From allegory to conceptualization, hypothesis and finally evidences: Alzheimer’s dementia, Parkinson's disease "gut–brain axis" and their preclinical phenotype

Researchers are constantly trying to develop therapeutic targets in neurodegenerative disorders like Alzheimer's dementia and Parkinson's disease.

Despite enormous endeavors, there are several unmet needs.

Several contradictory pathophysiological basis of neurodegenerative disorders are considered to be one of the most important cause underpinning.

"Gut–brain dysbiosis" has been considered as one of the most crucial link to explore.

Contemporary researches have suggested similar pathophysiological mechanisms underpinning Alzheimer's dementia and Parkinson's disease.

"Gut–brain dysbiosis" may be the missing thread connecting Alzheimer's dementia and Parkinson's disease prior to the expression of their overt clinical phenotype.

Recognition of preclinical phenotype of Alzheimer's dementia and Parkinson's disease have much broader perspective as it will help in building robust therapeutics at the earliest.

Authors herein critically analyze the pathophysiological basis of Alzheimer's dementia and Parkinson's disease in relationship with "Gut–brain dysbiosis" and also try to search the preclinical phenotype/s of Alzheimer's dementia and Parkinson's disease pivoting around the Freudian hypothesis.