Attention-Deficit/Hyperactivity Disorder is Related to Decreased Weight in the Preschool Period and to Increased Rate of Overweight in School-Age Boys

Abstract

Objective:

Previous studies have associated attention-deficit/hyperactivity disorder (ADHD) with growth deviations and obesity. However, available data regarding the growth of children with ADHD in their early childhood are insufficient. Therefore, we aimed to examine whether there are differences in body size between preschool boys with and without ADHD.

Methods:

The study used cross-sectional and retrospective longitudinal data concerning 112 boys with ADHD and a community-based sample of 308 boys without ADHD. The groups were homogeneous in terms of socioeconomic status, place of residence, term of birth, and birth weight. The average age of diagnosis was 8.3 years, and none of boys had been treated with stimulants before they were 7 years of age. Comparisons were made at the ages of 2, 4, and 6 years, for World Health Organization (WHO)-norm-standardized height, weight, body mass index (BMI), prevalence of underweight, overweight, and obesity. Separate analysis were made for the cross-sectional measurements of current body size.

Results:

Boys with ADHD at the age of 2 had significantly lower z scores for weight (t=−1.98, p=0.04) and BMI (t=−2.09, p=0.04), and at the age of 4 for weight (t=−2.05, p=0.04) than the boys from the control group. A significantly lower percentage of overweight/obesity was observed in boys with ADHD at the age of 2 in comparison with the control group. At the age of 6, boys with ADHD were underweight more often. Cross-sectional analysis of current body size showed that boys with ADHD had lower z scores for height (t=−3.08, p=0.002) and higher z scores (t=3.13, p=0.002) for BMI. Overweight was more frequent in this group.

Conclusions:

Preschool boys with ADHD (age of 2–6 years) have a tendency toward lower body weight than their peers. But in subsequent phases of development, they are shorter and more frequently overweight than boys without ADHD, when place of residence, socioeconomic status, term of birth, birth weight, comorbid conditions, and treatment are controlled.

Introduction

Attention-deficit/hyperactivity disorder (ADHD), manifesting with symptoms of three categories: Attention disorder, impulsiveness, and hyperactivity (American Psychiatric Association 2000), is one of the most often recognized neuropsychological disorders in school children. Its prevalence ranges from 5 to 10% among school children (Polanczyk et al. 2007). ADHD is often associated with learning disorders and impairment of academic and social functions (Wilens et al. 2002; Hanć and Brzezińska 2009). Stimulant medications, such as methylphenidate, are the first line of treatment for this disorder (Durston 2003).

Previous studies indicated that ADHD may be associated with deviations in biological development; however, the data are contradictory. Some studies characterized children with ADHD as having lower body weight and height (Spencer et al. 1998; Ptacek et al. 2009) or body mass index (BMI) (Heinonen et al. 2011). Other research shows that stimulant-naïve children with ADHD are characterized by greater body size: they are taller (Swanson et al. 2006, 2007; Hanć and Cieślik 2008b), and have higher body weight or BMI (Swanson 2006; Hanć and Cieślik 2008a; Hanć et al. 2012) than children without ADHD. Also, numerous recent studies suggested that ADHD is a risk factor of overweight and obesity in children, adolescents, and adults (Hanć and Cieślik 2008a; Cortese et al. 2013; Khalife et al. 2014; Hanć et al. 2015).

There is very little research focused on assessment of body size of preschool children with ADHD (Curtin et al. 2005; Ebenegger et al. 2011; Heinonen et al. 2011). This stage of development is especially interesting because it often precedes the moment of formal diagnosis of ADHD. The examination of preschool children enables the assessment of a relationship between ADHD and body size, while excluding the confounding influence of treatment with stimulants and the impact of disorders often co-occurring with ADHD, such as depression (Biederman et al. 2003), conduct disorder, or oppositional defiant disorder (Wilens et al. 2002), diagnosed in later years of life. Therefore, we aimed to analyze data on body size during preschool years of boys diagnosed and treated for ADHD, in a retrospective manner, and to compare trends in body size of preschool children with their subsequent body size, assessed in late childhood or adolescence, when many children with ADHD experience the psychological consequences of ADHD and are covered by pharmacological treatment. We have hypothesized that there are differences in height, weight, BMI, and prevalence of underweight, overweight, and obesity between boys with and those without ADHD.

Methods

Ethical and financial statements

Data used in the study were gathered in the framework of the project conducted in Poland in the years 2005–2008, whose aim was a multi-aspect assessment of biological development of children and adolescents with ADHD. The project was approved by the Ethics Committee of the Poznan University of Medical Sciences, Poland (Decision Number: 855/05) and conducted in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. The research was partialy sponsored by the Polish Ministry of Science and Higher Education (Grant Number: NN303 0175 33). Participants and their legal guardians were fully informed about research aims and procedures. Parents gave written consent for participation of their child in the study. Details and justification of the methods applied in the study as well as the general results of cross-sectional analyses of the body height, BMI, and the incidence of overweight/obesity in boys with ADHD were described elswhere (Hanć et al. 2015). At initial stage of this project, the growth in treatment-naive boys with ADHD was analyzed in comparison to the growth charts (Hanć et al. 2012). Next, the sample size was enlarged and compared with a community-based control group of children without symptoms of mental disorders. The following analyses were focused on the relationship between ADHD and nutritional status with controls for birth weight, stimulant treatment, comorbid disorders, and socioeconomic status (SES) (Hanć et al. 2015). There were differences in data gathered in the survey in the first and second stages of the project. For example, in the second stage of the project, the information concerning comorbid disorders was included. In the first stage, the parents of treatment-naive boys were asked about subjective estimation of income level (low, medium, high) and in the second stage, the questions about income were more detailed (the total net income of the family was estimated). The change in methods was necessary to allow comparison between the sample of boys with ADHD and the control group. Parents of boys examined in the first stage of the project were asked to supplement necessary information; nevertheless, complete data were obtained only for one third of the treatment-naive boys sample. Therefore only 52 of 150 (34.67%) boys examined in the first stage were included in the extended sample in the second stage of project, and constitute 17.57% of this sample (52 of 296 boys).

Selection criteria

Children and adolescents with ADHD were recruited among patients of psychiatric hospitals and university outpatient clinics. Inclusion criteria were: Male sex, age 6–18 years at the time of examination, intelligence quotient (IQ) ≥85, and providing consent to participate in the study. Participants had to have met the criteria for ADHD in the Diagnostic and Statistical Manual of Mental Disorders, 4th ed., Text Revision (DSM-IV-TR) (American Psychiatric Association 2000). Diagnosis was made by psychiatrists and psychologists on the basis of the Conners' Parent Rating Scale and the Diagnostic Structured Interview for ADHD, and Hyperkinetic Disorder According to International Statistical Classification of Diseases and Related Health Problems, 10th Revision (ICD-10) and DSM-IV-TR (Wolańczyk and Kołakowski 2005).

Data from children with ADHD were compared with those from individuals from the same population (community-based sample), without symptoms of ADHD and most other common psychiatric disorders, such as oppositional defiant disorder, conduct disorder, depression, and anxiety. Assessment of mental health in this group was based on parents' responses to the following questions: 1) “Has a doctor or health professional ever told you that your child has attention-deficit disorder, attention-deficit/hyperactivity disorder, that is, ADD or ADHD?” 2) “Has a doctor or health professional ever told you that your child has psychiatric disorders?” 3) “Has a doctor or health professional ever told you that your child has oppositional defiant disorder, conduct disorder, or emotional problems, such as depression or anxiety disorder?” Similar questions were used for identification of individuals with ADHD in earlier studies (Dey et al. 2004; Bandini et al. 2005; Warring and Lapane 2008). For the purpose of this study, we used data on boys for whom parents answered “no” to the above questions. The exclusion criteria in both compared groups were organic brain dysfunction, epilepsy, mental retardation, thyroid dysfunction, and fetal alcohol syndrome.

The variables such as SES (Ogden et al. 2010), place of residence (Davis et al. 2011), and perinatal factors such as birth weight (Rillamas-Sun et al. 2012; Szwed and Kosińska 2012), have well-documented impact on children's growth. Moreover, ADHD is associated with premature birth and low birth weight. Further, in Poland, ADHD is often diagnosed in large urban areas, in children whose parents are well educated and have higher income (Hanć et al. 2015). Therefore, in order to exclude the potential effect of confounding factors on the results of current comparisons of boys with and without ADHD the rigorous method of selection of respondents was used. The analyses conducted by us took into account only the data on boys living in cities of >100,000 inhabitants, born at term (between 37 and 42 weeks of gestation) and with normal birth weight (2500–4000g) both of whose parents had graduated from at least a high school, and who declared average or above-average income (total income of <4000 PLN net was classified as “below-average” based on the report of Central Statistical Office, which includes average salaries in Poland in 2005–2008) (General Statistical Office 2014). Similarly rigorous preselection method was used, among others, in the international projects aimed at establishing international standards of development from conception to birth (INTERGROWTH-21st) (Villar et al. 2013, 2014) and from birth to 5 years of age (World Health Organization [WHO] Multicentre Growth Reference Study [MGRS]) (Garza and de Onis 2004; World Health Organization 2006b), which evaluated the development of children of mothers selected because of optimum health, nutrition, education, and SES. By applying stringent selection criteria we were certain that SES and abnormalities of fetal development and pregnancy did not cause the differences in body size between the compared groups, and hence we could assume more likely that these differences were associated with the presence of symptoms of ADHD in boys in the sample and the absence of these symptoms in boys in the control group. The comparison of selection criteria between current and previously published analyses on the same data set is presented in Table 1.

Table 1.

Comparison of Selection Criteria Between the Current Study and Previous Analysis on the Same Data Set

Hanć et al. 2012	Hanć et al. 2015		Current study
Initial size of the sample: n=150 (treatment-näive boys with ADHD)	Initial size of the sample: n=296 (boys with ADHD with IQ≥85, for which the consent to participate in the study was obtained)	Initial size of the control group: n=784 (community-based sample)	Initial size of the sample: n=296 (boys with ADHD with IQ≥85, for which the consent to participate in the study was obtained)	Initial size of the control group: n=1575 (community-based sample)

Exclusion criteria
n of included boys	n of excluded boys	n of included boys	n of excluded boys	n of included boys	n of excluded boys	n of included boys	n of excluded boys	n of included boys	n of excluded boys
Step 1: The lack of parent's or children consent		Step 1: Incomplete data				Step 1: Mental retardation
143	7	288	8	657	127	296	0	1575	0
Step 2: Mental retardation		Step 2: Endocrine disorders				Step 2: Diseases: epilepsy, organic brain dysfunctions, thyroid dysfunctions, fetal alcohol syndrome
141	2	288	0	650	7	296	0	1550	10 (epilepsy) 15 (thyroid dysf.)
Step 3: Diseases: epilepsy, organic brain dysfunctions, thyroid dysfunctions		Step 3: Preterm birth						The risk of mental disorders
		Step 3: Preterm birth						1308	242
135	6	264	24	483	167
		Step 4: Postterm birth				Step 3: Incomplete retrospective growth measurements
		236	28	478	5	240	56	1086	222
		Step 5: Commorbid disorders		The risk of mental disorders		Step 4: Place of residence: <100 000 inhabitants
		219	6 (depression)11 (anxiety dis.)	396	82	203	37	750	336
						Step 5: The level of parents' education: at least 1 parent with only elementary education
						195	8	683	67
						Step 6: Family income below average
						170	25	481	202
						Step 7: Preterm birth
						157	13	392	89
						Step 8: Postterm birth
						142	15	375	18
						Step 9: Birth weight <2500 g
						133	9	359	15
						Step 10: Birth weight >4000 g
						112	21	308	51
Final size of the sample/control group
135/-		219/396				112/308

ADHD, attention-deficit/hyperactivity disorder; IQ, intelligence quotient.

Assessment of growth and nutritional status

The measurements of current height and weight were taken by trained medical staff with the use of anthropometer (Gneupel Praezision Mechanik, Swiss; instrument accuracy of ±1 mm) and a medical scale (Radwag PUE c/31; instrument accuracy of ±100 g) using the standard technique (de Onis et al. 2004). Two measurements of each boy were taken during the same visit. The arithmetic means were considered as the actual values of weight and height. Data on growth in earlier years of life were collected on the basis of medical registers. Retrospective longitudinal data of the range of 2–6 years of age, preceding the beginning of education in a primary school (children in Poland start school education at the age of 7), were obtained from medical registers and used for comparisons. These measurements of body weight and height were taken by pediatric nurses during periodic examinations with the use of medical scales (measurement accuracy±100 g) and stadiometers (measurement accuracy±1 mm). Only boys for whom all growth measurements were available, that is, current height and weight as well as height and weight in the 2nd (1.5–2.5), 4th (3.5–4.5), and 6th (5.5–6.5) years of life, were included in the final sample analyzed in current study. The age ranges for the preschool period adopted by us were justified by the fact that in the 2nd, 4th, and 6th years of life, children in Poland are subject to standard pediatric examination takings into account the assessment of growth. Therefore, it is possible to obtain growth data for these age ranges for many children.

BMI was calculated on the basis of body height and weight. Height, weight, and BMI were adjusted for sex and age on the basis of World Health Organization growth charts (World Health Organization 2006a; de Onis et al. 2007) and presented as z scores. International Obesity Task Force (IOTF) recommendations (Cole et al. 2000,2007) were used for an identification of individuals who were underweight, overweight, and obese. Underweight was diagnosed according to cutoffs for BMI, corresponding to BMI=17 at the age of 18, as proposed by Cole et al. (2007).

Statistical analysis

Height, weight and BMI were transformed into z scores with WHO Anthro and WHO AnthroPlus software. Two-tailed t test was used to compare z scores for height, weight, and BMI between boys with and those without ADHD in the following age classes. χ² test was used to assess the differences in prevalence of underweight, overweight, and obesity. Results were considered statistically significant at significance level of p<0.05. Statistical analyses were conducted using Statistica 10 software.

Results

Parental consent for participation in the study was obtained for 296 boys diagnosed with ADHD. The following individuals were excluded from the study group: 37 boys because their place of residence had <100,000 inhabitants, 8 boys from families where at least one parent had elementary education only, 25 boys from families with SES below average, 13 preterm-born and 15 postterm-born, 9 with low birth weight (<2500 g) and 21 with high birth weight (>4000 g). Fifty-six patients were excluded because of incomplete retrospective data. Of the 1575 boys applying for the control group, we excluded 242 because of the risk of mental disorders, 15 with endocrine disorders, 10 with epilepsy, 336 who were residents of a city of <100,000 inhabitants, 67 with at least one parent having only an elementary education, 202 boys from families with SES below average, 89 preterm-born and 18 postterm-born, 15 with low birth weight, and 51 with high birth weight, and 222 boys were excluded because of the missing retrospective data.

Analyses used longitudinal data on 112 boys with ADHD and 308 boys without ADHD (control group), who met our criteria. There was no statistical difference in the age of the boys from the two groups at the time of recruitment (ADHD: mean age=11.2, SD=2.9; control group: mean age=11.0, SD=2.6; t=0.65, df=418, p=0.52). Combined type of ADHD was the most frequent in the sample (79%). Boys with ADHD had at least one additional diagnosis (76%). Learning disorders (49%), speech disorders (23%), and opositional defiant disorder (34%) were the disorders most often diagnosed in this group. ADHD diagnosis was given to subjects in a wide age range (5–17 years) with mean of 8.3±2.4 years. Fifteen boys had been diagnosed before 7 years of age, and 47% of the sample was treated with stimulants at the time of recruitment. The mean age of treatment begining was 10.5±2.4 years. None of the subjects had been treated with stimulants before the age of 7.3 years (Table 2).

Table 2.

Characteristics of Boys with Attention-Deficit/Hyperactivity Disorder (ADHD)

Characteristics at the moment of examination	Categories	n	%
Type of ADHD
	ADD	10	8.9
	Hyperactive/impulsive	14	12.5
	combined	88	78.6
Comorbid disorders
	At least one additional diagnosed disorder	85	75.9
	Learning disorders	55	49.1
	Speech disorders	26	23.2
	Oppositional defiant disorder	38	33.9
	Conduct disorder	4	3.6
	Anxiety disorders	8	7.1
	Mood disorders	3	2.7
	Tic disorder	2	1.8
Treatment at the moment of examination
	Not currently medicated	59	52.7
	Currently medicated	53	47.3

	Mean	SD	95% CI	Min	Max
The age of ADHD diagnosis (years)	8.3	2.4	7.8–8.7	5	17
The age of initiation of stimulant treatment (y)	10.5	1.81	9.8–11.2	7.3	13

The examination of current body size (at the time of recruitment) showed z scores for height significantly lower (0.51 vs. 1.05, t=−3.08, df=418, p=0.002) and z scores for BMI higher (0.27 vs. −0.25, t=3.13, df=418, p=0.002) in boys with ADHD than in the control group. The prevalence of overweight and obesity was higher among boys with ADHD than in the cotnrol group (23.21% vs. 12.99%, χ² =6.32, df=1, p=0.01). The separate analyses showed significant difference only for overweight (17.86% vs. 8.77%, χ² =6.43, df=418, p=0.01) and not for obesity (5.36% vs. 4.22%, χ² =0.30, df=418, p=0.59). We did not find significant difference in the rate of underweight between the two compared groups (1.79% vs. 1.33%, χ² =0.10, df=418, p=0.76).

The next step was Student's t test performed in order to check the effect of the treatment status and the impact of oppositional defiant disorder and conduct disorder on the results obtained. The comparison of currently treated and untreated boys with ADHD did not show statistically significant differences for z scores for current height (0.46 vs. 0.57, t=0.55, df=110, p=0.58) or z scores for current BMI (0.22 vs. 0.34, t=−0.40, df=110, p=0.69). Also, a comparison of group of boys with oppositional defiant disorder or conduct disorder (n=42) with the boys without these comorbid disorders showed no statistically significant differences in current body size (z scores for height: 0.65 vs. 0.45, t = −1.01, df=110, p=0.31; z scores for BMI: 0.47 vs. 0.18, t=−1.00, df=110, p=0.32). Number of patients with mood (n=3) and anxiety (n=8) disorders was too small to perform a statistical evaluation of their effects on body size of studied boys.

Next we have estimated retrospectively weight, height and BMI in age of 2, 4, and 6 years for boys with ADHD and control group. Differences between z scores for height, weight, and BMI were assessed in subsequent age classes. Student's t test showed statistically significant differences for two age classes: 2 and 4. At the age of 2 years, boys with ADHD had significantly lower z scores for weight (difference=0.25 SD, t=−1.98, p=0.04) and BMI (difference=0.28 SD, t = −2.09, p=0.04), and at the age of 4 years they had lower body weight (difference=0.28 SD, t = −2.05, p=0.04) than the control group (Tables 3 and 4).

Table 3.

Raw Results of Height and Weight Measurements and BMI in the Following Age Classes in the Preschool Period

	Control group				ADHD
Classes of age (y)	n	Average weight in kg (SD)	Average height in cm (SD)	Average BMI (SD)	n	Average weight in kg (SD)	Average height in cm (SD)	Average BMI (SD)
2	308	13.69 (1.93)	89.50 (5.78)	17.10 (1.72)	112	12.60 (1.86)	90.25 (4.77)	16.67 (1.54)
4	308	18.39 (3.10)	106.29 (7.08)	16.22 (1.74)	112	17.74 (3.13)	105.17 (6.49)	15.95 (1.65)
6	308	22.76 (4.20)	119.08 (6.35)	15.96 (2.00)	112	22.91 (4.17)	119.17 (6.24)	16.03 (2.05)

ADHD, boys with attention-deficit/hyperactivity disorder; BMI, body mass index.

Table 4.

Comparison of Weight, Height and BMI Between Boys with and Without ADHD in the Following Age Classes in the Preschool Period; Two Tailed Student t Test Results

	Weight			Height			BMI
	Mean for z scores (SD)			Mean for z scores (SD)			Mean for z scores (SD)
Age (y)	Control group	ADHD	t (df=418)	Control group	ADHD	t (df=418)	Control group	ADHD	t (df=418)
2	0.75 (1.14)	0.50 (1.21)	−1.98 p =0.04	0.30 (1.66)	0.22 (1.50)	−0.46 p=0.64	0.82 (1.25)	0.54 (1.19)	−2.09 p =0.04
4	0.65 (1.23)	0.37 (1.17)	−2.05 p =0.04	0.44 (1.60)	0.19 (1.27)	−1.52 p=0.04	0.59 (1.24)	0.39 (1.19)	−1.48 p=0.14
6	0.50 (1.22)	0.53 (1.26)	0.27 p=0.79	0.46 (1.24)	0.50 (1.11)	0.32 p=0.75	0.29 (1.29)	0.33 (1.41)	0.22 p=0.83

Bold represents differences significant at the level of p<0.05.

ADHD, boys with attention-deficit/hyperactivity disorder; BMI, body mass index.

We have also assessed the differences in the prevalence of underweight, overweight, and obesity between boys with and those without ADHD. Statistically significant differences were observed in two age classes: 2 and 6. At the age of 2 years, boys with ADHD were overweight/obese less frequently than those in the control group (ADHD: 10.71%, control group: 20.13%, χ²=5.02, p=0.02). At the age of 6, boys with ADHD were statistically significantly more often diagnosed with underweight than boys without ADHD (8.93% vs. 3.25%, χ²=5.85, p=0.02) (Table 5).

Table 5.

Prevalence of Underweight, Overweight, and Obesity in the Following Age Classes in the Preschool Period – Comparison Between Boys With and Without ADHD; Results of the χ ² test

	Underweight			Overweight			Obesity			Overweight+obesity
Age (y)	Control group n (%)	ADHD n (%)	χ² (df=1)	Control group n (%)	ADHD n (%)	χ² (df=1)	Control group n (%)	ADHD n (%)	χ² (df=1)	Control group n (%)	ADHD n (%)	χ² (df=1)
2	9 (2.92)	6 (5.36)	1.41 p=0.23	42 (13.64)	8 (7.14)	3.30 p=0.07	20 (6.49)	4 (3.57)	1.30 p=0.25	62 (20.13)	12 (10.71)	5.02 p =0.02
4	13 (4.22)	8 (7.14)	1.48 p=0.22	43 (13.96)	17 (15.18)	0.10 p=0.75	16 (5.19)	3 (2.68)	1.20 p=0.27	59 (19.16)	20 (17.86)	0.09 p=0.76
6	10 (3.25)	10 (8.93)	5.85 p =0.02	30 (9.74)	18 (16.07)	3.25 p=0.07	18 (5.84)	4 (3.57)	0.85 p=0.35	48 (15.58)	22 (19.64)	0.97 p=0.32

Bold represents differences significant at the level of p<0.05.

ADHD, boys with attention-deficit/hyperactivity disorder.

Discussion

The main aim of this study was to assess the relationship between ADHD and body size in preschool boys. For this purpose, weight, height, BMI, and the prevalence of underweight, overweight, and obesity among boys with and without ADHD were compared retrospectively. Most of the comparisons showed no significant difference between the groups. However, at the age of 2, boys with ADHD had lower z scores for weight and BMI, and a lower incidence of overweight/obesity. At the age of 4 they had lower z scores for weight, and at the age of 6 more boys with ADHD were underweight compared with the boys from the control group. All statistically significant differences indicated a relationship of ADHD with a smaller body weight in preschool boys. Compared groups did not differ significantly in height.

Interestingly, these results were different to those obtained for the later years of life in the same sample. The analysis of the currently performed measurements (the average age of boys was 11 years) showed a lower body height, higher BMI, and a higher incidence of overweight in boys with ADHD than in the control group. The discrepancy between the results obtained for the preschool phase and later stages of development may have several causes. In our study, only 15 boys were diagnosed with ADHD before the age of 7, and none of them had been treated with stimulants in the preschool phase (on average, children began treatment between 10 and 11 years of age, with the minimum being 7.3 years). This allowed the exclusion of confounding variables, such as the growth-impairing effect of treatment when analyzing body size in preschool boys. What is more, the comorbid disorders of ADHD, which may additionally disturb growth, such as depression (Biederman et al. 2003; Reeves et al. 2008; Cortese et al. 2013), binge eating (Cortese and Peñalver 2010), or oppositional defiant disorder (Halfon et al. 2013), develop in subsequent stages of life. Differences in body size between the preschool phase and current measurements could then be related to the influence of drugs and comorbid disorders on the current height and weight in the study group. However, no differences between children currently treated and untreated as well as children with oppositional and defiant disorder/conduct disorder and children without these disorders were found in the study. We also assume that the number of boys with internalizing disorders (mood and anxiety disorders) in the sample was too small to affect the results obtained.

What is more, the rigorous selection of the studied sample and control group adopted in the study also allowed for the exclusion of the potential impact of variables whose effect on body size of children and youth has been well documented, such as birth body weight (Rillamas-Sun et al. 2012; Szwed and Kosińska 2012), family SES (Zawodniak-Szałapska et al. 2007; Ogden et al. 2010), and place of residence (Tambalis et al. 2013). This increases the probability that not only differences between the sample and the control group, but also differences between the preschool period and subsequent years of growth presented in the study were rather the effect of ADHD itself or other factors beyond control.

Our results are in accordance with earlier studies, which did not reveal an association between ADHD and excessive body weight in preschool children. One of them showed a lower BMI in children with greater manifestation of ADHD symptoms (estimated with the use of the Conners' Hyperactivity Index – parent version) at the age of 56 months (Heinonen et al. 2011). Results obtained by Ebenegger et al. (2011) indicated that a high level of hyperactivity/inattention was not associated with higher BMI and adiposity in preschool children. Another research with a separate analysis for preschool age (ranging from 2 to 5 years), showed no differences in the prevalence of overweight between children with ADHD and children in the general population (Curtin et al. 2005).

Several previous studies on older children and adolescents showed lower weight or height in participants diagnosed with ADHD (Spencer et al. 1998; Ptacek et al. 2009) or the lack of a relationship between ADHD and body weight, height, and BMI, and a lowered prevalence of overweight and obesity in that group (Dubnov-Raz et al. 2011). Nevertheless, many more studies indicated a relationship between ADHD and higher weight and BMI (Hubel et al. 2006; Hanć and Cieślik 2008a; Hanć et al. 2012) and a higher prevalence of overweight and/or obesity at school age, in adolescents, and in adults (Hanć and Cieślik 2008a; Cortese et al. 2013; Khalife et al. 2014; Hanć et al. 2015). In our study, boys with ADHD in school age and adolescence were shorter than the control group, which is consistent with some previous studies (Spencer et al. 1998; Ptacek et al. 2009), but were more likely to have higher BMI and be overweight, which is in line with the majority of studies evaluating the relationship of ADHD with body proportions and obesity (Hanć and Cieślik 2008a; Hanć et al. 2012; Cortese et al. 2013; Hanć et al. 2015). It suggests that the tendency toward excessive increase in body weight in boys with ADHD might rather be characteristic of the phases of development that follow the preschool period.

Our previous reports obtained from analyses of the same data set were focused on different analyses of body size in children with ADHD. In the first (Hanć et al. 2012) of two articles we have compared height, weight, and BMI of 135 boys with ADHD (mean age: 11.67 years) to the Polish growth charts recommended by the Polish Ministry of Health, attaching particular importance to measurement accuracy. The height of treatment-naïve boys with ADHD was not different from the norm. Nevertheless, the study revealed that weight and BMI were greater in boys with ADHD than was the norm. Weight was negatively related to the level of parents' education, and BMI was related positively to parents' BMI. The results of this first report suggested that ADHD is a risk factor for overweight. Therefore the aim of the second article (Hanć et al. 2015) was to assess the relationship between ADHD and overweight/obesity. We have compared height, weight, and the prevalence of overweight and obesity between the sample of boys with ADHD (n=219, mean age: 11.2 years) and the control group of boys without ADHD (n=396, mean age: 10.8 years). The research revealed that ADHD is strongly related to increased frequency of overweight irrespective of SES and education of parents, place of residence, ADHD type, term of birth, birth weight, or medication status, as well as comorbid disorders. What is more, there were no significant differences between compared groups in height and BMI. Current analyses on 112 boys with ADHD (mean age: 11.2 years) and 308 boys without ADHD (mean age 11.0 years), rigorously selected based on term of birth, birth weight, place of residence, SES, and parents' education, showed that ADHD was related to lower height and significantly increased rate of overweight irrespective of treatment status and comorbid disorders. Based on these three reports, the conclusion might be stated that school-age boys and adolescents with ADHD have normal or decreased height but they are more frequently overweight than boys without ADHD, and that these characteristics of body size are not related to the potential confounding factors enumerated.

The pattern of changes in body size in children with ADHD; that is, low birth weight revealed in many reports (Hanć et al 2015), smaller body size in preschool age, and increasing tendencies to be overweight from school age to adulthood could be explained by the fetal programming. According to this hypothesis, an alteration in fetal nutrition leads to developmental adaptation of the fetus' metabolism and physiology to an adverse environment (Barker et al. 1989). This could explain phenomenon that infants with low birth weight are more likely to have excessive growth that may cause obesity in later phases of development (Casey 2008). We have proposed this hypothesis as the explanation of the relationship between ADHD and lower birth weight as well between ADHD and increased risk of overweight in a previous article (Hanć et al. 2015). Nevertheless, the research found, the link between ADHD and overweight remained significant in analyses controlled for birth weight. Moreover, current analyses conducted on boys with birth weights between 2500 and 4000 g also showed that boys with ADHD had both a lower body weight in preschool and a higher frequency of overweight in school age and adolescence (mean age: 11.2). This indicates that the fetal programming hypothesis cannot be explanation of the link between ADHD and excessive weight.

In our previous (Hanć et al. 2012, 2015) and current analyses, we excluded the potential impact of SES, place of residence, date of birth and birth weight, treatment, and some comorbid disorders on the results. Therefore, we can conclude that the tendency to overweight in children with ADHD in school age and adolescence could be associated with psychological complications of ADHD. A higher percentage of overweight and obesity in school children with ADHD and in adolescents may result from low self-esteem, which is frequent in this group (Treuting and Hinshaw 2001; Hanć and Brzezińska 2009), and which may start to develop at that time (Barber et al. 2005), a well as from social exclusion (Diamantopoulou et al. 2005). Because of social isolation, children with ADHD may have fewer opportunities to participate in team sports associated with physical activity, and, therefore, spend more time watching television or playing on a computer than with their peers (Curtin et al. 2005). Peer rejection is often caused by the following behaviors characteristic of children with ADHD: Interrupting conversations, shouting out comments or answers to questions, failure of social cue recognition, and coping with frustration in an aggressive and impulsive way (Dumas and Pelletier 1999, Barber et al. 2005). Between 60 and 70% of children with a high level of ADHD symptoms do not have a friend in their class (Hoza et al. 2005), and their social problems increase as they progress from childhood to adolescence (Hoza et al. 2005; Cervantes et al. 2013). Additionally, a study by Ebenegger et al. (2011) showed that in spite of higher physical activity, preschool children with ADHD consume less healthy food or spend more time in front of the television. The beginning of school education (at 6–7 years of age, depending upon country of residence, and at 7 years of age in Poland for the examined cohort) also coincides with the onset of the critical phase for the development of self-esteem (Barber et al. 2005). At that very moment, children with ADHD may begin to realize their distinctness and the importance of social cues showing peers' dislike. Therefore, over time, because of social rejection, a decrease in physical activity and the establishment of unhealthy methods of compensation for emotional difficulties, children with ADHD might be steered toward more sedentary ways of spending time.

Also, shorter body height may be the result of the psychological consequences of ADHD. It seems that children with ADHD experience high levels of stress associated with school failure, in dealing with peers and adults, including parents. Previous studies revealed that the stressful events, adverse emotional atmosphere in the family, and negative emotional state of these children cause slower growth, and in extreme cases, result in so-called psychosocial dwarfism (Jovanović et al. 2003; Schooling et al. 2011; Hanć et al. 2014). Stressors cause an increased level of cortisol and a flattening of the daily cortisol profile (Vythilingam et al. 2010), which inhibits growth hormone (Pinto et al. 1999) and somatomedin (Thakore and Dinan 1994) secretion. Hyperactivity of the hypothalamic–pituitary–adrenal axis continues long after the stressor effect (Pervanidou et al. 2007); therefore, it might cause a long-lasting suppression of growth. In our study, both the lower body height and the higher incidence of overweight occurring in school-age children with ADHD and in adolescents may, therefore, be associated with high levels of stress, but this hypothesis has not been tested.

Limitations

The study has some limitations. One of them is its retrospective nature; we were not able to control the precision of the measurements of body weight and height (e.g., time of the day when children were measured, amount of clothing, techniques of measurement) at 2, 4, and 6 years of age. However, periodic examinations, during which the measurements were made, should be conducted in a standard way by trained health professionals with the use of certified measurement devices. It enables us to assume that the measurements were taken with satisfactory reliability.

The rigorous selection method applied in the study could be considered as a limitation because it caused the exclusion of a large number of boys from the ADHD group (62%) and the control group (80%). Nevertheless, by applying this method, we were certain that observed results of the comparison between groups were not affected by a few important confounding factors such as SES or the newborn's health, whose relationship with ADHD, or what was more important for us, with the body size of children, have been well documented previously. The use of a rigorous selection method could be then considered as an approach that significantly simplified the interpretation of the results obtained.

We did not have data on symptoms of comorbid disorders in the preschool period. Because the rate of additional diagnoses was high, there is a possibility that some of disorders might start to develop during preschool years. Therefore, we cannot exclude the possibillity that a few disorders such as depression, anxiety, or oppossitional defiant disorders could affect growth in some of patients (3%, 7%, and 34% respectively) in analyzed classess of age. However, the analysis of differences in current body size between boys with oppositional defiant disorder/conduct disorder, and those without these disorders did not reveal significant differences. Therefore, we concluded that these externalizing disorders, which frequently occured in the sample, did not affect growth in the preschool period.

In the present study, two factors definitely related to body weight, BMI, and body fat content, such as diet and physical activity, have not been studied, and therefore could not be used as control factors in the analysis. This is a limitation, as the current study of Ptacek et al. (2014) shows that boys with ADHD are more likely to have disruptive patterns of eating behavior, and the relationship among symptoms of ADHD, physical inactivity, and obesity has been shown in the birth cohort study of Khalife et al (2014). Additionally, in this article, we are putting forward hypotheses associating changes in body size over time with changes in psychosocial functioning and stress, whereas the examination did not include assessment of the psychological and social consequences of ADHD. Excessive body weight in patients with ADHD later in life may also be associated with specific genes (Albayrak et al. 2013), executive function deficits (Choudry et al. 2013), or daytime sleepiness (Cortese et al. 2008). The mentioned variables should be taken into account in subsequent researches on growth and the body fat content of children with ADHD.

Conclusions

Our results prove that boys with ADHD at the ages of 2–6 years do not differ distinctly from boys without ADHD in terms of height. However, at some ages a tendency can be observed for smaller body weight (age of 2 and 4), BMI (age of 2), lower prevalence of overweight/obesity (age of 2), or higher prevalence of underweight (age of 6). On this basis, it can be concluded that when SES, place of residence, term of birth, and treatment are controlled for, preschool boys with ADHD present a significant tendency toward lower body weight in comparison with a control group of children without mental disorders. In the later years of life, however, ADHD is associated with a lower body height and a higher incidence of overweight, which may be the result of psychological factors and is not related to treatment and the presence of comorbid disorders.

Clinical Significance

Earlier epidemiological and clinical studies, both cross-sectional and longitudinal studies, have shown a relevant relationship between ADHD and increased weight, BMI, overweight, and obesity. Our research showed that preschool boys with ADHD have a tendency toward a lower weight than their peers. Increased incidence of obesity in individuals with ADHD is characteristic of the later stages of development, and should be considered as the important comorbid condition of ADHD.

Footnotes

Disclosures

No competing financial interests exist.

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