Looking at diet as the culprit for modern society's epidemics of diabetes, heart disease, etc, without looking at exercise is only half the picture. Ancient people ate lots of meat in some parts of the world, lots of fish in other parts of the world, foraged for plants, berries, seeds in still other parts of the world..
I think that is exactly it . George Mann came to similar conclusions after studying indigenous groups including the Inuit, Masai, and some other tribe.

However, no ancient people were eating candy bars and soda pop.