Effects of therapeutic doses of lactulose vs. polyethylene glycol on isotopic colonic transit

Background Lactulose and polyethylene glycol are osmotic agents used to treat idiopathic chronic constipation. Aim To compare the effects of low doses of lactulose and PEG 4000 on transit time measured by scintigraphy in normal subjects. Methods For 5 days, 10 healthy subjects received either 10 g b.d. of lactulose or PEG 4000 in a randomized, double-blind, crossover study. On the evening of day 4, they took a capsule containing Amberlite resin pellets labelled with In-111. On day 5, after a 1000 kcal test meal labelled with 99Tcm, gastric, small bowel and colonic transits were measured. Results Gastric emptying and small bowel transit time were not different. Ascending colon emptying curve was significantly accelerated with lactulose in comparison with polyethylene glycol (P = 0.001) and, respectively, 50 +/- 18% vs. 35 +/- 18% of the radioactivity had left the ascending colon at the end of the study (P < 0.05). The descending colon filling curves, variations in the geometric centre and numbers of scintigraphic movements were not different. Conclusions In healthy subjects, in comparison to PEG 4000, usual therapeutic doses of lactulose significantly accelerate ascending colon emptying. This result supports a stimulating motor effect of colonic fermentation of lactulose.