All or any stress or infection may alter GIT environment. Lack of O2 in asthma as well as stess may cause some GIT imbalance. Parkinsons disease is also a mental stress. Moreover, The exact reason that the cells of the brain deteriorate is unknown & its some symptoms may be linked ti GIT disturbances:- Loss of fine motor skills Difficulty writing, may be small and illegible Difficulty eating Difficulty with any activity that requires small movements Uncontrolled, slow movement Frequent falls Decline in intellectual function (may occur, can be severe) A variety of gastrointestinal symptoms, mainly constipation. http://www.nlm.nih.gov/medlineplus/ency/article/000755.htm If we study deeply, we may find the relation of GIT imbalances with all or most of the symptoms of all or most of the diseases. Digestive & repiratory systems are our most basic & prominents input systems( other senses hearing, skin sensations & visual are bit differant non physical inputs so can't be thought for physical inputs) which can be linked to root causes or basic understanding for all body systems( but not given much importance).