Both prunes and prune juice have a laxative effect that is not entirely understood. A 100-gram serving of prunes — about 10 prunes — contains 6 grams of fiber, but “this alone would not be enough fiber to explain the laxative effect,” said Lisa Cimperman, a dietitian and spokeswoman for the Academy of Nutrition and Dietetics.
What may be doing the trick is the presence of a sugar alcohol called sorbitol. There are 15 grams of sorbitol in a serving of prunes and 6 grams in a serving of juice. Sorbitol is incompletely digested and fermented by bacteria in the gut. It also draws water into the gastrointestinal tract and, Ms. Cimperman said, “both of these effects can help promote bowel movements.”
Sorbitol occurs naturally in dried fruit and raisins and many fresh fruits, and is also manufactured as a sweetener for use in dietetic candy and sugar-free gum and mints. Consumption can cause gas, abdominal cramps and diarrhea, and The Academy of Nutrition and Dietetics warns against consuming more than 50 grams of sorbitol a day.
Studies that have evaluated the laxative effect of prunes and prune juice have generally examined the effects of a 100-gram serving of either prunes or prune juice. Though plums may have a similar laxative effect, “similar amounts of plums would be difficult to consume in a day,” Ms. Cimperman said.