What is the deal with nightshades? Why are they unpaleo?
Nightshades are plants in a family that includes tomatoes, potatoes, bell peppers, etc. Technically, I think they're "unpaleo" because they're New World plants from the Americas, and the Americas weren't settled by humans until after (or at the very end of) the paleolithic era. So paleolithic humans generally didn't eat them.But by that standard, almost no modern foods are paleo. Modern beef cattle did not exist during the paleolithic era, or modern chicken, or modern pork, or modern bananas or strawberries, etc.
So the real question is whether nightshades contain substances that humans are not completely adapted to. As with dairy, legumes, etc., the answer varies from individual to individual.
Plants in the nightshade family contain certain alkaloids that some people are sensitive to. Some people's arthritis or autoimmune diseases, for example, can be aggravated by consuming fruits from nightshades. (Note that potatoes, however, are not fruits. The fruit from the potato plant is rather toxic to humans, but the potato part of the plant, the tuber, does not contain the toxic alkaloids in question.) Other people, however, appear to consume nightshades with no problems.
As always, people need to find out what works for them. If you have arthritis, for example, you may want to try avoiding tomatoes and peppers for 30 days and see if it gets better.
I've never personally met anybody who told me they had a problem with nightshades, so as far as I know, it doesn't seem to be a major deal.