A Pilot Participant-blind Study of the Effect of Bran, Psyllium and Nopal on Intestinal Water Content Using Magnetic Resonance Imaging

BACKGROUND AND RATIONALE

The role of dietary fibre in human health is a topic of both debate and research. Interpreting evidence is complicated by changes in the definition of fibre. In the UK the Scientific Advisory Committee for Nutrition has recently proposed a definition of "all carbohydrates that are neither digested nor absorbed in the small intestine and have a degree of polymerisation of three or more monomeric units, plus lignin". They have also called for further research into the physiological effects of fibre of the gastrointestinal (GI) tract.

The Nottingham GI MRI group has been at the forefront of elucidating the effects of poorly digested carbohydrates on gastrointestinal physiology. The investigators have published techniques to measure free water in the small bowel and assessment of the physical form of chyme in the colon using MR relaxometry. This includes the demonstration that fructose ingestion on its own leads to increased free water in the small bowel compared to co-ingestion with glucose.

Bran is often confused with dietary fibre. Although it contains fibre, the term actually refers to the outer husk of seeds and grains. It has been shown to accelerate oro-caecal transit (OCT) with scintigraphy. Bran can also be shown to increase small bowel water content, possibly by mechanical stimulation, one of the most powerful stimuli for small intestinal secretion, which may be a mechanism to prevent blockage. In the past bran's high fibre content led to the hypothesis that its effect derived from holding water in the lumen but a comparison study of 17 different fibres found an inverse relationship between their water-holding properties and effect on faecal weight, with bran holding the least water but inducing the largest weight changes. Rather, weight change appeared to relate to the amount of undigested fibre present in the faeces and the associated changes in bacterial numbers, which account for most of dry stool weight.

In contrast, psyllium husk contains a fibre that binds water effectively. On this basis it has been used as a laxative for thousands of years. In fact, there is little evidence that psyllium accelerates whole gut transit time as might be expected of a laxative. Work within the investigating group has shown that psyllium increases water in both the small bowel and the faeces, leading to softer, looser stool.