Effects of lactulose and polyethylene glycol on colonic transit

Background: The effects of lactulose and polyethylene glycol on colonic transit are poorly established. Aim: To assess the effects of these laxatives on colonic transit in normal subjects. Methods: Colonic transit (mean residence time, cumulative counts in stool, counts remaining in the proximal or distal colon) was measured scintigraphically in normal subjects on the second and third day of a 3-day ingestion of 67-134 g/day lactulose, or 59 g/day polyethylene glycol. Results: At similar stool weight (lactulose: 653 +/- 120 g/day; polyethylene glycol: 522 +/- 66 g/day), transit was significantly slower during 99 g/day lactulose when compared with 59 g/day polyethylene glycol; this was most pronounced in the distal colon (mean residence time: lactulose - 403 +/- 55 min; polyethylene glycol - 160 +/- 41.9 min). Short chain fatty acid concentration in 24-h stool correlated significantly with counts remaining in the distal colon at 12 h(r = 0.79, P = 0.001). Increasing lactulose doses were significantly associated with increasing stool weight (r = 0.79) and shorter mean residence time in the total (r = -0.56) and distal colon (r = -0.64). The sum of faecal carbohydrates plus short chain fatty acids was associated with stool weight (r = 0.95, P < 0.001). Conclusion: Lactulose accelerates colonic transit. However, compared with polyethylene glycol, transit during lactulose is prolonged.