It is thought that prunes, also called dried plums, aid in reducing constipation due to a high content in dietary fiber as well as polyphenols and sorbitol. It is likely that these have a combined effect.
Sorbitol is known to have natural laxative properties. Prunes contain both water soluble and insoluble fiber. Water soluble fiber helps the body absorb more nutrients while insoluble fiber adds bulk to stools to help them pass more easily. One cup of pitted, uncooked prunes contains approximately 12 grams of fiber, or about 6 grams of fiber per 100 grams of prunes. The daily recommended fiber intake is 21 to 25 grams for women and 30 to 38 grams for men.
In clinical studies with patients who had chronic constipation, prunes had a greater laxative effect than that of psyllium, a dietary fiber commonly used in commercially prepared laxatives. For this reason, researchers believe that the benefits observed with subjects who ate prunes is due to ingredients other than fiber, such as the sorbitol. Dried plums are also well tolerated by most people even when taking other emergency laxative preparations. Prunes are considered to be an effective first-line treatment for mild to moderate constipation.