Iron absorption from plant sources
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Iron Absorption from Plant Sources: Forms and Bioavailability
Non-Heme Iron in Plant Foods
All iron in plant foods—such as legumes, cereals, vegetables, and fruits—is in the form of non-heme iron, which is generally less well absorbed by the human body compared to heme iron found in animal products. Non-heme iron makes up about 90-95% of the total daily iron intake in vegan diets, but its absorption is relatively poor due to its chemical form and the presence of inhibitors in plant foods 710.
Factors Affecting Iron Absorption from Plants
Inhibitors: Phytates and Polyphenols
Plant-based diets often contain high levels of phytates and polyphenols, compounds that bind iron and significantly reduce its absorption in the gut. This is a major reason why iron deficiency is more common in populations relying heavily on plant-based diets 410.
Enhancers: Ascorbic Acid and Citric Acid
Certain organic acids, especially ascorbic acid (vitamin C) and citric acid, can greatly enhance the absorption of non-heme iron from plant sources. These acids form soluble chelates with iron, making it more available for absorption. Including fruits or foods rich in these acids alongside plant-based meals can markedly improve iron uptake .
Iron Absorption from Specific Plant Sources
Legumes
Studies measuring iron absorption from common legumes such as soybeans, black beans, lentils, mung beans, and split peas in humans have found that the mean percentage absorption is uniformly low, ranging from 0.84% to 1.91%. This indicates that these legumes are generally poor sources of bioavailable iron for humans .
Plant Ferritin
Ferritin is a protein that stores iron in plants, especially in legumes. Research shows that iron from soybean ferritin is absorbed as well as iron from ferrous sulfate (a common supplement) in non-anemic women, suggesting that ferritin-bound iron is a promising, bioavailable form of plant iron. However, the natural concentration of ferritin iron in most edible seeds is not high enough for significant dietary impact unless plants are biofortified 69.
Mechanisms of Iron Uptake in Plants
Plants absorb iron from the soil primarily as Fe2+ (ferrous) ions. They have developed mechanisms to solubilize and take up iron, including the release of reductants by roots and the use of iron chelates. While chelates help move iron to the root surface, plants mainly absorb the iron itself and not the chelating agents 158.
Iron Complexes and Soil Interactions
Iron in soil is often bound in forms that are not easily available to plants. Humic substances, such as Fe-humate complexes, can serve as good sources of iron for plants, as shown in long-term studies with barley, sunflower, and other species. These complexes are absorbed and transported to plant shoots, supporting plant growth .
Conclusion
Iron from plant sources is mostly in the non-heme form, which is poorly absorbed due to inhibitors like phytates and polyphenols. However, absorption can be improved by consuming vitamin C or citric acid-rich foods with plant-based meals. While legumes are generally poor sources of bioavailable iron, ferritin-bound iron in plants shows promise for better absorption. In plants, iron uptake from soil involves specialized mechanisms, and certain iron complexes like Fe-humate can enhance availability. Overall, strategies to improve iron absorption from plant sources are important for populations relying on plant-based diets 12345678+2 MORE.
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