Effect of acute dietary fibre supplementation on colonic pH in healthy volunteers.

Dietary fibre and undigested starch are fermented to short chain fatty acids by colonic bacteria with acidification of the colon. It has been suggested that acidification of the colon by these fatty acids inhibits bacterial metabolism, but this concept has been disputed. The aim of this study was to investigate the short term effect of a dietary fibre load on colonic metabolism. Colonic pH and breath hydrogen was measured in healthy omnivorous British male volunteers following ingestion, in turn and at weekly intervals, of 15g lactulose, wheat bran biscuits, oat bran biscuits and ispaghula husk. All the test meals caused a reduction in caecal pH and an increase in breath hydrogen production. The changes were greatest with lactulose. Lactulose and wheat bran caused acidification of the right and left colon whereas oat bran and ispaghula husk caused acidification of mainly the right colon. An inverse correlation between right colonic pH and breath hydrogen was observed in only the oat bran study. This study has demonstrated the ability of dietary fibre to lower right colonic pH and to increase breath hydrogen excretion. The changes were greater with soluble fibre than with insoluble fibre but the change in luminal pH was persistent all round the colon with insoluble fibre.