Originally Posted by
bakerjw
The human body is quite adaptable especially with the intermixing of people as travel increased.
But the extent of that intermixing has increased very fast in terms of evolutionary scale, so we're fairly closely descended from people who probably never travelled more than 100 miles from their birth place. When you scramble those genetics up in myriad ways, who knows what the best diet is going to be for your metabolism?
Also, keep in mind that metabolism is largely determined by mitochondrial DNA, which is entirely matrilineal, and therefore requires mutation to change from generation to generation. You could have a widely varied geographic heritage on your father's side and even on your mother's father's side, but have matrilineal DNA from a population isolated in a village high up in the Alps, for example.
Last point on evolution--success in adaptation is defined as surviving to reproduce, so it's really a very low bar in determining a good diet. Almost anything will allow you to do that. That's a far cry from answering the question "what diet will keep me healthy into my old age?" not to mention "what won't make me fat?"