Iron - Health Benefits - Improving Iron Absorption
Iron is an essential trace element that exists in the body in the form of divalent iron (heme iron; Fe2+) and trivalent iron (non-heme iron; Fe3+). Therefore, iron must be obtained through diet. An adult human stores approximately 3 to 4 grams of iron in the body 1. Other scientists estimate 4.8 grams of iron in the human body 2. Approximately 65 to 80% of that is found in red blood cells 3.
A large portion of iron is stored in the form of ferritin and hemosiderin, the iron storage proteins, in the bone marrow and liver. A smaller amount of the storage proteins is found in the spleen, intestinal mucosa, and muscles. Through storage, the body is continuously supplied with iron. Iron is primarily needed for blood formation and oxygen transport. There are over 500 proteins in the body that contain iron 4.
In plants and many processed vegan foods, iron is exclusively present in the form of non-heme iron. Adequate amounts of iron can be supplied to the body through a balanced vegan diet. By implementing measures to improve absorption, the uptake of iron from food can be further enhanced.
Functions and Benefits
Iron is an essential component of hemoglobin, myoglobin, and cytochromes 5.
Hemoglobin (red blood pigment) is a protein and a component of red blood cells (erythrocytes). It binds the oxygen taken up through the lungs and transports it to all cells. Cells require a regular supply of oxygen to produce enough energy. As the energy metabolism takes place, hemoglobin picks up the resulting waste product, carbon dioxide, and transports it to the lungs, where it is exhaled.
Myoglobin (red muscle pigment), also a protein, is responsible for taking up oxygen from the blood. Additionally, myoglobin is needed for holding and storing oxygen in muscle cells. Cytochromes are involved in electron transfer. Cytochrome P450 assists in the breakdown of endogenous compounds and environmental toxins in the liver and intestines 6.
Additional important functions and tasks: Iron
- is involved as a cofactor (essential for enzyme function) in collagen formation 7; Collagen is the most abundant structural protein in the body, found in skin, bones, muscles, and tendons, ensuring cohesion, elasticity, and regeneration.
- is needed as a component of the enzyme NADH dehydrogenase for energy-generating bodily functions 8
- Proteins that require iron are involved in DNA replication, DNA repair, and cell cycle control 9
- supports the growth of eukaryotic cells 10; Eukaryotes are organisms whose cells contain a nucleus and organelles.
- is a component of iron-sulfur clusters that serve as cofactors for proteins involved in enzyme catalysis, electron transport, and regulation of gene expression 11
- supports the immune system 12
- is required for the synthesis of carnitine 13; Carnitine plays an important role in energy metabolism.
Preventive Benefits of Iron
Adequate iron prevents anemia, which can manifest as pallor, fatigue, and decreased performance.
In a study, it was demonstrated that non-heme iron, unlike animal heme iron, helps lower blood pressure 14. According to the study, meat consumption was associated with higher blood pressure.
Iron for Athletes
An adequate oxygen supply to the muscles through hemoglobin and myoglobin is crucial, especially for athletes, to achieve maximum muscle performance with the help of oxygen.
If the body doesn't have enough iron available during exercise, it can lead to deficiencies such as muscle fatigue and exhaustion.
A summary of a 2014 study demonstrates that the use of iron supplements can enhance performance in women. 15. Additionally, women who took supplements had a lower heart rate.
Essentials of Iron Absorption
The absorption of iron from food depends on the bioavailability of iron. The non-heme iron (Fe3+) found in plants is insoluble and not bioavailable 16. It needs to be converted from Fe3+ to Fe2+ through the body's own ferrireductase or dietary reducing agents such as ascorbic acid (vitamin C) - see section: Improving Iron Absorption.
Heme iron (Fe2+) of animal origin is therefore better absorbed than non-heme iron found in plants. The absorption rate for non-heme iron varies from 0.7 to 22.9%, and it strongly depends on the current iron status, plasma ferritin level 17. Individuals with sufficient iron reserves absorb less non-heme iron from food compared to those with inadequate reserves 18. Hemoglobin concentrations and the risk of iron deficiency anemia are similar for vegans compared to omnivores and other vegetarian groups 19. In individuals with low iron status, non-heme iron is absorbed almost as well as heme iron 20.
A study conducted in 1991 demonstrated the impact of various food components on non-heme iron absorption 21. Three groups were formed for this purpose - normal diet, a diet with absorption enhancers, and a diet with iron inhibitors. On average, the first group absorbed 6.1%, the second group absorbed 13.5%, and the third group absorbed only 2.3% of the iron.
Improving Iron Absorption from Food
For significantly better absorption of plant-based non-heme iron, simultaneous consumption of vitamin C-rich foods and beverages can be ensured. 22 23. Vitamin C is capable of binding to non-heme iron, converting trivalent iron (Fe3+) to divalent iron (Fe2+), thereby enhancing the absorption of dietary iron in the small intestine (increased bioavailability) 24. Ascorbic acid effectively counteracts the inhibitory effects of phytates (found in grains, nuts, seeds, and legumes) on iron absorption 25 26 27. At the same time, it stimulates storage in the iron storage protein ferritin. For example, orange juice and apple juice contribute to improved absorption 28. Some iron-rich foods like broccoli and tomatoes already contain vitamin C, allowing the intestine to optimally absorb the iron. Citric acid, which is mainly found in citrus fruits, is similarly effective 29 30. Here's the link to the citric acid foods.
The two amino acids methionine (methionine-rich foods) and cysteine (cysteine-rich foods) also enhance the absorption of plant-based iron 31 32. They enhance the reduction of trivalent iron to divalent iron, making it more absorbable.
Sugar also improves the bioavailability of non-heme iron 33. Fructose increased the production of ferritin in cells induced by iron.
Regarding improved absorption through vitamin A, there are different study results. Study results from 1998 and 2000 indicate improved iron absorption through vitamin A from 34 35. For foods containing vitamin A, click here. Vitamin A seems to bind to iron released during digestion and acts as a complexing agent, reducing the inhibitory effect of polyphenols and phytates on non-heme iron absorption 36. However, a more recent study from 2014 showed that vitamin A did not improve iron absorption 37. Instead, it was shown that carotenoids improved iron absorption (foods with beta-carotene).
Possibly prebiotic fibers like inulin can positively influence iron absorption. 38.
What impairs iron absorption?
However, food components can also cause reduced iron absorption.
So mainly, phytates (phytic acid), which are classified as secondary plant compounds and are mainly found in unrefined grains, nuts, oilseeds, and legumes, reduce absorption. They tightly bind to iron and make its uptake in the digestive tract impossible 39. Therefore, foods high in iron like spinach and rhubarb, which also have a high phytic acid content, are not the best sources of iron. However, the bioavailability of iron can be improved through cooking, roasting, fermenting, and sprouting seeds. This breaks down phytic acid and allows for better iron absorption 40. However, it should be noted that heating and processing of food can degrade ascorbic acid, which in turn diminishes the enhancing benefit of vitamin C on iron absorption 41.
Also, oxalic acid binds to iron. Plants rich in iron such as Swiss chard and red beets are not particularly good sources of iron because they contain high amounts of oxalic acid. Study results from 2008 suggest that the effects of oxalic acid may not be as significant as previously thought. 42.
Coffee, cocoa (both contain chlorogenic acid), but especially green and black tea (contains tannins), can impair the absorption of iron from food due to the presence of phenolic acids (which belong to the group of polyphenols). 43 44. Tannins (tannic acid) bind to non-heme iron, ultimately reducing the absorption of iron. 45 46. So, drinking tea during a meal can reduce iron absorption by up to 50% 47. Therefore, if you already suffer from iron deficiency, you should wait one to two hours after eating before drinking tea. Polyphenols also have positive properties such as preventing certain types of cancer and improving bone density 48. Therefore, they should not be completely avoided.
Zinc and manganese compete with iron for absorption due to their similar chemical properties, which can impair uptake 49 50. However, scientists suggest that the potential negative effects of zinc should only be considered in the context of zinc supplementation.
A deficiency in vitamin B2 also negatively affects iron metabolism 51. Flavoproteins formed from vitamin B2 (riboflavin) act as cofactors in the absorption and utilization of iron 52. To learn about foods high in vitamin B2, click here.
Certain protein compounds can negatively affect iron absorption. For example, soy proteins inhibit absorption, as shown in 53. Additionally, proteins of animal origin such as egg protein, casein, and whey also inhibit iron absorption, as mentioned in 54 and 55.
Furthermore, studies have shown that calcium also reduces the absorption of heme and non-heme iron, as indicated in 56 and 57. The effect of calcium on iron absorption (especially when taking calcium supplements) may be short-lived, and the body may adapt over time, as discussed in 58.
What do omnivores need to consider?
Study findings suggest that dietary heme iron may increase the risk of colon cancer in postmenopausal women, especially those who consume alcohol. 59.
Are there any considerations for vegans?
According to studies, vegans tend to have higher iron intake compared to the general population. 60 61 62 63. This is primarily due to the high consumption of vitamin C-rich foods among vegans, which significantly improves iron absorption. The daily iron requirement can be easily met through a vegan diet. Women should consider their higher iron needs due to menstruation.