The objective of the study is to investigate the efficacy of motivational interview intervention with adolescent students at a school environment on the adhesion to a therapeutic plan to loose weight.

Motivational interview (MI) is a style of communication collaborative, objective-directed, with special attention to change talk.

The present study's main objective is to investigate the efficacy of a MI intervention in adolescents with weight excess, in a school setting, over their motivation to adhere to a health lifestyle plan of dietary habits and physical exercise.

Secondary objectives are to investigate the effect of MI over self-concept, quality of life, depressive symptoms, anxiety symptoms, body mass index, abdominal circumference and blood pressure.

Registry Procedures Data on anthropometric parameters is recorded in data collection sheet and stored in individual files, which are available for monitoring procedures and for auditing by the Metabolic and Food Behavior Disorders Scientific Committee.

Data Checks Data checks will be performed to compare data entered into the registry against predefined rules for range or consistency with other data fields in the registry

Source Data Verification All the data will be checked to assess accuracy, completeness, or representativeness of registry data by comparing the data to external data sources: audio-taped interviews, paper case report forms and paper case self-report forms. All the registries will be stored during five years after study completions according to the Medical Faculty of Lisbon norms.

Variables:

Motivational Interview Audio-taped clinical interview record Motivational Interviewing Treatment Integrity (MITI) version 3.1.1

Motivation to adhere to a physical exercise plan Self-report by a paper questionnaire Self-regulation and perceived competence questionnaire to begin or maintain an healthy diet

Motivation to adhere to a diet plan Self-report by a paper questionnaire Self-regulation and perceived competence questionnaire to begin or maintain regular physical exercise

Self-concept Self-report by a paper questionnaire Self Perception Profile for Children

Quality of life Self-report by a paper questionnaire Pediatric Quality of Life Inventory 4.0 (PEDSQL 4.0)

Depressive symptoms Self-report by a paper questionnaire Children Depression Inventory (CDI)

Anxiety symptoms Self-report by a paper questionnaire State-Trait Anxiety Inventory for Children (STAIC C-2)

Body mass index Calculated by the ratio of weight in kilograms over the square of height in meters^2.

Weight Direct measurement by the research team Measured with Omron Body Composition Monitor with Scale BF 11® (Omron®, Kyoto, Japan) in kilograms

Height Direct measurement by the research team Measured with Seca® 213 Portable Stadiometer Height-Rod® (Seca@, Hamburg, Germany) in meters

Abdominal circumference Direct measurement by the research team Seca 203 Circumference Measuring Tape® (Seca@, Hamburg, Germany) in centimeters

Blood pressure Direct measurement by the research team Riester Risan Sphygmomanometer® (Riester®, Jungingen, Germany) in mm Hg

Inclusion Criteria:

• Age between 14 and 19 years old
• Body mass index percentile equal or above 85th percentile according to World Health Organization charts (de Onis 2010)

Exclusion Criteria (one or more of the following):

• recent weight loss of 10% or more of body weight
• pregnancy
• breastfeeding
• endocrine disease
• present therapy with antidepressant or hypoglycemic drugs
• present treatment for food behavior disease or depression
• cognitive impairment of the student or his/her legal tutor.