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Increasing stool weight promotes laxation and bowel regularity by stimulating colonic motility to propel residue distally. Most sources of insoluble fiber and fiber provided by mixed-food diets significantly increase stool weight, although they do not always do so with equal effectiveness. Concentrates of soluble fiber, with the exception of psyllium seed husk, have little effect on bowel regularity. It is recommended for healthy individuals that fiber be provided by foods, which would include cereal and grain products, legumes, fruits and vegetables. The major form of fiber in these foods is insoluble, particularly in grain products. In addition, grain products are major sources of fiber in the US diet. When food intake is inadequate, such as during disease or in the older adult, it may be necessary to consume a fiber concentrate to achieve bowel regularity. The additional stool mass that occurs with large fiber intakes is plant material, bacteria and water, and different fiber sources differ in the way in which they alter the amounts of these constituents.