Biomedical subjects
Publications and source records attributed to D F Evans.
A sodium ion carrier transport system contains the antibiotic monensin in the membrane and uses a pH gradient as an energy source. The experimental results are in accord with theoretical predictions based on a mechanism which is understood on a molecular level.
An analysis of fluid mechanics and diffusion in the gallbladder predicts that there is a thin layer of high bile-constituent concentration near the gallbladder wall. Cholesterol may precipitate in this layer, even when the average cholesterol concentration in the gallbladder is below saturation. The amount precipitated increases with time, with increased average cholesterol concentration, and with decreased average lecithin concentration. If the gallbladder does not empty completely, the precipitated cholesterol particles may grow over many cycles of gall bladder filling and emptying. The analysis explains why cholesterol-gallstone formation is not correlated with bile-constituent concentration alone, why a flaccid, noncontractile, gallbladder has a greater chance of forming gallstones, why small stones are frequently found near the gallbladder wall, and why stones may be found in only one limb of a double gallbladder.
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.
Dietary fibre supplementations studies to evaluate the effect of dietary fibre on colonic and faecal pH have relied on the use of the unabsorbed disaccharide and osmotic laxative lactulose. We studied the effect of chronic dietary fibre supplementation on colonic pH in healthy British volunteers who consumed a normal mixed diet but who consumed a normal mixed diet but who were asked to double their daily intake of dietary fibre from their usual fibre sources. The median dietary fibre intake of the 5 male volunteers was 17.5 g/day. This was doubled to 34.4 g/day. The pH in all the parts of the colon decreased progressively during the period of study, being lowest at the end of two weeks of fibre supplementation. The pH returned to the presupplementation values within two weeks of cessation of fibre supplementation. This study has shown that the effect of dietary fibre on colonic pH is short-lived and therefore its use for dietary intervention in large bowel diseases should be indefinite.