The goal of this clinical trial is to learn if a fiber (psyllium) can change the way bacteria use fructans (a type of sugar) and whether psyllium can help decrease childhood irritable bowel syndrome (IBS) symptoms when eating fructans. The main questions it aims to answer are:

Aim 1: The effect of psyllium at two doses given with a fructan meal on microbial fructan fermentation (intracolonic pH; H2 gas production; gut microbiome composition; fecal short-chain fatty acids, lactate, glycomics).

Aim 2: Determine the impact of psyllium given with a fructan meal on fructan-induced GI symptoms.

Participants will first be asked to eat a specific diet over two three-day periods to determine if fructans worsen their IBS symptoms. Those with worsening symptoms with fructans will be asked to participate in the second part of the study. This includes two weeks of baseline (no change in diet) and two weeks of eating a specific diet with fructans with either psyllium or glucose. Participants will be asked to complete pain and stool diaries, submit stool specimens, swallow a pill to capture gut acid levels, and give breath samples.

Inclusion Criteria:

• Children between the ages of 12-17 years meeting pediatric Rome IV criteria for IBS

Exclusion Criteria:

• Children who have had previous bowel surgery, have documented GI disorders (e.g., ulcerative colitis), or a serious chronic medical condition (e.g., diabetes)
• weight and/or height are > or < 2 SD for age
• have chronic conditions with GI symptoms (e.g., cystic fibrosis)
• have been on antibiotics or probiotics within 3 months (because of potential alterations to the GI microbiome0
• girls who are pregnant (tested with urine beta-human chorionic gonadotropin at the initial visit)