The heart attack myth dates back to early forum posts claiming from the game itself that if Carl Johnson became fat enough, he would run the risk of having a heart attack.
While at first the myth seems likely, it has been found that no heart attack code exists in the game. Players might confuse Carl Johnson starving to death from a heart attack, which is the likely source of this myth. It may also be because at the start of the game, it warns the player not to eat too much or they will die, although this never happens in the game at all.
A rare glitch in the PC version can be misinterpreted as a heart attack. It happens when CJ is trying to climb something and then he randomly dies, this is not a heart attack and is more likely the game confusing the climb for fall damage. This glitch is a lot more likely to occur if the frame limiter is disabled.
The Meat Business
However, in the mission The Meat Business, Johnny Sindacco gets hit by a heart attack after confronting Carl Johnson. Therefore, heart attack is known to exist in the game, only with regards to a cutscene.