What makes these "athletes" die so young? I just explained it in the post above yours. A 2003 study published in the Journal of the American Medical Association, said that 97 percent of NFL players were overweight and 56 percent qualified as obese. Again, while we look at them and think "athlete", what's going on in their arteries, veins, and heart is something quite different. Add to that the "drug" factor (i.e. steroids, HGH, recreational), the physical abuse of the game, not to mention the dangerous lifestyles most lead off field(fast cars, fast women, drinking, partying)...it all becomes pretty clear where that 20 years comes from.
Just to add 1 more point...even the "obese" part is worse than normal because players routinely gain/lose weight, which is about the worst thing you can do healthwise. They'll balloon up in the offseason, then drop back down during the season (or vice-versa), they'll gain wait to move from OLB to DE. That all takes a toll on the body.
A 2003 study published in the Journal of the American Medical Association, said that 97 percent of NFL players were overweight and 56 percent qualified as obese. Well, truth be told, I'm always leary of terminology like 'overweight' - because typically these are simply measures against a national (or worse imaginary) mean. For example: a 6'3" 235 pound wide receiver is 'overweight'. Averages and BMI ratings don't take into account muscle mass, body fat percentage, etc. Would a Shawn Merriman (6'3", 275) qualify as obese? (I think the criteria is different for 'obese' but I haven't read the study.) So while you told me, I'm not buying it as the answer. Kevin Mitchell wasn't exactly a lineman, though he may have been obese in retirement - the link doesn't say, and I can't seem to find out. However, Kevin Mitchell could very easily have died from other causes, I'm simply wondering out loud about the larger picture.