The prevention of overweight and obesity in adolescents is a public health priority given the impact of obesity on both short- and long-term health. Scientific evidence has shown that interventions in diet and physical exercise can reduce the risk of obesity in children and young people since adolescence is an ideal stage for educating on a healthy lifestyle and correcting the habits that may have been acquired in childhood. Smartphone applications (apps) can provide a useful alternative to overweight and obesity prevention measures.

The objective of this study was to evaluate, through a randomized controlled clinical trial, the effect of an intervention based on a mobile health application (m-Health) on improving the degree of sport and nutrition knowledge, eating habits, and level of physical activity of adolescents.

The sample consists of 305 adolescents, 154 in the intervention group and 151 in the control group that are evaluated at the beginning and 6 months later regarding sociodemographic, eating habits, food knowledge, level of physical activity, body mass index, and waist circumference. The educational intervention was carried out using an m-Health tool, a mobile phone application. The outcomes were changes in the mentioned variables to a six-month follow-up between the two groups.

Design: A randomized parallel-group, controlled clinical trial was performed on adolescents from public secondary schools in the province of Cadiz, Spain.

Randomization and Blinding: The randomization and allocation to each group (1:1, intervention, and usual care) are based on computer-generated random numbers stratified in the three courses of secondary education. The researchers responsible for the study do not participate in the allocation of the participants. Due to the kind of intervention, blinding is not possible when the participants are allocated to groups. To minimize any bias, objective clinical variables are measured during the evaluation visit and the analyses are performed by blinded researchers.

Study Sample: The estimated sample size to detect a mean difference of at least one point in the level of healthy eating habits, measured by the KidMed questionnaire, with a standard deviation of the values in the questionnaire known from previous studies of 2.5 points, a level of confidence of 95% and statistical power of 90%, it was 126 participants in each group, intervention, and control. Assuming a 20% loss to follow-up, 305 participants were recruited, 154 in the intervention group and 151 in the control group.

Recruitment: All participants, in the intervention and control groups, received an Intervention on healthy habits consisting of four sessions developed by a nurse over two months. Adolescents who voluntarily decided to participate in the mobile Health educational intervention were recruited by a nurse. Both the adolescents and their parents signed the informed consent.

Intervention: The nurse installs the application on the mobile phone of the participants in the intervention group. It is a friendly, easy-to-use application for smartphones and tablets that favors the process of learning while playing.

Application Contents: Through the application, the adolescent participates in five activities: food, exercises, tests, learning more, and games that are different from each other but interconnected and that allow progress to be seen. The application "Care your habits" has educational materials on nutrition and physical activity. The participant records the food eaten and the physical activity performed, in addition to answering health questionnaires.

Technical Data of the Application:

The application has been developed for Android mobiles with versions between API 16 and API 28, which guarantees user compatibility. This app uses Firebase, a cloud platform that enables certain features, such as a secure login for app users. The application uses an initial Activity, which is responsible for managing the Login. The application is structured in different Fragments. Each of these Fragments is made up of a layout that is the graphical interface that the user sees and a class in the Java programming language that is the part in charge of processing the user's interaction with the interface and performing the appropriate action.

The app needs a server that manages interactions with the database where the data that users provide voluntarily from the app itself is stored. The part of the server that supports and complements the Android application is made up of a RESTful API developed in the NodeJS programming language. This server communicates with Firebase in order to verify the identity of the user who is providing data at all times, so that a user could not falsify another user's data.

All of this information is persistently stored in a NoSQL database, specifically a MongoDB database. The information provided by users is encrypted to prevent data theft.

Ethical Considerations: The study was conducted in agreement with the guidelines and protocols established in the Helsinki Declaration as revised in Fortaleza (Brazil) in October 2013, and complies with Law 14/2007 on Biomedical Research and with European Data Protection Regulations. The Biomedical Research Ethics Committee of Cádiz, Andalusian, approved the project with the reference: 11_02_2019. The informed written consent of all the participants and their parents was requested. The application guarantees security measures regarding the current General Data Protection Regulations in Europe. It also includes data encryption mechanisms.

Inclusion Criteria:

• Adolescents who were studying the 1st to 3rd grades of secondary education in public institutes
• Have a smartphone or tablet with an Android operating system and internet access

Exclusion Criteria:

• Adolescents with barriers that prevented them from understanding the characteristics of the study: Comprehension, visual or auditory problems
• Adolescents with disease or treatment that affect the variables studied (diagnosed diseases that affect body weight such as liver or kidney disease or regular treatment with drugs such as corticosteroids)