Could a daily serving of grapes alter your gut bacteria and immune defenses?

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria * Healthy adults between the ages of 45 and 65 years old * Body Mass Index (BMI) within the range of 18.5 to 25.9 kg/m² * Willing and able to provide informed consent * Agreement to maintain usual dietary patterns throughout the study, except for the provided study powder * Agreement to avoid exclusion criteria (below) during the study period * Able and willing to collect and mail a stool sample during the microbiome screening phase and, if enrolled in the intervention phase, and at the three defined study timepoints * For enrollment into the intervention phase, meeting pre-specified microbiome eligibility criteria based on the screening stool sample, as defined in the study protocol. Exclusion Criteria * Antibiotic use within 6 months prior to enrollment, or any planned antibiotic use during the study * Recent initiation of or dose change in any medication or supplement within 3 months prior to enrollment, or planned initiation or dose change during the study * Initiation or change of probiotic, prebiotic, or synbiotic supplements during the study. Existing long-term stable probiotic use is permitted * Current or recent (within 3 months) systemic corticosteroid, biologic, DMARD, chemotherapy, or other immunosuppressive therapy; regular NSAID use (more than 2 doses per week); or planned initiation of any of the above during the study * Active inflammatory bowel disease, irritable bowel syndrome with current symptoms, celiac disease, microscopic colitis, active diverticulitis, or acute gastrointestinal infection within 3 months; any history of C. difficile infection; or prior bariatric surgery or bowel resection * Uncontrolled or unstable chronic disease, including but not limited to uncontrolled diabetes (HbA1c \>8%), chronic hepatitis B or C, cirrhosis, moderate-to-severe chronic kidney disease (eGFR \<60), active or recent (within 5 years) malignancy, solid organ or bone marrow transplant, or uncontrolled cardiovascular or psychiatric illness * Planned major change in diet, physical activity, or lifestyle during the study period; recent significant weight change (\>5% in 3 months); or current adherence to an extreme dietary pattern (strict vegan, ketogenic, carnivore, or very-low-carb) maintained for more than 3 months * Habitual high polyphenol intake, including daily consumption of more than 2 servings of berries, more than 500 mL of red wine, more than 4 cups of tea, or regular consumption of pomegranate, grape juice, or dark chocolate more than 3 times per week, as assessed by screening food-frequency questionnaire * Heavy alcohol use (more than 14 drinks per week), current tobacco use, or daily cannabis use * Known grape allergy or sensitivity; pregnancy, planned pregnancy, or breastfeeding during the study; current participation in other interventional clinical trials; or any condition that, in the investigator's judgment, would compromise study participation, compliance, or data interpretation