It’s long been known that fibre in the diet is essential for health. Even the somewhat cranky health farms of the early 20th century were obsessed with this.
Since then the explanation for these benefits was that fibres acted as a kind of ‘bulking agent’ for stools, allowing the large bowel to ‘push against something’ when it comes to proper bowel movements.
It’s only recently that the real reason has been identified: fibres are important because certain types of very beneficial bacteria in the large bowel need them.
Simple as that!