Dietary iron is contained in two forms namely non-heme and heme. Though heme contributes to a minor portion, its bioavailability is better as compared to non-heme iron [10] [17] . Commercially, heme iron is available as heme iron polypeptide (HIP) for oral supplementation. In HIP, the heme porphyrin ring is utilized to supply iron to the sites of absorption. HIP represents a promising strategy for supplementation of iron with higher bioavailability and lower gastrointestinal irritability [18] . HIP is produced by hydrolysis of animal source hemoglobin with proteolytic enzymes. Digestion conditions, enzymes used, and drying method alters the properties of heme-iron [19] . The technique used for synthesis of HIP leaves the peptides from the hemoglobin subunits covalently bound to the heme ring. This increases solubility at low pH [10] . Solubility of heme iron governs its binding to heme receptor and transport into the cell which is reflected in achievement of higher bioavailability. Compared to non-heme iron, a 10 times higher bioavailability of heme iron have been reported in normal volunteers [20] .

Non-selective iron channels called divalent metal transporters assist in the transport of iron salts into the body. Besides, this iron channel is not regulated, which means that the iron salt intake needs to be strictly controlled to avoid iron overload. Unlike, iron salts regulation of absorption of heme iron polypeptide does occur through receptors. Thus, continuous supplementation does not lead to iron overload [19] .

DMT1 (divalent metal transporter 1) at the microvillus membrane of the enterocytes allows uptake of non-heme iron. Ferric iron is reduced to ferrous form by duodenal cytochrome b protein (DCytB) present in enterocyte membrane. Absorbed iron joins the labile iron pool inside the enterocyte. From this labile pool, ferrous iron is absorbed into the bloodstream via ferroportin (FPN1) transporter which is again oxidized to ferric form. The ferric iron then binds to apo-transferrin and is transported in blood. Heme iron absorption is postulated to be by two mechanisms (Figure 1). In the first mechanism, heme iron is taken up by heme transporter protein coupled folate transfer/heme carrier protein 1 (PCFT/HCP1) directly into the cytoplasm of enterocyte. The intact heme is then transported across basolateral membrane by cell surface receptor for feline leukemia virus subgroup C (FLVCR) protein and then binds to hemopexin in the circulation. Also, some heme iron may be catabolized by heme oxygenase 1 (HO-1) to release non-heme iron and biliverdin. With second mechanism, heme iron is believed to be internalized by receptor mediated endocytosis which is degraded by HO-2 releasing non-heme iron and biliverdin. The non-heme iron then follows the path described previously [19] [21] .

Compared to the non-heme iron, polymerization of heme iron is minimal in alkaline condition. Humic substances like tannins, phytates and chelators like desferrioxamine do not reduce heme bioavailability. This factor is important in Indian scenario where dietary iron bioavailability is poor due to phytate rich diets, inadequate dietary iron, and folate intake [22] . Presence of protein enhances

absorption of heme iron. [4] This differentiation of iron as heme iron and non-heme iron is important. Seligman et al. [20] administered HIP, ferrous fumarate, and placebo with breakfast to patients. HIP led to increased iron absorption even with meal as compared to ferrous fumarate and placebo (p < 0.03 and p < 0.02 respectively). Due to anticipated lack of gastrointestinal side effects, compliance might be better.

Nam et al. [19] studied absorption efficiency of heme iron polypeptide (HIP) and iron salt preparation in Korean healthy volunteers. The objective was to investigate the absorption efficiency of heme-iron polypeptide and iron salt preparations available. They gave five different iron salts to volunteers in a double blind randomized fashion [Group I: HIPk12 (HIP produced in Korea - 12 mg Iron), Group II: HIj12 (HIP produced in Japan - 12 mg Iron), Group III: Ferrous aminoacetate - 12 mg Iron, Group IV: Ferrous aminoacetate - 100 mg Iron, Group V: Placebo group (12 mg glucose)]. Absorption of different salts was then compared. Intake of HIPk but not iron salt increased serum levels suggesting better absorption of HIP than iron-salts. Also, absorption of iron correlated with serum iron concentration. Significant increase in serum iron level was reported at 3 and 5 hours in patients with low serum iron levels (<80 μg/dL) than those with normal serum iron. But, whether such a benefit persists during the pregnancy was evaluated by Young et al. [23] . Intrinsically labelled ⁵⁸Fe-heme and non-heme ⁵⁷Fe (ferrous sulphate) were administered to pregnant women in third trimester. Maternal and cord blood was collected at delivery. ⁵⁸Fe and ⁵⁷Fe levels were assessed as a function of maternal/neonatal iron status. Maternally absorbed ⁵⁸Fe tracer present in the neonates (2.7 ± 1.3) compared to the ⁵⁷Fe tracer (2.2 ± 1.4) was significantly greater (p = 0.02) (Figure 2). Also, transfer of 58Fe to the fetus was inversely associated with maternal Hb and maternal ferritin levels whereas total body iron significantly correlated with heme iron transfer. Further, net transfer of ⁵⁸Fe was directly associated with neonatal Hb and erythropoietin levels. This suggests that heme-iron administered to the mother may be favorably transported across the placenta to the fetus.

Young et al. [8] studied iron utilization with two stable oral isotopes (⁵⁷Fe and ⁵⁸Fe) in pregnant and non-pregnant women. Women were randomly assigned to receive both an animal-based heme meal (intrinsically labelled ⁵⁸Fe pork) and labelled ferrous sulphate (⁵⁷Fe) fed on alternate days. Blood samples obtained 2-week post dosing were used to assess iron status indicators, serum hepcidin and iron utilization based on RBC incorporation of iron isotopes. Heme-iron utilization was significantly higher relative to utilization of ferrous sulphate among both pregnant (47.7% ± 14.4% vs. 40.4% ± 13.2%, p = 0.04) and non-pregnant women (50.1% ± 14.8% vs. 15.3% ± 9.7%, p < 0.0001).

In another clinical study, Suzuki et al. [24] evaluated the efficacy of BioIron-150 (Heme Iron Polypeptide 3 mg) in pregnancy and in females with bleeding disorder. Twenty-four pregnant women in third trimester (27 to 40 weeks of

gestation) were administered BioIron 150 mg (equivalent to HIP 3 mg), two or three times a day depending on Hb and hematocrit levels. A gradual rise in Hb levels was observed over a 14-week period. Most patients’ Hb reached above 11 gm/dL with twice daily regime. Figure 3 shows the changes in Hb levels from baseline to 14 week. HIP improved and/or maintained Hb as well as hematocrit levels. Authors have suggested that average dose of 6.5 mg elemental iron as HIP or maximum dose of 9 mg elemental iron as HIP is sufficient to improve anemia in pregnant women when administered in the third trimester [24] .

Eskeland et al. [25] conducted randomized, double blind trial to evaluate the effects of low dose iron supplementation (27 mg of elemental iron daily), with and without a heme component, given in the second-half of pregnancy, on iron status in pregnancy and postpartum. Participants were randomized into three groups as follows. First group, the heme-iron (HI) group (n = 31) received three tablets containing 1.2 mg heme-iron plus 8 mg Fe²⁺ as iron fumarate per tablet and one placebo tablet, second group, the non-heme iron (Non-HI) group (n = 30) received one tablet containing 27 mg Fe²⁺ as iron fumarate with 100 mg vitamin C and three placebo tablets and third, the placebo group (n = 29) received four placebo tablets. Postpartum, Hb of significant (p < 0.05) number of women fell below 11 gm/dL in placebo group (52%) than in the supplemented group (25%). In the postpartum period, Hb < 10 gm/dL was not found in any women in iron supplemented group as against in 14% women receiving placebo. Also, empty iron stores in heme-iron supplemented group in postpartum phase were found in only 8% women than those in 14% women in pregnant state whereas there was significant increase in number of women with empty iron stores in postpartum period in non-heme iron group (from 3% to 27%) as well as in placebo group (from 21% to 52%). Ten percent of the HI, 20% of the Non-HI and 45% of the placebo group, failed to maintain an acceptable hematologic status. This consolidates the findings of other studies. Heme iron supplementation not only improves iron status but also helps in maintenance of iron levels in the post-partum period.

Heme iron absorption is regulated by heme receptor and body iron levels as against non-heme iron salts. Nam et al. [19] in their study have observed that there was significant increase in serum iron levels in subjects supplemented with HIP who had low initial serum iron levels (<80 μg/dl). But same was not evident in those who had normal initial iron levels. Seligman et al. [20] has noted similar findings but they had taken serum ferritin levels as a marker of body iron store. They observed that continual supplementation of heme-iron may not lead to iron overload. This is an important finding wherein the possibility of iron overload with higher intake or overuse of conventional iron salt can be avoided with use of heme-iron supplements instead. Findings of Young et al. [8] suggest that heme iron utilization is not significantly influenced by iron stores or serum hepcidin. Iron utilization correlated with serum hepcidin levels but role of hepcidin in heme-iron utilization is not as clearly understood as compared to non-heme iron utilization. A recent evaluation from Abdelazim et al. [26] compared HIP (oral Proferrin^®-ES) and intravenous iron saccharate complex (Ferrosac) in 206 pregnant women with iron deficiency anemia (Hb < 10 gm/dL). At the end of 3-months treatment, Hb increased from 8.5 ± 3.5 to 11.3 ± 1.3 gm/dl in HIP treatment and from 8.7 ± 2.5 to 11.7 ± 0.9 gm/dl in IV iron treatment. Ferritin level also increased in two groups. There were no significant differences in two groups for change in Hb or ferritin levels. GI upset was reported in 1.6% of patients receiving oral HIP. Study reported oral HIP as effective and tolerable treatment. Thus, HIP can be considered as an alternative to intravenous iron saccharate complex for iron deficiency anemia of pregnancy.

Suzuki et al. [24] reported HIP as a safe and effective supplement in pregnant women. None of the subject experienced any unexpected adverse events and there were no drop outs due to the same. Hoppe et al. [17] investigated the efficacy of heme iron in the form of blood-based crisp bread as a diet-based treatment program against non-heme iron supplementation to improve the iron status of women of reproductive age. They reported that diet-based treatment has few side effects and worth to be used to improve iron status of women. Seligman et al. [20] checked for safety of heme-iron supplementation with questionnaire based investigation. No patients from heme-iron group had any side effects. Eskeland et al. [25] also has noted similar findings in terms of compliance. Less than 50% compliance with treatment was 4% in heme-iron group, 12% in non- heme iron group and 5% in placebo group.

Essentially, any adverse drug reaction can directly affect compliance of the patient. Ekstrom et al. [27] observed limited adherence to iron supplementation as a major reason for the lower effectiveness of anemia prevention programs. They found that among women at 21 - 26 week of gestation, adherence in gastric delivery system (GDS) iron supplement group was 61% compared with 42% for the conventional iron supplement group. In both groups, women experiencing side effects had about one third lower adherence. Habib et al. [28] in their study compared the outcome of Hb levels in 308 pregnant women near term with their compliance to iron supplementation during initial pregnancy. Hb levels improved significantly only amongst strictly compliant pregnant women. Anemia was significantly associated with noncompliance with iron supplementation, the adjusted OR was 6.19 (95% CI 2.55 - 15.02, p < 0.0001). Known association of oral non-heme iron therapy with gastro-intestinal intolerance and interaction with food hampering its absorption are limiting factors to implement guidelines of iron supplementation in pregnant women. [29] [30] Good tolerance to heme- iron supplementation would maximize compliance to its routine administration and hence improve significantly the maternal and fetal iron store and Hb levels. Table 1 summarizes current evidence from these studies.

Heme-iron is used as supplement to treat iron deficient states and to improve and maintain iron stores in pregnant women. Higher bioavailability compared to non-heme iron makes it a suitable choice as a supplement for iron deficiency anemia. It is especially useful in patients who have low compliance to the non-heme iron therapy (iron salts like ferrous sulphate) due to gastro-intestinal intolerance. Also, it is useful where parenteral iron is contraindicated or cannot be used for any other reasons. Superior tolerability provides added advantage with heme-iron. Compliance to the iron therapy is the single-most important factor that has been identified as obstacle to effective treatment of iron deficiency anemia. Thus, it can be recommended and used as a first choice of iron supplementation in iron deficiency anemia in pregnancy to improve iron and maintain maternal iron stores, to improve neonatal iron stores, through effective and safe therapy with sustained compliance over long term. The Korean food

*see text for details.

and drugs administration, Korea food additives code identifies heme iron as a natural additive and is available in Korea [31] . Health Canada approved heme- iron in 2007 [32] .