One has to remember that our bodies' ability to cope with all sorts of diets and environmental factors was most probably a very beneficial thing for prehistoric man. For them it wasn't possible to just change their diet or lifestyle, if their tribe didn't catch game in some time, coping with less nutrient food was essential. And vice versa, when they got some meat, it was good that the body stored fat so they could survive until the next kill. For us, it could be nice if bad lifestyle choices manifested themselves faster in our condition, but that would have killed our ancestors.