We propound that low GDNF levels cause more easily impaired intestinal mucosal barrier function, more difficult intestinal mucosal repair, changes in intestinal permeability, and a more susceptible intestinal nervous system to intestinal flora, all of which may be linked to the “second hit” of Braak theory, which is one of the causes of PD constipation. This evidence concerns the gene GDNF and Parkinson disease.