Some chemicals in coffee, like caffeic acid and quercetin, might make a harmful protein clump together in the brain at high doses, which could make Parkinson’s disease worse — but other coffee chemicals don’t do this.

A compound in coffee called quercetin might stop some harmful protein clumps linked to Alzheimer’s from forming, but it doesn’t stop another type linked to tau, and at high doses, it might even make a different protein clump more — so its effects are mixed and not simple.

Quercetin, a natural compound, might help stop harmful clumps linked to Alzheimer’s, but barely affects another type linked to Alzheimer’s, and could actually make clumps linked to Parkinson’s worse at higher doses — so it’s not a safe all-purpose brain protector.