*3.1. Iron Homeostasis*

The pathophysiology of IDA is closely related to the iron balance in the body. Iron is a vital trace mineral that plays a crucial role in many body functions, including oxygen transport, immune regulation, and enzyme catalyse reactions [106]. An overview of the iron cycle is depicted in Figure 2.

The total iron content in the body for a 70 kg adult male is about 3500 mg to 4000 mg, corresponding to a per kg bodyweight content of approximately 50 mg to 60 mg [106]. Around 65% of iron is contained within the Hb of RBCs to carry oxygen around the body [107]. RBCs have a typical life span of about 120 days. Old and worn RBCs are phagocytosed by the reticuloendothelial system, particularly the macrophages, in the spleen, liver, and bone marrow to release iron. In ferrous ion (Fe2+) form, about 3 mg of free iron is

carried in the bloodstream by transferrin to the bone marrow to be recycled into making new RBCs. Approximately 300 mg of iron is kept in bone marrow for erythropoiesis [108,109]. Excess iron of up to 1000 mg is stored in the liver hepatocytes in complex hemosiderin. As ferritin, about 300 mg of iron is also contained in cells and tissues of organs and muscles. Together, transferrin, ferritin, and hemosiderin store approximately 20 to 30% of the iron in the body as spare [108,109].

The body loses about 1 to 2 mg of iron each day through blood loss or peeling of epithelial cells. Such loss is unregulated as humans and mammals have no iron excretion mechanism and must be replenished via controlled uptake from diet to maintain iron balance [109]. Diet is the only source of iron supply for the body after birth, excluding exogenous therapeutic means. There are two types of dietary iron, animal-derived haem iron and non-haem iron, the dominant form of iron in plants and abundant in both animalbased and plant-based foods [110]. The organic haem iron, which accounts for about 10% of the dietary iron, is more readily absorbable by the body [108], but its uptake mechanisms remain unclear, with receptor-mediated endocytosis and direct haem transporters of intestinal enterocytes being the two most prevailing hypotheses [108,110–112].

**Figure 2.** An overview of the iron cycle in humans depicting iron absorption, transportation, functioning, recycling, storage, and regulation.

The inorganic non-haem iron, which makes up 90% of the total iron in food, is absorbed in the duodenum and upper jejunum section of the small intestines through a complex process [108]. Non-haem iron can be in Fe2+or ferric (Fe3+) forms. First, Fe3+ must be reduced to Fe2+ ion by duodenal cytochrome B (DcytB), a trans-plasma membrane ferric reductase enzyme. Fe2+ can then traverse into the cytoplasm of the duodenal enterocytes by the divalent metal transporter 1 (DMT-1), the hydrogen-driven metal transporter residing at the apical membrane. An acidic micro-environment and the presence of ascorbate can facilitate the reduction of Fe3+ to Fe2+ and iron uptake across the apical membrane [106]. Iron in the enterocytes can be sequestered within the iron-buffering protein ferritin for later use. When iron is required by the body, it can be exported across the enterocyte basolateral membrane via ferroportin (FPN), an iron-regulated transporter protein. Iron stored as ferritin will be lost if not released by FPN as the enterocyte is sloughed after several days [110].

The body maintains its iron content in a tight balance; even though iron is physiologically essential, too much iron is toxic and detrimental to health. Systemic iron homeostasis is controlled through hepcidin, a liver peptide serving as the principal regulator [112]. Hepcidin induces FPN degradation to prevent export from iron-absorptive enterocytes and iron-recycling macrophages. High iron loading and inflammation will transactivate the hepcidin antimicrobial peptide (HAMP) gene in hepatocytes to encode hepcidin to reduce iron entry into circulation. In contrast, low iron status and increased erythropoietic demand will inhibit hepcidin transcription and promote iron export via FPN [109,112].
