GUT-FEELINGS: Using Citizen Science for Large-Scale Prebiotic Clinical Trials for Mental Health in Adults

Criteria: 3.1 Inclusion Participants must meet all of the following criteria: * Adults aged 21-80 years (inclusive) at the time of electronic informed consent, of any ethnicity, race, or gender identity * Assigned sex at birth determines sex-specific PRO branching (e.g., menstrual interference) where applicable * Reside in the contiguous 48 states in the United States and is able to provide a valid US shipping address and and 10-digit mobile phone number * Per state regulations, individuals residing in New York, New Jersey, and Rhode Island will be excluded * Capable of providing voluntary, electronic informed consent and willing to comply with study procedures, including remote biospecimen collection * PROMIS Anxiety 8A T-score between 55.0 and 69.9 (inclusive) at screening, corresponding to mild-to-moderate anxiety symptoms. * On a stable medication regimen for ≥30 days prior to screening and willing to maintain that regimen for the duration of the trial, unless changes are deemed medically necessary by the participant's healthcare provider. Stable, prescribed psychotropic medications (including SSRIs, SNRIs, and other antidepressants or anxiolytics) are not exclusionary. * Willing to take a study product and not know the intervention arm assignment (active or placebo) or the product identity. * Reliable daily access to the internet via mobile device or computer 3.2 Exclusion Individuals who report any of the following during screening may be excluded from participation: * Currently pregnant, planning to become pregnant during the trial, or breastfeeding * Unable to read and understand English at a 7th-grade reading level * Currently enrolled in another interventional clinical trial or planning to enroll in one during the trial timeline * Heavy alcohol consumption (defined for female sex assigned at birth \>/= 2 alcoholic beverages/day, male sex assigned at birth \>/=3/day) or use of recreational or illicit drugs other than cannabis * History of bariatric or weight-loss surgery (e.g., Roux-en-Y gastric bypass, sleeve gastrectomy, adjustable gastric banding, or biliopancreatic diversion). * Reports a current diagnosis of inflammatory bowel disease (ulcerative colitis or Crohn's disease). * Current or recent (within 3 months) major illness, surgery, or hospitalization posing a known significant safety risk * Diagnosis of cardiac dysfunction, severe liver disease, or severe kidney disease that presents a known contraindication to study product ingredients or substantial safety risk: * NYHA Class III or IV congestive heart failure, atrial fibrillation, uncontrolled arrhythmias, cirrhosis, end-stage liver disease, stage 3b or 4 chronic kidney disease, or kidney failure * Significant immunocompromise, including diagnosed primary or acquired immunodeficiency, current chemotherapy, current immunotherapy, current use of immunosuppressive medications, or daily corticosteroid dose \>5 mg prednisone-equivalent * Antibiotic use (oral, IV) currently or within 30 days prior to screening * Plans to start a new structured dietary regimen (including new probiotic or prebiotic supplementation outside the study product) during the trial period * Use of medications with a well-established interaction posing a substantial safety risk with study product ingredients * Active suicidality as determined by a response of 3 in item 9 of the PHQ-9 score at screening * Known hypersensitivity to any component of the study products * Wave-specific exclusion criteria per Subprotocol A (Wave 1) or Subprotocol B (Wave 2) Note on medication exclusions: routine prescription medications (including antihypertensives, anticoagulants at standard outpatient doses, antiseizure medications, and medications carrying a grapefruit-consumption warning) are not categorically excluded. Specific medications posing a documented safety risk in combination with the food ingredients (products) are addressed by the study screening Q-bank. * Participants residing in the states of New York, New Jersey, or Rhode Island are not eligible to participate in the study. This restriction is due to state regulatory requirements governing direct-to-consumer at-home laboratory specimen collection and processing that cannot be accommodated within the fully decentralized design of the study. No alternative collection pathway is available under the current protocol. * To ensure compliance, state-of-residence logic is built into the enrollment platform. Participants who indicate a residential address in New York, New Jersey, or Rhode Island will not be eligible. This automated screening ensures that ineligible participants are not inadvertently enrolled in the study.