To join the study, children must be between 12 and 17 years old and have irritable bowel syndrome based on special rules called Rome IV. Children cannot join if they have had bowel surgery, other stomach diseases like ulcerative colitis, or serious health problems like diabetes. They also cannot join if their height or weight is far from normal for their age. Children with other long-term illnesses that affect the stomach, like cystic fibrosis or kidney disease, cannot join either. Children who took antibiotics or probiotics in the last 3 months cannot join because these change the stomach bacteria. Children who are pregnant or have conditions like autism, major depression, or bipolar disorder cannot join because these make it hard to report symptoms. Children taking certain medicines, like diabetes or heart medicines, also cannot join.