Interactions between iron deficiency and lead poisoning: epidemiology and pathogenesis

Science of the Total Environment 330 (2004) 21–37

Interactions between iron deficiency and lead poisoning: epidemiology and pathogenesis Wilson T. Kwong, Phyllis Friello, Richard D. Semba* Department of Ophthalmology, Johns Hopkins University School of Medicine, 550 North Broadway, Suite 700, Baltimore, MD 21205, USA Accepted 18 March 2004

Abstract Iron deficiency and lead poisoning are common among infants and children in many parts of the world, and often these two problems are associated. Both conditions are known to cause anemia and appear to produce a more severe form of anemia when in combination. Although the nature of their relationship is not completely elucidated, characterization of a common iron–lead transporter and epidemiological studies among children strongly suggest that iron deficiency may increase susceptibility to lead poisoning. Recent human studies suggest that high iron intake and sufficient iron stores may reduce the risk of lead poisoning. Future clinical trials are necessary to assess the effect of iron supplementation in the public health prevention of lead poisoning and the kinetics of lead in the body. 䊚 2004 Elsevier B.V. All rights reserved. Keywords: Iron; Lead; Divalent metal transporter 1; Anemia; Erythropoiesis; Iron supplementation

1. Introduction Iron deficiency is the most common nutritional deficiency in the world, affecting as much as 66– 80% of the world’s population (UNICEFyUNUy WHOyMI Technical Workshop, 1998) and is the leading nutritional cause of anemia (Allen and Casterline-Sabel, 2001). Infants and young children are especially vulnerable to iron deficiency because of their rapid growth and increased physiologic demands for iron. Similarly, lead is a major hazard to children in industrialized countries (UNICEF, 1998; Kaul, 1999). Despite continued *Corresponding author. Tel.: q1-410-955-3572; fax: q1410-955-0629. E-mail address: [email protected] (R.D. Semba).

reductions in blood lead concentrations in the United States, lead poisoning remains prevalent among young children living in urban settings (Adams et al., 1998). Lead poisoning is also a problem among young children in some developing countries where exposure may be high due to continued use of leaded gasoline, hyperurbanization, and industrial pollution. Both iron deficiency anemia and lead poisoning adversely affect child development and human health. Lead poisoning is associated with impaired neurobehavioral development, lower intelligence, reduced birth-weight, and decreased nerve conduction velocity (Landrigan et al., 1976; CDC, 1991). Iron deficiency is associated with lower cognitive test scores in infants, impaired scholastic perform-

0048-9697/04/$ - see front matter 䊚 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2004.03.017

22

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

ance, shortened attention span, reduced muscle function and physical activity, and impaired mental acuity in older children and adults (Fairbanks and Beutler, 1995; CDC, 1998; Lee, 1999). In pregnant women, severe iron deficiency anemia can increase the risk of death during the perinatal period. During the last three decades, cross-sectional studies have linked iron deficiency with lead toxicity and provided evidence that these two conditions are related. In addition to epidemiological associations, shared aspects of their pathogenesis and a common iron–lead transporter make it likely that an interaction or synergism exists between the two disorders. A recent longitudinal study of children has demonstrated an association of iron deficiency with subsequent lead poisoning, suggesting an increased susceptibility to lead poisoning in iron-deficient individuals (Wright et al., 2003). There is growing evidence that high iron intake and iron-replete status may reduce lead absorption in children (Hammad et al., 1996; Kim et al., 2003; Wright et al., 2003; Wolf et al., 2003), thus, the prevention of iron deficiency may represent a potential public health intervention for reducing lead exposure in humans (Mahaffey, 1995; CDC, 2002a). However, currently it is unclear how effective such a nutritional intervention would be against lead absorption. The purpose of this review is to examine the epidemiological association between iron deficiency and lead poisoning, propose biological mechanisms for a synergism in their pathogenesis, and identify major gaps in scientific knowledge. 2. Epidemiology of iron deficiency and elevated blood lead concentrations An estimated four to five billion people worldwide are iron-deficient and two billion people are anemic, mainly from iron deficiency (UNICEFy UNUyWHOyMI Technical Workshop, 1998). High risk groups for iron deficiency and iron deficiency anemia include young children, women of childbearing age, and pregnant women. The main factor that contributes to iron deficiency among young children in urban populations in the US is a low level of bioavailable iron in the diet. Infants born to iron-deficient mothers are also at

higher risk of iron deficiency (De Pee et al., 2002). Data from the most recent National Health and Nutrition Examination (NHANES) surveys indicate that approximately 6% of children ages 1 to 5 years old in the United States are irondeficient and less than 5% are anemic (CDC, 2002b). In developing countries, where an estimated 53% of school-age children suffer from anemia (United Nations, 2000), the prevalence is much higher. Like iron deficiency, lead poisoning is especially prevalent in young children. Over half a million children aged 1 to 5 years old in the US have blood lead concentrations higher than the 10 mgydl (Kaufmann et al., 2000) designated by the Centers for Disease Control and Prevention (CDC) as the cutoff for lead poisoning (CDC, 1991). The highest risk group consists of 1- to 2year-old non-Hispanic black children, in which 21.6% have elevated blood lead concentrations (Brody et al., 1994). Iron deficiency and lead poisoning share many of the same risk factors including low socioeconomic status, residence in the inner city, and minority race. Thus it would be expected for both conditions to be concentrated in similar populations. Several studies have found significant associations between iron deficiency and lead poisoning with the vast majority of the studies centering on children ages 1 to 6 years old. Children are at higher risk for lead exposure because they have more hand-to-mouth activity and absorb more lead than adults (CDC, 1991). The link between iron deficiency and elevated blood lead concentrations has been reported to be strongest in children between the ages of 1 and 2 years, weaker in older children, and not significant in adults (Yip and Dallman, 1984). Although several studies report significant associations (Yip et al., 1981; Yip and Dallman, 1984; Wright et al., 1999, 2003; Bradman et al., 2001; Kim et al., 2003), there are also a number that did not find significant associations between the two conditions (Hershko et al., 1984; Wolf et al., 1994; Hammad et al., 1996; Serwint et al., 1999). Part of the discrepancy may lie in the inconsistent criteria for the designation of iron deficiency among the studies. Various measures of iron deficiency and anemia in these studies included trans-

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

ferrin saturation, ferritin concentrations, hematocrit, hemoglobin concentrations, serum iron, red cell distribution width, and ironytotal iron-binding capacity (FeyTIBC) (Table 1). Some of these measures are more sensitive to changes in the body’s iron stores, while others are not affected until late in the stages of iron deficiency. In the study by Hershko, the correlation between iron deficiency and lead poisoning was measured in older children between the ages of 10 and 18. It is likely that the interaction between iron deficiency and lead poisoning may be strongest in young children. It is notable that the studies with the largest number of subjects for analysis (Yip and Dallman, 1984; Wright et al., 1999, 2003; Kaufmann et al., 2000) all found significant associations between iron deficiency and elevated blood lead levels, indicating that, in some cases, failure to detect association may be attributed to limited sample size and low statistical power. Hammad found an inverse relationship between serum iron and blood lead and acknowledged that the lack of statistical significance may be due to inadequate power to detect the association. Although Serwint did not find a statistically significant correlation between ferritin and blood lead levels, a negative correlation existed between ferritin and low and moderate levels of lead poisoning with the correlation coefficient for moderate lead poisoning much closer to statistical significance. In two analyses by Yip et al. (1981) and Yip and Dallman (1984), iron deficiency was significantly associated with lead poisoning. Their examination of data from NHANES II showed 3.4% of children (ages 1–12 years) with FeyTIBC-10% had lead poisoning, while only 1.1% of children in the same age group with FeyTIBC)30% showed lead poisoning (Yip and Dallman, 1984). The correlation of FeyTIBC and blood lead was strongest in children between 1 and 2 years of age. In a cross-sectional study of urban preschool children, Hammad et al. (1996) did not identify an association between serum iron and blood lead, but found higher dietary intake of iron in the highest quartile to be associated with lower lead concentrations. Serwint’s analysis of NHANES III data for urban children did not detect a difference in iron status between children of low and mod-

23

erate blood lead concentrations (Serwint et al., 1999). The study defined iron depletion as ferritin -10 mgyl, which is a more stringent cut-off than for similar studies that used ferritin as a measure of iron status. In another study of urban children, Wright (1999) found a significant association between low-level lead poisoning and iron deficiency as measured by mean corpuscular volume (MCV) and red cell distribution width (RDW). Both conditions were found in 1.1% of the 3650 children studied, implying that thousands of inner city children across the US are affected by both conditions. Bradman conducted a study of children in lead contaminated environments, dividing his analysis into tertiles of lead exposure. Iron-deficient children had higher concentrations of blood lead for every tertile than their iron-replete controls. The disparity was greatest in children living in the most contaminated settings (Bradman et al., 2001). Until recently, the epidemiological association between iron deficiency and lead poisoning was based on cross-sectional studies from which it was difficult to infer the nature of a possible interaction between the two conditions. However, an increased susceptibility to lead poisoning in iron-deficient individuals has been suggested since iron-deficient rats absorb a greater fraction of ingested lead (Six and Goyer, 1972; Ragan, 1977; Robertson and Worwood, 1978). Human data from radioactivetracer experiments have been less consistent (Watson et al., 1980; Flanagan et al., 1982; Watson et al., 1986). Recently, Wright et al. (2003) conducted a longitudinal analysis of iron deficiency and lead poisoning in 1050 urban children. Children who were iron-deficient had a greatly increased risk for subsequent lead poisoning (Odds Ratio wORx 4.12, 95% Confidence Interval wCIx 1.96 B 8.65) compared to iron-replete children. The existence of such a temporal relationship provides strong evidence of increased lead absorption in iron-deficient children. Most investigations have focused on children ages 6 years old and younger, with much fewer studies examining the relationship in older children and adults. Among older children, Yip and Dallman (1984) have reported a weak correlation in children aged 3–12 years, and Hershko et al.

24

Table 1 Studies showing epidemiological association between iron deficiency and lead toxicity Subjects

N 191

Children, 1–2 year

7724

Observations

Refs.

Transferrin -16%, Hb, ferritin -16 mgyl FeyTIBC -16%

Significant association between iron deficiency and elevated blood lead Significant association between iron deficiency and elevated blood lead No correlation of ferritin and blood lead levels No association between iron deficiency and elevated blood lead No correlation between blood lead and ferritin No correlation of Hb or iron deficiency anemia with blood lead No significant association between serum iron and blood lead No correlation between ferritin and blood lead Significant association between iron deficiency and low level lead poisoning Significant inverse correlation between blood lead and blood Hb Increased likelihood of elevated blood lead concentrations in children with history of anemia Significant association between iron deficiency and higher blood lead Significantly higher blood lead in infants with iron deficiency anemia Higher occurrence of iron deficiency in lead workers Significant association between irondeficient status and subsequent lead poisoning

Yip et al., 1981

Children, 0–7 year

51

Transferrin -14% ferritin -16 mgyl Transferrin -16%

Children, 1–7 year

112

Ferritin -15 mgyl

Children, 1–2 year

184

Children, 0–5 year

299

Hb, iron deficiency anemia Serum iron

Children, 0–3 year Children, 0–4 year

787 3650

Ferritin -10 mgyl MCV, RDW

Children, 10–18 year

Children, 2–15 year

558

88

Hb

Children, 1–5 year

4043

History of anemia

Children, 1–6 year

319

Ferritin -12 mgyl

Infants, 9 months

141

Men

118

Hemoglobin -110 gyl ferritin -10 mgyl Hematocrit -41%

Children, 1–4 year

1050

MCV, RDW

Yip and Dallman, 1984 Hershko et al., 1984 Clark et al., 1988 Markowitz et al., 1990 Wolf et al., 1994 Hammad et al., 1996 Serwint et al., 1999 Wright et al., 1999 Counter et al., 2000 Kaufmann et al., 2000

Bradman et al., 2001 Willows and Gray-Donald, 2002 Kim et al., 2003 Wright et al., 2003

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

Children, 1–6 year

Laboratory measures

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

(1984) reported no relationship in schoolchildren aged 10–18 years. Yip’s and Dallman’s analysis (1984) also revealed no significant relationship among adults. Recently, Kim et al. (2003) have published results on Korean lead workers in which the prevalence of iron deficiency was associated with elevated blood lead concentrations. A significant negative relation was also found between dietary iron and zinc protoporphyrin, a parameter used to measure lead’s hematologic toxic effects. Kim’s observation is consistent with Hammad’s finding that dietary intake of iron in the highest quartile is associated with lower lead concentration. These results, taken with Wright’s observation that pre-existing iron deficiency is associated with subsequent lead poisoning, provide evidence for the possible benefits of dietary iron intervention and iron-replete status as protective measures against lead toxicity. 3. Biological basis for synergism of iron deficiency and lead poisoning Both iron deficiency and lead poisoning are capable of independently producing microcytosis and anemia. In the erythrocyte, both conditions affect so many of the same cellular processes and result in such similar outcomes that their combined effects on hematology are an interesting yet complicated area of study. Several investigators have found evidence for a synergistic relationship, noting that the anemia present in iron deficiency with concomitant lead poisoning is more severe than the anemia found in pure iron deficiency (Watson et al., 1958; Six and Goyer, 1972; Clark et al., 1988). Other than their combined effects on heme synthesis, the biological mechanisms of the interaction between iron deficiency and lead poisoning have not been well studied. Three physiological processes in which the two conditions are thought to interact are iron–lead absorption, red cell metabolism, and erythropoiesis (Fig. 1). 3.1. Gastrointestinal iron and lead absorption via the DMT1 transporter The increased absorption of ingested lead in iron-deficient rats was first demonstrated by Six

25

and Goyer (1972) and has been corroborated by others (Angle et al., 1977; Ragan, 1977; Robertson and Worwood, 1978; Flanagan et al., 1979; Wright et al., 1998). Efforts to demonstrate the same effect in humans have produced mixed results. Watson et al. (1980, 1986) conducted two radioactive tracer experiments, finding in his larger study that 50% of the subjects showing increased iron absorption also showed increased lead absorption. However, in a similar experiment by Flanagan et al. (1982), gastrointestinal absorption of lead was not correlated with iron absorption or iron stores in the body. A common iron–lead transporter had been postulated by many for decades after the aforementioned studies in rats. In 1997, Gunshin reported the discovery of a metal transporter in mammalian cells that exhibited conductance for ferrous iron, lead, and other metal cations. The metal transporter was shown to transport lead and iron in human intestinal cells (Tallkvist et al., 2000; Bannon et al., 2003) and was later named Divalent Metal Transporter 1 (DMT1). Expression of the transporter is high in the duodenum (Gunshin et al., 1997), consistent with the site of iron and lead absorption in humans. Further characterization of the metal transporter revealed biological mechanisms that enabled increased lead absorption during iron deficiency. During periods of low iron stores, expression of DMT1 in the duodenum is greatly increased (Gunshin et al., 1997), allowing not only increased iron absorption but also lead absorption. In one experiment, over-expression of DMT1 in human cell lines resulted in a more than 7-fold increase in lead transport compared to controls (Bannon et al., 2002). It appears that a high level of DMT1 expression is necessary for a substantial increase in lead absorption. Watson et al. (1986) observed that lead absorption is not raised until iron absorption is well above the normal range. Bannon et al. (2003) also reported that at low to normal levels of DMT1 expression, lead absorption is not affected by changes in DMT1 expression. Therefore a plausible system could exist where variations of iron stores within the normal range do not cause a significant increase in lead absorption, but during periods of

26

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

Fig. 1. Proposed mechanisms for iron deficiency and lead toxicity.

iron deficiency, the level of DMT1 becomes high enough to allow for increased lead absorption. Adequate iron intake may serve a dual function in preventing the absorption of lead (Bannon et al., 2002). First, intake of iron lowers the number of lead transporters in the gut since DMT1 regulation in the duodenum is sensitive to levels of iron uptake (Tallkvist et al., 2000; Morgan and Oates, 2002). Second, since DMT1 has a much higher affinity for iron over lead, the presence of iron in the gut can competitively inhibit the uptake of lead. Iron has been shown capable of completely inhibiting lead uptake by DMT1 (Bannon et al., 2002). The mentioned biological mechanisms appear in accord with studies suggesting the pro-

tective effects of iron uptake against lead poisoning. Conversely, it would appear that iron-deficient individuals consuming low iron diets are at greatest risk for elevated blood lead levels. Although iron can competitively inhibit lead uptake, it does not appear that lead can do the same to iron uptake (Iturri and Nunez, 1998). Instead, lead may limit iron absorption through a different mechanism, one similar to cadmium. Cadmium, like lead, is a divalent metal transported by DMT1 with no known physiological benefit in humans. Interestingly, cadmium has been shown to downregulate the expression of DMT1 to 70% of controls when added to human intestinal cells (Tallkvist et al., 2001). Further study is needed to

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

determine whether lead can elicit a similar response. 3.2. Effects of iron deficiency and lead on the red cell The anemia of iron deficiency is microcytic and hypochromic. The anemia of lead poisoning is usually described as normocytic and hypochromic, although microcytosis is commonly observed. Since both conditions produce similar hematological consequences, it is likely that they impact many of the same cellular processes, producing synergistic effects. Indeed, separate studies have found that the degree of microcytosis for a given reduction in hemoglobin appears to be greater when both conditions are present than in iron deficiency alone (Watson et al., 1958; Clark et al., 1988). Yet another study has noted the anemia accompanying the combination of both conditions is more severe and more hypochromic than uncomplicated iron deficiency anemia (Six and Goyer, 1972). Also, the concentrations of ZPP, a measure of heme synthesis inhibition, are dramatically higher in the presence of both conditions than in either iron deficiency or lead poisoning alone. The hematological effects of each condition may be exacerbated by the presence of the other condition. Red cells are central in the pathogenesis of both conditions: in iron deficiency anemia, the hematocrit is lowered and in lead poisoning, over 99% of blood lead is intraerythrocytic. During anemia, lead becomes even more concentrated in red blood cells. According to one study, at a blood lead concentration of 40 mg percent and a hematocrit of 45%, the intraerythrocytic concentration of lead is 85 mg percent; and at a hematocrit of 30%, intraerythrocytic lead increases to 160 mg percent (Angle and McIntire, 1974). It seems reasonable that the increased toxicity to erythrocytes during iron deficiency anemia and lead poisoning is partly due to an increased concentration of lead in erythrocytes. Once inside erythrocytes, lead can have deleterious effects on several essential cellular processes, including inhibition of protein synthesis. One study in reticulocytes found that lead decreased incorporation of 14C labeled amino acids (Borsook et

27

al., 1957). Interestingly, iron was found to have the opposite effect in the same study. Based on these observations, low cellular iron concentrations during iron deficiency may enhance lead’s inhibition on synthetic reactions. Lead has a high affinity for the side chains of at least five amino acids, which might explain why lead has been found to bind to numerous proteins crucial for the uptake, transport, and utilization of iron. These proteins include transferrin (Qian et al., 1997), mucin (Conrad et al., 1991), mobilferrin (Conrad et al., 1990), DMT1 (Gunshin et al., 1997), hemoglobin (Barltrop and Smith, 1972), and enzymes of heme synthesis (Goldberg, 1972). In a state where iron levels are inadequate, lead may disrupt proper iron utilization and exacerbate iron deficiency. Such is the case in the synergistic inhibition of heme synthesis. The combined effect of lead poisoning and iron deficiency on heme synthesis is the best characterized interaction between the two conditions. When heme synthesis is inhibited at the final step, zinc instead of iron is incorporated into protoporphyrin resulting in elevated levels of zinc protoporphyrin (ZPP). Both iron deficiency and lead poisoning are each capable of inhibiting heme synthesis at this final step. In the presence of both conditions, studies find particularly defective heme synthesis. Clark et al. (1988) noted a dramatically elevated ZPP level of over 500 mgydl in iron deficiency with concomitant lead poisoning. The upper limit of elevated ZPP levels in iron deficiency is thought to be 200 mgydl and the level found for lead poisoning was 240 mgydl (Clark et al., 1988). Similar effects were noted in rat studies (Hashmi et al., 1989). These findings provide clear evidence for a synergistic effect between iron deficiency anemia and lead poisoning that inhibits heme synthesis to a greater extent. To explain this observation, it has been noted that ferrochelatase, the enzyme catalyzing the last step in heme synthesis, is especially sensitive to low iron levels in the presence of lead poisoning (Mahaffey and Annest, 1986; Kapoor et al., 1984). Lead, which accumulates at mitochondria, apparently interferes with transport of iron in the mitochondria, the site of ferrochelatase. As a result, what little iron is available cannot efficiently reach ferrochelatase.

28

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

There is a similar synergistic effect on the urinary excretion of d-aminolevulinic acid (ALA), another precursor in the synthesis of heme. When ALA dehydratase is inhibited, ALA builds up and is excreted in the urine. The levels of urinary ALA are usually not affected by iron deficiency but are elevated in lead poisoning due to lead’s inhibition of ALA dehydratese. The simultaneous occurrence of both iron deficiency and elevated lead levels results in an almost two-fold increase in urinary ALA than with the presence of elevated lead levels alone (Six and Goyer, 1972; Hashmi et al., 1989). The ability of lead to hinder heme synthesis at several points and also disrupt globin synthesis (White and Harvey, 1972) may explain the higher degree of hypochromia observed in iron deficiency anemia with co-existent lead poisoning (Six and Goyer, 1972). 3.3. Lead intoxication and sickle cell disease Lead’s inhibition of heme synthesis and disruption of protoporphyrin utilization also appears to have a role in the demyelination of neuronal axons and the development of peripheral neuropathy. Peripheral neuropathy is not unusual in lead intoxicated adults, but is an exceedingly rare manifestation of lead toxicity in children. In 19 documented cases of peripheral neuropathy secondary to lead poisoning in children, nine of the children also possessed sickle cell disease (SCD) (Imbus et al., 1978; Nelson and Chisolm, 1986). This high association is suggestive of an increased susceptibility to lead-induced peripheral neuropathy in children with sickle cell disease, as SCD alone is not known to cause peripheral neuropathy. The mechanism for the increased susceptibility is unknown and little is known about the interaction of lead poisoning and sickle cell disease. Lead poisoning is known to cause demyelination. Lead intoxicated individuals have a high concentration of blood protoporphyrins and excrete an excess of protoporphyrins that are thought to be precursors of a substance essential for myelin maintenance (Goldberg et al., 1954; Lambert and Schochet, 1968; Imbus et al., 1978). Similarly, children with SCD frequently have elevated concentrations of blood protoporphyrins (Tigner-Weekes et al.,

1979) that may play a role in the increased susceptibility to lead-induced neuropathy. Lead has also been shown to prevent the normal localization of porphyrins to the myelin sheaths of axons, resulting in demyelination (Whetsell et al., 1984). Addition of normal heme to the myelinated neurons prevented the demyelinating effects of lead. Although iron is a cofactor for enzymes involved in myelination and a component of heme, it is unknown how the presence of sickle cell disease affects the interplay between iron and lead. 3.4. Ineffective erythropoiesis During iron deficiency anemia, physiological processes attempt to compensate for the decreased oxygen carrying capacity of blood. In response to hypoxia, the kidney secretes erythropoietin (EPO), which in turn, stimulates erythropoiesis in bone marrow through a classic feedback loop. Lead poisoning has the opposite effect, depressing EPO concentrations either through direct inhibition of production or indirectly through its toxic effects on renal cells (Graziano et al., 1991; Romeo et al., 1996; Liebelt et al., 1999; Porcelli et al., 2002). The inhibition of EPO production by lead would be expected to decrease if not prevent the exponential increases of EPO seen in anemia. One study finds evidence for such an effect in male manufacturing workers chronically exposed to lead. Workers with lead-induced anemia failed to produce an increase of EPO concentrations, while non-exposed controls and various anemic patients (causes of anemia were unspecified) with normal renal endocrine function demonstrated the expected exponential rise of EPO (Osterode et al., 1999). Although lead’s inhibitory effects of EPO production were studied in patients with lead anemia and not iron deficiency anemia, it is quite possible that lead’s inhibitory effects disrupt similar compensatory mechanisms in the EPO production of iron deficiency anemia. This possible interaction will need to be investigated in future work. Erythroid progenitor cell counts are reduced in lead poisoning. Lead poisoning may reduce erythroid burst-forming unit (BFU-E) cell counts by increasing apoptosis (Osterode et al., 1999), but the specific mechanisms for this reduction are

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

29

Table 2 Potential consequences of lead toxicity and iron deficiency on childhood development Lead toxicity

Iron deficiency

Lower IQ Poor eye-hand coordination Poor fine motor skills Impaired hearing Decreased stature or growth Reduced gestational age Reduced weight at birth Reading disability Deficits in vocabulary Aggression Abnormal postural balance Longer reaction times Sleep disturbances Poor scholastic performanceyschool failure Increased hyperactivity Impaired attentionyvigilance Poor speech performance Disturbed social behavior Impulsivity Poor organization Lack of persistence Increased daydreaming Antisocialydelinquent behaviors Increased fear Withdrawal Disinterest Anemia

Lower mental test scores Lower cognitive function Decreased social interaction Vasomotor disturbances Behavioral problems Irritability Anorexia Less pleasure and delightyless playful More wary Increased hesitance Decreased curiosity Pica Impaired immune functions Increased breath holding spells Decreased exercise capacity Inability to maintain body temperature Anemia

unclear. Osterode et al. (1999) suggest the possibility that lead’s inhibition on heme synthesis resulted in apoptosis because other reagents inhibiting the process have shown to induce apoptosis in human BFU-E cells. Another possibility involves lead’s depression of EPO production. EPO is a glycoprotein necessary for the maturation of BFU-E into erythroblasts. In the absence of EPO, BFU-E cells undergo apoptosis and are destroyed. The effect of lead on the differentiation of erythroid progenitors in the presence of iron deficiency anemia has not been well studied. Future studies may provide insight on the greater microcytosis observed in the presence of both conditions (Watson et al., 1958; Clark et al., 1988). 4. Effects of iron and lead on neurodevelopment 4.1. Adverse effects on neurodevelopment Both iron deficiency and lead poisoning are associated with a host of developmental abnor-

malities in children (Table 2). The adverse effects of iron deficiency include lower cognitive function, impaired motor abilities, behavioral problems, and decreased social interaction (CDC, 1998). Many of same abnormalities are also seen in children with elevated blood lead concentrations. Given the common occurrence of iron deficiency with lead poisoning in children, it is often difficult to discern which defects are attributable to solely lead poisoning. In a cross-sectional study that compared children from a smelter town to children from a non-lead exposed town, lead poisoning and iron deficiency each had independent effects on mental development (Wasserman et al., 1992). Among children at 18 months of age, a decrease in hemoglobin from 12 to 10 gmydl was associated with a 3.4 point decrease in Mental Development Index (MDI) while an increase in blood lead concentration from 10 to 30 mgydl was associated with a 2.5 point decrease in MDI (Wasserman et

30

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

al., 1992). The association of decreased hemoglobin with lowered MDI was present in children from the smelter town as well as the non-leadexposed town, suggesting that iron deficiency had a negative effect on cognitive development in children that was independent of lead exposure. The same authors later reported in a longitudinal study that the developmental delays of iron deficiency in 2-year-old children were reversed by age four through iron supplementation (Wasserman et al., 1994). Despite this reversal, lead concentrations remained independently associated with decreases in mental test scores at age four and with cumulative losses in cognitive function. Another study suggests that cognitive improvements due to changes in blood lead concentrations are dependent on a child’s iron status (Ruff et al., 1996). In children with adequate iron levels, each 1 mgydl decrease in blood lead resulted in a 1.2 point increase on tests of cognitive development. In iron-deficient children, a similar relationship was not observed. While some of the cognitive deficits of iron deficiency may be reversible, those of lead poisoning do not appear reversible, even with chelation treatment. Although treatment with succimer, an oral lead-chelating agent, reduces blood lead, a randomized clinical trial on children with moderate lead poisoning (20–44 mgydl) reported no greater improvement on cognitive, neuropsychological, or behavioral tests after treatment than in placebo (Rogan et al., 2001). The current cut-off for lead poisoning is 10 mgy dl (CDC, 1991). However, this value may be of little significance in assessing the adverse effects of lead on cognitive functioning. Epidemiologic studies have reported impaired neurobehavioral development at 10 mgydl of lead and some suggest harmful effects below this level (CDC, 1991). Poorer fine motor skills, slower reaction times, and increased reading disabilities can be found in children exposed to low levels of lead without the presentation of any clinical symptoms (Kaul, 1999). The blood lead concentration and the level of exposure to lead are not the only determinants of lead’s effects. Different individuals at the same levels of lead poisoning may vary in their presentation of neuropsychological effects. The manifestation of lead toxicity depends on several factors,

including the timing and frequency of exposure, the child’s age at the time of assessment, and the context of the assessment. The effects of lead poisoning have been shown to persist into adulthood. Early exposure to lead is associated with a seven-fold increase in the rate of high school failure and six-fold increase in reading disability (Needleman et al., 1990). Antisocial behavior and juvenile delinquency have been linked with lead toxicity in retrospective cohort (Needleman et al., 1996) and controlled studies (Dietrich et al., 2001; Needleman et al., 2002). In adults, higher levels of lead and lead exposure have been associated with criminal behavior (Denno, 1990; Pihl and Ervin, 1990; Needleman et al., 2002), violent crime (Nevin, 2000), and homicide (Stretsky and Lynch, 2001). 4.2. Possible cellular mechanisms for neurotoxicity of lead The mechanisms of lead neurotoxicity are no doubt complex and numerous. Lead traverses the blood brain barrier, accumulates in the brain, and preferentially damages the prefrontal cerebral cortex, hippocampus, and cerebellum. Several cellular targets of lead have been identified, but the mechanistic effects are not completely understood (Marchetti, 2003). The observed learning deficits in lead exposed children may partly be attributed to lead’s disruption of N-methyl-D-aspartate (NMDA) glutamate receptors (Marchetti, 2003) and nitric oxide synthase (Chetty et al., 2001). Both proteins are integral to long-term potentiation (LTP) in the developing nervous system. Myelination and neurotransmitter receptors also have been identified as significant targets of lead (Roncagliolo et al., 1998; Marchetti, 2003). Proper function of neurotransmitter systems is vital in regulating emotional responses, memory, and learning (Finkelstein et al., 1998). 4.3. Possible cellular mechanisms for neurotoxicity of iron deficiency Iron deficiency during childhood has multiple consequences for neurochemistry, as expressed in the adverse effects on cognitive development and

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

behavior. Iron is required as a cofactor for proteins involved in myelination and neurotransmitter metabolism. Oligodendrocytes, which produce myelination, possess proteins requiring iron to synthesize fatty acids and cholesterol for myelin production. During iron deficiency, myelination is irregular (Beard, 2003). Neurobehavioral changes in iron-deficient children also have a basis in altered neurotransmitter metabolism. Iron-deficient children excrete more urinary norepinephrine than do iron-replete children (Fairbanks and Beutler, 1995). Monoamine oxidase is involved in the synthesis and catabolism of important neurotransmitters such as norepinephrine, dopamine, and serotonin. In iron deficiency, the activity of monoamine oxidase is decreased (Prpic-Majic et al., 2000). Inappropriate catecholamine metabolism can be associated with decreased attentiveness, restricted perception, and impaired reaction times (Prpic-Majic et al., 2000). Dopamine receptors are altered in iron deficiency, further disrupting normal transmission (Beard, 2003). 5. Iron deficiency and iron supplementation alter the kinetics of lead excretion The presence of iron deficiency and treatment with iron supplements have a significant effect on the kinetics of lead in the human body. Evidence exists to suggest that iron supplementation slows the rate of lead excretion (Table 3). Ruff studied children with levels of lead between 25 and 55 mgydl while providing iron supplements to those who were iron-deficient. The children receiving iron supplements demonstrated a slower decline in blood lead level over a period of 6 months than iron-replete children not receiving supplements. In a placebo-controlled clinical trial, Angle and McIntire (1974) also found that children given iron supplements maintained higher levels of blood lead over a similar period of time. Studies of lead distribution in rats provide insights on the observed decreases in lead excretion. Iron supplementation of iron-deficient rats caused a redistribution of lead out of the kidneys (Wright et al., 1997) and a decrease in urinary lead excretion. Wright postulated that an accompanying rise in hemoglobin and red blood cell mass increased the lead binding

31

capacity of erythrocytes. The redistribution of lead from the tissues to the interior of red blood cells reduces the urinary excretion of lead and results in a slower decline of blood lead. Although the natural excretion of lead may be inhibited by iron supplementation, sufficient iron status appears to aid lead excretion induced by chelation. Separate studies have found higher iron status to be correlated with higher urinary lead excretion during chelation with CaNa2EDTA (Post et al., 1978; Markowitz et al., 1990). Although the reported increase in lead excretion is small, these findings suggest iron supplementation followed by chelation may improve induced lead excretion in iron-deficient children. However, such a strategy should only be considered in children with blood lead levels between 25 and 44 mgydl, whose blood lead concentrations do not necessarily warrant immediate chelation therapy. More study is needed on the effects of iron supplementation on lead kinetics before such a course of treatment is employed. It has also been suggested that iron supplements should only be given to iron-deficient children with lead poisoning and iron-replete children chronically exposed to lead. Iron supplementation has not been shown to benefit iron-replete children with pre-existing lead poisoning and may reduce lead excretion (Wright, 1999). 6. Prevention of lead poisoning The removal of lead from gasoline and household paint has resulted in dramatic reductions of lead levels in the general US population. According to NHANES data, blood lead levels have dropped by 80% since the 1970s (CDC, 1997). Recommendations from the CDC regarding lead abatement focus on household paint, dust, and soil, which comprise the most common sources of childhood lead poisoning. Contamination of drinking water from lead soldered joints is also of concern since lead is more readily absorbed from water than in food. Given that the neurotoxic effects of low level lead poisoning are irreversible even after treatment with pharmacological therapies, lead abatement must remain the primary approach in the public health management of lead poisoning. In 1991, the Public Health Service

32

Subjects Children, 2–5 year

N 30

Intervention groups a

ID children given placebo; ID children given FeSO4 Subjects given CaNa2EDTA

NyA

110

Children, 1–7 year

112

Lead-intoxicated children given CaNa2EDTA

Children, 2–4 year

332

Children, 1–3 year

42

Children, 6–8 year

602

Lead exposed children given iron supplements; Non-lead exposed children given iron supplements Iron-replete children with lead intoxication; ID children with lead intoxication given iron supplements. Lead exposed children given placebo; Lead exposed children given ferrous fumarate

a

Iron-deficient. Mental Development Index. c General Cognitive Index. b

Findings

Refs.

Greater increase in blood lead concentrations in iron-supplemented group Correlation between serum iron and CaNa2EDTA-induced lead excretion Iron status children with moderate intoxication has a small but significant effect on CaNa2EDTA-induced lead diuresis Consequences of iron deficiency anemia on MDIb are reversible with iron supplementation. Lead is independently, adversely associated with GCIc scores Decline in blood lead concentrations was slower in iron supplemented children over a 6 month period.

Angle and McIntire, 1974

Iron reduced blood lead in children by a small but significant amount

Post et al., 1978 Markowitz et al., 1990

Wasserman et al., 1994

Ruff et al., 1996

Kordas et al., 2004

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

Table 3 Intervention studies of iron deficiency and lead toxicity

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

(PHS) drew up a plan to eliminate childhood lead poisoning on a national level, but the project never was derailed and never reached fruition for a variety of political reasons (Needleman, 1998). Although past efforts for complete lead abatement have been unsuccessful, the eradication of childhood lead poisoning requires the continued pursuit of this course of action. At this time, lead is still commonly found in paint from homes built before 1980 (CDC, 1991). Total removal of lead paint from the home is necessary because lead painted surfaces, even those in good condition, are prone to deteriorate eventually (Rosen and Mushak, 2001). An obvious barrier to the complete removal of lead paint from all homes is the prohibitive cost of such a massive undertaking. However, several economic analyses including one from the PHS demonstrate the monetary benefits of lead abatement far exceed its costs (Binder and Falk, 1991; Grosse et al., 2002; Landrigan et al., 2002; Needleman, 2004). The analyses take into account the costs of hospitalization, special education, in addition to the costs of lead removal. The most recent analysis by Grosse et al. (2002) estimates the benefits to range into the hundreds of billions of dollars. The economics of lead abatement are an incentive rather than an obstacle in attaining the public health benefits of lead abatement. The societal benefits may be even greater as several studies have found epidemiological associations between criminal, antisocial behavior and lead exposure (Stretsky and Lynch, 2001). Additional approaches to prevention have included lead education, lead screening programs, and dietary enhancement of nutrients such as iron. Although recent studies appear to support iron intervention in preventing lead poisoning (Hammad et al., 1996; Wright et al., 2003), randomized clinical trials are necessary to assess any benefits (CDC, 2002a). A recent clinical trial conducted among lead-exposed first-grade children in Mexico showed that iron supplementation reduced blood lead concentrations by a small but significant amount (Kordas et al., 2004), suggesting that iron supplementation is a potential strategy to reduce blood lead concentrations in children.

33

7. Future directions Despite the high comorbidity of iron deficiency and lead poisoning, additional work is needed to characterize the nature of their relationship and their biological interactions. Further randomized clinical trials are necessary to assess the effects of iron supplementation on the public health prevention of lead poisoning and the kinetics of lead in the body. Iron supplementation may prove beneficial to both iron-deficient and iron-replete children constantly exposed to lead by decreasing lead absorption. However, the decrease in lead absorption should be weighed against the drawbacks of iron supplementation such as slowed excretion and increased retention of lead (Wright, 1999). Other than their combined inhibition of heme synthesis, the biological mechanisms of interaction between iron deficiency and lead toxicity remain to be elucidated. Further work is necessary to characterize the anemia found in combined iron deficiency and lead poisoning with focus on aspects particularly aggravated by the presence of both conditions. Finally, lead’s inhibition of increases in EPO during lead-induced anemia raises the possibility that EPO production in the presence of iron deficiency anemia and lead poisoning will also be blocked. Future investigation of this interaction may explain the increased microcytosis observed in the presence of both conditions (Watson et al., 1958; Clark et al., 1988). 8. Conclusions In young children, the prevalence of iron deficiency has been reduced by the iron fortification of certain foods. Reductions in lead poisoning have resulted from the removal of lead from household paint and gasoline. The comorbidity of iron deficiency and lead poisoning remains unusually high in children in some urban areas and in developing countries (Yip et al., 1981; Clark et al., 1988; Wright et al., 1999). Direct evidence for an increased susceptibility to lead poisoning in iron-deficient individuals has been scanty, but a growing number of studies support the interaction between iron deficiency and lead poisoning (Watson et al., 1980, 1986; Bannon et al., 2002; Wright

34

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

et al., 2003; Kordas et al., 2004). Biochemical studies on the DMT1 transporter reveal that the iron–lead transporter exhibits the necessary characteristics to mediate increases in lead absorption during iron deficiency. It is well documented that lead poisoning and iron deficiency adversely affect many of the same cellular processes (Borsook et al., 1957; Mahaffey and Rader, 1980), especially those of the red cell (Griggs, 1964; Vallee and Ulmer, 1972; Hashmi et al., 1989; Lubran, 1980). It has also been observed by many that the anemia of combined lead poisoning and iron deficiency is more microcytic and hypochromic (Watson et al., 1958; Six and Goyer, 1972; Clark et al., 1988). The increases in severity likely result not only from the synergistic cellular effects of the two conditions, but also effects on physiological processes such as erythropoiesis. Because lead affects virtually every system in the body (CDC, 1991) and iron is a required cofactor in numerous cellular processes, the effects of iron deficiency and lead poisoning likely intersect at several pathways yet to be discovered. Although the CDC currently recommends plentiful dietary iron intake as an intervention against lead absorption, the effectiveness of such dietary recommendation has not been well characterized. The prospect of providing iron supplementation to limit lead poisoning is a potential strategy, especially in areas where complete lead abatement is not economically feasible. Iron supplementation may be complicated because iron-supplemented children with pre-existing lead poisoning have been shown to have a slower decline in blood lead (Ruff et al., 1996), and further clinical trials are needed to clarify these issues. Acknowledgments This study was supported by a grant from the National Institutes of Health, National Institute for Drug Abuse (RO1 DA15022). The authors wish to thank their project officer, Dr Jag Khalsa, for his continuing support.

References Adams WG, Geva J, Coffman J, Palfrey S, Bauchner H. Anemia and elevated lead levels in underimmunized innercity children. Pediatrics 1998;101:E6. Allen L, Casterline-Sabel J. Prevalence and causes of nutritional anemias. In: Ramakrishnan U, editor. Nutritional anemia. Boca Raton: CRC Press, 2001. p. 7 –21. Angle CR, McIntire MS. More on the relevance of the concentration of lead in plasma vs. that in blood. J Pediatr 1974;85:286 –287. Angle CR, McIntire MS, Brunk G. Effect of anemia on blood and tissue lead in rats. J Toxicol Environ Health 1977;3:557 –563. Bannon ID, Portnoy ME, Olivi L, Lees PS, Culotta VC, Bressler JP. Uptake of lead and iron by divalent metal transporter 1 in yeast and mammalian cells. Biochem Biophys Res Commun 2002;295:978 –984. Bannon DI, Abounader R, Lees PS, Bressler JP. Effect of DMT1 knockdown on iron, cadmium, and lead uptake in Caco-2 cells. Am J Physiol Cell Physiol 2003;284:C44 – C50. Barltrop D, Smith A. Lead binding to human haemoglobin. Experientia 1972;28:76 –77. Beard JL. Iron deficiency alters brain development and functioning. J Nutr 2003;133(Suppl 1):1468S–1472. Binder S, Falk H. Strategic plan for the elimination of childhood lead poisoning. Atlanta, Ga: Centers for Disease Control, 1991. Borsook H, Fischer EH, Keighley G. Factors affecting protein synthesis in vitro in rabbit reticulocytes. J Biol Chem 1957;229:1059 –1070. Bradman A, Eskenazi B, Sutton P, Athanasoulis M, Goldman LR. Iron deficiency associated with higher blood lead in children living in contaminated environments. Environ Health Perspect 2001;109:1079 –1084. Brody DJ, Pirkle JL, Kramer RA, Flegal KM, Matte TD, Gunter EW, Paschal DC. Blood lead levels in the US population B Phase 1 of the Third National Health and Nutrition Examination Survey (NHANES III, 1988 to 1991). J Am Med Assoc 1994;272:277 –283 (Erratum in: J Am Med Assoc 1995;274(2):130). Centers for Disease Control and Prevention. Preventing lead poisoning in young children. Atlanta, US Department of Health and Human Services, 1991. Centers for Disease Control and Prevention. Update: blood lead levels–United States, 1991–1994. MMWR 1997;46:141 –146. Centers for Disease Control and Prevention. Recommendations to prevent and control iron deficiency in the United States. MMWR 1998;47(RR-3):1 –29. Centers for Disease Control and Prevention. Managing elevated blood lead levels among young children: recommendations from the Advisory Committee of Childhood Lead Poisoning Prevention. Atlanta, Centers for Disease Control, 2002a.

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37 Centers for Disease Control and Prevention. Iron deficiency– United States, 1999–2000. MMWR 2002b;51:897 –899. Chetty CS, Reddy GR, Murthy KS, Johnson J, Sajwan K, Desaiah D. Perinatal lead exposure alters the expression of neuronal nitric oxide synthase in rat brain. Int J Toxicol 2001;20:113 –120. Clark M, Royal J, Seeler R. Interaction of iron deficiency and lead and the hematologic findings in children with severe lead poisoning. Pediatrics 1988;81:247 –254. Conrad ME, Umbreit JN, Moore EG, Peterson RD, Jones MB. A newly identified iron binding protein in duodenal mucosa of rats. Purification and characterization of mobilferrin. J Biol Chem 1990;265:5273 –5279. Conrad ME, Umbreit JN, Moore EG. A role for mucin in the absorption of inorganic iron and other metal cations. A study in rats. Gastroenterology 1991;100:129 –136. Counter SA, Buchanan LH, Ortega F, Rifai N. Blood lead and hemoglobin levels in Andean children with chronic lead intoxication. Neurotoxicology 2000;21:301 –308. Denno DW. Biology and violence: from birth to adulthood. New York, Cambridge: University Press, 1990. De Pee S, Bloem MW, Sari M, Kiess L, Yip R, Kosen S. The high prevalence of low hemoglobin concentration among Indonesian infants aged 3–5 months is related to maternal anemia. J Nutr 2002;132:2215 –2221. Dietrich KN, Ris MD, Succop PA, Berger OG, Borschein RL. Early exposure to lead and juvenile delinquency. Neurotoxicol Teratol 2001;23:511 –518. Fairbanks VF, Beutler E. Iron deficiency. In: Beutler E, Lichtman MA, Coller BS, Kipps TJ, editors. Williams hematology, 5th ed. New York: McGraw-Hill, 1995. p. 490 – 511. Finkelstein Y, Markowitz ME, Rosen JF. Low-level leadinduced neurotoxicity in children: an update on central nervous system effects. Brain Res Brain Res Rev 1998;27:168 –176. Flanagan PR, Hamilton DL, Haist HJ, Valberg LS. Interrelationships between iron and lead absorption in iron-deficient mice. Gastroenterology 1979;77:1074 –1081. Flanagan PR, Chamberlain MJ, Valberg LS. The relationship between iron and lead absorption in humans. Am J Clin Nutr 1982;36:823 –829. Goldberg A, Paton W, Thompson J. Pharmacology of the porphyrins and porphobilinogen. Brit J Pharmacol 1954;9:91 –94. Goldberg A. Lead poisoning and heme biosynthesis. Br J Haematol 1972;23:521 –524. Graziano JH, Slavkovic V, Factor-Litvak P, Popovac D, Ahmedi X, Mehmeti A. Depressed serum erythropoietin in pregnant women with elevated blood lead. Arch Environ Health 1991;46:347 –350. Griggs RC. Lead poisoning: hematologic aspects. Prog Hematol 1964;4:117 –137. Grosse SD, Matte TD, Schwartz J, Jackson RJ. Economic gains resulting from the reduction in children’s exposure to lead in the United States. Environ Health Perspect 2002;110:563 –569.

35

Gunshin H, Mackenzie B, Berger UV, Gunshin Y, Romero MF, Boron WF, Nussberger S, Gollan JL, Hediger MA. Cloning and characterization of a mammalian proton-coupled metal-ion transporter. Nature 1997;388:482 –488. Hammad TA, Sexton M, Langenberg P. Relationship between blood lead and dietary iron intake in preschool children. A cross-sectional study. Ann Epidemiol 1996;6:30 –33. Hashmi NS, Kachru DN, Tandon SK. Interrelationship between iron deficiency and lead intoxication (Part 1). Biol Trace Elem Res 1989;22:287 –297. Hershko C, Konijn AM, Moreb J, Link G, Grauer F, Weissenberg E. Iron depletion and blood lead levels in a population with endemic lead poisoning. Isr J Med Sci 1984;20:1039 – 1043. Imbus CE, Warner J, Smith E, Pegelow CH, Allen JP, Powars DR. Peripheral neuropathy in lead-intoxicated sickle cell patients. Muscle Nerve 1978;1:168 –171. Iturri S, Nunez MT. Effect of copper, cadmium, mercury, manganese and lead on Fe2q and Fe3q absorption in perfused mouse intestine. Digestion 1998;59:671 –675. Kapoor S, Seaman C, Hurst D, Matos S, Piomelli S. The biochemical basis of the clinical interaction of Fe Deficiency and lead intoxication. Pediatr Res 1984;18:242A. Kaufmann RB, Clouse TL, Olson DR, Matte TD. Elevated blood lead levels and blood lead screening among US children aged one to five years: 1988–1994. Pediatrics 2000;106:E79. Kaul B. Lead exposure and iron deficiency among Jammu and New Delhi children. Indian J Pediatr 1999;66:27 –35. Kim HS, Lee SS, Hwangbo Y, Ahn KD, Lee BK. Crosssectional study of blood lead effects on iron status in Korean lead workers. Nutrition 2003;19:571 –576. Kordas K, Lopez P, Rosado JL, Alatorre J, Ronquillo D, Stoltzfus RJ. Long-term effects of iron and zinc supplementation on cognition of lead-exposed Mexican children wabstract 噛343.8x. Experimental Biology 2004: April 17– 21, Washington, D.C., 2004, p. A483. Lambert RW, Schochet SS Jr. Demyelination and remyelination in lead neuropathy: electron microscopic studies. J Neuropathol Exp Neurol 1968;27:527 –545. Landrigan PJ, Baker EL Jr, Feldman RG, Cox DH, Eden KV, Orenstein WA, Mather JA, Yankel AJ, Von Lindern IH. Increased lead absorption with anemia and slowed nerve conduction in children near a lead smelter. J Pediatr 1976;89:904 –910. Landrigan PJ, Schechter CB, Lipton JM, Fahs MC, Schwartz J. Environmental pollutants and disease in American children: estimates of morbidity, mortality, and costs for lead poisoning, asthma, cancer, and developmental disabilities. Environ Health Perspect 2002;110:721 –728. Lee GR. Iron deficiency and iron deficiency anemia. In: Lee GR, Foerster J, Lukens J, Paraskevas F, Greer JP, Rodgers GM, editors. Wintrobe’s clinical hematology, 10th ed. Baltimore: William and Wilkins, 1999. p. 979 –999. Liebelt EL, Schonfeld DJ, Gallagher P. Elevated blood lead levels in children are associated with lower erythropoietin concentrations. J Pediatr 1999;134:107 –109.

36

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37

Lubran MM. Lead toxicity and heme biosynthesis. Ann Clin Lab Sci 1980;10:402 –413. Mahaffey KR. Nutrition and lead: strategies for public health. Environ Health Perspect 1995;103(Suppl 6):191 –196. Mahaffey KR, Rader JI. Metabolic interactions: lead, calcium, and iron. Ann NY Acad Sci 1980;355:285 –297. Mahaffey KR, Annest JL. Association of erythrocyte protoporphyrin with blood lead level and iron status in the second National Health and Nutrition Examination Survey, 1976– 1980. Environ Res 1986;41:327 –338. Marchetti C. Molecular targets of lead in brain neurotoxicity. Neurotox Res 2003;5(3):221 –236. Markowitz ME, Rosen JF, Bijur PE. Effects of iron deficiency on lead excretion in children with moderate lead intoxication. J Pediatr 1990;116:360 –364. Morgan EH, Oates PS. Mechanisms and regulation of intestinal iron absorption. Blood Cells Mol Dis 2002;29:384 –399. Needleman HL, McFarland C, Ness RB, Fienberg SE, Tobin MJ. Bone lead levels in adjudicated delinquents. A case control study. Neurotoxicol Teratol 2002;24:711 –717. Needleman HL, Riess JA, Tobin MJ, Biesecker GE, Greenhouse JB. Bone lead levels and delinquent behavior. J Am Med Assoc 1996;275:363 –369. Needleman HL, Schell A, Bellinger D, Leviton A, Allred EN. The long-term effects of exposure to low doses of lead in childhood: an 11-year follow-up report. N Engl J Med 1990;322:83 –88. Needleman HL. Childhood lead poisoning: the promise and abandonment of primary prevention. Am J Public Health 1998;88:1871 –1877. Needleman HL. Lead poisoning. Annu Rev Med 2004;55:209 – 222. Nelson MS, Chisolm JJ Jr. Lead toxicity masquerading as sickle cell crisis. Ann Emerg Med 1986;15:748 –750. Nevin R. How lead exposure relates to temporal changes in IQ, violent crime, and unwed pregnancy. Environ Res Sec A 2000;83:1 –22. Osterode W, Barnas U, Geissler K. Dose dependent reduction of erythroid progenitor cells and inappropriate erythropoietin response in exposure to lead: new aspects of anaemia induced by lead. Occup Environ Med 1999;56:106 –109. Pihl RO, Ervin F. Lead and cadmium levels in violent criminals. Psychol Rep 1990;66:839 –844. Porcelli B, Terzuoli L, Frosi B, Pagani R, Montomoli L, Romeo R, Sartorelli P. Blood concentrations of lead and erythropoietin. J Lab Clin Med 2002;139:125. Post EM, Levin M, Oski FA. The influence of iron on lead mobilization by EDTA wAbstractx. Pediatr Res 1978;12:426. Prpic-Majic D, Bobicc J, Simicc D, House DE, Otto DA, Jurasovicc J, Pizent A. Lead absorption and psychological function in Zagreb (Croatia) school children. Neurotoxicol Teratol 2000;22:347 –356. Qian ZM, Xiao DS, Wang Q, Tang PL, Pu YM. Inhibitory mechanism of lead on transferrin-bound iron uptake by rabbit reticulocytes: a fractal analysis. Mol Cell Biochem 1997;173:89 –94.

Ragan HA. Effects of iron deficiency on the absorption and distribution of lead and cadmium in rats. J Lab Clin Med 1977;90:700 –706. Robertson IK, Worwood M. Lead and iron absorption from rat small intestine: the effect of dietary Fe deficiency. Br J Nutr 1978;40:253 –260. Rogan WJ, Dietrich KN, Ware JH, Dockery DW, Salganik M, Radcliffe J, Jones RL, Ragan NB, Chisolm JJ Jr, Rhoads GG. Treatment of Lead-Exposed Children Trial Group. The effect of chelation therapy with succimer on neuropsychological development in children exposed to lead. N Engl J Med 2001;344:1421 –1426. Romeo R, Aprea C, Boccalon P, Orsi D, Porcelli B, Sartorelli P. Serum erythropoietin and blood lead concentrations. Int Arch Occup Environ Health 1996;69:73 –75. Roncagliolo M, Garrido M, Walter T, Peirano P, Lozoff B. Evidence of altered central nervous system development in infants with iron deficiency anemia at 6 mo: delayed maturation of auditory brainstem responses. Am J Clin Nutr 1998;68:683 –690. Rosen JF, Mushak P. Primary prevention of childhood lead poisoning–the only solution. N Engl J Med 2001;344:1470 – 1471. Ruff HA, Markowitz ME, Bijur PE, Rosen JF. Relationships among blood lead levels, iron deficiency, and cognitive development in two-year-old children. Environ Health Perspect 1996;104:180 –185. Serwint JR, Damokosh AI, Berger OG, Chisolm JJ Jr, Gunter EW, Jones RL, Rhoads GG, Rogan W. No difference in iron status between children with low and moderate lead exposure. J Pediatr 1999;135:108 –110. Six KM, Goyer RA. The influence of iron deficiency on tissue content and toxicity of ingested lead in the rat. J Lab Clin Med 1972;79:128 –136. Stretsky PB, Lynch MJ. The relationship between lead exposure and homicide. Arch Pediatr Adolesc Med 2001;155:579 –582. Tallkvist J, Bowlus CL, Lonnerdal B. Functional and molecular responses of human intestinal Caco-2 cells to iron treatment. Am J Clin Nutr 2000;72:770 –775. Tallkvist J, Bowlus CL, Lonnerdal B. DMT1 gene expression and cadmium absorption in human absorptive enterocytes. Toxicol Lett 2001;122:171 –177. Tigner-Weekes L, Pegelow C, Lee S, Powars D. Lead screening in sickle cell disease. J Pediatr 1979;95:738 –740. UNICEF. The progress of nations. New York: UNICEF, 1998. p. 1998 UNICEFyUNUyWHOyMI Technical Workshop. Preventing iron deficiency in women and children: background and consensus on key technical issues and resources for advocacy, planning, and implementing national programmes. New York: UNICEF, 1998. United Nations. Fourth Report on the World Nutrition Situation, January 2000: Nutrition Throughout the Life Cycle. Geneva: Administrative Committee on CoordinationySubCommittee on Nutrition (ACCySCN) in collaboration with IFPRI, 2000.

W.T. Kwong et al. / Science of the Total Environment 330 (2004) 21–37 Vallee BL, Ulmer DD. Biochemical effects of mercury, cadmium, and lead. Annu Rev Biochem 1972;41:91 –128. Wasserman G, Graziano JH, Factor-Litvak P, Popovac D, Morina N, Musabegovic A, Vrenezi N, Capuni-Paracka S, Lekic V, Preteni-Redjepi E, Hadzialjevic S, Slavkovich V, Kline J, Shrout P, Stein Z. Independent effects of lead exposure and iron deficiency anemia on developmental outcome at age 2 years. J Pediatr 1992;121:695 –703. Wasserman GA, Graziano JH, Factor-Litvak P, Popovac D, Morina N, Musabegovic A, Vrenezi N, Capuni-Paracka S, Lekic V, Preteni-Redjepi E, Hadzialjevic S, Slavkovich V, Kline J, Shrout P, Stein Z. Consequences of lead exposure and iron supplementation on childhood development at age 4 years. Neurotoxicol Teratol 1994;16:233 –240. Watson CJ, Decker E, Lichtman HC. Hematologic studies of children with lead poisoning. Hematologic studies in children with lead poisoning. Pediatrics 1958;21:40 –46. Watson WS, Hume R, Moore MR. Oral absorption of lead and iron. Lancet 1980;2:236 –237. Watson WS, Morrison J, Bethel MI, Baldwin NM, Lyon DT, Dobson H, Moore MR, Hume R. Food iron and lead absorption in humans. Am J Clin Nutr 1986;44:248 –256. Whetsell WO Jr, Sassa S, Kappas A. Porphyrin-heme biosynthesis in organotypic cultures of mouse dorsal root ganglia. Effects of heme and lead on porphyrin synthesis and peripheral myelin. J Clin Invest 1984;74:600 –607. White JM, Harvey DR. Defective synthesis of and globin chains in lead poisoning. Nature 1972;236:71 –73. Willows ND, Gray-Donald K. Blood lead concentrations and iron deficiency in Canadian aboriginal infants. Sci Total Environ 2002;289:255 –260.

37

Wolf AW, Jimenez E, Lozoff B. No evidence of developmental III effects of low-level lead exposure in a developing country. J Dev Behav Pediatr 1994;15:224 –231. Wolf AW, Jimenez E, Lozoff B. Effects of iron therapy on infant blood lead levels. J Pediatr 2003;143:789 –795. Wright RO. The role of iron therapy in childhood plumbism. Curr Opin Pediatr 1999;11:255 –258. Wright RO, Hu H, Maher TJ, Amarasiriwardena C, Chaiyukul P, Woolf AD, Shannon MW. Effect of iron supplementation on lead excretion and distribution in iron-deficient, lead poisoned rats wabstractx. J Toxicol Clin Toxicol 1997;35:556. Wright RO, Hu H, Maher TJ, Amarasiriwardena C, Chaiyakul P, Woolf AD, Shannon MW. Effect of iron deficiency anemia on lead distribution after intravenous dosing in rats. Toxicol Ind Health 1998;14:547 –551. Wright RO, Shannon MW, Wright RJ, Hu H. Association between iron deficiency and low-level lead poisoning in an urban primary care clinic. Am J Public Health 1999;89:1049 –1053. Wright RO, Tsaih SW, Schwartz J, Wright RJ, Hu H. Association between iron deficiency and blood lead level in a longitudinal analysis of children followed in an urban primary care clinic. J Pediatr 2003;142:9 –14. Yip R, Dallman PR. Developmental changes in erythrocyte protoporphyrin: roles of iron deficiency and lead toxicity. J Pediatr 1984;104:710 –713. Yip R, Norris TN, Anderson AS. Iron status of children with elevated blood lead concentrations. J Pediatr 1981;98:922 – 925.