Neurodegenerative diseases are a major health concern due to their growing societal implications and economic costs. The identification of early markers of pathogenic mechanisms is one of the current main challenges. The gut-brain axis has become a primary target because of its transversal role across the neurodegenerative spectrum and its effect on cognition. However, despite recent progress, how changes in the gut-microbiota composition can affect the human brain is still unclear. The goal of this observational study is to characterise the gut-microbiota composition associated with alterations in brain structure and function during the ageing process and across neurodegenerative disorders. This is based on recent studies showing that changes in the human brain and in the microbiota composition, can indicate very sensitively and in a predictive way pathological development and, consequently, be used as markers of neurodegenerative diseases. The main questions it aims to answer are: * How variation in the gut-microbiota composition correlates with the normal brain ageing trajectory? * How dysregulation in the gut-microbiota correlates with pathological changes in brain regions in specific neurodegenerative disorders? * Can the impact of the gut-microbiota on the brain be modulated by blood biomarkers? The investigators will recruit 40 young healthy participants, 40 old healthy participants, 40 participants with prodromal Alzheimer's Disease, 40 participants with Parkinson's Disease and 40 participants with Multiple Sclerosis. Participants will undergo the following examinations: * Magnetic Resonance Imaging * Analysis of a stool sample * Analysis of a blood sample * Neuropsychological assessment * Questionnaires on eating habits