Grape Impact on Human Gut Microbiome

The human gut microbiome consists of trillions of microorganisms that are essential to host metabolism, immune system regulation, and the maintenance of disease resistance. Recent findings indicate that dietary changes can rapidly and significantly alter the composition of these microbial communities, leading to substantial effects on the host's overall physiology. Foods rich in polyphenols are highly effective at modulating this environment because they possess selective antimicrobial properties and exert beneficial prebiotic effects on the resident microbiota.

Polyphenol-rich foods beneficially modulate the human gut microbiome with downstream metabolic and immune effects. California table grapes contain a complex mixture of phenolics, resveratrol, flavans, flavonols, and anthocyanins, making them ideal candidates for investigating gastrointestinal and immune health. However, previous grape-microbiome studies have been limited by small sample sizes (n=21-29), short durations (2 weeks), low-resolution microbiome technologies (16S profiling), and failure to measure corresponding inflammatory or immune biomarkers.

This project will first conduct a microbiome screening phase in up to 300 healthy adults to identify individuals who meet pre-specified microbiome and other study eligibility criteria. Eligible individuals may then be invited to participate in a rigorous, multi-omic pilot study using standardized freeze-dried grape powder to determine how daily grape consumption modulates the gut microbiome-immune axis in up to 40 healthy adults. The study team will employ state-of-the-art metagenomic sequencing to achieve high-resolution microbiome profiling; metabolomic analysis; and immune biomarker profiling to assess gastrointestinal inflammation.

Specific Aims

Overarching Goal: To build on our unique expertise in high-resolution microbiome analyses to conduct a rigorous, multi-omic pilot study determining how daily consumption of California table grape powder modulates the human gut microbiome-immune axis in healthy adults.

Hypothesis: Compared to a placebo, daily consumption of California table grape powder will beneficially modulate the gut microbiome by: (1) altering microbial composition; (2) shifting the fecal metabolome to increase grape-derived and microbial-derived anti-inflammatory compounds; and (3) reducing baseline biomarkers of gastrointestinal inflammation.

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