Second, adaptiveness will shift, given exposure to new diets. But over longer periods, a population can become better suited in a positive fashion to the new diet. So what's bad for you may not be quite so bad for me. Finally, economic issues are still important. We might be able to improve a individual's diet, but it would probably be quite difficult to, for example, shift the bulk of the world's 6 billion people from diets with lots of grain to diets with equivalent absolute-fresh ancestral fruit varieties instead.
Some balance seems called for.
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