Feasibility and Acceptability of Metacognitive Training for Eating Disorders in Adolescents With Anorexia Nervosa

Recruitment rate

Proportion of eligible participants who consent to participate in the study, calculated as the number recruited divided by the number eligible and approached Minimum Progression Criterion: ≥ 50% of eligible participants consent to participate

Retention/Completion rate

Proportion of participants who completed the full intervention and all planned outcome assessments Minimum Progression Criterion: ≥ 70% of participants complete the intervention and assessments

Intervention engagement / adherence

Number of synchronous sessions attended (out of six), which determines exposure to the six intervention modules, and level of engagement during sessions. Engagement with recommended between-session exercises, which are not mandatory, will also be recorded as an indicator of optional participation rather than required adherence Minimum Progression Criterion: ≥83% of participants attend at least 5 of the 6 synchronous sessions and demonstrate adequate in-session engagement, defined as participation in at least one core exercise per attended session. Engagement with recommended workbook exercises related to the core modules (cognitive flexibility and perfectionism) will be recorded descriptively through therapist review of the workbook and will not constitute a mandatory criterion for progression.

Safety and adverse events

Adverse events are defined as any unexpected or clinically significant occurrences potentially related to the intervention, including: * Clinical deterioration: marked increase in core AN symptoms (e.g., restrictive eating, purging, excessive exercise) compared to baseline; * Suicidal ideation or self-harm: thoughts, plans, or behaviours indicating risk of self-injury; * Need for urgent medical intervention: emergency visits, hospitalization, or other urgent care; * Other unexpected adverse events potentially linked to the study participation. Intervention suspension criteria: * Severe clinical deterioration in AN symptoms * Emergence of suicidal ideation or self-harm behaviours Mild or moderate adverse events do not automatically lead to suspension but are monitored and managed in coordination with the treating clinician. Minimum Progression Criterion: No severe adverse events attributable to the intervention.

ED behaviours and attitudes: Eating Disorder Examination Questionnaire -(EDE-Q) (Fairburn & Beglin, 1994); Portuguese validated version EDE-Q 5.2. (Machado et al., 2014)

The EDE-Q is a self-report instrument consisting of 28 items across four subscales: Restraint, Eating Concern, Shape Concern, and Weight Concern. Items are rated on a 7-point Likert scale (0-6), with higher scores indicating greater concern with eating, weight, and body shape and/or more maladaptive eating-related behaviours. The questionnaire assesses both cognitive-attitudinal features and the frequency of key disordered eating behaviours over a 28-day reference period. A higher global score reflects greater overall severity of eating disorder symptoms. To obtain a subscale score, the ratings for the relevant items are added together and the sum divided by the total number of items forming the subscale. To obtain an overall or 'global' score, the four subscales' scores are summed and the resulting total divided by the number of subscales (i.e. four).

Perfectionism: Frost Multidimensional Perfectionism Scale (FMPS) (Frost et al., 1990). The Portuguese version of the FMPS validated by Carmo et al. (2017)

The Organization subscale consists of six items. Each item is rated on a 5-point Likert scale (1 = strongly disagree, 5 = strongly agree). The total FMPS score was calculated by summing five subscales (Personal Standards, Concern over Mistakes, Doubts about Actions, Parental Expectations, and Parental Criticism), excluding the Organization subscale due to its weaker correlations with the other dimensions. The Organization subscale is reported separately. All items are scored in the same direction, with higher scores indicating greater levels of perfectionism in each domain. Elevated scores reflect stronger perfectionistic tendencies, including heightened concern about mistakes, higher personal standards, perceived parental pressure, doubts about performance, and preference for organization.

Cognitive Flexibility/Body Image Flexibility: Body Image Acceptance and Action Questionnaire (BI-AAQ) (Sandoz et al., 2013); Portuguese validated version: Ferreira, C., Pinto-Gouveia, J., & Duarte, C. (2011)

The BI-AAQ is a self-report instrument consisting of 12 items that measure body image-related psychological flexibility, including body image acceptance and experiential avoidance. Each item is rated on a 7-point Likert scale (1 = never true, 7 = always true). All items are reverse-scored so that higher scores reflect greater body image-related psychological flexibility and acceptance, whereas lower scores indicate higher inflexibility and experiential avoidance. The total score is calculated as the sum of all 12 items, resulting in a possible range of 12 to 84.