I eat lots of other high fibre foods: oatmeal, wheat bran, whole grain breads and pastas, salads. No issues.

Oligosaccharides are something different. They’re in between simple sugar and fibre. Medium chain sugars basically. They need to be broken apart to be absorbed and used by the body. But we don’t produce the enzyme needed to cleave them.

Bacteria can break them down but they live in the colon (like most other digestive bacteria). This breakdown in the colon essentially results in a bunch of simple sugars being introduced into a bacteria-rich environment. More bacteria grow and feast on the sugars, producing gases such as methane.