Dietary Iron Uptake and Absorption

Abstract

Iron is an essential element that participates in many physiological roles in the human body, including oxygen transport, DNA synthesis, cell division and differentiation, immunity, and electron transport. Iron absorption takes place primarily on the duodenum of the small intestine through the enterocyte cell. Its levels in the human body are controlled only by absorption since unlike most other essential nutrients, it has no regulated excretory system in humans. Hence, maintenance of iron level is critical to avoid adverse physiological consequences of iron deficiency or overload. Dietary iron exists in two forms; haem and non-haem; the bioavailability of these forms of iron is dependent on dietary inhibitors (calcium, phytates, and polyphenols) and enhancers (ascorbic acid). The mechanism that regulates iron absorption occurs in three stages; (i) luminal uptake and transport of iron across the apical membrane (ii) transfer of iron to the basolateral membrane and (iii) transport of iron across the basolateral membrane into circulation. The proteins that mediate iron uptake at the various stages include divalent metal transporter I, ferroportin, hephaestin, and hepcidin. This chapter will elaborate on dietary iron in its different forms, factors that enhance and inhibit iron absorption, the mechanism by which it is absorbed and iron supplementation and fortification.

1. Introduction

Healthy living requires an adequate supply of essential metals, and one such is iron. Iron plays a crucial role in enzyme catalysis, oxygen transport, electron transport, cell division and differentiation, and regulation of gene expression [1, 2, 3]. Despite its indispensability, when it is excessively present in diets, it can be toxic as it is capable of catalyzing the formation of reactive oxygen species [1]. Given the dual tendencies of iron, the concentration of the essential metal in the body must be kept within bounds [1]. The levels of iron in the human body are regulated mainly by the amount absorbed per time [4]. The mechanism of iron excretion is an unregulated process, and it occurs via loss of sweat, menstruation, shedding of hair and skin cells, and rapid turnover and excretion of enterocytes [4]. In general, iron is present in diets as either haem or non-haem. Of the two forms, the non-haem form is by far the most prevalent accounting for 80–90% of the iron in a standard diet, but the less absorbed [5, 6]. The haem form accounts for 10–20% of the iron in a standard diet and is the more absorbed of the two forms [5, 6]. The sources of the two forms of iron are diverse. The haem form is derived from hemoglobin and myoglobin of animal food sources such as meat, fish, and poultry; whereas, the non-haem form is derived from plants such as fruits, vegetables, dried beans, nuts, and grain products and iron-fortified foods [4, 7].

The absorption of iron is influenced by certain dietary factors including the type of food consumed and the phytochemical constituent. Inadequate iron absorption leads to iron deficiency anemia, and excessive iron absorption leads to iron overload, a condition often caused by transfusion therapies used to treat thalassemia and other anemias [8, 9]. Iron levels are controlled mainly by regulating iron absorption in the duodenum. This mechanism allows for an increase or decrease in iron levels depending on the physiological demand.

2. Haem and non-haem iron

Dietary iron exists as haem and non-haem iron. Haem iron can be obtained mainly from hemoglobin and myoglobin of meat. Haem iron is well absorbed, and it is not influenced by other dietary constituents [10]. It can be sourced from meat, poultry, liver, spleen, and boiled egg yolk (Figure 1) [11]. Non-haem iron contains largely ferrous iron and can be obtained from both animal and plant sources. Unlike heam iron, the absorption of non-haem iron is low and is influenced by other constituents of the diet [10]. Sources of non-haem iron include whole grains, cereals, pulses, green leafy vegetables like pumpkin leaves, radish leaves, mustard leaves, spinach, curry, mint, parsley, coriander, and drumstick (Figure 1) [11].

Figure 1.

Dietary sources of iron (A) sources of haem iron (B) sources of non-haem iron.

3. Dietary iron absorption

Iron absorption takes place in the duodenum and proximal jejunum, and it is essential for maintaining iron balance in the body as the body does not have an active excretory system [12]. The absorption of iron is dependent on many factors, one of which is the physical state of iron. Iron exists in the ferric state (Fe³⁺) at physiological pH; however, for it to be absorbed, it must be in the ferrous state (Fe²⁺) because ferrous ion does not undergo polymerization and their solubility is greater than ferric ions [13]. Also, the quantity of iron stores has an impact on its absorption. It has been gathered from previous studies that iron absorption has an inverse relationship to the quantity of iron stored in the body [14, 15].

3.1 Factors influencing dietary iron absorption

3.1.1 Factors influencing haem iron absorption

• Amount of dietary haem iron,

• Iron status of the subject: Haem iron from animal sources is well absorbed in humans and contributes to more than 10% of the total absorbed iron [16]. The absorption varies from 15 to 25% in normal subjects and 25 to 35% in iron-deficient subjects [11].

• Content of calcium in meal (e.g., milk and cheese)

• Food preparation (time and temperature): Studies have shown that baking and prolonged frying reduces absorption of haem iron by about 40% [11, 15]

3.1.2 Factors influencing non-haem iron Absorption

• Amount of potentially available non-haem iron

• Iron status of the subject: The absorption of non-heam iron varies between 2 and 20%. Absorption of non-haem iron is higher in individuals who are severely iron deficient compared to individuals with normal level. Individuals who are anemic and pregnant have the highest absorption rates (5–13%)

• Other constituents in the diet: The type of food ingested along with plant food determines the rate at which the non-haem iron in the plant is absorbed. If the food contains reducing substances like ascorbic acid, the iron will be kept in the reduced ferrous form and will be better absorbed [11, 15].

3.2 Inhibitors and enhancers of iron absorption

The duodenal pH-dependent process of iron absorption is modulated by certain dietary compounds [4]. These dietary compounds can either be inhibitors or enhancers. Figure 2 shows the main enhancers and inhibitors of iron absorption. One primary enhancer of iron absorption is ascorbic acid. As an enhancer, ascorbic acid in the diet increases the absorption of non-haem iron [17]. Mechanistically, ascorbic acid chelates ferric ions in the stomach at an acidic pH, keeping the ferric ion soluble until it reaches the duodenum of the small intestine. At the duodenum, ascorbic acid donates an electron to a ferric ion, acting as a free radical scavenger and reducing it to a ferrous ion, the absorbable form of iron that crosses the brush border membrane of the enterocytes [18]. It is observed that the effect of ascorbic acid is dose-dependent and can increase the absorption of iron only when both nutrients are consumed together [19, 20, 21]. More still, ascorbic acid can overcome the effects of all dietary inhibitors of iron absorption when it is included in a diet with high non-haem iron [4]. Another enhancer of iron absorption is animal tissues such as those of beef, chicken, fish, pork, and lamb [22]. A study carried out by [23] observed that meat increases iron absorption by inactivating the luminal factors that prevent iron absorption. The authors further asserted that meat forms a luminal transporter that conveys iron from the intestinal lumen to the mucosal cell membrane and finally to the enterocyte cytoplasm.

Figure 2.

Main enhancer and inhibitors of iron absorption. Ascorbic acid is the main enhancer of iron absorption, while phytate, polyphenols, oxalate, and calcium ion act as inhibitors of iron absorption. Proteins can either act as inhibitors or enhancers of iron absorption depending on the source.

In contrast, inhibitors of iron transport prevent the bioavailability of the essential metal. The most prominent inhibitors are phytate and polyphenols found in plant-based foods [24]. Phytate is a naturally occurring component found in plants, and it has an inhibitory effect on the bioavailability of most minerals [25]. Phytate cannot be degraded by humans due to the lack of endophytases, and it is not absorbed in the small intestine. Hence, minerals chelated in phytate are not bioavailable [26, 27]. Polyphenols are abundantly found in human diets, namely vegetables, cereals, spices, tea, coffee, red wine, and cocoa and work by forming a complex with iron [28, 29, 30]. However, the precise mechanism by which polyphenols reduce the bioavailability of iron is still to a large extent unclear [31, 32]. Also, calcium is a known inhibitor of iron absorption. Whereas, calcium inhibits both haem and non-haem iron, phytate, and polyphenols inhibit only non-haem iron [33]. Although the exact mechanism of calcium inhibition is still unknown, however, certain possibilities have been proposed Ref. [33] suggested that calcium inhibition of iron absorption occurs during the efflux of iron into the portal circulation. Conversely, [34] proposed that calcium inhibition of iron absorption occurs during the initial entry of iron into the enterocytes. In either case, it is multiply attested that calcium is an inhibitor of iron absorption. Depending on their source, certain proteins have been reported to be inhibitors and enhancers of iron absorption [32]. While proteins from meat were reported to be enhancers of iron absorption, other proteins such as those from egg, soybean, and casein were reported to be inhibitors of iron absorption [23, 35, 36, 37]. To a large extent, oxalate is an inhibitor of mineral bioavailability [38, 39, 40]. However, its impact on iron absorption is debatable. Studies on rats have shown that the effect of oxalate on ferric ion absorption is insignificant; however, when the iron is in the ferrous form, oxalate has been observed to limit its absorption by forming insoluble ferrous oxalate [41].

4. Mechanism of iron absorption

4.1 Haem iron absorption

As asserted earlier, haem iron is more efficiently absorbed than non- haem iron. Before haem iron is made available for absorption, it must first be released from proteins such as hemoglobin and myoglobin by proteolytic cleavage in the intestinal lumen [42]. Afterward, the haem moiety, with its bound iron, is absorbed intact through the brush border membrane of the intestinal enterocytes [6]. Afterward, the haem moiety, with its bound iron, is absorbed intact through the brush border membrane of the intestinal enterocytes [6]. The fact that iron is not released from haem before absorption makes haem iron more efficiently absorbed than non- haem iron, which readily binds to a variety of substances in the diet reducing its bioavailability [43, 44, 45, 46]. Haem carrier protein (HCP1) mediates the absorption of haem iron and transports it across the apical membrane [47]. Upon arrival at the cytoplasm, iron is released from the protoporphyrin moiety most likely by haem oxygenase 1 (HO-1) [45, 48, 49]. Afterward, the protoporphyrin moiety is degraded, and the iron released joins the pathway observed in non-haem iron efflux into the portal circulation [1].

4.2 Non-haem iron absorption

Figure 3 shows the major pathways of non-haem iron absorption.

Figure 3.

Major pathways of non-heme iron absorption. Ferric ion is converted to ferrous ion by the ferric reductase enzyme, duodenal cytochrome B (Dcytb) on the brush border membrane of the enterocytes. Upon the reduction of Fe (III) to Fe (II), an integral membrane protein of the enterocytes called divalent metal cation transporter 1 (DMT1) transports Fe (II) across the membrane into the enterocyte cytoplasm. If iron is required by the body, it forms a complex with hephaestin and is rapidly transported across the basolateral membrane by the protein ferroportin 1 (FPN), and then, it binds to transferrin for distribution into the body through portal circulation. If iron is not required by the body, it is stored as ferritin in the enterocyte cytoplasm.

4.2.1 Ferric ion reduction

A large percentage of the non-haem iron present in diets is in the oxidized or ferric form, Fe (III), which must first be reduced to the ferrous form, Fe (II) before it can be taken up by the duodenum and proximal jejunum of the intestinal epithelial cells or enterocytes [1, 4]. A principal candidate for facilitating this reduction is the ferric reductase enzyme, duodenal cytochrome B (Dcytb) on the brush border membrane of the enterocytes (Figure 3) [3]. This enzyme is pH-sensitive and requires the low pH of the gastric acid in the proximal duodenum for activation.

4.2.2 Iron uptake across the apical membrane

Subsequent to reduction of Fe (III) to Fe (II), non-haem iron makes its entry into the enterocytes through a transporter known as divalent metal transporter (DMT1) (Figure 3) [50]. DMT1 is an apical brush border membrane transporter, and its expression is increased by the hypoxia- inducible factor-2α during iron deficiency [51]. DMT1 imports iron into the cytosol of absorptive cells after which it is either transferred across the basolateral membrane or bound to ferritin for storage; the body’s demand for iron determines the pathway it follows.

4.2.3 Iron efflux across basolateral membrane

When iron is in low demand, the body stores it as ferritin (a cytosolic iron storage protein). However, when required, ferrous iron from both haem and non-haem iron is transferred from the apical side to the basolateral side [47]. In a study by Das et al. [52], it was reported that a nuclear receptor coactivator NCOA4 is required for the transport of iron within the enterocytes. The NCOA4 is commonly expressed as a “coiled-coil” domain responsible for the promotion of iron release from ferritin. This occurs by aiding ferritin delivery to lysosomes for degradation through a process called ferritinophagy [53]. It was observed that the induction of NCOA4 messenger RNA and protein in the duodenum of mouse occurred as a form of response to iron deficiency induced by diet and hemolytic anemia induced chemically [52].

The proteins ferroportin 1 [(FPN1) (a transmembrane protein with 12 domains that can only transport ferrous iron)] and hephaestin [(HEPH), a multicopper ferroxidase] act to transport iron across the basolateral membrane (Figure 3) [54, 55]. After the exportation of ferrous iron from the enterocytes, hephaestin converts ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) by oxidation. The ferric iron is then bound to transferrin for onward distribution throughout the body through plasma circulation [56]. Each transferrin has the ability to bind two Fe³⁺ to form TF-[Fe³⁺]2 complex which binds to transferrin receptor (TFR1) on the cell surface and is absorbed into cells to form endosome [57]. Subsequently, a six transmembrane prostate epithelial antigen 3 (STEAP3) reduces Fe³⁺ to Fe²⁺ and then transported by DMTI to the cytoplasm to perform physiological functions [50, 58].

4.3 Regulation of iron absorption

Iron homeostasis is a tightly regulated process. The regulation of non- haem iron absorption has been studied extensively, and since haem iron absorption shares many similarities with non-haem iron absorption, the regulatory mechanism employed in the latter can be extrapolated on the former. Most studies have shown that iron deficiency can stimulate the absorption of both haem and non-haem iron [43, 46, 49, 59, 60, 61, 62]. Furthermore, erythropoiesis is a factor that affects the rate of absorption of haem and non-haem iron. However, the influence of erythropoiesis on iron absorption is specie specific.

At the molecular level, the efflux of iron across the basolateral membrane is regulated almost exclusively by hepcidin levels [4, 63, 64]. Hepcidin is a twenty-five (25) amino acid peptide hormone, synthesized in the liver whose level decreases when body iron requirements are high and increases when iron requirements decline [54].

When iron levels are high, that is body iron requirements decline, hepcidin binds the ferroportin/hephaestin complex resulting in its internalization and degradation (Figure 4), effectively shunting cellular iron into ferritin stores, and preventing its absorption into the portal circulation [4]. The binding of ferroportin/hephastin to hepcidin results in the ubiquitination of major lysine residues, endocytosis, and degradation in lysosomes, inhibiting iron entry into the bloodstream [65].

Figure 4.

Regulation of iron absorption (A) increased hepcidin expression by the liver results in the internalization and degradation of ferroportin. Loss of ferroprotin on the cell surface results in low plasma iron level. (B) Normal hepcidin levels respond to iron demand and regulate iron import into the plasma. (C) Iron overload results from reduced levels of hepcidin causing an efflux of iron into the plasma.

Ferroportin undergoes ubiquitination by the oxidation and demethylation of 5-methylcytosines in DNA, carried out by an iron- and 2- oxoglutarate-dependent dioxygenase Tet. This leads to the induction of the ferroportin’s endocytosis and proteolysis. Ubiquitination is carried out by a flow of enzymes E1, E2, and E3, this they do by activating ubiquitin, transferring it, and ultimately ligating it to a lysine side chain in the target protein, respectively. Jiang et al. [66] reported that in the absence of Rnf217 in macrophages, there was a slower hepcidin-dependent ferroportin degradation, and this suggests that some other E3 ligases may also be involved in the regulation of the degradation of ferroportin [66]. In situations when body iron requirements are high and the concentration of hepcidin is decreased, ferroportin/hephastin complex readily signals the translocation of iron into the portal circulation (Figure 4).

Another protein involved in iron homeostasis is hereditary hemochromatosis protein (HFE). The HFE protein is expressed by the hepatocytes, and it competes with transferrin binding to TFR1, hence lowering iron uptake into cells [67]. Alternately, binding of transferrin to TFR1 releases HFE to bind to TFR2 in hepatocytes, thereby increasing hepcidin transcription [68].

Furthermore, iron homeostasis is regulated by iron-responsive element-binding protein (IRPs) which consists of both IRP1 and IRP2. In order to mediate homeostasis, IRP binds to the corresponding IRE on the untranslated region of mRNA encoding the protein essential for cellular iron regulation (TFR1, DMT1), thereby participating in iron uptake (TFR1), storage (ferritin), redistribution, and efflux (FPN1) [69]. When intracellular iron is low, IRP can inhibit the translation of FPN1 and ferritin but increase the synthesis of TFR1. On the contrary, when intracellular iron is surplus, synthesis of FPN1 and ferritin is increased while TFR1 is degraded [70].

Another mechanism contributing to systemic iron regulation is the hypoxia-inducible factor (HIF) prolyl hydroxylases, and they have been discovered to not only be sensitive to hypoxia but also concentrations of iron. These enzymes sense oxygen and iron, thereby regulating HIF, and through this regulation, they control some specific gene transcriptions which are responsible for the absorption, distribution, and utilization of iron [71].

5. Iron supplementation and fortification

The high prevalence of iron-deficiency anemia among pregnant women and young children has become an issue of global concern [72]. This prevalence is highest in resource-poor countries that can rarely afford diets rich in iron. Moreso, the poor diet alongside high levels of infection limits the uptake of iron and results in many pathological conditions [73]. Hence, prevention of iron deficiency and anemia has become a major public health goal. It is therefore a necessity to design ways of delivering additional iron effectively to the affected population. Therefore, WHO recommended that governments of countries with high prevalence implement programs that can help in the reduction of iron deficiency anemia. One of such programs is supplementation for young children and pregnant women [74, 75].

Oral administration of iron supplement is the most frequently used method for iron delivery because it is convenient, not expensive, and not restricted by external conditions [76]. The first generation of iron supplementation is ferrous sulfate (FeSO4) tablets. Although FeSO4 is easy to handle and affordable, it has the capability of reacting with sulfide and polyphenol to cause food decoloration and deterioration. This has a negative impact on the gastrointestinal tract, leading to poor absorption. FeSO4 and other inorganic iron can also trigger nausea, vomiting, and diarrhea in individuals that use them [77, 78]. The absorption of iron from the second-generation iron supplements (ferrous citrate, ferrous gluconate, ferrous fumarate, and ferrous lactate) had an improvement over the first generation. The supplements are organic acids iron salt chelated with a small molecule; therefore, the organic acid and iron interact together, and this helps in the gradual release of iron in the acidic gastric stage avoiding rapid surges [79]. However, despite this improvement, these second-generation iron supplements still caused adverse gastrointestinal reactions [80, 81]. The problem was overcome by developing chelated and encapsulated iron which improved the stability of iron in the intestinal tract. Therefore, iron microcapsules that are nanoparticle-based which demonstrated better bioavailability and safety have now emerged as novel oral iron supplements [79].

Another strategy for improving iron status is iron fortification. This process involves the addition of iron-containing substances to the product recipe, either as isolated compounds (e.g., iron salts or chelates) or as iron-rich ingredients (e.g., meat or its derivatives) [73]. It is believed that if iron is delivered through the natural matrix of food the adverse effects experienced with supplementation will be reduced. Iron fortification therefore involves delivering small doses of iron into a food vehicle at the point of manufacturing [73]. Although this method raises body iron levels slowly compared with iron supplementation, however, it is considered to be safer [82].

Iron fortification is usually done with staple food items like rice, oil, and wheat; condiments such as fish sauce, soy sauce, and lentils; salt and sugar; processed food items like infant complementary food, dairy products, and noodles [83]. However, the addition of iron to different food item has produced technical challenges over the years [84]. One of such is induction of organoleptic changes that renders the taste of the food item unacceptable to consumers. Since iron has the ability to transit between two oxidation states, it creates chemical instability with the food matrix, thereby resulting in changes in the taste [85]. Albeit, this technical obstacle was overcome by using insoluble, poorly soluble, or strongly chelated iron compounds which have limited chemical activity [86].Therefore, the recommended iron salts for fortification by WHO are ferrous sulfate, ferrous fumarate, ferric pyrophosphate, and electric iron powder [87, 88].

Fortification of cereal flour with iron is considered to be cost-effective and sustainable way of improving iron status in deficient populations [89]. Since wheat is widely consumed staple by about one-third of the world’s population [90], it will be a suitable vehicle for iron fortification. Moreso, WHO and other relevant stakeholders have endorsed NaFeEDTA as the only iron salt suitable for use in fortification of high-extraction flours [91]. This is because NaFeEDTA binds strongly to ferric iron at gastric juice pH in the stomach and exchanges it for other metals in the duodenum as the pH increases, thereby protecting iron from the phytic acid present in food consumed [73]. NaFeEDTA has also been used effectively as fortificant in food vehicles like curry powder, sugar, fish sauce, and maize flour [72, 88, 92, 93].

Technology has also been developed for the fortification of salt with iron since it is also widely consumed. The National Institute of Nutrition has developed standards whereby 1 g of salt is fortified with 1 mg of iron and 15 μg of iodine. This provides about 30–60% of RDA of 17 mg of an adult man consuming 5–10 g of salt per day [93]. This platform of fortification of salt with iron is preferred to fortification of cereals because a relatively smaller volume of the foodstuff is required. In addition, iodization of salt is nearly universal, and therefore, it will be easy to suggest a double-fortified salt (iron-fortified iodized salt) to the populace [11].

6. Conclusion

Iron is an element that is critical for human life, and due to its ability to exist in different oxidation state, it partakes in oxygen transport, DNA synthesis, energy production, and many more physiological functions. The regulation of iron levels in human is unique compared to other essential dietary nutrients because the body lacks a regulated excretory process for getting rid of excess iron. Therefore, an efficient regulatory mechanism has been developed to maintain body iron content at the level of absorption in the duodenum.

In view of the efficient absorption of haem iron, a precise mechanism of iron absorption is necessary to adequately manage diseases that bother on defects in the iron signaling pathway. Also, given the role of dietary factors in determining iron bioavailability and absorption, it is essential for individuals experiencing iron deficiency and anemia to pay keen attention to their dietary intake to forestall undesirable health implications. In order to achieve this, iron supplements and iron-fortified products can be included in the diet.

Conflict of interest

The authors declare no conflict of interest.