The aim of the current study was to examine the relationship between each subtype of EDs and ASD tendency regarding the presence or absence of self-induced vomiting. We first compared the illness duration, BMI, EDE-Q scores, and AQ scores in the four groups (BN, AN-BP, AN-R, and BED) that were diagnosed by a psychiatrist as per the DSM-IV and the DSM-5 (Table 1). Next, the four subtypes were divided as per the presence or absence of self-induced vomiting; the illness duration, BMI, age, EDE-Q, scores and AQ scores were compared (table2). Illness duration could be a confounding factor; therefore, we performed additional analysis to confirm it. Furthermore, assuming that BMI is a confounding factor, we compared EDEQ and AQ scores between anorexic group (AN-BP and AN-R) and bulimic group (BN and BED). (Table 3) Finally, we examined the ratio of the subtypes and the number of patients who exceeded the cut-off value of the AQ score (Table 4). Here, BMI could be confounding factor, and additional analyses were performed to confirm this.
We had hypothesised that patients without self-induced vomiting had a higher tendency for ASD; however, there was no difference in the AQ between AN-BP and AN-R. The significant difference in the AQ score between BN and BED was clear. Of the four ED subtypes, BED had the highest ASD tendency. In the four groups, AN-BP had the longest duration of illness and was significantly longer than AN-R.
The significantly short illness duration in AN-R indicates that rigid food restrictions cannot continue for a long time, with most patients ultimately switching to other ED subtypes. Previous studies have shown that most patients switch to BN  because the patient’s ability to control or restrict food lessens after a period of severe food restriction (i.e., as a normal physiological response to starvation). In turn, this leads to increased food consumption accompanied by self-induced vomiting in a belief that this will prevent weight gain while still being able to eat. However, the reasons why some AN-R patients maintain strict food restrictions and do not transition to other ED subtypes are less well understood. Considering the characteristics of ASD, it is possible that some patients do not transit from AN-R because they follow strict dietary rules, such as only eating certain items at all times, due to preservation of sameness. (or Considering the characteristics of ASD, it is possible that people who follow strict dietary rules, such as only eating certain items at all times, due to preservation of sameness are included in AN-R.)
On the other hand, some patients say that they ‘would never want to vomit’ and are tolerant of weight gain even if they start overeating. In these BED patients, the absence of self-induced vomiting means that they tend to be obese , and it is unclear why they do not vomit in the face of weight gain . If considering the characteristics of ASD, it may be possible that some patients with BED do not vomit because another obsessive compulsion arising from ASD is stronger than the core psychopathology of the ED, i.e. fear of being fat and the failure of severe restriction leads to acceptance of weight gain. This is seen clinically in our practice with comments from patients such as “I am scared to vomit,” “My life is over when I am vomiting,” or “Looking at vomit disgusts me.” In such instances, the fear or aversion to vomiting might be stronger than the desire not to gain weight. In addition, patients who were absorbed in dietary restrictions were able to postpone the desire to lose weight due to the disgust of vomiting along with the failure of restrictions, and to endure the weight gain somehow could not be explained by the psychopathology of EDs. In some cases, from the experience of vomiting once in the past, people have a strong sense to visceral sensations and/or disgust of vomiting; they are unable to forget the trauma and find it difficult to eat food because they do not want to vomit again. People with hyperesthesia within the autism spectrum are reluctant to self-induce vomiting. In addition, some patients cannot eat because they are afraid of vomiting. Therefore, detailed assessment of vomiting in each patient should be needed.
If there is an ASD tendency, it is easy to fall into maladaptation to environment because of lack of flexibility, and the behaviour is likely to become a routine. Social skill and communication skills scores of AQ in patients without self-induced vomiting were higher than those in patients with self-induced vomiting in this study. In a further analysis, patients with BED had by far the highest AQ score among patients without self-induced vomiting, and there was no difference in the AQ scores between AN-BP and AN-R.
The maladaptation to environment may lead to sameness behaviors, such as routine dietary patterns of patients, and environmental adjustment is first necessary. Therefore, evaluating patients for the presence of self-induced vomiting when assessing them for EDs may help us understand the association with ASD tendencies.
To the best of our knowledge, this is the first study to compare the predisposition for ASD by ED subtype and the presence or absence of self-induced vomiting.
For patients with ED coexisting ASD, long-term treatment may be needed. If they are maladaptive to environment, adjustment of environment is needed. For ED patients it may be necessary to prioritize the identification of characteristics over diagnosis. When assessing ED patients who never vomit, it is important to clarify the reason for that, i.e. what they are afraid of as a consequence of vomiting.
This study had some limitations. Of note, the sample size was small, there were differences in the number of participants in each subtype, and we only assessed the tendency for ASD using the AQ. There were no data for healthy subjects to compare patient data with standard values. Besides the above, there was a large age spread. In general, it is known that AN-R is much more common in the younger ages, and self-induced vomiting usually appear later on in adolescence/young adulthood. The differences of subjects' age, BMI and illness duration may always be a confounder and should be controlled it with bigger sample size.
Symptoms of depression and anxiety disorders, irritability, emotional lability, and obsessional features are frequent accompaniments in ED. Typically, these features worsen with weight loss and improve with weight regain . Interest in the outside world also declines as patients become underweight, with the result that most patients become socially withdrawn and isolated. Since we did not measure anxiety or depression in this study, it is unclear how these were associated with ED and AQ scores. Since ED has a high incidence of anxiety and depression, it should be added to the evaluation index in the future study.
This study used AQ for evaluation of ASD. We should have used AQ-10 because a few adolescent patients were included in our subjects. However, AQ is a self-completed scale and not the assessment measure to diagnose ASD. Using Autism Diagnostic Observation Schedule(ADOS) or Autism Diagnostic Interview (ADI) for ASD evaluation and excluding patients with extremely low body weights and including control subjects in a larger overall sample are needed in the future research [9,21].