These considerations have led the authors of this study to discuss the findings of neuroimaging techniques used to investigate the neurobiology of BI distortion in the ED population, but without losing sight of the repercussions that the brain can have on mental and behavioral functions [66].
Prevalence data is vital for planning health care and the demand for this service [67]. The global prevalence of EDs is moderately high. Additionally, many individuals affected by this psychopathology do not seek treatment, which can worsen the condition [24, 67].
The DSM-V [6] states that AN’s 12-month prevalence (period prevalence) in young American females is approximately 0.4%. In Keski-Rahkonen and Mustelin [68], the lifetime prevalence of AN in European women is between < 1% and 4% [67, 69, 70].
As for the prevalence of the BN diagnosis, the DSM-V [6] indicates that, over 12 months, the prevalence is 1 to 1.5% among young American females. The lifetime prevalence among European women is similar to that reported by the DSM-V and falls between < 1% and 2% [67–71].
The 12-month prevalence for BED in American adult men and women is 0.8% and 1.6%, respectively. Lifetime prevalence in European women is approximately 1–4% [67–70]. Kessler et al. [71] reported similar findings in a study involving data from 14 countries within the WHO Global Mental Health Research Initiative, including Brazil, emphasizing its main urban area: the city of São Paulo.
Hay et al. [72] found that the prevalence of any ED was 16.3% in a three-month point prevalence assessment of the Australian female population. The diagnoses of AN, BN, and BED together amounted to a prevalence of 6.7%. Regarding each specific pathology, the following prevalence data is cited: AN = 0.46%; BN = 0.66%; BED = 5.58%.
So far, the moderate to high prevalence of AN, BN, and BED, as well as the 25% global increase in all EDs [24], highlights the need for research and healthcare attention directed at this psychopathology, which presents a high risk of suicide, especially in AN and BN diagnoses [6, 67, 68]. AN is the psychiatric disorder with the highest mortality rate among all mental disorders, considering that suicide comprises a significant portion of deaths (according to DSM-V [6], the rate would be 12 per 100,000 per year) [67].
Given that EDs have multifactorial causes and the diagnosis is only made official when all the criteria are met, it is worth noting that the increase in prevalence is quite alarming since many individuals can experience some of the symptoms for months or years before reaching all the criteria to be diagnosed. For this reason, studies into the factors that lead to ED, such as body image, need to be considered relevant when developing public policies.
The findings regarding the countries from which the research in this study originated [Europeans (54.8%), Asians (19.3%), Americans (6.45%), and Oceania (6.45%)] are compatible with the prevalence figures for ED, as well as the high mortality rate. This data indicates that when faced with high ED prevalence, these countries invest heavily in research related to the subject. Our findings also corroborate this information with the significant increase in research using neuroimaging techniques, especially between 2010 and 2013, to investigate BI in EDs.
It is worth noting that no evidence involving neuroimaging techniques has been found in Latin American and African countries. Therefore, some inferences can be drawn: due to the precarious economic conditions of some African countries, little investment has been made in mental health research and care; research that uses neuroimaging techniques is high cost and often not compatible with the countries' investment possibilities; epidemiological data on ED are scarce and difficult to access; Some responses from voluntary participants could correspond to diagnostic criteria for ED, however, when taking into account the socioeconomic and cultural context, the diagnoses are not valid [73]. In the review by van Hoeken et al. [73] no cases of AN were found in the epidemiological data but the prevalence of BN was 0.87% (similar to that described in the DSM-V [6]). On the other hand, the prevalence of unspecified eating disorders (including the diagnosis of BED according to the DSM-IV reference) was 4.45%, which, when considering BED, would be close to the prevalence described in the DSM-V [6].
When it comes to evaluating the ethnic variable, the data on the population of young African women is very similar to that found in American women of African descent, as well as in Latin American countries, which is to say that this population seems to be more affected by BN and BED diagnoses [6, 73]. Some scholars stipulate that this fact is associated with the local standard of beauty and cultural factors that become protective (e.g., Black North American women are more likely to be satisfied with their bodies when compared to Caucasian North American women) [74, 75]. However, it is also hypothesized that the diagnosis of ED is underdiagnosed due to the lack of specialized professionals [67, 73, 75].
As for AN diagnosis, according to the prevalence data, Caucasian women are known to be more affected. However, it is also worth noting that in countries with a higher concentration of Caucasian women, there is greater investment in mental health and research on the subject, as was verified in the results indicating the countries of origin of the articles collected in this review [73].
The increase in the prevalence of ED in developing Western countries, which includes Latin American countries, has been detected recently. For BN and BED diagnoses, the data is similar to that found in American, European, and Asian studies [71, 76–79]. In Brazil, specifically, the prevalence of BN is 1–2%, and BED is 4.7% [70, 71, 75]; concerning AN, there is no record of Brazilian data on the prevalence of this diagnosis, which cannot be understood as being inexistent. Thus, some studies stipulate that data from Latin America, including Brazil, for AN prevalence would range between 0.1–0.5% [6, 75, 79, 80].
In conjunction with the absence of more advanced research, the data above demonstrates a gap in Brazilian scientific production. It should be noted that the prevalence of ED in Brazil is close to the data found in the areas mentioned in the previous paragraphs, which would justify greater financial incentives for national research with advanced technology, as would be the case with research using neuroimaging techniques. Also greater collaboration between countries for conducting research as well as for potential findings and treatment development that would benefit everyone.
Both the literature on prevalence and the literature assessing the risk of developing ED point to the importance of conducting research focusing more specifically on body image, given that BID is a central symptom in EDs, even acting as a factor in maintenance or recurrence. Furthermore, it is worth noting that body dissatisfaction is a risk factor that has already been well-documented in the literature [6, 30, 76, 81–85].
Upon observing the samples of studies in this review, most papers had an AN diagnosis (58%), with volunteer patients who had a mean BMI of 16.36 kg/m². The mean age of these participants was 24.84 years, consistent with the indication in the literature that this psychopathology is more common in young women and becomes rare in women over 30 [24]. Considering this data on BMI and age, we could classify this public as having a moderate severity level of the disease if we apply the benchmarks set out by the DSM-V [6]. Namely, AN is a psychopathology of greater severity due to the patient’s low weight. It is also a psychopathology in which BID is found more concretely, often evidenced by emaciated bodies.
The number of studies conducted with BN diagnoses (19%) was lower than those comparing AN and BN diagnoses (23%). This result could indicate that, since these patients do not have a significantly/radically low weight as in cases of AN, research has focused less on this population, even though the prevalence of BN is higher than that of AN. The data on mean BMI (22.11 kg/m²) and mean age (24.65 years) are compatible with the literature. In other words, BMI in BN pictures tends to fall between eutrophy and overweight [6], and adolescents and young adults are at risk of developing an ED [24].
In this study, there was no evidence of purging disorder. Likewise, there were no studies with BED cases, although the study by Beato-Fernández et al. [60] included the former diagnosis of non-purging bulimia (still present in the DSM-IV), which would currently qualify as a BED diagnosis. As such, the results presented in the study by these researchers allow us to consider certain data on this diagnosis and its differences in the context of BI compared to AN and BN.
All the selected articles were conducted with female participants, which points to another critical gap and, therefore, calls for further research involving the male population. It could be inferred that this result is associated with the underdiagnosis of this psychopathology in men because it is treated as exclusive to women. However, as explained in the introduction, the prevalence ratio between men and women regarding this disorder is 3:1 [86–88], which shows that there are more cases of ED in women. It also demonstrates that the number of men is considerable and has been increasing recently [69].
In studies with AN patients, three brain areas have been emphatically highlighted as those that may show some alteration and, consequently, affect the processing of somatosensory, interoceptive, and emotional signals: the insula, the rostral anterior cingulate cortex, and the prefrontal areas (premotor cortex and left medial orbitofrontal cortex). Seven articles refer to the reduction in insular activity and its association with body image distortion because the impairment of this area leads to losses in the integration between visual and body perception and emotions [37–43].
Among these seven articles, Gaudio et al. [38] and Horndasch et al. [39] assessed the adolescent population. The mean disease duration was 4 months (+/- 1.8) and 16.86 months (+/-15) respectively. Interestingly, these articles that assessed the early stages of the condition are associated with the brain structures in which some alteration in functioning was detected, potentially showing data that is more closely related to what seems to occur at the disorder’s beginning and onset. Based on the identified neural correlates, it is impossible to indicate causes. However, it can be inferred that the affected areas are related to an excess of self-referential thoughts (rostral anterior cingulate cortex); difficulty recognizing the internal body signals, such as interoception and emotions (insula); impaired learning about taste, smell, and visual recognition of objects (left medial orbitofrontal cortex); motor dysfunction, excessive movement, or postural inability – proprioception (paracentral lobe and left cerebellum). The superior occipital cortex is responsible for visual processing, and the cerebellum is involved in eye movement, while the orbitofrontal cortex is where information on abdominal distension and body weight is integrated.
Therefore, based on these altered correlates in the condition’s early stages, self-awareness appears to be lagging, starting with interoceptive awareness and the recognition of emotions. Frank [89], Frank [26], and Mishra et al. [28] cite alterations in the insula, which corroborates what was found in the present study: deficiencies in the insula are associated with problems in interoceptive awareness, reward signals, and body satisfaction, both in AN and BN pictures.
Due to alterations in the orbitofrontal cortex, the learning association between stimulus and reinforcement may be impaired, as well as the correction of this association with the alteration in the contingency of the environment, meaning that when the reward becomes reinforcing or punishing according to the environment, the subject must be able to decode the stimulus [90]. This characteristic seems to be contradictory since the act of eating is seen as punitive, while the radicalization of body emaciation is seen as reinforcing.
In addition to this possible deficiency in learning associated with the orbitofrontal cortex, it is well known that the visual pathways from the occipital cortex are directed to the orbitofrontal cortex. In other words, the relationship and altered structures at the very beginning of the disorder seem to lead to the idea that there is an error in judgment in EDs. Rachlin [91] defines judgment as a reference for decision-making, i.e., judgment is an internal and subjective process through which the individual interacts with the environment. Some judgment mistakes can be perception mistakes since perception is not a concrete or factual representation of reality but has an adaptive character that aims to allow the selection of certain behaviors in favor of survival [92–94].
Comparing data on the early development of AN in adolescents with data from remitted patients shows that, in remission, the angular gyrus/supramarginal areas still undergo some alteration in their functioning. The angular gyrus is located in the posterior inferior parietal lobe and is related to higher cognitive functions such as: attention, spatial cognition, conceptual representation, semantic processing, language, reasoning, social cognition, episodic memory, mentalization, emotional regulation, processing the judgment of some characteristic concerning one’s image or mental concept of oneself.
Although no results of alteration in the connectivity of the default mode network were found in this study with the AN adolescent population, it is known that the angular gyrus/supramarginal area is related to this network of areas with high metabolic activity and blood circulation at rest [43]. Other functional networks involved with the angular gyrus are the cingulo-opercular, frontoparietal, and attention-ventral networks. Therefore, in AN, initially, one of the networks of altered brain areas is associated with the functioning of the self, with the integration of learning about which stimulus can be reinforcing or punishing, with motor/postural ability (proprioception), with visual processing and with body image distortion. In remitted pictures, the alteration in the angular gyrus is maintained, which seems to perpetuate excessive thinking about oneself in concept and relation to self-image. Finally, body image distortion seems to be related to perceptual error in self-assessment (judgment).
The article by Domakonda et al. [52], involving adolescents diagnosed with BN (mean duration of illness 29.7 months), mentions alterations in the default mode network in conjunction with the ventral attention network. In BN cases, but not in AN pictures, alterations in the default mode network and the attention-ventral network appear to have contributed to the explanation of attention biased towards self-referential thoughts with an emphasis on body shapes and weight [89].
Carhart-Harri and Friston [95] indicate that the default mode network is consistent with Freudian descriptions of ego functions (secondary processes – reception and containment of endogenous excitation, i.e., the ego is the psychic entity through which the drive is invested/destined). It is worth highlighting that egoic development and functioning resonates with the default mode development and functioning, which occurs with an intense increase in connectivity during puberty. Knowing that ED pictures usually occur concomitantly with early adolescence, it can be inferred that the alteration of the ventral attention networks and the default mode begin to take place in this period, generating persistent self-referential thoughts [95, 96].
In BN diagnoses, the insula is also an area that appears altered. All the articles included in this review associate alterations in insular functioning with tasks that measure body satisfaction; that is, satisfaction with the body seems to be related to interoceptive awareness, the recognition of affect, and reward signals. Both increased and decreased insular activity were detected in accordance with the type of task and exposure to emaciated or bulky bodies.
Two articles that evaluated AN patients and involved level and satisfaction tasks found greater insular activation when patients were confronted with emaciated and slender bodies. These findings indicate an experience that evokes stronger emotions, and this form of modification of insular functioning can lead to distortion between the perceived body and the ideal body [37, 41]. However, when the patients were confronted with their appearance, the insula responded with suppression, as did the attentional system. These results suggest that alterations in this connectivity may be related to BID in AN.
In research comparing the acute AN and remitted AN groups, it was observed that the reduction in gray matter volume and auditory network connectivity (including the right insula) were recovered in the remitted AN group due to the renutrition of these patients [43]. Therefore, the connectivity between the insula and the post-central gyrus in deficit in acute AN is recovered with regaining weight (renutrition) [43]. Recovery from malnutrition leads to the restoration of gray matter volume and improved self-perception in a social context, according to the original selected articles [40, 43].
According to Craig’s studies [97, 98], the different sensations coming from the body first activate the insula, which contains selective modality components and includes a wide variety of sensations ranging from pain to taste, as well as subjective sensations and feelings of all kinds. Craig [97] expresses the view that integration (including interoceptive activity) within the insula generates a model for a feeling, which can be understood as a neural representation of sensory-motor homeostatic conditions (amount of energy used – CNS economy and, therefore, psychological). In the author’s opinion, these conditions would support the evolution of human consciousness (sentience of being and observation of the feeling of knowing).
In accordance with the findings of this study and the literature, particularly the evidence found in Craig [97–99], there are alterations in the integration of the insula lead to sensory, sentimental, emotional, perceptual, cognitive (the decision-making process), and subjective (introspection) impairments, which possibly modulate self-awareness. Thus, it could be considered that the insular alteration in ED patients is related to body image distortion, symptoms of not recognizing internal signals (interoception), excessive thoughts about oneself, and difficulty in perceiving one’s feelings (which are commonly associated with discomfort with body size, dimension, and shape).
The joint activation of the insula and the anterior cingulate cortex is detected when experiencing emotions, which would justify the lack of recruitment of these areas in the AN group since this psychopathology identifies a delay in the perception of one’s own emotions, in interoceptive awareness, and in somatosensory signals processing. The inclusion of areas such as the amygdala and medial prefrontal cortex in the insula and anterior cingulate cortex generates the emotional salience network, which seems to be the basis for emotional awareness of cognitive functions (influence of emotions in cognition). In EDs, alterations in this network and these areas have been identified [66, 96, 97, 99]. Increased connectivity in these areas results in excessive self-referential thinking. BN pictures show increased connectivity between the anterior cingulate cortex and the precuneus, pointing to a possible connection with high concern about the body [66, 96, 97, 99]. These alterations lead to emotional responses such as body dissatisfaction. With the prefrontal areas involved, the behavioral response is greater body checking.
It is known that proprioception has been mentioned a few times throughout the text due to the observed alterations, which are reflected in postural inabilities and difficulty in recognizing posture. Therefore, a brief explanation is in order about proprioception being a sense that implies the feeling of position and movement of the body [18, 100]. Although this sense is not conscious, it affects self-perception awareness, i.e., the learning process involves self-awareness. From this, it can be inferred that the way one learns involves the self-perception of each subject, which is closely linked to body perception (formation of the body schema) that arises in the light of one’s own senses, including proprioception[18, 101–103].
Castellini et al. [46], one of the collected articles in this study, showed that the activations of brain areas increased or decreased according to exposure to images of bodies, from which it was argued that altered connectivity of some brain areas affects self-representation emotionally and cognitively. According to Craig’s hypothesis [97, 99], the insula is involved in receiving and integrating various emotional and behavioral conditions, as well as being a fundamental basis for human consciousness. Therefore, the formation of cognition associated with the self would affect the perception of one’s image.
The caudate nucleus is also cited as an affected area in AN and BN. In AN, the reduced activation of the caudate nucleus seems to be associated with processing images of the human body, perceiving touch, and self-representation [39, 47]. In BN, this area is related to impulsive behaviors (binge eating) [58]. Other areas connected to the caudate nucleus, such as the occipital cortex, the extrastriate body area, and the fusiform area, are associated with the actual perception of the body in terms of size and shape. Temporal and parietal areas, with specificities reported in the results, are related to body movement, size, and comparison behavior. All these alterations would justify the distorted recognition of the body in ED cases. All these alterations would justify the distorted recognition of the body in ED cases.
Analyzing the results of this study, the authors can say that an important part of body image distortion – the feeling of being inadequate and body dissatisfaction – is related to the insula, and the distortion and perception parts are related to the occipital, temporal, and parietal areas. From the moment we get to know the world and experience life through the body, a distorted conception of the self as a whole, and especially through the body, occurs in these individuals. Regarding the severity of symptoms, from the sample collected in the articles, all patients diagnosed with AN were severe based on the average BMI of 16.36 kg/m², but it was not possible to determine whether neuroimaging techniques could indicate/diagnose the severity of BID.
In summary, given that ED patients, whose central symptom is BI distortion, seem to have an error in judgment that affects their perception of sizes and dimensions according to the evaluation and comparison among the reference options found in their environment, it can be suggested that further research evaluating the judgment process in ED patients concerning body image should be carried out in order to provide more evidence in this area.
Some guidelines regarding treatments can be outlined, suggested, and reaffirmed according to what is already known in terms of therapeutic practices, so that they can be applied in an associated and interdisciplinary manner.
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Neuropsychological rehabilitation that considers the judgment mechanism ecologically, meaning it is adapted to treat body image distortion in patients with eating disorders, as well as other distortions associated with the size and dimensions of objects (as found altered in the ED group) [104];
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Psychotherapies in which the psychologist is attentive to integrating the patient's emotions and perception of self-image, and establishes connections between control behaviors and overeating related to body image misjudgment. It is also worth noting that knowledge about body image and its neurological and behavioral repercussions is crucial for psychological interventions concerning perception, judgment, and implications for the individual's processes of evaluation, decision-making, and choice (self-observation);
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Promoting body reconnection and satisfaction through intuitive exercise. This model encourages mindful practice that focuses on recognizing bodily signals, thereby fostering a deeper connection with oneself during exercise, preferably with the aim of promoting body satisfaction and maximizing pleasure in physical activity. Intuitive exercise serves as a preventive measure against dysfunctional exercise driven by body dissatisfaction or body image distortion [105–107];
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The intervention technique on body dimension perception applied by physiotherapy aims to improve the accuracy of recognizing the actual size and shape of the body [108]. This intervention approach, developed through scientific research and clinical experience at AMBULIM-IPq-HC-FMUSP [108, 109], involves exercises in judgment regarding size and dimension. These activities confront the patient with their perceived size and their actual size, allowing for comparison and verification of the difference between distortion and reality [108, 109];
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Meditative practices can contribute to interoceptive awareness and emotion recognition [110].