Irritable Bowel Syndrome and Control Volunteers: Diet Challenge

During the study: Screening/Visit 1 * Participants will be asked to read and sign this informed consent after all questions about the study have been answered to satisfaction. * This is a screening visit, and we may determine that a participant is ineligible to continue to participate in the study. * The study team will collect medication and medical history. * A physical exam will be performed by the physician. Vital signs and body measurements will be collected. * Participants will be provided with a take-home lasagna meal (vegetarian and non-vegetarian option) and dessert. * Participants will receive a stool collection kit and instructions for the collection, storage, and transportation of stool. * Instructions will be provided for a low fiber, high fat diet for Days 2-4. * Participants will be asked to complete diet recall using the ASA24 and a bowel pattern diary to record bowel symptoms over the course of the study. * IBS participants will complete the irritable bowel syndrome severity scoring system questionnaires (IBS-SSS). Day 2-4 (at home): * Participants will record stool symptoms and patterns in a diary daily. * Participants will be instructed to consume a low fiber diet (maximum one piece of fruit and 100 g of vegetables a day, and white flour instead of whole grain products), avoid alcohol, and start a 4-day 100 g fat diet. * On the evening before Day 5, participants will be instructed to collect a stool sample at home and return during Visit 2. Day 5 (Visit 2): * Participants will return to the clinic on Day 5 after an overnight fast. * Participants in the reproductive age range will be given a urine pregnancy test prior to inulin ingestion and the study team will confirm a negative test result. * Participants will receive breakfast onsite. The non-vegetarian option will consist of pancakes, butter, syrup, hard-boiled eggs, and bacon. The vegetarian option will substitute the bacon with a cheese snack (approximately 790 to 840 kcal) along with 10 g inulin Orafti® powder) thoroughly mixed in 200 mL of water. * Take-home meals will be provided for lunch and dinner and are to be consumed at 4 and 8 hours from breakfast. * IBS participants will complete the irritable bowel syndrome severity scoring system (IBS-SSS) questionnaire. * A stool sample will be collected on Day 5. * Participants will continue to complete diet recall using the ASA24 and a bowel pattern diary to record bowel symptoms over the course of the study. Day 6 * Participants will be instructed to collected a final stool sample (Day 6). * Participants will continue to complete diet recall using the ASA24 and a bowel pattern diary to record bowel symptoms over the course of the study. Day 7 (Visit 3) • Participants will return to the clinic to return stool collections (Day 5 and Day 6), the daily diaries, and the IBS-SSS forms (if applicable). Day 30-90 (Optional dietary follow-up): To further assess the utility of the 24 hour dietary recall, willing participants will be invited to participate in a an optional follow-up activity during which they will complete the Automated Self-Administered Dietary Assessment Tool with or without assistance of a trained interviewer. Interested volunteers will be contacted by phone and provided instructions on how to complete the tool online by the study team.