Iron metabolism, nutrition and absorption

Iron is an essential micronutrient that is involved in numerous metabolic functions including redox-stress resistance, respiration and DNA synthesis^{(Reference Lieu, Heiskala and Peterson13)}. It is best known as a key constituent of haemoglobin (Hb), which is involved in the delivery of oxygen to tissues throughout the body. Iron functions as a co-factor in many metabolic pathways and is required by almost all organisms^{(Reference Lieu, Heiskala and Peterson13)}. There are two main routes through which iron enters the body naturally: via the maternal bloodstream through the placenta, which is relevant during gestation; and from the diet by intestinal absorption, which occurs throughout life after birth. During the first ∼6 months of life, dietary iron intake in breastfed infants is relatively low because of the low amount of iron (0.4 mg/l) in breast milk^{(Reference Cai, Harding and Friel14)} However, newborn infants generally possess significant iron stores (250–300 mg), accumulated from the mother during the last trimester of pregnancy, which largely meet iron requirements for the first 6 months of life^{(Reference Fuqua, Vulpe and Anderson15)}. Thus, in infants born at full term to mothers who were iron-sufficient during pregnancy, there is little requirement for dietary iron in the first 6 months^{(Reference Fuqua, Vulpe and Anderson15)}.

Dietary iron presents in two major forms, haem (‘organic’) and non-haem (‘inorganic’) iron. Dietary haem iron is largely derived from haemoglobin and myoglobin in meat; hence, the primary dietary source of haem iron is from the consumption of animal-based food products, including meat and fish, whilst non-haem iron is mainly obtained from plant-based products such as cereals, vegetables, fruits, legumes and pulses. The dietary absorption of haem iron is relatively high (between 15% and 35%); however, it generally accounts for a relatively small proportion (5–10%) of total dietary iron^{(Reference Monsen, Hallberg and Layrisse16)}. On the other hand, the absorption rate (1–10%) of non-haem iron is far lower, although it represents the main source of dietary iron in both the developed and developing world^{(Reference Sharp17)}. Non-haem iron is absorbed from the diet in the reduced (ferrous) state, mainly in the duodenum and ileum^{(Reference Tondeur, Schauer and Christofides18,Reference Zimmermann and Hurrell19)} . The details of haem absorption remain unclear^{(Reference Geng, Duan and Wang20)}. One major reason for the relatively high absorption of haem iron is that its uptake is largely unaffected by other dietary components, unlike non-haem iron which is strongly influenced by dietary composition. For instance, ascorbic acid and gastric acid improve the absorption of non-haem iron by reduction and solubilisation^{(Reference Gulec, Anderson and Collins21)}, whereas phytate, polyphenols (e.g. tannins) and oxalate, all derived from plants, act as iron-chelating agents that reduce iron bioavailability and, thus, restrict iron absorption. An unavoidable consequence of the poor absorption of dietary iron is that it mostly remains unabsorbed and so passes into the lower gut where it is potentially available to the gut microbiota.

Prevalence of iron deficiency

According to the World Health Organization (WHO)⁽²²⁾, IDA affects approximately 2 billion people worldwide, representing almost 30% of the world’s population. The WHO⁽²³⁾ also estimates that about 62.3% of preschool children in Africa are anaemic, of which ∼50% is attributable to ID^{(Reference Kassebaum, Jasrasaria and Naghavi24)}. ID is not only a problem of the developing world as it is also the single most prevalent nutrient deficiency in developed countries^{(Reference Fuqua, Vulpe and Anderson15)}. ID affects the development and function of key organs, including the brain^{(Reference Prado and Dewey25–Reference Beard27)}. Children and women are those most susceptible to IDA and its deleterious consequences, especially in developing countries where the situation is further worsened by the high burden of disease and infection in resource-poor areas and communities. In infants and children, IDA impairs cognitive development and function^{(Reference Lozoff28)}. This has serious consequences for health and is considered to be a major inhibitor of economic development and productivity by reducing the work capacity of the population^{(Reference Alderman, Hoddinott and Kinsey29,Reference Horton and Ross30)} .

Prevention and treatment of iron deficiency

There are a number of intervention strategies for the prevention and/or treatment of ID in children. These include the consumption of iron-rich foods, fortification of food with iron, iron supplementation and general health-supporting measures such as deworming programmes. Deworming prevents hookworm infection in children; such infections have been linked to blood loss which drives anaemia^{(Reference Bauleni, Tiruneh and Mwenyenkulu31)}. For iron-deficient infants and children in populations where the prevalence of ID is high, iron supplementation (as recommended by the WHO) has been the most effective intervention strategy to prevent and/or treat ID and, hence, reduce anaemia^{(Reference Pasricha, Hayes and Kalumba32,Reference Brunken, Muniz and da Silva33)} as well as ID^{(Reference Berglund, Westrup and Domellöf34)}. Home-based food fortification with ‘multiple micronutrient powders’ (MNP) is another effective strategy for the prevention/treatment of ID in children below 5 years^{(Reference Salam, MacPhail and Das35,Reference Teshome, Andang’o and Osoti36)} . MNP can reduce anaemia and ID, providing a similar impact to that of iron supplementation, but MNP also carry the added advantage of provision of other critical micronutrients (such as zinc and vitamins) that are beneficial to child growth and development^{(Reference Zlotkin, Schauer and Christofides37)}. For instance, in a randomised controlled trial in Ghanaian infants between the ages of 6 and 12 months, the home-based use of MNP resulted in a significantly three-fold lower level of ID in the intervention group^{(Reference Adu-Afarwuah, Lartey and Brown38)}. A recent Cochrane systematic review has highlighted the benefits of home-based fortification of food with MNP containing iron as an effective strategy for the prevention and treatment of anaemia and ID in infants below 2 years of age, indicating that the resulting improvement in anaemia and ID are comparable to those achieved using iron supplements^{(Reference Suchdev, Jefferds and Ota39)}.

However, the application of iron supplementation and MNP is not without risk. A controversy arose from the publication of a randomised controlled trial of iron supplementation in 24 076 preschool children in Zanzibar where the intervention group (who received a daily iron dose of 12.5 mg) were 11% more likely to be hospitalised and 12% more likely to die from severe illness (including sepsis, measles, pneumonia, meningitis and pertussis) compared with those who did not receive the iron treatment; this led to the termination of the trial^{(Reference Sazawal, Black and Ramsan40)}. In this study, children were given either: iron (12.5 mg), folic acid (50 µg) and zinc (10 mg); iron and folic acid; zinc alone; or placebo. It was the groups that received the iron and folic acid (with or without zinc) that were withdrawn from the study because of high morbidity and increased mortality rates^{(Reference Sazawal, Black and Ramsan40)}. The increased morbidity and mortality risk in the children receiving iron supplements could be attributed to the likelihood that most of the iron supplement received would remain unabsorbed and thus could favour the proliferation of pathogenic gut bacteria leading to increased risk of illness. Additionally, iron is an important micronutrient for the growth and proliferation of the malaria parasite^{(Reference Neuberger, Okebe and Yahav41,Reference Clark, Goheen and Cerami42)} , implying that iron supplementation in malaria-endemic areas would most likely increase the prevalence and disease severity within such populations^{(Reference Sazawal, Black and Ramsan40)}. Another randomised controlled trial in Africa, involving 1956 infants and young children from Ghana, reported increased diarrhoea-related hospital admissions following intake of 12.5 mg of elemental iron per day over 5 months^{(Reference Zlotkin, Newton and Aimone43)}, although there was a reduction in the incidence of anaemia in the iron group. Though not investigated, this study implies that iron supplementation in this population might have caused disruption to the gut microbiota, leading to diarrhoea. These findings are similar to the earlier larger population group randomised controlled trial in Zanzibar, as discussed above^{(Reference Sazawal, Black and Ramsan40)}. Thus, even though iron supplementation may be an effective strategy to combat ID, it may cause adverse effects in populations at increased risk of infection.

Importantly, subsequent studies indicate that iron supplementation does not result in an increased risk of infection or death as long as there is regular infection control and surveillance^{(Reference Neuberger, Okebe and Yahav41,Reference Clark, Goheen and Cerami42,Reference Okebe, Yahav and Shbita44,Reference Ojukwu, Okebe and Yahav45)} . Indeed, iron supplementation has been shown to improve immune defence in children by increasing CD4⁺ cell counts and CD4:CD8 ratios, which is a crucial aspect of cell-mediated immunity^{(Reference Das, Saha and Mukhopadhyay46,Reference Frost, Tan and Abbas47)} .

Impact of iron on the gut microbiota

Bacterial colonisation of the gut depends on the ability of bacteria to acquire iron and other essential nutrients. It has been shown that almost all gut bacteria, except lactobacilli, are iron requiring, which indicates that iron is a growth-limiting nutrient for the gut microbiota^{(Reference Pandey, Bringel and Meyer48)}. Thus, the degree of iron availability is expected to influence gut microbiota composition. Indeed, evidence from both animal and human trials has shown that iron supplementation results in alterations in gut microbiota composition^{(Reference Zimmermann, Chassard and Rohner49–Reference Jaeggi, Kortman and Moretti51)}.

In rats, iron depletion significantly decreased Bacteroides and Roseburium spp., whilst Lactobacillus, Leuconostoc, Pediococcus spp. and Enterobacteriaceae were significantly increased^{(Reference Dostal, Chassard and Hilty50,Reference Werner, Wagner and Martínez52)} . Similarly, in a randomised controlled trial with 147 school children (aged 6–14 years) in Africa (Cote d’Ivoire), the intake of 20 mg doses of electrolytic iron per day for 6 months in iron-fortified biscuits led to a negative alteration in gut microbiota composition where the number of Enterobacteriaceae were increased while lactobacilli decreased. The change in gut microbiota composition was also associated with raised faecal calprotectin, a marker of gut inflammation^{(Reference Zimmermann, Chassard and Rohner49)}. Furthermore, there was no improvement in anaemia or iron status compared with the control (non-iron-supplemented group). This is an important study because ID is prevalent in children under 5 years in developing countries and, consequently, such children often receive iron supplements. However, the results indicate that iron supplements can have a negative impact on gut health, whilst not improving iron status, which raises concerns regarding the efficacy of iron fortification programmes for children in developing nations.

As indicated, iron supplementation has also been linked to increased diarrhoea incidence in malnourished children^{(Reference Zimmermann, Chassard and Rohner49)}. In a recent study, the effect of high and low iron fortification doses on the gut microbiota was studied in 6-month-old Kenyan children^{(Reference Jaeggi, Kortman and Moretti51)}. In this double-blinded randomised controlled trial, 115 children were fed MNP for 4 months with either: no additions; 2.5 mg iron per day, as an ethylenediaminetetraacetic acid (EDTA) complex; or 12.5 mg iron per day, as ferrous fumarate. The results showed a reduction in Bifidobacteriaceae (the predominating bacterial group) after iron supplementation, which shifted the gut microbiome from domination by beneficial bacteria, including Bifidobacterium and Lactobacillus, to one containing potentially harmful pathogenic bacteria such as Enterobacteriaceae. This effect was more significant with the higher ferrous iron dose. The treatment also resulted in increased intestinal inflammation (as measured by faecal calprotectin) and no decrease in anaemia or ID for children who were iron deficient prior to the intervention. These findings are thus very similar to those described above^{(Reference Zimmermann, Chassard and Rohner49)}.

In contrast, Dostal et al.^{(Reference Dostal, Lacroix and Pham53)} reported potentially positive effects of iron (ferrous sulphate) supplementation on the gut microbiota, characterised by increases in Bacteroides spp., and butyrate producers such as Faecalibacterium prausnitzii and Coprococcus spp. These effects were observed in a rat model system carrying faecal microbiota from a healthy 6-year-old child. In addition, there were significant increases in caecal propionate, butyrate and acetate concentration in the iron supplemented rats compared with control (iron-deficient group), suggesting that iron promotes a beneficial microbial community and positive metabolite production in a healthy rat model. In addition, a 38-week randomised placebo-controlled iron supplementation trial on ID South African children (6–11 years old) found no effect of iron supplementation on the relative abundance of the gut microbiota population^{(Reference Dostal, Baumgartner and Riesen53a)}. It was thus concluded that iron has no clear effect on the gut microbiota and inflammation unless there are associated risk factors such as pathogen infection caused by poor sanitation and hygiene conditions, inadequate access to good drinking water and poor health systems^{(Reference Dostal, Baumgartner and Riesen53a)}. Similarly, a recent clinical dose–response study in Nigerian toddlers showed that a multi-nutrient-fortified dairy drink containing iron in three different daily doses (2.24, 4.48 and 6.72 mg) had no adverse effect on the gut microbiota but was effective in treating anaemia^{(Reference Owolabi, Senbanjo and Oshikoya54)}. The findings of the above two studies are in contrast to those described earlier in this section^{(Reference Zimmermann, Chassard and Rohner49–Reference Jaeggi, Kortman and Moretti51)}. This likely reflects the differences in the approaches employed, including the use of healthy children and populations with low risk of infections as compared with populations at high risk of ID and infections. This also highlights the need for further research to fully understand the factors that influence the impact of dietary-iron intervention strategies on gut health in children.

The nature of prebiotics

Prebiotics are defined as ‘substrates that are selectively utilized by host microorganisms conferring health benefits to the host’^{(Reference Gibson, Hutkins and Sanders55)}. Prebiotics have been extensively studied for their beneficial impact on gut microbiota and health. According to Gibson et al., prebiotics must possess the following characteristics to be classified as such: be a selective substrate for specific bacteria; exhibit resistance to low colonic pH, hydrolysis by intestinal enzymes and gastrointestinal absorption; and result in positive effects on host health and wellbeing^{(Reference Gibson, Hutkins and Sanders55)}. Common, well-studied prebiotics include galacto-oligosaccharides (GOS), inulin and fructo-oligosaccharides (FOS)^{(Reference Roberfroid, Gibson and Hoyles56)}. Inulin and FOS are both fructans consisting of a linear chain of fructose with β(2→1) linkages. The degree of polymerisation of inulin is up to 60, whilst that of FOS is below 10. GOS is classified into two groups: GOS that is derived from lactose through enzymatic trans-glycosylation with galactose in β(1→6), β(1→3) and β(1→4) linkages; and GOS with excess galactose at C₃, C₄ or C₆ ^{(Reference Davani-Davari, Negahdaripour and Karimzadeh57,Reference Gibson, Scott and Rastall58)} . The effect of prebiotics is mediated through their conversion into short-chain fatty acids (SCFA) during fermentation by specific members of the colonic gut microbiota. SCFA serve as a source of energy to gut epithelia cells, lowering the pH of the intestinal lumen thus restricting growth of some pathogens and providing an anti-inflammatory effect to the host^{(Reference Miqdady, Al Mistarihi and Azaz59)}. The enhancement of Bifidobacterium and Lactobacillus spp. in the gut is a common outcome of prebiotic application with subsequent positive effects on host health^{(Reference Adamberg, Sumeri and Uusna60)}. These bacteria have been observed to limit the growth of pathogens, improve intestinal barrier function and enhance immune function^{(Reference Parnell and Reimer61,Reference Wasilewski, Zielińska and Storr62)} through their production of SCFA.

Health benefits of prebiotics in children

Breast milk is usually the first food taken after birth. A unique characteristic of human milk is the presence of ‘human milk oligosaccharides’ (HMO), which are an important source of prebiotics for the infant^{(Reference Bode, Contractor and Barile63)}. HMO consist of over 200 distinct structures composed of a range of monomer units including galactose, glucose, N-acetylglucosamine, fucose and sialic acid^{(Reference Ninonuevo and Lebrilla64)}. In vitro studies have shown that the utilisation of specific HMO by the infant gut microbiota is species specific with the predominating Bifidobacterium spp. acting as the major HMO fermenters^{(Reference Jost, Lacroix and Braegger65)}. Interestingly, the HMO composition of breast milk is affected by multiple maternal factors including body mass index (BMI), mode of delivery, age, diet, duration of lactation and geographical location^{(Reference McGuire, Meehan and McGuire66–Reference Davis, Lewis and Krishnan68)}. These factors are also expected to influence the composition and functional potential of the infant gut microbiota. Breast milk has been reported to contain high amounts of HMO (5–20 g/l)^{(Reference Uwaezuoke, Eneh and Ndu69)}. Indeed, there is an association between the diversity and homogeneity of HMO composition, and the growth and development of healthy children in their first 6 months of life^{(Reference Alderete, Autran and Brekke70)}. However, studies in Gambia and Malawi have shown an inverse relationship between HMO and undernutrition in infants^{(Reference Davis, Lewis and Krishnan68,Reference Charbonneau, O’Donnell and Blanton71)} .

Prebiotics are now being considered as a promising therapeutic tool for the improvement of health as well as treatment and prevention of a number of diseases in children^{(Reference Miqdady, Al Mistarihi and Azaz59)}. There is increasing evidence suggesting that prebiotics offer a broad spectrum of positive effects on child health, mediated primarily through the gut microbiota^{(Reference Umu, Rudi and Diep72)}, as depicted in Fig. 1. In a randomised controlled double-blind clinical trial that assessed the effect of FOS on constipation in infants, Souza et al. reported significantly higher relative abundance of Bifidobacterium spp. in the faecal microbiota compared to the control^{(Reference Souza, Tahan and Weber73)}. Further, the study reported softer stools in the FOS group compared to the control. The consumption of FOS has been associated with significant increases in the relative abundance of colonic Bifidobacterium spp. according to a recent meta-analysis of the impact of FOS consumption on the gut microbiota^{(Reference Dou, Yu and Luo74)}. As indicated above, the fermentation of these prebiotics by the gut microbiota generates SCFA (in particular, propionate, butyrate and acetate). SCFA exert a range of benefits on child health including provision of about 10% of the total body energy^{(Reference Koh, De Vadder and Kovatcheva-Datchary75,Reference den Besten, van Eunen and Groen76)} . Lactate and succinate are also produced by fermentation of prebiotics. Such fermentation products lower the pH of the gut which helps to prevent the growth of pathogens^{(Reference Ansell, Parkar and Paturi77)}. Prebiotics also enhance the absorption of micronutrients, especially iron^{(Reference Rivière, Selak and Lantin78–Reference de Vrese and Schrezenmeir80)}. Prebiotics have been proposed to increase iron absorption by the production of SCFA during colonic microbiota fermentation which results in a lower colonic pH. This, in turn, would be expected to raise ferric iron solubility and availability, which would increase the potential for reduction of Fe³⁺ to Fe²⁺, thus supporting iron absorption^{(Reference Ahmad, Ahmed and Iqbal81)}. However, Husmann et al. did not observe any significant effect of GOS on iron absorption in women, although, in in vitro, GOS supported a two-fold increase in iron solubility^{(Reference Husmann, Stierli and Bräm82)}.

Fig. 1. Effect of prebiotics and iron supplementation on the gut microbiota and health outcomes in children. ↑ and ↓ indicate increase or decrease, respectively, in microbiota factors and health outcomes upon iron (red font), prebiotic (brown font) or iron plus prebiotic (blue font) supplementation.

There is strong evidence that prebiotics (GOS/FOS) in infant formula support healthy weight gain^{(Reference Mugambi, Musekiwa and Lombard83)}. In addition, intake of prebiotic supplements among obese and overweight 7–12-year-old children resulted in a significant weight loss compared with the placebo control^{(Reference Nicolucci, Hume and Martínez84)}. Thus, prebiotics can support a healthy body weight in children. However, little is understood concerning the effect of prebiotics on the gut microbiota during undernutrition and how this might affect malnutrition treatment in children. For such reasons, the International Scientific Association for Probiotics and Prebiotics (ISAPP) recommends that more research should be conducted on the mechanism by which the gut microbiota and prebiotics/probiotics influence undernutrition in specific age groups, especially children^{(Reference Sheridan, Bindels and Saulnier85)}.

Prebiotics can also confer direct antimicrobial effects by adhering to the binding site of bacteria on the enterocyte surface which blocks the attachment of pathogenic bacteria to the intestinal epithelium^{(Reference Gibson, McCartney and Rastall86,Reference Shoaf, Mulvey and Armstrong87)} . The beneficial effects of prebiotics during infancy have also been reported in a systematic review that showed a reduced incidence of gastrointestinal infections, an improved stool consistency, reduced frequency of vomiting and reduced regurgitation among infants who received formulae containing FOS/GOS, compared with control formulae^{(Reference Braegger, Chmielewska and Decsi88)}. These findings further support the health promoting effects of prebiotics in children.

Effect of prebiotics on gut microbiota in infants and young children receiving iron supplementation

Children in developing countries are at increased risk of ID, and therefore the WHO recommends routine iron supplementation in areas with high prevalence of ID⁽²³⁾. With the potential adverse effects of iron supplementation on the gut microbiota in such populations, numerous studies have looked at the mitigating impact of prebiotics during iron supplementation. A recent study in Kenyan infants suggests that provision of prebiotic (GOS) during iron supplementation not only improved anaemia but also offset the adverse effects caused by iron supplementation^{(Reference Paganini, Uyoga and Kortman89)}. In this study, 155 infants of 6.5–9.5 months were enrolled and randomised into three groups. The first (control) group received MNP containing 30 mg of ascorbic acid and other vitamins, and 10.5 g of maltodextrin; the second (Fe group) received the same but with 5 mg of Fe (2.5 mg as sodium iron ethylenediaminetetraacetate and 2.5 mg as ferrous fumarate); and the third (FeGOS) group received the same as the second group but with GOS (7.5 g) instead of maltodextrin (placebo). After 4 months of the intervention, a significant decrease (about 50%) in anaemia was observed in the Fe-only and FeGOS groups compared with the control. In addition, there was a significant increase of bifidobacteria and lactobacilli in the FeGOS group, but these bacteria were less abundant in the Fe-only group. The results indicate that combining a prebiotic (GOS) with Fe supplementation offsets the negative effects of iron on the gut microbiota whilst improving anaemia outcomes^{(Reference Paganini, Uyoga and Kortman89)}. Supplementing iron with prebiotics may also improve iron absorption. For instance, Paganini et al. found that GOS supplements resulted in a 62% increase in iron absorption in Kenyan infants consuming ferrous fumarate (FeFum) and ferric sodium EDTA^{(Reference Paganini, Uyoga and Cercamondi90)}. However, another study reported that a single GOS dose added to the meals of Kenyan infants did not increase iron absorption^{(Reference Mikulic, Uyoga and Paganini91)}.

The inclusion of prebiotics has now become an important practice in formula milk production due to the growing evidence of the beneficial impact on the gut microbiota in early life and throughout the life cycle. Most formula milk is now fortified with GOS and/or chicory root derived inulin (FOS)⁽⁹²⁾. The beneficial impact of these prebiotics (GOS and FOS) in formula milk has been indicated by several studies. For instance, the growth of bifidobacteria and lactobacilli was stimulated by FOS and GOS when incorporated into infant formula milk^{(Reference Civardi, Garofoli and Longo93)}. Increased abundance of these bacteria (bifidobacteria and lactobacilli) is associated with reduced diarrhoea incidence in children^{(Reference Monira, Shabnam and Alam94–Reference Solano-Aguilar, Fernandez and Ets96)}, culminating in improved nutrient absorption and health. Supplementing formula milk with FOS also resulted in a beneficial impact on the development of the gut microbiota in preterm infants^{(Reference Kapiki, Costalos and Oikonomidou97)}. In this study, healthy preterm infants were randomly fed with infant formula supplemented with FOS or standard infant formula for 14 d after birth. The FOS treatment resulted in significantly higher bifidobacteria and lower Escherichia coli levels in the faecal microbiome^{(Reference Kapiki, Costalos and Oikonomidou97)}. Other studies have shown that the addition of prebiotics to infant formula can increase the abundance of faecal bifidobacteria in infants, such that levels resembling those of breastfed infants are achieved. Such effects are associated with increased production of SCFA along with softer and more consistent stools^{(Reference Vandenplas, Zakharova and Dmitrieva98–Reference Holscher, Faust and Czerkies100)}. Another study showed significant increases in faecal concentrations of lactate, propionate and butyrate in formula-fed infants who received whey protein with prebiotics, compared with breastfed infants^{(Reference Holscher, Faust and Czerkies100)}. Other work showed that inulin-type oligosaccharide supplementation of infant formula milk increases bifidobacteria abundance and improves stool consistency (softer stools) and frequency^{(Reference Lohner, Jakobik and Mihályi101)}. In addition, GOS supplementation in infants resulted in a reduced incidence of gastroenteritis compared to the non-supplemented group indicating GOS lowers the incidence of intestinal infections in infants^{(Reference Bruzzese, Volpicelli and Squeglia102)}. In a double-blind study on formula-fed newborn infants, FOS/GOS prebiotic caused a significant reduction in the levels of faecal clostridia and an increase in bifidobacteria, thus positively impacting the composition of the gut microbiota^{(Reference Costalos, Kapiki and Apostolou103)}. In summary, there is now considerable evidence supporting the favourable effect of prebiotics on the gut microbiota and gut health of infants.

Furthermore, in a randomised placebo-controlled trial on the influence of prebiotics on the gut health of young children, Soldi et al.^{(Reference Soldi, Vasileiadis and Lohner104)} reported that inulin induces a significant increase in bifidobacteria levels and mitigates the gut microbiota-related adverse effects caused by antibiotics in children (aged 3–6 years). Similarly, a double-blind, randomised controlled trial assessing the effect of prebiotics on body fat and intestinal microbiota in Canadian overweight or obese children (7–12 years) also found that prebiotic treatment significantly increases levels of Bifidobacterium spp. and led to a significant reduction in body fat. Similar treatment also resulted in a significant reduction in gut inflammation (as indicated by interleukin 6 levels)^{(Reference Nicolucci, Hume and Martínez84)}. All this evidence suggests that prebiotics can have a beneficial impact on the gut microbiota, gut inflammation and body fat levels of young children, and may also improve iron absorption^{(Reference Miqdady, Al Mistarihi and Azaz59,Reference Paganini, Uyoga and Kortman89)} .

In summary, ID remains a public health concern, with children and women in developing countries being the most vulnerable. Iron supplementation may be useful for treating and preventing ID in children, but there are concerns of the adverse effects this may have on the health of infants and children. There is a growing body of evidence suggesting that prebiotics provide positive effects on the gut microbiota and gut health of infants and children. Such effects include prevention of childhood infections such as diarrhoea, and the promotion of infant health, growth and development. Furthermore, it is now becoming apparent that prebiotics can also support the application of iron supplementation in the treatment of ID in children even though much evidence is still needed to support its use. Thus, prebiotics have much potential in countering the negative gut health impact of iron supplements in nutritionally deprived children with ID, allowing an improvement both in overall health and in the effectiveness of iron supplementation strategies.