The effect of prunes on stool output, gut transit time and gastrointestinal microbiota: A randomised controlled trial

Abstract

Background & aim: Prunes (dried plums) are perceived to maintain healthy bowel function, however their effects on gastrointestinal (GI) function are poorly researched and potential mechanisms of action are not clear. We aimed to investigate the effect of prunes on stool output, whole gut transit time (WGTT), gut microbiota and short-chain fatty acids (SCFA) in healthy adults Methods: We conducted a parallel group, randomised controlled trial with three treatment arms in 120 healthy adults with low fibre intakes and stool frequency of 3–6 stools/wk. Subjects were randomised to 80 g/d prunes (plus 300 ml/d water); 120 g/d prunes (plus 300 ml/d water) or control (300 ml/d water) for 4 weeks. Stool weight was the primary outcome and determined by 7-day stool collection. Secondary outcomes included stool frequency and consistency (stool diary), WGTT (radio-opaque markers), GI symptoms (diary), microbiota (quantitative PCR) and SCFA (gas liquid chromatography). Group assignment was concealed from the outcome assessors. Results: There were significantly greater increases in stool weight in both the 80 g/d (mean + 22.2 g/d, 95% CI −1–45.3) and 120 g/d (+32.8 g/d, 95% CI 13.9–51.7) prune groups compared with control (−0.8 g/d, 95% CI −17.2 to 15.6, P = 0.026). Stool frequency was significantly greater following 80 g/d (mean 6.8 bowel movements/wk, SD 3.8) and 120 g/d (5.6, SD 1.9) prune consumption compared with control (5.4, SD 2.1) (P = 0.023), but WGTT was unchanged. The incidence of flatulence was significantly higher after prune consumption. There were no significant differences in any of the bacteria measured, except for a greater increase in Bifidobacteria across the groups (P = 0.046). Prunes had no effect on SCFA or stool pH. Conclusions: In healthy individuals with infrequent stool habits and low fibre intake, prunes significantly increased stool weight and frequency and were well tolerated. Prunes may have health benefits in populations with low stool weight. Clinical trial registry number and website: ISRCTN42793297 http://www.isrctn.com/ISRCTN42793297.