What does drinking prune juice daily do for constipation?

Laxative effect. Prune juice is a well known laxative. It is not completely clear why Sorbitol, a sugar in it may be part of the reason, as it can cause water to be absorbed into the intestine, which softens the stool, Dried prunes are even better for you because they have a lot of fiber which helps with constipation among other things. I think dried prunes are delicious! See: http://tinyurl.com/ktqm2ng.
It's fiber. . Fiber isn't absorbed and passes through the GI tract, and out.On a daily basis this gives bulk and increases the water content thereby softening the stool.This makes it easy to pass and relieves mild to moderate constipation. .
Keeps you regular. Many people find that daily prune juice promotes regular bowel movements. The same may be accomplished by apricot juice. Increasing dietary fiber (e.g., granola for breakfast) may help too. A person who has a sudden persistent change in bowel habits, however, should consult a physician.
Should help. Prune juice could help manage chronic constipation by increasing the frequency of bowel movement, it may take up to 3 days before you notice improvement and it may not work for everybody. .