Wheat bran  

The useful role of wheat bran in promoting regularity in colonic function and preventing constipation is generally accepted. In addition, growing research has focused its protective effect against colon and breast cancers.
 
Amount of fibre in the diet has an effect on colonic function, the type of fibre and its digestibility or fermentability also play a  significant role. Both soluble and insoluble fibres have value in promoting regularity  in colonic function, as measured by stool weight and transit time, but they promote regularity via different mechanisms. Insoluble fibres, such as those from wheat bran, are  resistant to fermentation by colonic bacteria and increase fecal bulk by retaining water. 
 
Among the different sources of dietary fibre  as fecal bulking agents, wheat bran is probably the most studied and among the most effective. Wheat bran ranked among the highest in fecal bulking, exceeding fibres from fruit and vegetables, gums and mucilages, cellulose, oats, corn, legumes and pectin.
 
From a food processing perspective, the range of particle size in commercially available wheat bran offers many functional benefits.  While fiber particle  size may affect its colonic effects, the range of particle size typically found in commercially available wheat bran (coarse bran > 1400 µm to very finely ground bran <500 µm) is well within that reported to be associated with fecal bulking effects.