Aim:
Reduced wellbeing and low mood also apply to young people and may lead to serious mental problems. The purpose of the study was to analyze the prevalence of depression threat symptoms depending on the level of physical activity of young people, taking into account environmental factors and the quality of social relations.

Material and methods:
The survey conducted within the framework of the last round of international HBSC studies in the academic year 2017/2018 covered 5225 students aged 11-15 years from all over Poland. Mental wellbeing of young people measured by the WHO-5 screen test was the dependent variable. 16 independent variables were taken into account, in blocks related to the assessment of demographic features, physical activity, social and economic position and social relations.

Results:
The study demonstrated that 14.2% of respondents have depression threat symptoms. After a multidimensional analysis the main risk factors were low level of family support (OR=3.20), maladjustment to school (OR=2.79), inability to cope in social situations (OR=2.52) and low physical activity (OR=2.15). Factors associated with living conditions do not have a direct effect on depression risk but moderate the impact of physical activity on its symptoms.

Conclusions:
The protective effect of physical activity on the mental health of teenagers is maintained after taking into account several other predictors of reduced wellbeing. Factors associated with living conditions do not have a direct influence on depression risk but moderate the effect of physical activity on its symptoms. The protective effect of physical activity is particularly visible in single parent families, in poor families and in large cities. It is important to design programs providing equal opportunities to children growing up in a less favourable environment by providing young people from the poorest families with the ability to participate in free of charge sports activities, particularly team sports.