Iron Status in Toddlerhood Predicts Sensitivity to Psychostimulants in Children

Abstract

Objective: Iron deficiency is associated with impaired dopaminergic signaling and externalizing behavior. The authors examine, whether iron stores in toddlerhood influence later response to psychostimulants. Method: Youth participating in a study monitoring the long-term safety of risperidone were included in this analysis if they had received psychostimulant monotherapy for at least 3 weeks and had a complete blood count obtained before psychostimulant treatment. Sensitivity to psychostimulants was defined based on the weight-adjusted dose during the 1st year of treatment. Regression analysis examined whether the hematological tests based on the characteristics of red blood cells were associated with sensitivity to psychostimulants. Results: A total of 29 participants (93% men; 76% Whites), primarily with ADHD (93%), comprised the current sample. The hematological tests were obtained, on average, 3 years before the initiation of psychostimulants monotherapy that occurred at 5.8 years of age and continued for a median of 0.85 years, at an average daily dose of 0.98 mg/kg (SD = 0.38) in methylphenidate equivalent. Compared with those who were poorly sensitive to psychostimulants, after adjusting for age, mean corpuscular volume was significantly higher in the highly and moderately psychostimulants sensitive groups. Conclusions: If replicated, these findings suggest that more attention should be paid to optimizing body iron in early childhood.

Keywords

ADHD Stimulants Treatment Iron

ADHD is one of the most common neuropsychiatric disorders in children (American Psychiatric Association, 2000). Psychosocial interventions effectively reduce symptom severity and impairment though psychotropics are often prescribed, especially in severe cases (Pliszka, 2007). Most of the proven pharmacological agents alter the dopaminergic system, consistent with research implicating dopamine in the various neuropsychological processes impaired in ADHD (Pliszka, 2007). In particular, psychostimulants, which represent the first-line treatment for ADHD (Pliszka, 2007), block the dopamine transporter and release catecholamines from storage granules in the presynaptic neurons (Ford, Greenhill, & Posner, 2003). However, although many studies have established the efficacy of psychostimulants for ADHD, predictors of response remain somewhat elusive (Pliszka, 2007). Among the most consistent findings, brain abnormalities have been associated with decreased clinical efficacy.

Iron is a critical nutrient involved in brain development in general and that of the dopaminergic system in particular. In fact, it is a cofactor for tyrosine hydroxylase, the rate-limiting enzyme for catecholamine synthesis (Sachdev, 1993). In addition, iron deficiency (ID) in rats results in reduced density of the dopamine transporter as well as the D1 and D2 dopamine receptors in the basal ganglia (Beard, Chen, Connor, & Jones, 1994; Burhans et al., 2005; Erikson, Jones, & Beard, 2000; Erikson, Jones, Hess, Zhang, & Beard, 2001; Nelson, Erikson, Pinero, & Beard, 1997). This effect appears more pronounced in males (Burhans et al., 2005; Erikson et al., 2001). Moreover, the response to cocaine, a potent inhibitor of the dopamine transporter, is attenuated in ID rats (Erikson et al., 2000; Nelson et al., 1997). Additional support comes from studies in children with ID showing reduced prolactin response to a clonidine challenge, again reflecting dysfunctional dopaminergic signaling (Felt, Jimenez, et al., 2006).

With iron playing a key role in neurotransmission, dendritogenesis, synaptogenesis, and myelination, concerns have been raised about the impact ID might have on cognitive, behavioral, and emotional development (Lozoff & Georgieff, 2006). This is particularly true as up to 7% of young children and 16% of female adolescents in the United States are affected by ID (Centers for Disease Control and Prevention [CDC], 2002). This is an age when the brain is undergoing substantial growth and maturation. In fact, ID anemia has been linked to socioemotional and academic impairment (Felt, Beard, et al., 2006; Lozoff & Georgieff, 2006; Lozoff, Jimenez, Hagen, Mollen, & Wolf, 2000; Wachs, Pollitt, Cueto, Jacoby, & Creed-Kanashiro, 2005). In addition, studies in patients with ADHD have reported an inverse association between serum ferritin, a measure of body iron, and the severity of inattention, hyperactivity and impulsivity, or sleep disturbances (Cortese, Konofal, Bernardina, Mouren, & Lecendreux, 2009; Konofal, Lecendreux, Arnulf, & Mouren, 2004; Oner, Alkar, & Oner, 2008).

Moreover, it is particularly disturbing that prospective studies have found persistent cognitive and neurochemical abnormalities, long after the resolution of ID identified postnatally (Burhans et al., 2005; Felt, Beard, et al., 2006; Lozoff et al., 2000). For example, more than 10 years after their iron stores had been replenished; children with a history of ID were more likely to have repeated a grade and/or received educational services, compared with children with a normal iron status (Lozoff et al., 2000). They also exhibited increased behavior problems, including inattention (Lozoff et al., 2000). Furthermore, these children had altered prolactin response to a clonidine challenge (Felt, Beard, et al., 2006). These findings suggest that, at least to some extent, ID in infancy might have a long-lasting effect on brain development in general and on the dopaminergic system in particular. This would also be consistent with some studies in ID rats showing only partial recovery of neurotransmission following iron supplementation (Burhans et al., 2005). In fact, the severity of ID and the timing of iron repletion appear crucial in determining the extent to which recovery will be complete (Lozoff & Georgieff, 2006).

In sum, preclinical and human research suggests that ID, inattention or ADHD, and impaired dopaminergic signaling may be interrelated, both acutely, during ID and, chronically, after it has been corrected. However, to our knowledge, only one published study has investigated the association between serum ferritin and response to treatment in ADHD (Millichap, Yee, & Davidson, 2006). In that study, there was no difference in clinical response to psychostimulants between children with low (<20 ng/ml) versus high (>60 ng/ml) serum ferritin concentration. This question is particularly pertinent in light of animal studies showing that ID dampens the inhibitory effect of cocaine on the dopamine transporter (Erikson et al., 2000; Nelson et al., 1997). Such evidence raises the possibility of a modulatory effect of iron status on the efficacy of psychostimulant treatment in patients with ADHD because, as noted earlier, they act by blocking the dopamine transporter (Ford et al., 2003).

Therefore, we here examine whether iron status in infancy and toddlerhood predicts response to psychostimulant treatment. We used hematological tests based on characteristics of red blood cells (i.e., hemoglobin [Hb], hematocrit [Hct], mean corpuscular volume [MCV], and red blood cell distribution width [RDW]) as proxies for iron status and tested the hypothesis that these parameters are associated with the dose of psychostimulants used to successfully treat ADHD symptoms. The dose of psychostimulants, we presumed, reflects the integrity of the central dopaminergic system.

Method

Participants

This analysis uses data collected in an ongoing study evaluating the safety of risperidone during long-term treatment. The study aims, rationale, and initial findings have been described elsewhere (Calarge, Acion, Kuperman, Tansey, & Schlechte, 2009, Calarge, Zimmermann, Xie, Kuperman, & Schlechte, 2010). Briefly, 7- to 17-year-old youths were enrolled if they had been taking risperidone, irrespective of indication, for a minimum of 6 months. Concomitant psychotropic treatment was allowed, except for other antipsychotic medications. Patients with intellectual disability, traumatic brain injury, other neurological disorders, or significant medical conditions were excluded as were pregnant females and those receiving hormonal contraception.

Procedures

This study was approved by the local institutional review board. Written assent was obtained from children aged 11 years or younger and written consent from adolescents and parents or guardians.

All medical and psychiatric records were obtained. In the majority of the cases, records were available since birth. We extracted all relevant clinical information, including the psychiatric diagnoses and the medication history. We recorded the start and stop dates of each medication as well as changes in the dosage and formulation (Calarge et al., 2009). This documentation, confirmed by a physician, also reflected any potential deviation from the prescribed treatments. All dosages of psychostimulants were expressed in methylphenidate (MPH) equivalents for amphetamines (× 2; Calarge et al., 2009; Swanson et al., 2007).

During the chart review, we also extracted all the results of hematological tests based on characteristics of red blood cells as well as the weight of the participants at each time point. The latter allowed us to adjust the dose of MPH for weight (i.e., computing a mg/kg/day dose). A time point for data entry was created in the data set every time a weight measurement was available in the medical record or a medication change took place.

At enrollment, the parent was asked to complete a questionnaire about the pregnancy, including the age of the parent at the time of the pregnancy as well as the gestational age at birth, from the last menstrual period. We defined prematurity as birth before 37 weeks. These variables are relevant to ruling out the possibility of anemia resulting from gestational factors, including prematurity (Marks, Dietz, Holloway, & Dean, 1998).

The psychiatric diagnoses were also obtained from the medical record. However, to evaluate the validity of these clinical diagnoses, a best-estimated diagnosis was generated in 96 participants based on a review of all available clinical data, including a standardized interview of the parent and the youth (if 11 years old or older) using the National Institute of Mental Health (NIMH) Diagnostic Interview Schedule for Children (DISC-IV; Shaffer, Fisher, Lucas, Dulcan, & Schwab-Stone, 2000). Agreement between the clinical diagnosis and the research-generated, best-estimate diagnosis was compared, using κ statistic (Cohen, 1960), for five diagnostic categories: (a) ADHD, (b) disruptive behavior disorders, (c) pervasive developmental disorders, (d) tic disorders, and (e) mood disorders. The κ values and 95% confidence intervals [CIs] were 0.71, 95% CI [0.50, 0.93]; 0.78, 95% CI [0.60, 0.97]; 0.84, 95% CI [0.72, 0.96]; and 0.39, 95% CI [0.17, 0.61], respectively, reflecting fair to substantial agreement. The lower agreement for mood disorders likely reflects the relapsing and remitting nature of this disorder, making it less likely that the symptoms would remain unaltered by the time participants underwent the standardized assessment at study enrollment.

Statistical Analysis

Our overall hypothesis was that iron status, as reflected by the hematological indices, will be associated with “sensitivity to MPH” treatment. Due to the exploratory nature of this analysis, the absence of standardized measures for clinical response in this retrospective study, and the anticipated variability in the dose and duration of psychostimulant treatment in a clinical setting, we defined “sensitivity to MPH” as follows: First, we computed the average daily dose of MPH and adjusted it for weight (i.e., mg/kg/d). We then determined, during the 1st year of treatment, the percentage of time spent taking a low (i.e., ≤0.6 mg/kg/d), high (i.e., >1.2 mg/kg/d), or medium daily dose of MPH (Nigg, Hinshaw, & Halperin, 1996). This variable captures the dose and duration components of response to psychostimulants, which are important to consider when assessing responsiveness to treatment. However, their distributions were severely skewed, which prevented us from using these percentages as dependent variables (the regression diagnostics revealed severe violations of model assumptions). Therefore, we defined three groups reflecting declining sensitivity to MPH: The first group, referred to as “highly sensitive,” included those participants who took a low dose of MPH for at least 50% of the time but took a high dose for less than 20% of the time. The “low-sensitive” group included those who took a high MPH dose for at least 50% of the time but a low dose for less than 20% of the time. The “moderately sensitive” group included the rest of the participants. We have assumed that the treating clinicians modified the dose of psychostimulants based on clinical response and that maintenance treatment reflected patient and parental satisfaction.

We restricted the analysis to those participants whose hematological tests were obtained before a psychostimulant was initiated and who had received the medication for at least 3 weeks before it was stopped or another psychotropic was added (Figure 1). When multiple measurements for the same hematological parameter were present, we selected the one that was obtained closest to 1.5 years of age. This is because children are at the highest risk for ID between the ages of 9 and 18 months, coinciding with their rapid rate of growth (Marks et al., 1998). We also excluded measurements that were obtained before 3 months of age as these usually reflect iron stores accrued in utero (Marks et al., 1998).

Figure 1.

Time course for a prototypical patient

As noted earlier, we used the participants’ weight at each time point to compute a daily, weight-adjusted dose of MPH. However, as clinicians do not measure or record weight at every clinic visit, weight information was missing in 27.5% of all the time points in the entire data set (n = 8,933). In those instances, we used the last weight available before the missing one and the first one available afterward to estimate the missing weight. To confirm the accuracy of this linear interpolation method, we conducted a simulation analysis whereby we randomly excluded 30% of the nonmissing weight values obtained after age 4 and interpolated them using the same linear method. The percentage difference between the interpolated and observed values was then computed (i.e., [observed weight – interpolated weight] × 100% / observed weight; Bland & Altman, 1999). The mean (SD) of this percentage was −0.02 (3.56). The 95% CI for the percentage bias was −6.96, 6.96 (Figure 2; Bland & Altman, 1999). In other words, 95% of the interpolated weight values fell within ±7% of the actual values. This suggests that our linear interpolation method is appropriate to estimate missing weight values in the data set.

Figure 2.

Percentage difference between observed and interpolated weight measurements plotted against observed weight

Demographic and clinical variables across the three MPH-sensitivity groups were compared using ANOVA for continuous variables and the Fisher’s exact test for categorical variables. To test our primary hypotheses, we used ANCOVA with each hematological index modeled as the dependent variable and the MPH-sensitivity grouping serving as the predictor variables. As the age at which the blood test was obtained and the age at which psychostimulants were initiated varied across the participants, though not across the MPH-sensitivity groups, we controlled for these two variables in the analyses.

All the statistical tests performed were two-tailed. Given the exploratory nature of this analysis, we did not adjust statistical significance for the number of comparisons we conducted. We performed all the analyses using SAS version 9.2 for Windows (SAS Institute Inc., Cary, NC).

Results

Of the 144 participants with complete data recruited into the original study, 29 had at least one hematological test obtained before psychostimulants were started and received psychostimulant monotherapy for at least 3 weeks during their 1st year of treatment. We found no differences between the 29 participants included in this analysis and those not included (n = 115) based on sex (93% men vs. 89%, p > .7), racial/ethnic composition (76% Whites vs. 84%, p > .2), age (11.2 years ± 2.8 vs. 12.0 ± 2.8, p > .1), or age when psychostimulants were started (5.8 years ± 1.7 vs. 6.2 ± 3.2, p > .4). There were also no differences in the clinical diagnoses or in the indication for risperidone treatment across the two groups (all p > .2).

Table 1 shows the demographic and clinical characteristics of the 29 participants included in this analysis divided into three groups based on the percentage of time, during the 1st year of psychostimulant treatment, they received a low, medium, or high weight-adjusted MPH dose. By study design, all participants were taking risperidone, at recruitment, to target aggression and irritability in 93% (n = 27) of the cases, impulsivity in 3% (one case), and tics in another 3%. The most common clinical diagnosis, at enrollment, was ADHD present in 27 (93%) children. However, comorbidity was the rule with 22 (76%) children having a disruptive behavior disorder, 13 (45%) an anxiety disorder, 4 (14%) a depressive disorder, 5 (17%) a tic disorder, and two children (7%) carrying a diagnosis of pervasive developmental disorder.

Table 1.

Differences in Demographic, Treatment Characteristics, and Gestational Data Across the Three MPH-Sensitivity Groups

Characteristic	High-MPH sensitivity (n = 8)	Medium-MPH sensitivity (n = 9)	Low-MPH sensitivity (n = 12)	Statistic	p value
Age at enrollment, years	10.8 ± 3.0	10.4 ± 2.8	12.0 ± 2.8	F = 0.90	>.4
Male, n (%)	8 (100)	8 (89)	11 (92)	Fisher’s	>.99
Age MPH started, years	5.3 ± 1.8	5.8 ± 1.5	6.2 ± 1.8	F = 0.58	>.5
Duration of MPH treatment, years	0.6 ± 0.4	0.7 ± 0.4	0.7 ± 0.4	F = 0.22	>.8
Mean MPH dose, mg/kg/d	0.64 ± 0.22	0.91 ± 0.14	1.26 ± 0.38	F = 12.1	<.0002
Max MPH dose, mg/kg/d	0.93 ± 0.46	1.05 ± 0.24	1.64 ± 0.34	F = 11.9	<.0002
MPH start to enrollment interval, years	5.4 ± 3.1	4.6 ± 2.2	5.8 ± 2.3	F = 0.58	>.5
Maternal age at birth, years^a	25.3 ± 3.9	26.4 ± 7.4	25.3 ± 4.7	F = 0.11	>.8
Gestational age, weeks^a	39.2 ± 1.8	40.0 ± 1.1	39.4 ± 2.1	F = 0.44	>.6
Prematurity, n (%)^a	1 (17%)	0	1 (10%)	Fisher’s	>.7

MPH = methylphenidate. High-, medium-, and low- MPH sensitivity was based on the relative percentage time spent, during the 1st year of treatment, receiving a high, medium, or low weight-adjusted dose of MPH (see Methods section for details).

Birth information was available for 27 participants, the other two having been adopted.

As the aim of the current investigation is to explore the association between iron status in toddlerhood and sensitivity to psychostimulants, we restricted the analysis to the initial psychiatric treatment period when the participants received only psychostimulants (Figure 1). On average, the participants were 5.8 years old (SD = 1.7) when they started taking psychostimulants. Their average daily dose was 0.98 mg/kg (SD = 0.38) and the median duration of treatment was 0.85 years (interquartile range = 0.72). As noted earlier, the sample was divided into three groups based on the percentage of time each participant took MPH at a low, medium, and high dose. Using this definition, 8 participants were considered to have high sensitivity to MPH, 9 having medium sensitivity, and 12 having low sensitivity. No demographic or clinical differences were significant across the three groups, except those related to MPH dosage (Table 1).

The hematological tests were obtained at a mean age of 2.8 years (SD = 1.8), on average, 3.0 years (SD = 1.5) before the onset of psychostimulant treatment. The mean Hb concentration was 11.9 g/dl (SD = 1.1, range = 10-14.3), Hct was 34.4% (SD = 2.7, range = 26.5-38.4), MCV was 76.3 femtoliters (SD = 5.8, range = 63.4-84.4), and RDW was 13.9 (SD = 1.9, range = 10.8-17.3). Using the cutoffs for the different hematological tests, as defined by Marks et al. (1998) based on age and sex, low Hb was present in 18% (n = 5) of the sample, low Hct in 22% (n = 5), microcytosis in 48% (n = 10), and large RDW in 42% (n = 5). The indication to obtain the tests included a workup for a medical illness (n = 16, 55%), screening during yearly physicals (n = 8, 28%), a workup for anemia (n = 3, 10%), and screening during a psychiatric evaluation (n = 2, 7%). There were no significant differences across the three “MPH sensitivity” groups in the indications for the test (p > .7).

Table 2 presents the results of the hematological tests across the three MPH-sensitivity groups. MCV was significantly different across the groups with the high-sensitivity group having the largest value, followed by the medium-sensitivity group. The low-sensitivity group had the smallest MCV as would be expected if ID did, as hypothesized, reduce the susceptibility of the dopaminergic system to psychostimulants. Though none of the other hematological parameters was significantly different across the three groups, all the observed differences were in the expected directions. The rates of microcytosis and large RDW were numerically different across the three groups (Table 2). However, these differences did not reach statistical significance, likely due to the small sample size.

Table 2.

Differences in Hematological Tests Across the Three MPH-Sensitivity Groups

Characteristic	High-MPH sensitivity (n = 8)	Medium-MPH sensitivity (n = 9)	Low-MPH sensitivity (n = 12)	Statistic	p value
Age test obtained	3.0 ± 2.2	2.8 ± 1.8	2.7 ± 1.6	F = 0.07	>.9
Test to MPH start time interval, years	2.3 ± 1.2	2.9 ± 1.7	3.5 ± 1.6	F = 1.44	>.2
Hemoglobin, g/dl	12.0 ± 1.1	12.0 ± 1.2	11.7 ± 1.0	F = 0.33	>.7
Low hemoglobin, n (%)^a	2 (25)	1 (11)	2 (18)	Fisher’s	>.8
Hematocrit (%)	34.3 ± 4.2	35.3 ± 1.6	33.8 ± 2.0	F = 0.59	>.5
Low hematocrit, n (%)^a	2 (28)	0 (0)	3 (33)	Fisher’s	>.3
MCV, fL	79.2 ± 2.7	78.7 ± 1.0	71.9 ± 7.2	F = 5.3	<.02
Microcytosis, n (%)^a	3 (43)	1 (17)	6 (75)	Fisher’s	>.1
RDW (%)	14.3 ± 1.5	13.0 ± 2.0	14.6 ± 1.9	F = 0.90	>.4
Large RDW, n (%)^a	1 (25)	1 (17)	3 (75)	Fisher’s	>.4

MPH = methylphenidate; MCV = mean corpuscular volume; RDW = red blood cell distribution width. High-, medium-, and low- MPH sensitivity was based on the relative percentage time spent, during the 1st year of treatment, receiving a high, medium, or low weight-adjusted dose of MPH (see Methods section for details).

Cutoffs were based on Marks, Dietz, Holloway, and Dean (1998; Tables 6 and 8). They varied by age and sex.

ANCOVA was then used to predict the four hematological parameters, after adjusting for the age when a psychostimulant was started as well as the age when the hematological test was obtained. Table 3 lists the beta estimates of the models predicting the four hematological parameters along with their standard errors. Compared to those that were poorly sensitive to psychostimulants, after adjusting for age, mean MCV was higher by 8.6 fL (p < .002) in the highly sensitive group and by 7.1 fL (p < .006) in the moderately sensitive one. An alternative analysis, with the three MPH-sensitivity groups, rank ordered (low, moderate, and high), also showed a significant association of MCV with MPH sensitivity (partial Spearman’s r = .56, p < .01).

Table 3.

Multiple Linear Regression Models Predicting Individual Hematological Tests As a Function of MPH Sensitivity, Adjusting for the Age When Psychostimulants (MPH) Were Started and When the Hematological Test Was Obtained

Test	Predictor variables	β estimates	SE	t	p value
Hemoglobin	High- MPH- sensitivity group	0.43	0.53	0.8	>.4
	Medium- MPH- sensitivity group	0.37	0.49	0.8	>.4
	Age MPH initiated	0.12	0.17	0.7	>.4
	Age Hb measured	0.13	0.15	0.9	>.3
Hematocrit	High- MPH- sensitivity group	0.62	1.55	0.4	>.6
	Medium- MPH- sensitivity group	1.56	1.47	1.1	>.3
	Age MPH initiated	0.15	0.47	0.3	>.7
	Age Hct measured	−0.01	0.49	−0.0	>.9
MCV	High- MPH- sensitivity group	8.55	2.32	3.7	.002
	Medium- MPH- sensitivity group	7.07	2.25	3.1	.006
	Age MPH initiated	1.20	0.73	1.6	.12
	Age MCV measured	0.31	0.73	0.4	>.6
RDW	High- MPH- sensitivity group	−0.69	1.38	−0.5	>.6
	Medium- MPH- sensitivity group	−2.09	1.25	−1.7	.14
	Age MPH initiated	−0.50	0.37	−1.4	>.2
	Age RDW measured	0.06	0.39	−0.1	>.8

Discussion

ID has been associated with altered dopaminergic signaling, cognitive impairment, inattention, and externalizing symptoms, including ADHD. In this analysis, we explored whether tests based on characteristics of red blood cells obtained, on average, 3 years before the onset of treatment with psychostimulants were correlated with response to stimulant treatment. MCV were significantly associated with sensitivity to psychostimulants as indexed by receiving a lower dose of psychostimulants during the 1st year of treatment.

Past research has linked reduced serum ferritin with the severity of inattention, hyperactivity, and impulsivity (Konofal et al., 2004; Oner et al., 2008). However, serum ferritin was measured around the same time the clinical ratings were obtained. Although such findings contribute to establishing an association between iron stores and ADHD, they fail to address the essential point raised by preclinical and human studies that ID, early during development, may lead to persistent changes in dopaminergic signaling and behavioral and cognitive sequelae (Burhans et al., 2005; Felt, Beard, et al., 2006; Lozoff et al., 2000). The alteration in dopaminergic neurotransmission affects various brain regions, including the basal ganglia, a structure consistently implicated in ADHD (Dickstein, Bannon, Castellanos, & Milham, 2006). Due to the nature of our database, we were able to specifically test whether iron stores in the toddler years were associated with a differential sensitivity to psychostimulants in grade school. Although MCV was strongly associated with MPH sensitivity, no other index reached significance, likely due to the small sample size. Nevertheless, the trends were in the expected duration with Hb and Hct being positively associated with MPH sensitivity and RDW negatively so.

In the only other published study, which we are aware of, that investigated the association between serum ferritin and response to psychostimulant treatment, Millichap et al. (2006) found no difference in the response rate to psychostimulants based on ferritin concentration. It was not clear, however, how response to treatment was defined. Moreover, patients with low serum ferritin were given iron supplements before treatment with psychostimulants was initiated. Therefore, it is possible that iron repletion masked the potential link between serum ferritin and sensitivity to psychostimulants by attenuating ADHD symptoms as has been shown in a recent double-blind, placebo-controlled trial (Konofal et al., 2008). By contrast, using a different sample in a multiphase clinical trial, our group has recently found that baseline ferritin concentration was negatively correlated with the weight-adjusted dose of amphetamine necessary to optimize clinical response in youths with ADHD (Calarge, Farmer, DiSilvestro, & Arnold, 2010, in press).

This being a retrospective study, we could not evaluate how many of the participants had frank anemia on the initiation of psychostimulants. These data were not available. However, a number of participants did have abnormalities in their hematological parameters before treatment onset, as described in Table 2. Still, only one child with ID anemia appeared to have received iron supplementation. It is possible that the clinicians did not deem the abnormalities severe enough to treat, especially given that the blood tests were obtained to specifically evaluate for anemia in only three of the children. Nevertheless, the absence of information regarding anemia at the start of psychostimulants should not detract from the significance of our findings as ID has been shown to affect cognitive and behavioral functioning even in the absence of frank anemia (Grantham-McGregor & Ani, 2001; Halterman, Kaczorowski, Aligne, Auinger, & Szilagyi, 2001; Lozoff et al., 2000). This is likely because, when intake is limited, iron is diverted toward Hb synthesis, resulting in a reduction in brain iron concentration before anemia becomes manifested (Lozoff & Georgieff, 2006).

Though our findings contribute to the growing literature linking ID, ADHD, and dopaminergic signaling, a number of limitations are worth noting. First, the retrospective design of the study could have biased the results in several ways. It is possible that the indication for laboratory testing was itself the primary cause underlying the differences in MPH sensitivity. Although we cannot rule out this possibility due to the small sample size, we believe it is unlikely as the indication for the blood test did not differ across the groups. Moreover, in several participants, the test was obtained as part of a general medical screen, in the absence of any particular medical condition. In addition, our findings would have been stronger if we had symptom ratings at the initiation of psychostimulants and later on during the course of the treatment. Furthermore, our sample is not representative of children with ADHD as comorbidity was prevalent and as, by study design, all the participants eventually received an antipsychotic medication. Therefore, it is necessary to replicate our findings in a larger sample of children whose ADHD was well controlled with psychostimulants alone and where both genders are equally represented. This would also minimize the risk for Type 1 errors. Nevertheless, it is of interest that in the vast majority of our participants, risperidone was prescribed to treat irritability and aggression. In utero iron deprivation in monkeys leads to impulsivity whereas postnatally it results in increased emotionality. Similarly, perinatal iron stores in humans are correlated with negative emotionality (Wachs et al., 2005). This temperamental trait is often observed in aggressive children with disruptive behavior (Briggs-Gowan, Carter, Bosson-Heenan, Guyer, & Horwitz, 2006). Finally, this being a convenience sample, we used hematological tests based on the characteristics of red blood cells as markers of iron stores. Although the combination of low MCV and high RDW is an accepted index of ID, additional parameters could have included serum iron concentration, iron binding capacity, transferrin, and ferritin.

In sum, with food enrichment, ID has significantly declined in the developed countries, including the United States. However, in at-risk populations, such as minorities and individuals from disadvantaged backgrounds, ID in infancy and the toddler years remains common (CDC, 2002; Marks et al., 1998). These groups are also at an elevated risk for externalizing disorders. This combination of risks not only makes them vulnerable to developing ADHD but also, based on our findings reported here and elsewhere (Calarge et al., in press), might attenuate their response to the gold standard treatment. Therefore, it is essential to optimize efforts to prevent ID early on in development while avoiding the adverse events associated with excessive iron supplementation (Iannotti, Tielsch, Black, & Black, 2006).

Footnotes

Acknowledgements

The authors would like to thank the patients and their families for their commitment to this research, the staff of the University of Iowa child psychiatry division, the research coordinators, and the Clinical Research Unit Staff.

Aspects of this work have been presented at the annual meeting of the American Academy of Child & Adolescent Psychiatry, October, 2009, Honolulu, HI. Dr. Bridget Zimmerman supervised the statistical analysis.

The author(s) declared no potential conflicts of interests with respect to the authorship and/or publication of this article.

The author(s) disclosed receipt of the following financial support for the research and/or authorship of this article: This study was funded by a 2005 Young Investigator Award, through a Medical Student Summer Research Fellowship from the American Academy of Child & Adolescent Psychiatry sponsored by the Campaign for America’s Kids, and by the National Institute of Health (RR024979, R21MH080968, and K23MH085005). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Center For Research Resources or the National Institutes of Health.

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