Obesity, Exercise, and Cardiovascular Health

It’s kind of a cliche that regular exercise is good for the heart. At the same time, I have been told that with fat people, the heart works harder because it has to pump more blood around. And that obese people have a tendency to have thicker heart muscles than they otherwise would have.

It seems that extra exercise is not automatically a positive thing.

Common sense says that the main difference between (1) the thin athlete ; and (2) the fat dude whose main exercise is stuffing his face with donuts, is that the athlete’s heart gets to rest and recover from the extra exercise it gets. Another possible difference is that the fat guy is likely to be eating foods which will clog up his arteries.

Of course, one can also look at anecdotal examples like Ron Daws, a world class distance runner who died at the age of 55 from a heart attack. Examples like this provide evidence for the “too much exercise” hypothesis.

So my question is this:

Why does the heart strain from regular, moderate exercise seem to be a lot better for your heart than the heart strain from being obese? If it is mainly the difference between constant and intermittent strain, is there any research to back this up? If so, what is the ideal amount of exercise for cardiovascular health?

Also, how much of a missing variable is diet? In other words, to what extent are fat people more likely to have heart attacks simply because their obesity is a proxy for eating a lot of junk food?