What does fiber do?
Dietary fiber comes from the thick cell wall of plants. It is an indigestible complex carbohydrate. Fiber is divided into two general categories-water soluble and water insoluble.
Soluble fiber lowers cholesterol.1 An analysis of many trials of soluble fiber reveals it has a cholesterol-lowering effect, but the degree of cholesterol reduction in many studies was quite modest.2 For unknown reasons, diets higher in insoluble fiber (mostly unrelated to cholesterol levels) have been reported to correlate better with protection against heart disease in both men and women.3 4
Soluble fibers can also lower blood sugar levels in people with diabetes, and some researchers find that increasing fiber decreases the body’s need for insulin—a good sign for diabetics.5 However, a research review reveals that just how much moderate amounts of soluble fiber really help people with diabetes remains unclear.6 As with heart disease, a clear mechanism to explain how insoluble fiber helps diabetics has not been identified. Nonetheless, diets high in insoluble fiber (from whole grains) associate with protection from adult-onset diabetes.7
Insoluble fiber softens stool, which helps move it through the intestinal tract in less time. For this reason, insoluble fiber is partially effective as a treatment for constipation.8 The reduction in "transit time" has also been thought to partially explain the link between a high fiber diet and a reduced risk of colon cancer as found in some studies,9 though anticancer effects unrelated to "transit time" have also been reported.10
The true relationship between fiber and colon cancer risk has recently been clouded by data coming from several directions. In animal research, wheat bran is proving to be more protective than other diets containing equal amounts of insoluble fiber, suggesting that fiber in wheat may not be the primary cause of protection sometimes associated with wheat.11 In human research, a recent well respected study found no significant link between fiber and colon cancer prevention.12 A trial from South Africa found that avoidance of meat and dairy, and not the presence of fiber, appears to be primarily responsible for a low risk of colon cancer.13 As a result of these negative findings some researchers and doctors have begun to question the idea that insoluble fiber protects against colon cancer, a concept that had arisen from a large body of older research.
Fiber also fills the stomach, reducing appetite. In theory, fiber should therefore reduce eating, leading to weight loss. However, at least some research has found increased fiber to have no effect on body weight despite decreasing appetite.14
Lignan, a fiber-like substance, has mild antiestrogenic activity. Probably for this reason, high lignan levels in urine (and therefore dietary intake) have been linked to protection from breast cancer in humans.15
Where is fiber found?
Whole grains are particularly high in insoluble fiber. Oats, barley, beans, fruit (but not fruit juice), psyllium, and some vegetables contain significant amounts of both forms of fiber and are the best sources of soluble fiber. The best source of lignan, by far, is flaxseed (not flaxseed oil, regardless of packaging claims to the contrary).
Fiber has been used in connection with the following conditions (refer to the individual health concern for complete information)