The chestnut tree (Castanea sativa Mill.) is a species widely cultivated in Portugal, which is a major producer of chestnuts. Nuts are nutritionally interesting, not only because of their content of vitamins, minerals, and phytochemicals but also of their high fiber content.

Fiber, as it is not digested by humans, has a preponderant role in the intestinal microbiota, for its maintenance, and, consequently, has an impact on metabolic status.

The inclusion of foods rich in these components, and with extensive local production, can be an excellent strategy for improving the metabolic parameters of the population.

The main objective of this single group assignment clinical trial is to evaluate the effect of including roasted chestnuts in the daily diet on the composition and diversity of the intestinal microbiota. It is also intended to evaluate metabolic parameters to determine the impact of this intervention.

Inclusion Criteria:

• Caucasian.
• Filling informed consent.
• Body mass index (BMI) between 18,5 kg/m2 and 25 kg/m2.

Exclusion Criteria:

• Chestnut sensibility.
• Chestnut daily consumption in the month before the study initiation.
• Having taken antibiotics within the 6 months prior to beginning the study;
• Use of pro/prebiotics or fiber as a dietary supplement or any food/molecule that modifies intestinal transit time 6 weeks before recruitment.
• Use of laxative 6 weeks before recruitment.
• Specific dietary regimen (e.g., vegan).
• Specific nutritional therapy (e.g. high protein).
• Excessive alcohol consumption.
• Smoking.
• Diagnosis of gastrointestinal pathology, hormonal or thyroid pathology, autoimmune diseases, chronic use of corticosteroids, psychiatric disease or Diabetes Mellitus.
• Use of proton pump inhibitors, antidiabetic drugs, insulin, or statins.
• Pregnant or breastfeeding.
• Participation in another clinical trial within the last 3 months.