A plausible-sounding mechanism with much thinner evidence behind it than sugar or processed food.
Tomatoes, potatoes, peppers and eggplant — the nightshade family — get flagged in some autoimmune-protocol and elimination-diet circles as inflammatory triggers. It's worth separating the plausible mechanism from the evidence actually supporting it.
Nightshades contain compounds called alkaloids (solanine is the most discussed), and the claim is that these trigger gut permeability and inflammation in sensitive individuals.
Robust clinical evidence for nightshade sensitivity as a widespread, general population issue is thin — most of what circulates is anecdotal or based on very small, preliminary studies. Compare this to sugar or ultra-processed food, where the evidence base is far larger and more consistent.
Nightshade elimination protocols typically remove processed foods at the same time (tomato-based sauces, potato chips, etc.) — the improvement people notice may often be explained by that broader change rather than the nightshades specifically.
If you suspect a personal sensitivity, a structured elimination and reintroduction — ideally with a dietitian — is the way to actually find out, rather than assuming the popular claim applies to you.