Composition Of Faeces From Human Subjects Consuming Diets Based On Conventional Foods Containing Different Kinds And Amounts Of Dietary Fibre

This 1990 Swedish study examined how different types of dietary fiber from regular foods affect what comes out in our stool and why fiber helps with bowel movements. The researchers fed volunteers four different diets: one rich in fiber from whole grains, two rich in fiber from beans and vegetables, and one low-fiber diet. They then carefully analyzed the participants' stool samples to understand how well different fibers were digested and what remained.

The key finding was that fiber from whole grains was much less digestible (only 62% broken down) compared to fiber from vegetables and legumes (88-90% broken down). Interestingly, people eating the grain-based fiber diet didn't have significantly more beneficial gut bacteria in their stool, which challenged the common belief that highly digestible fiber always promotes more bacterial growth in the intestines.

The researchers discovered that the stool-bulking benefits of fiber likely come from undigested soluble fiber that holds water, rather than just from increased bacterial growth. This suggests that different types of fiber work through different mechanisms - some by feeding gut bacteria, and others by physically adding bulk and retaining water in the digestive system.

This research helps explain why eating a variety of fiber sources (whole grains, vegetables, fruits, and legumes) may be more beneficial than focusing on just one type. In clinical practice, this supports recommending diverse, whole-food sources of fiber rather than concentrated fiber supplements for optimal digestive and metabolic health.