This is an observational case-control add-on study to an investigator-initiated clinical trial (IICT) (ClinicalTrials.gov Identifier: NCT03167307): Omega-3 fatty acids as firstline treatment in pediatric depression. A 36-week multi-centre, double-blind, placebo-controlled randomized superiority study.

This project will recruit a healthy control group matched for age and sex to a sub-group of patients with diagnosed pediatric major depressive disorder (pMDD) enrolled in the IICT. The aim is to investigate the relationship of n-3 FA intake and status with mental health in children and adolescents with and without diagnosed pMDD, and explore potential biochemical mechanisms underlying this relationship by measuring biomarkers related to n-3 FA metabolism, mental health and cognitive function.

According to a recent school survey, one out of ten adolescents in Zurich have moderate to marked depressive symptoms. Early onset of paediatric major depressive disorder (pMDD) is a risk factor for chronic and recurrent forms of depression in adulthood (MDD). MDD is associated with difficulties in relationships, impaired school and work performance, and an increased risk of substance abuse. Furthermore, MDD is a major contributor to the burden of suicide and poor long-term health late in life. The emerging potential of n-3 polyunsaturated fatty acids (n-3 PUFAs), also known in short as omega-3 fatty acids (n-3 FAs), for the treatment of MDD is being investigated in several studies. Previous observational studies suggest a link between the consumption of n-3PUFA-rich food and the level of depressive symptoms. However, studies that investigated the potential beneficial effects of n-3 PUFA supplementation for the treatment of MDD compared with placebo showed inconsistent findings.

Primary objectives of this project are:

1. To determine the relationship of n-3 FA intake and status with mental health in children and adolescents with and without diagnosed pMDD.
2. To explore biochemical mechanisms underlying this relationship by measuring biomarkers related to n-3 FA metabolism, mental health, cognitive function, and potential pathways linking n-3 FA intake/status and brain health (e.g. immune system, gut microbiome-brain-axis).

Participants meeting the following criteria are eligible for the study:

Inclusion criteria:

• Participants of female and male sex
• Children aged 8 to ≤ 13 years or teenagers 13 to < 18 years at time of study entry
• Written informed assent of the subject and written informed consent from the subject's parents/legal representatives
• No present primary diagnosed psychiatric disorder according to the M.I.N.I. KID Test

Exclusion criteria:

• More than 4 weeks of regular n-3 FA supplementation
• Women who are self-reported pregnant or breast feeding
• Pre-existing neurological or medical conditions likely to be a risk factor for developing depressive symptoms

In a first step participants will be screened for in- and exclusion criteria. If participants can be included into the study we will

• Measure participant's mental health with behavioural and cognitive assessments
• Collect biological samples from participants (blood, urine, saliva, hair, and stool)

This study aims to recruit a total of 200 participants: 100 cases (from IICT) and 100 controls (recruited for this study).

There will be a total recruiting period of 18 months.

For cases: participants enrolled in the IICT and therefore meeting the criterie for participation in the IICT will be selected.

For controls, participants fulfilling all of the following inlclusion criterie are eligible for the study

Inclusion Criteria:

• Participants of female and male sex
• Children aged 8 to ≤ 13 years or teenagers 13 to < 18 years at time of study entry
• Written informed assent of the subject (appendix informed consent forms) and written in formed consent form from the subjects' parents/legal representatives
• No present primary diagnosed psychiatric disorder according to the M.I.N.I. KID Test.
• No use of chronic medication
• Able to communicate in German; degree of understanding sufficient to comply with trial procedure

Exclusion Criteria:

• More than 4 weeks of regular n-3 FA supplementation (>2 daily capsules standard strength providing > 600 mg combined EPA/DHA) within the last 6 months.
• Women who are self-reported pregnant or breast feeding.
• Self-reported pre-existing neurological (such as e.g. brain tumour, temporal lobe epilepsy, HIV encephalopathy) or medical conditions (ICD-10 F06-F07) likely to be a risk factor for developing depressive symptoms
• Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc. of the participant.

Children/adolescents in German speaking Switzerland

A BioBank will be established. Access to the Biobank can be requested upon completion of the IICT and the add-on study contacting the principal investigator Dr. Jeannine Baumgartner

upon study completion of the IICT and the add-on study.