The goal of this clinical trial is to evaluate whether a dietitian-guided nutritional intervention can improve constipation symptoms in people with Parkinson's disease (PD). The main questions it aims to answer are:

• Can a dietitian-guided nutritional intervention increase the number of weekly bowel movements in individuals with PD and functional constipation?
• Can this intervention positively influence gut microbiota composition, dietary intake, and nutritional status?

Researchers will compare the intervention group to a control group that will receive general dietary guidance only after the study period, to see if the intervention leads to improvements in bowel function and related health outcomes.

Participants will:

• Follow a diet plan developed by a dietitian, based on dietary reference intakes and tailored to the needs of individuals with PD and constipation
• Participate in follow-up sessions with the dietitian for 3 months
• Complete assessments at baseline, midpoint, and end of the intervention to evaluate bowel function, constipation symptoms, gut microbiota, nutritional status, and diet quality

Inclusion Criteria:

• Adults with a previous diagnosis of Parkinson's disease
• On a stable dose of levodopa for at least 3 months
• Diagnosis of Functional Constipation by the Rome IV protocol

Exclusion Criteria:

• Diagnosis of atypical or secondary parkinsonism
• Hoehn and Yahr stage greater than 2
• Presence of severe neurological or psychiatric disorders that impair the ability to participate in study procedures
• Previous diagnosis of dementia
• Presence of comorbidities such as active cancer, chronic obstructive pulmonary disease (COPD), heart failure, and/or chronic kidney disease
• History of gastrointestinal cancers, inflammatory bowel diseases, or surgeries involving the gastrointestinal tract
• Constipation secondary to clinical conditions such as hypothyroidism or diabetes mellitus
• Use of opioids
• Use of probiotics or antibiotics in the past 8 weeks
• Continuous use of laxatives in the past 8 weeks
• Presence of severe dysphagia according to the Functional Oral Intake Scale (FOIS)
• Individuals receiving enteral or parenteral nutrition
• Individuals under nutritional follow-up in the past 3 months