The medical definition of a laxative is any food or medicine that makes it easier for solid waste to pass through the body, according to the Merriam-Webster online dictionary. The two types of fiber, soluble and insoluble, that make up dietary fiber -- the kind of fiber you eat -- do have a laxative effect, albeit for different reasons. A healthful diet consists of both types of fiber, as they both help promote intestinal health and aid in preventing bowel diseases.
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All the plant-based foods contain a mixture of soluble and insoluble fiber, according to the University of Maryland Medical Center. Most people in the U.S. don't get enough dietary fiber in their diet, which, according to the U.S. Department of Agriculture, should be about 38 grams of fiber per day for men and about 25 grams a day for women. About a quarter of that consumption should include soluble fiber. Consuming a diet rich in both types of fiber help maintain digestive health and promote regular bowel movements. Fiber can also help with the prevention of bowel disease. For example, a long-term follow-up study published in the April 1998 "Journal of Nutrition" found that consuming dietary fiber, particularly insoluble fiber, reduced the risk of diverticular disease in male health professionals.
Insoluble fiber does not dissolve in water and so passes through the digestive tract virtually intact while absorbing water, which creates a bulky but soft stool -- making it a natural laxative, as a bulkier, softened stool moves more easily through the digestive tract. A smoother movement of bodily waste through the digestive tract can help prevent constipation and straining when having a bowel movement, which can help prevent hemorrhoids and other bowel diseases. Another advantage of insoluble fiber is that it does not get consumed much by intestinal bacteria, which means it produces less intestinal gas.
The gel-like stool created by soluble fiber does not bulk up the stool, but it can act as a laxative if you have particularly hard and dry stools -- a common condition in those who are constipated. Many over-the-counter laxatives for constipation contain soluble fiber for this reason, according to the University of Maryland Medical Center. Soluble fiber also has other benefits, including helping to reduce intestinal absorption of sugars and starch, which can ultimately lower cholesterol levels. One study published in the March 2008 edition of "Phytomedicine" found that patients taking psyllium husks, which are high in soluble fiber, experienced a significant decrease in cholesterol levels after three weeks of treatment.
Whole foods, such as whole grains, fresh fruits and vegetables, legumes and nuts, are the best sources of fiber, as they have vital nutrients and minerals that fiber supplements typically do not. Taking a fiber supplement can be beneficial, however, particularly if you have a digestive problem, but ensure you drink plenty of water to avoid constipation. If you have constipation or an impacted stool, the University of Maryland Medical Center recommends consulting your doctor before taking a fiber supplement. Dietary fiber can cause gas and bloating if taken in large quantities, particularly if you aren't used to fiber in your diet. Gradually increasing the amount of fiber in your diet can usually help prevent any problems.