Hmm... I guess one really needs to properly define "healthy". My unprofessional take on it - We've tens of thousands of years of evolutionary history behind us where food was not really plentiful or guaranteed. This is why our bodies crave high energy foods like sugars and fats. These are considered "bad" these days, but a couple of hundred years ago, when food was more scarce, would probably properly be considered "healthy" for the energy they provided alone. .
Of course, now there is an abundant supply of such, available with no more work than a swipe of a plastic card or exchange of a piece of paper. Therein, I believe, lies the problem - the increased amount of food we intake vs. the decreasing average activity. Since this situation is so recent in human history, evolution hasn't had a chance to play catch up yet.
Of course (also in my unprofessional opinion) we've stopped natural evolution - at least in the proper direction. Evolution dictates that those with undesireable traits die off, for the most part, before they have a change to pass those traits on. Modern medicine, along with our current social programs, pretty much keeps everyone alive to childbearing age, even those genetically undesireable. And I've seen some pretty ugly and stupid mofo's reproduce - and they usually find someone just as ugly and stupid to do it with, so the offspring are probably likely to be that much worse off. Perhaps a bit of a twisted way to look at it, but hey, that seems to be pretty much the way evolution works.
Rocko