Dietary fiber is a group of complex carbohydrates that are not a source of energy for human beings. Because human digestive enzymes cannot break the bonds that hold fiber's sugar units together, fiber adds no calories to your diet and cannot be converted to glucose.
Ruminants (animals, such as cows, that chew the cud) have a combination of digestive enzymes and digestive microbes that enable them to extract the nutrients from insoluble dietary fiber (cellulose and some hemicelluloses). But not even these creatures can pull nutrients out of lignin, an insoluble fiber in plant stems and leaves and the predominant fiber in wood. As a result, the U.S. Department of Agriculture specifically prohibits the use of wood or sawdust in animal feed.
But just because you can't digest dietary fiber doesn't mean it isn't a valuable part of your diet. The opposite is true. Dietary fiber is valuable because you can't digest it!
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