Fiber refers to indigestible plant matter consumed with food. Insoluble fiber is not water-soluble, and so passes through the intestine and directly into the stool. By increasing stool volume, it speeds the time to defecation. This is thought to be healthy because when the stool moves through faster, toxins and metabolic byproducts have less time to hang around and cause colon cancer. Recent studies have brought this into question, but the jury is still out.

One thing that isn't questioned is that by trapping water, insoluble fiber softens stool and may relieve constipation. This is the basis for products like Metamucil™. By adding bulk to the gastric load, insoluble fiber also slows sugar absorption and reduces blood sugar fluctuations.

Good sources of insoluble fiber include wheat bran or whole-wheat breads, and some vegetables.

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