The sample comprised 80 adolescents (40/40: experimental/control groups), aged between 12 and 18 (M = 14.6; SD = 1.88), 52.9 % females.
Sample size was calculated according to the power analysis, with BMI, quality of life, treatment adherence and lifestyle used as starting points. We aimed at being able to show differences between the Next.Step participants and the control group with a standardized effect size (Cohen’s d) of 0.4 or larger [9–11]. Assuming a 30 % drop-out rate (in the literature systematic review, the drop-out rate ranged between 4.9  and 30 % ), an alpha of 0.05 and a statistical power (1-Beta) of 80 %, we came to a need for recruiting at least 75 adolescents.
Body Mass Index (BMI) and percentage of fat mass
BMI and percentage of fat mass were measured by trained health professionals from the Paediatric Obesity Clinic (POC), Hospital de Santa Maria, Lisbon, and were extracted from the adolescents’ clinical file. Height was measured to the nearest 0.1 cm, without shoes, with the participant back to the stadiometer, in the Frankfurt position and after an expiratory phase (height stadiometer, SECA 217, Hamburg, Germany). Body weight and body composition (bioelectrical impedance scale InBody 230, Seoul, Korea). Body weight will be measured to the nearest 0.1 kg, in the anthropometric position (with the palms turned into thighs), with the subjects wearing as few clothes as possible, and without shoes or socks. BMI was calculated as body weight in kilograms divided by the square of height in meters [BMI = weight (kg)/height2 (m)]. Subjects were classified into overweight (BMI ≥ 85th percentile) according to the age- and sex-specific percentiles proposed by the World Health Organization (WHO).
Self-efficacy/adherence behavior and perceived benefits
Adolescents reported on their self-efficacy/adherence behavior to weight control using the Adherence to Weight Control Questionnaire (AWCQ) . The AWCQ is a screening tool that includes 36 items, rated on a 1–5 Likert scale (“Do not agree” to “Totally agree”), organized in two scales: Treatment adherence to weight control and Risk of non-adherence to weight control. The former scale includes four subscales: Self-efficacy/adherence behavior (SEA); Parents and providers’ influence; School and Friends’ influence; and Perceived benefits (PB). In this study, we have only used the SEA and the PB dimensions as we were mostly interested in internal/individual sources of behavioral influence. The SEA dimension includes 12 items such as ‘I accomplish the treatment even if I’m not in the mood’. This dimension represents the perception of self-ability and commitment to organize and perform a particular health behavior, i.e., self-confidence in successfully engaging in the weight management program, intention to carry out the treatment and ability to identify the specific strategies to succeed. The PB dimension includes four items (e.g., ‘I want to lose weight to be healthier’) related to perceptions of the positive or reinforcing consequences of engaging in the weight management program.
In the previous validation study with a sample of Portuguese adolescents with obesity , the SEA and PB dimensions have shown good internal consistency (α = .89 and α = .77, respectively), similar to the ones found in the current study (α = .84; α = .75, respectively).
Quality of life (QoL)
Adolescents completed the Impact of Weight on Quality of Life-Lite (IWQOL-Lite) , which assesses obesity-specific quality of life. The IWQOL-Lite is a 31-item questionnaire scored on five-point Likert scale ranging from 1 (never) to 5 (always). This instrument assesses four dimensions: Body self-esteem; Social life; Physical comfort; and Relation with the family. Engel and colleagues  found a good value of internal consistency for the total score. In the current study, we have found a similar value (α = .85).
Positive life perspective and Health responsibility
Adolescents completed the Adolescent Lifestyle Profile (ALP-R2; adapted and validated for the Portuguese population ), which measures the frequency of health promoting behaviors in adolescents. The ALP-R2 includes 36 items rated on a four-point Likert scale ranging from 1 (never) to 4 (always). This instrument assesses seven dimensions: Health responsibility (HR), Physical activity, Nutrition, Interpersonal relationships, Spiritual health, Positive life perspective (PLP), and Stress management. In this study, we have only focused on the PLP and HR dimensions. PLP dimension includes four items (e.g., ‘I’m excited about the future’) and HR dimension includes six items (e.g., ‘I usually ask questions about how to improve my health to my doctor/nurse’). We opted to focus only on these two dimensions because we were essentially interested in internal/individual sources of behavioral change.
Sousa et al.  found satisfactory values of internal consistency for the HR and PLP dimensions. In the current study, we have found good values of internal consistency (α = .74 and α = .80, respectively).
Participants were selected from the population of adolescents followed at the POC. All eligible adolescents with appointments between 2014 April 1 and 2015 January 31 were included in the study. Adolescents were assigned to the control or experimental groups by alternating patients sequentially. Participants were required to be overweight, aged between 12 and 18, willing to participate in the study and have internet access at least once a week. Exclusion criteria were the presence of severe psychopathology, pregnancy or having been proposed for bariatric surgery. The control group followed the clinical standard intervention, including individual appointments with the paediatrician, dietician and exercise physiologist every 3 months. In addition to the standard intervention, the experimental group was invited to access the e-therapeutic platform, which includes a set of resources, such as educational resources (videos, brochures, menus, weekly tips, access to other links), self-monitoring (food, weight and physical activity records), social support (chats, discussion forums and personalized messages), interactive training and motivational tools (personal goals planning, treatment progression registry, positive reinforcement) [8, 17]. The intervention length was 12 weeks and was based on a case management methodology. So, the program had the direct support of an interdisciplinary team (including paediatrician, nutritionist, exercise physiologist, and psychologist) who intervened when requested by the case manager (nurse). Full presentation of the platform features can be found in detail in the study protocol .
The study protocol was approved by the Ethics Committee of the Faculty of Medicine, University of Lisbon. Adolescents and their parents signed an informed consent. The voluntary nature of their participation was explained and confidentiality was assured.
Anthropometric measurements and self-reported questionnaires were collected at 0 months (baseline assessment) and at 3 months (post-intervention assessment).
All statistical analyses were performed using Statistical Package for Social Science (SPSS) version 22. Descriptive statistics were calculated, and the groups were compared regarding their baseline characteristics by Mann-Whitney U test (U) for continuous variables and Chi-Square test (χ
2) for nominal variables. To analyze the effectiveness of the Next.Step program compared to the standard intervention, in what concerns a set of anthropometric and psychosocial variables, we conducted multiple repeated-measures factorial ANOVAs. We further performed multiple comparisons using Bonferroni adjustment at p < .017, to reduce the probability of type I error. We opted to use this analytic strategy to analyze the two main effects of the factors (time and group) and the interaction effect between them.