Background: Non-alcoholic fatty liver disease (NAFLD) is rapidly increasing alongside overweight and obesity, not only in adults but also in children and adolescents. It is unknown what impact the development of NAFLD in childhood may have in later life. The importance of early detection and treatment lies in its potential for progression to cirrhosis, liver cancer and liver-related death, as well as its associated extrahepatic comorbidities. Vibration-Controlled Transient Elastography (VCTE) with Controlled Attenuation Parameter (CAP) is an effective, non-invasive and safe diagnostic method to estimate the degree of fibrosis and steatosis in the liver, but little is known about its applicability in the paediatric population. Objectives: 1) To assess the prevalence of significant liver fibrosis (LSM≥6,5kPa) using VCTE, and that of non-alcoholic fatty liver disease (≥225dB/m) using CAP in children and adolescents. 2) To determine the optimal cut-off points of the CAP to achieve maximum concordance with the Magnetic Resonance Imaging (MRI) findings in the diagnosis of mild, moderate and severe NAFLD in children and adolescents. Methods: cross-sectional population-based study which will include 2.866 subjects aged ≥9 to ≤16 years. Participants will undergo: anamnesis, physical examination, blood extraction, VCTE, MRI and questionnaires on socio-demographic data, personal and family medical history and lifestyle assessment. Applicability and relevance: the study aims to establish the foundations for the use of VCTE in children and adolescents in order to achieve early diagnosis of NAFLD. Moreover, it will serve to understand in further detail the disease and to identify the risk groups of children and adolescents who may be at risk of developing it. Ultimately, this will help determine to which subgroups of the population we need to target resources for prevention and early detection of this entity, as well as possible intervention for its treatment.

Inclusion Criteria:

• Subjects aged between 9 and 16 years.
• Subjects whose parents/legal guardians have signed the informed consent.
• For subjects over 12 years: it will also be necessary to have their own signed informed consent.

Exclusion Criteria:

• Subjects with previously diagnosed liver disease.
• Subjects receiving hepatotoxic drug treatment.
• Subjects with diagnosed eating disorders.

The study will be carried out among children and adolescents aged between 9 and 16 years who are enrolled in the Spanish compulsory education system in the territory of Mataró. Since schooling at state level is compulsory in this age group, more than 90% of this population will be accessible from the schools themselves.