Our bodies can only use iron in two forms: Heme-Iron and Non-Heme Iron. A free iron ion will need a carrier to survive. Natural hemoglobin protein contains so-called heme iron. Non-heme in the regular iron supplements has the iron molecule chelated or bound to for instance salts, starch, citrates or other chemical compounds. Iron in vegetables has a very low uptake, only 2-4 % compared to heme iron: 20 - 40 % uptake.
Iron deficiency is common. Low iron levels can cause a variety of vague symptoms such as fatigue, weakness, difficulty concentrating and moodiness. At most risk are growing children and those who are pregnant, but there are cases in all groups.
Regular iron tablets often give side-effects. This is because they leave reactive and toxic free iron ions in the gut. Artificial/synthetic non-heme iron must disintegrate in the gut before any uptake can happen. Since uptake for non-heme is low, reactive free irons remain in the gut.
Heme iron tablets typically have a tolerance close to placebo level. Compared to this, non-heme oral iron has a therapy failure rate due to unpleasant and potentially therapy-ruining side effects of around 30 %