This study aims to assess the feasibility and acceptability of Metacognitive Training for Eating Disorders (MCT-ED) in adolescents with anorexia nervosa and atypical anorexia nervosa.

Anorexia nervosa is the eating disorder associated with the highest mortality rate, with peak incidence occurring during adolescence. Recent studies indicate that, globally, the incidence of adolescent anorexia nervosa has increased by approximately 17% over the past 30 years, with a further rise observed during and following the COVID-19 pandemic. The consequences of untreated anorexia nervosa extend into adulthood, significantly affecting both physical and mental health, resulting in long-term morbidity and a substantial burden on society. Elevated levels of perfectionism and cognitive inflexibility are commonly observed in this population and are thought to contribute to the maintenance of eating disorder symptoms as well as resistance to treatment. Accordingly, the MCT-ED aims to reduce rigid and inflexible thinking styles, decrease perfectionism, and address cognitive biases that perpetuate the symptoms of the disorder.

The primary aim of this feasibility study is to assess key process indicators, including recruitment, retention, adherence to the intervention, and safety. To ensure reproducibility, a session checklist will be used to document both intervention delivery and participant engagement for each session. A minimum fidelity plan will include routine supervision of intervention delivery by a senior team researcher with expertise in metacognitive training, optionally supported by an additional research team member with experience in eating disorders and adolescent populations.

Potential participants are referred to the study by the treatment clinical team (child and adolescent psychiatrist/psychiatrist) during eating disorder consultations, only after the adolescent and/or their parent/guardian have provided explicit consent for their contact details to be shared with the research team. For recruitment purposes, a member of the research team contacts potential participants by telephone to schedule an initial videoconference meeting with the adolescent and their legal guardians.

All participants will be invited to take part in an optional brief interview at the end of the intervention and completion of all study assessments. Interviews are conducted via videoconference by a member of the research team who is independent from the therapist who delivered the intervention. The semi-structured interview explores participants' perceptions of the intervention, including overall evaluation, perceived benefits, most and least useful components, age-appropriateness, comprehension, session format and structure, exercises and workbooks, study questionnaires, potential involvement of caregivers, and suggestions for improvement. This approach allows systematic collection of feedback on acceptability, practicality, and potential areas for refinement. Participants who choose not to participate will still have their study participation preserved, including attendance at intervention sessions and completion of study questionnaires.

For CONSORT reporting purposes, the number of participants allocated to the intervention, those who received the intervention, and those who did not receive the intervention, together with reasons, will be documented. In addition, losses to follow-up and discontinuation of the intervention, including reasons where possible, will be recorded. Finally, the number of participants invited to take part in the qualitative interview and the number who completed the interview will be documented.

Progression decisions in this feasibility and acceptability study are guided by predefined criteria using a three-tiered traffic-light system to support interpretation and decision-making.

Inclusion Criteria:

• Adolescents aged 13 to 19 years;
• Diagnosis of Anorexia Nervosa or Atypical Anorexia Nervosa; confirmed by the treating child and adolescent psychiatrist according to DSM-5-TR or ICD-11 criteria;
• Currently followed in an Eating Disorder Outpatient Consultation;
• Fluent in Portuguese.

Exclusion Criteria:

• Acute suicidal ideation or behaviour requiring immediate clinical intervention;
• Imminent risk of hospitalization due to clinical deterioration;
• Current diagnosis of psychotic disorder, bipolar disorder, or substance dependence;
• Any clinical condition that, in the judgment of the treating clinician, would make participation in a videoconference-based intervention inappropriate.