They are made of compounds of carbon, hydrogen, and oxygen called sugars or saccharides. Molecules of these simple sugars attach together to make long branching chains that are called complex carbohydrates. These large carbohydrate molecules are also commonly referred to as starch. Once you eat them, digestion by intestinal enzymes disassembles these chains back into the simple sugars, which then pass easily through the intestinal wall into the bloodstream, where they journey to the body's tissues. Metabolic processes change these simple sugars into energy, which provides fuel for the body's activity.
Dietary fibers are even longer chains of complex carbohydrates.
Unlike starch molecules, these fibers resist digestion because of their chemical configurations. Therefore, most fibers eventually end up in the colon and form the bulk of your stool. Most people think that fibers are only the husks of grains and the long stringy components in fruits and vegetables, but actually, dietary fibers are present in all plant tissues. For example, after a potato is peeled, the white matter we eat has plenty of relatively indigestible fibers in it. Fats too are complex molecules made up of carbon, oxygen, and hydrogen.