Introduction

Attention deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder (NDD) that has been estimated to occur in 5% of children in the United States, according to the Diagnostic and Statistical Manual-5 (DSM-5).^1,2) Specifically, the prevalence of ADHD is around 5.3% in the pediatric population and 3.4% in the adult population.^3,4 The persistence of the condition into adulthood is increasingly acknowledged, with data reporting that over half of children diagnosed with ADHD have symptoms that persist into adult years.⁵⁾ In DSM-5, ADHD is defined by symptoms of persistent inattention and/or hyperactivity and impulsivity out of proportion to the degree of inattention/hyperactivity or impulsivity seen in similarly-aged peers.¹ The relationship between ADHD and eating disorders (EDs) has been recently repeatedly suggested. Both ADHD and EDs can be chronic and persist into adulthood.⁶ EDs most frequently reported to be associated with ADHD are binge eating disorder (BED) and bulimia nervosa (BN).^7)-(12 BED is defined by recurrent episodes of binge eating associated with guilt, disgust, and marked distress but without compensatory behavior. BN is characterized by binge eating episodes in a short period of time (approximately 2 hours) followed by inappropriate compensatory behaviors. The diagnostic criteria of anorexia nervosa (AN) includes three essential aspects: persistent restriction of energy intake and/or purging behavior, intense fear of weight gain, and disturbance in the self-perception of one’s body shape.²⁾ Although the link between AN and ADHD is controversial, some studies suggest an association between both.^13,14 DSM-5 also includes other eating and feeding disorders occurring earlier in childhood, such as pica, rumination disorder, and avoidant/restrictive food disorder. In contrast to AN and BN, these disorders are not associated with concerns about body weight and shape. AN and BN typically manifest during adolescence and are strongly tied to maturation processes during later developmental phases.¹⁾ Patients with ADHD and EDs share common features, such as impulsive behavior, disturbed reward encoding, low self-esteem, and a neuropsychological profile including deficits in attention and executive functions, which lead to altered motivational control and attentional biases.¹⁵ Additional insights into the comorbidity and overlap between ADHD and EDs is of major relevance, as it may contribute to the understanding of possible pathological mechanisms underlying both conditions and have important implications for the management of patients with comorbid ADHD and disordered eating patterns. Common effective therapeutic strategies may be particularly relevant, given the personal and social adverse outcomes that these comorbidities may elicit.

Given these considerations, the objectives of this study were to critically review the prevalence and overlap of ADHD and EDs, as well as evidence in the literature of an association between both conditions, seeking to clarify some of the possible mechanisms underlying their comorbid relationship.

Methods

A narrative review was conducted based on studies retrieved from PubMed database using the following combined MeSH terms: “attention deficit hyperactivity disorder” OR “ADHD” AND “eating disorders” OR “anorexia nervosa” OR “bulimia nervosa” OR “binge eating disorder”. The “related articles” function of PubMed, bibliography of selected articles, and Google Scholar database were also used to identify additional studies. Pica, rumination disorder, and avoidant/restrictive food disorder were not object of this review. Inclusion criteria comprised primary sources specifically focusing the association between ADHD and EDs. To enable a comprehensive literature review, no constraints were considered regarding year of publication, language, or methods used in studies.

Results

Based on predefined inclusion criteria, a total of 22 articles were eligible for the review.

Results showed that several studies have focused on the prevalence of EDs in ADHD, with the opposite association also being investigated (Table 1). The prevalence of EDs in patients with ADHD has been reported across a range up to 12 %.⁶ The literature describes an association between the development of ADHD during childhood and later development of EDs during adolescense.¹⁶ In a systematic review and meta-analysis, Nazar et al. showed that patients with ADHD have a higher risk of comorbidity with EDs, reporting a 3.82 odds ratio of developing EDs in the context of ADHD. People with EDs also have higher levels of comorbidity with ADHD, with an odds ratio of 2.57 of ADHD diagnosis in individuals with EDs.¹⁷ In a study with 4719 subjects, Zibrowski et al. reported that lifetime ADHD was strongly and significantly associated with lifetime BN, BED, and any ED in unadjusted models, but not with AN or subthreshold BED. After adjusting for demographic variables and psychiatric comorbidities, all associations between lifetime ADHD and EDs were substantially attenuated, with only the association between ADHD and BN remaining statistically significant.¹⁸ Similarly, Surman et al. conducted a retrospective systematic analysis to evaluate BN rates among patients with ADHD using data from four case-control studies (two in adults [n=742] and two in children [n=522]). The prevalence of BN in the pediatric ADHD group (n=280) was 1% and in the pediatric control group (n=242) was 0%. Adult women with ADHD had significantly higher rates of BN than those without ADHD: 11.2% (n=124) versus 2.0% (n=191). These findings suggest that patients with ADHD may have a higher risk of developing BN compared to non-ADHD counterparts.¹⁹ Conversely, the prevalence of ADHD symptoms in patients with EDs has been reported to range from 5 to 17%.²⁰ A systematic review reported a prevalence of ADHD in EDs between 1.6 and 18.0%, being more common in the AN purging subtype (AN-BP) and BN than in the AN restrictive subtype (AN-R).²¹ Svedlund et al. reinforced these results in a study assessing the prevalence of ADHD in 1165 adults with Eds, which also described a higher frequency of ADHD symptoms in patients with binge eating/purging eating disorders, suggesting that the frequency of ADHD symptoms in AN purging subtype is as high as in BN.²² Sala et al. assessed 73 females with longstanding history of EDs, 37 of whom with AN-R, 28 with AN-BP, and 8 with BN. A total of 13 patients evidenced comorbidity with ADHD in the three ED subgroups globally: three in the AN-R, nine in the AN-BP, and one in the BN subgroups. The remaining 60 patients presented only ED diagnosis. Anxiety differed significantly between patients with and without ADHD, with the first showing higher scores. These results suggest that previously observed associations of ADHD with EDs might be due to additional psychiatric disorders that are often in comorbidity with both conditions.^15,22 The study by Yates and colleagues included a sample of 189 individuals (55 classified as having the AN-R subtype, 97 the AN-BP subtype, and 37 BN) and found that 5.3% of participants met full criteria for ADHD during childhood. Overall, 6.7% of individuals with diagnostic criteria of ADHD belonged to the AN-BP or BN groups and only 1.8% belonged to the AN-R group. Over one-fifth of the participants (21.2%) met criteria for at least six ADHD symptoms in at least one ADHD domain.²⁰ In addition, Seitz et al. showed that 21% of patients with BN met clinical criteria for childhood ADHD compared to only 2.5% of healthy controls. Adult ADHD was also more prevalent in patients with BN, with an odds ratio of 4.2. Patients with BN and previous childhood ADHD were more impulsive and inattentive than patients only with BN, with the authors suggesting an additive effect regarding impulsivity, with the occurrence of depression and anxiety causing more emotional stress, which would worsen both ADHD and BN symptoms. These patients also displayed more severely disordered eating patterns and more general psychopathological symptoms compared with those without ADHD. The severity of ED symptoms was better explained by inattentiveness than by impulsivity or hyperactivity.²³⁾ Conversely, a five-year prospective study found that children with ADHD showed higher levels of BN-type eating pathology at follow-up than those without ADHD, with impulsivity being the strongest predictor of ED behaviors.⁸ The literature describes common neurobehavioral circuits between ADHD and binge eating behaviors, such as those involved in response inhibition, emotion regulation, and reward processing.^15,24,25) When analyzing shared traits, it should be considered that additional similar comorbidities might result in clinical and symptomatic overlap. For example, anxiety disorders and mood disorders are frequently reported as ED and ADHD comorbidities.^22,26,27 In addition, impulse control problems have been reported in patients with EDs (especially BN and BED) and with ADHD.²³ Alcohol and drug abuse in particular has been observed in patients with EDs and with ADHD.^28,29

Discussion

This study assessed the prevalence of ADHD in EDs and vice-versa. It could be discussed whether ADHD itself can be a predisposing or vulnerability factor for the development of EDs, which mainly occur during adolescence.³⁰ Developmental changes, such as puberty and stressful events, could trigger ED behaviors, especially in patients with ADHD.³¹ In addition, patients with NDDs have an increased risk of other mental disorders beyond those in the ED spectrum.³² As such, EDs might be regarded as one of several possible progressive and comorbid conditions in patients with NDDs. The literature suggests the potential benefit of screening for ADHD as part of the overall assessment of EDs, in particular BN, BED, and AN-BP.²¹ Impulsive behaviors represent a core ADHD symptom, and deficits in their regulation have been shown in ED patients, especially in those with binge eating and purging behaviors.⁶ The association between ADHD and EDs may also be explained by a combination of genetic mechanisms. Genetic variability in dopaminergic genes related to reward processing has been described in patients with comorbid ADHD and BED.^33,34) On the other hand, the number of ADHD symptoms correlate with ED symptom severity in all binge/purge ED subtypes, and ADHD symptoms have been found to predict binge eating severity and bulimic symptoms.^35)-(37 These findings may have important implications for prevention and early intervention programs, which could focus on mood regulation in individuals with ADHD symptoms at risk for developing disordered eating. Future studies should address whether patients with these comorbid conditions have different prognosis, course of disease, and treatment response compared to those with either disorder alone.

Conclusion

Only few studies in the literature have addressed a direct causal relationship between ADHD and EDs, suggesting an overlap between both conditions. However, these studies have mostly included small sample sizes and used disparate approaches, which limits the generalization of results. EDs and ADHD share several key features that may explain the overlap of both diagnoses, including attention deficits, impulsive mechanisms, and low self-esteem. Future studies are required to gain additional insights into the association between both disorders and improve the clinical management and treatment of patients with this comorbid association.