One of the most common misconceptions about encopresis (functional constipation) even among healthcare specialists, is that it is caused by not eating enough fiber and/or not drinking enough liquid. The truth is that diets low in dietary fiber or liquid do not cause functional constipation. Functional constipation (encopresis) is caused by stool withholding and subsequent rectal distention. However, for prevention, fiber-rich diets are recommended for all children, especially for those who have a history of occasional or functional constipation.
Dietary sources of fiber contain both soluble and and insoluble fiber (see Chapter 17 in my book for the important differences between the two.) For children, the daily amount of fiber recommended is the age of the child plus five grams. The recommended liquid consumption from all sources for children 1-8 years of age is 4 to 5 cups (32 to 40 fluid ounces) a day.