Bran does not work on horse's like it does on humans. As a laxative it wouldn't be my first choice, not enough fiber and you'd have to feed to much to have a huge effect. Yes it's true you don"t want to be feeding it large volumes, it's very high in Phosphorus which can cause an imbalance in the calcium/PH levels.
The amount of fiber in bran is only about 11%. Whereas something like beet pulp is about 20%.
This a great article written by the Alberta AG. http://www1.agric.gov.ab.ca/$departm...l/hrs3243#Bran