing ferroportin to transfer iron into the blood stream.  The consumption of alcohol has been shown to enhance the effects of hemochromatosis.  Wine is especially rich in polyphenols and tannins and may have less of an effect than other spirits.7,8,11

Garlic

Garlic has demonstrated to increase iron absorption by over 50% by down regulating hepcidin activity on ferroportin.  Sulphur-containing cystine residues appear to competitively inhibit hepcidin binding on the domain binding site on ferroportin.20,21

Probiotics

Specific strains of probiotics have demonstrated to enhance iron absorption in some preliminary studies.  Supplementation with Lactobacillus fermentum, Lactobacillus plantarum and Streptococcus thermophilus have shown to increase iron absorption and bioavailability.  While the exact mechanism of their effects is not known, they are speculated to improve iron solubility in the intestinal lumen, increase conversion of ferric iron to ferrous iron, produce metabolites that increase iron absorption or act as physical iron carriers and transporters.7,8,23  

Phytates

Phytates or phytic acid also known as inositol hexaphosphate are typically found in whole grains, beans, legumes, seeds, nuts, and other selective plants.  Wheat bran and oat bran are particularly high in phytates. Phytates are a rich reservoir of phosphate and have antioxidant properties.  Phytates are strong chelators of different minerals, including iron.  Phytates in food can inhibit iron absorption up to 50%.7,8,11,16

Polyphenols

Polyphenolics are compounds found in a wide variety of foods that share a common phenolic ring structure. These include flavonoids, lignans, phenolic acids and stilbenes.  Tannins in beverages like tea are rich in polyphenolic compounds.  Polyphenols are ubiquitous in distribution and occur widely in fruits, vegetables, nuts, grains, seeds and legumes.  Polyphenolic compounds from foods can bind iron by greater than 50%.  Polyphenols in foods can inhibit iron absorption by 50 to 70% and polyphenols in beverages like black tea, coffee, and chocolate can inhibit iron absorption by 50 to 90%.11,16,17,22

Calcium

Calcium, in both foods and as supplements, has been shown to inhibit iron absorption.  Calcium has demonstrated to inhibit both the absorption of heme and non-heme iron.  Calcium does not necessarily increase intestinal luminal absorption, but rather inhibits ferroportin transfer into the bloodstream.  Both calcium-rich foods and calcium supplements have demonstrated iron absorption inhibition especially when consumed within two hours of iron supplements.  A dose of 300 milligrams showed the maximal inhibition from both food and supplementation when take together with iron. Other studies show that a small to moderate does of calcium-rich foods does not affect iron absorption as significantly.7,8,11

Dietary Fiber

Dietary fiber from food and supplements may impact iron absorption.  Insoluble dietary fiber from wheat bran has been shown to slightly decrease iron absorption.  Soluble fiber from oat bran has also demonstrated to slightly decrease iron absorption.  Other studies show only a marginal or non-significant effect on iron absorption.7,8,11 

Oxalates

Oxalates are small organic acids found in certain foods, including certain fruits, vegetable, nuts and whole grains.  Oxalates are particularly high in dark green leafy vegetables like spinach, chard and kale.  The oxalic acids can bind to several divalent minerals like calcium and ferrous iron and form precipitates in the digestive system.  Consuming iron with these foods may inhibit its bioavailability and absorption.7,8,11,17

Protein, Fats, and Carbohydrates

Different proteins can enhance or diminish iron absorption.  Animal-sourced protein from red meat, fish, and poultry can increase iron absorption.  Whey protein may increase iron absorption slightly. Egg protein and soy protein can decrease iron absorption.  It is theorized that other plant proteins may decrease iron absorption. A high fat meal and rich carbohydrate meal can also decrease iron absorption.7,8,24,25

Phytoestrogens

Several phytoestrogens, including naringenin, quercetin, and resveratrol have demonstrated to increase hepcidin expression.  Genestein and iprivalone from soy products have been shown to increase hepcidin activity and decrease iron absorption.  A polysaccharide found in Dong quai (Angelica sinensis) has been shown to decrease hepcidin levels.  Vitamin D supplementation can decrease hepcidin levels. Testosterone and estrogen can decrease hepcidin levels, while progesterone can increase hepcidin levels.7,8,25

Turmeric

Turmeric (Curcuma longa) has shown to both inhibit and promote iron absorption.  The active ingredient in turmeric root is a group polyphenolic compounds that can directly bind iron molecules.  However, the curcuminoids are also anti-inflammatory and can decrease hepcidin production in the liver.  Lower doses of turmeric appear to increase iron absorption while higher doses tend to decrease iron absorption.  Also the timing of iron and turmeric compounds may be conflicting.  If you consume iron at the same time as turmeric, the polyphenols may have a direct effect on binding iron and prevent it from being absorbed.  Conversely if you take turmeric at a different time than iron, there is no direct conflict and it may decrease hepcidin production in the liver and thereby increase ferroportin activity.26,27

Iron Supplementation

If patient has iron deficiency and iron-deficiency anemia as determined by generally accepted lab tests and dietary requirements are not or cannot be met, then iron supplementation can be recommended.

The RDA (recommended dietary allowance) of iron for adults between the ages of 19 to 50 years is 8 milligrams daily for men, 18 milligrams daily for females, 27 milligrams daily for pregnancy and 9 milligrams per day for lactation. Other specific doses can be found online.28,29 

Choose an iron supplement based on the results of the patient’s lab test, type of iron, the elemental dose, dietary requirements, digestive capacity, price point, the clinician’s experience, and generally what is in the patient’s best interest.28,29

Start by taking iron supplements on empty stomach and not with food.  Take at least ½ to 1 hour before a meal or after 2 hours following a meal. Do not take with other supplements or medication unless suggested by the clinician.28,29

Take iron supplement once per day or if taking more than one dose and the patient gets nausea or upset stomach then take twice per day.28,29

Take consistently for 2 to 3 months before retesting iron levels in a lab test that includes CBC, ferritin and iron.28,29

Take vitamin C with the iron supplement in general ratio of 10:1, that is 10 milligrams of vitamin C to every 1 milligram of elemental iron. If the elemental iron dose is 10 milligrams, then take with at least 100 milligrams of vitamin C.  If the elemental iron dose is 25 milligrams, then take with 250 milligrams of vitamin C. If the elemental iron dose is 100 milligrams, then take with 1000 milligrams of vitamin C.  Take vitamin C at the same time is iron supplement.18,20,28,29

If the patient will not take vitamin C with iron, then encourage the patient to take the supplement with a glass of fruit juice, preferably high in acidity like orange juice, tomato juice, or other fruit.28,29

Make adjustments to iron dose based on the patient’s results in blood test and physical well-being following the introductory trial.28,29

If patient complains of nausea, constipation, or other digestive disturbance, have the patient take the iron supplement with food.  Try to encourage the patient to avoid foods high in calcium, phytates, and polyphenols when taking iron supplements.  Taking iron supplement with a protein should be suggested.28

If no change in iron levels after several months, decide on a different iron supplement or different strategy.  All the while be mindful of the potential cause of low iron caused by illness, loss, or poor absorption.28,29

Again, if no change in iron levels, then suggest to the patient about taking iron every second day instead of every day. The patient could try take a double dose or two-day supply of iron at every second day.  Again wait 2 to 3 months before evaluating how successful this recommendation is. Some studies show that this regimen can be effective for some patients who are resistant to every day dosing.  Taking iron daily can increase hepcidin levels for up to 24 hours following the ingested dose.30,31 

Also, the optimal daily elemental iron dose is probably patient specific.  In clinical practice, a dose between 25 to 50 milligrams of elemental iron seems to be adequate for most people who are iron deficient.  Occasionally some patients require a dose of 100 milligrams per day or more of elemental iron. Higher doses can be recommended for patients who do not respond to the lower dose.  Be mindful that a higher daily dose of iron can have a negative feedback on iron absorption mechanism.  High iron can trigger increased hepcidin expression thereby having the opposite effect and decreasing iron absorption.28,29

Ensure adequate levels of vitamin D are maintained.32

Check for resistance caused by other supplements or drugs especially calcium, antacids and protein pump inhibitors.28,29 

The patient can also consume garlic as food or supplement daily to help decrease hepcidin levels and encourage iron absorption.20,21

Turmeric or curcumin in a lower dose may also be suggested to help reduce inflammation, lower hepcidin, and encourage iron absorption.26,27

Encourage the consumption of fermented foods like sauerkraut and kefir.  As a precaution do not consume calcium-rich fermented products at the same time as iron supplementation.7,8,23

If iron levels are still depressed after 6 months of supplementation or longer and the patient has not improved subjectively, then a re-evaluation of the treatment strategy is on order.  Remember some patient’s lab values may not improve no matter what you do.  In this case a referral for further evaluation may be in order.  And sometimes iron shots or intravenous iron may be suggested if nothing else helps.

References

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Published September 23, 2023

About the Author

Douglas G. Lobay, ND, is a practicing naturopathic physician in Kelowna, British Columbia. Dr. Lobay graduated with a Bachelor of Science degree from the University of British Columbia in 1987. He then attended Bastyr College of Health Sciences in Seattle, Washington, and graduated with a Doctor of Naturopathic Medicine in 1991. While attending Bastyr College, he began to research the scientific basis of natural medicine. He was surprised to find that many of the current medical journals abounded with scientific information on the use of diet, nutrition, vitamins, and botanical medicines. Besides practicing naturopathic medicine Dr. Lobay enjoys research, writing and teaching others about the virtues of good health and nutrition. He has authored several books, numerous articles, and papers and has taught many courses at seminars and colleges throughout his career. He is married to Natalie and has two daughters, Rachel and Jessica. He also enjoys hiking, hockey, skiing, tennis, travelling and playing his guitar.