They said it could be just from his diet that he was on and lost 10 pounds in 3 days before the combine. It could be somthing small.
As a doctor, I have a hard time trying to figure out a way that a diet (short of a lack of a few specific essential vitamins, but even that takes months/years to develop; or alcoholism) would cause the heart muscle of the L ventricle to get bigger. Unless by "diet" they mean PEDs that are stimulants or other medications that he could have been on. Those are known to cause ventricular hypertrophy and stiffening which would decrease the ejection fraction.
Saying it was "just diet" sounds to me like some PR guy at his agent's office trying to save his draft stock.
Then again, I am just speculating on what I have read, which is speculation. Without any first hand knowledge, no one knows what exactly was wrong with him.
Romeo would say "he has a heart"