"Parents need to be vigilant and look for signs of stress, depression or anxiety that is beyond the normal teenage, seek help if there seems to be a persistent problem," said study author Cecilia Bergh of Orebro University in Sweden.

There are indications that mental well-being is declining in young people.

The study investigated whether conditions like depression in adolescence (age 18 or 19) are associated with a higher risk of cardiovascular disease in adulthood. The team also examined the possible role of stress resilience (ability to cope with stress in everyday life) in helping to explain any associations.They included 238,013 men born between 1952 and 1956 who underwent extensive examinations in late adolescence and were then followed into middle age (up to the age of 58 years).

The assessments at the age of 18 or 19 years included medical, psychiatric, and physical examinations by physicians and psychologists. Stress resilience was measured by an interview with a psychologist and a questionnaire, and based on familial, medical, social, behavioural and personality characteristics.

A total of 34,503 men were diagnosed with a non-psychotic mental disorder (such as depression or anxiety) at conscription. Follow-up for cardiovascular disease was through hospital medical records. The study found that a mental disorder in adolescence was associated with the risk of having a myocardial infarction (heart attack) by middle age.

Compared to men without mental illness in adolescence, the risk of myocardial infarction was 20 per cent higher among men with a diagnosis - even after taking into account other characteristics in adolescence such as blood pressure, body mass index and parental socioeconomic status.

The association between mental illness and heart attack was partly - but not completely - explained by poorer stress resilience and lower physical fitness in teenagers with a mental illness.

"Better fitness in adolescence is likely to help protect against later heart disease, particularly if people stay fit as they age," the study authors wrote.

The study was recently presented at the 'ESC Congress 2020 - The Digital Experience' event.