Everything.. everything we eat, has a negative impact in some way, shape, or form. If you want to diet, good for you. If others choose not to, that's their choice too. I still firmly believe the best diet going is the common sense diet, but its alll opinions from people who eat. Nothing more.
And our "core base" diet argument could be disputed with devolution. I mean, there was a time we ate raw animals we killed, and various berries we could gather. That doesn't mean it was the right way for humans to eat.