What food comes to mind when you think of alleviating constipation?
Prunes.
Is it the fiber?
Surprisingly, no.
Prunes - which are now euphemistically called "dried plums" - have some fiber, about 8 grams per 1 cup of pitted plums; but their famous "laxation effect" comes from sorbitol, a naturally occurring sugar alcohol which you find also added to sugar-free candy, sugar-free chocolate and other sugar-free foods that can yield some nasty GI side effects.
Sorbitol absorbs water and makes foods heavier as they transit through your gut fighting and reversing constipation; the fiber in prunes helps - but it's the sorbitol doing most of the work.
If you're interested in prunes and sorbitol, nationally syndicated nutrition columnist Ed Blonz just did a great article on the topic - you can check it out here. If you're not familiar with Blonz, don't be put off by his website - he is no graphic designer - but he is most certainly a very well-respected authority in nutrition science with a great knack for answering complicated nutrition questions in a straightforward and evidence-based manner!
The California Dried Plum Board has more information about the relationship between prunes and digestive health on their website www.tummywise.com.