Roughly 5 grams of iron are present in the human body and is the most abundant trace metal. It is absorbed in the intestine as heme or non-heme iron depending on the food source. Heme iron is derived from the digestion of hemoproteins in meat. Non-heme iron is mainly derived from plants and exist as iron(II) or iron(III) ions.
Iron is essential for more than 500 hemeproteins, the likes of which include hemoglobin and myoglobin, and account for 80% of iron usage. The other 20% is present in ferritin, hemosiderin, iron-sulfur (Fe/S) proteins, such as ferrochelatase, and more.