[Barrin92]

>A close friend of mine, when we were 21, had heart attack and passed away. No family history of heart attacks, he was in excellent shape (cycling was his life) and just randomly out of the blue.

Not sure if this is the case here, but fatal heart attacks in healthy physically active young adults are often the result of Hypertrophic cardiomyopathy.

This is a fairly common (~ 1 in 500) inheritable condition that can be easily diagnosed by ultrasound. Everybody do yourself a favour and get at least a routine checkup in every few years. It prevents deaths.