Fiber is a type of carbohydrate that the body cannot digest. Unlike most carbohydrates, which are broken down into sugar molecules called glucose, fiber cannot be broken down into sugar molecules, and instead passes through the digestive system relatively intact. The National Academy of Medicine defines fiber as dietary fibers (nondigestible carbohydrates and lignans) that occur naturally in plants, and functional fibers that are extracted from plants or synthetically made and are nondigestible with a beneficial health effect in humans. Some types of fiber fall into both categories, such as oligosaccharides and resistant starches that may be naturally occurring or synthetically made.
There are many different kinds of fiber, and they have different effects on the body. Some types of fiber influence how quickly food moves through the gut, adding bulk to stools and keeping the digestive system healthy. Other types of fiber can impact blood cholesterol and how quickly the body absorbs sugar from foods and drinks. Soluble fibers are water-soluble and therefore digestible, and are present in fruits, vegetables, oats, and barley and may help manage blood sugar and cholesterol levels. Insoluble fibers are not digestible and come from the outer skins of plants, and can help prevent constipation.
It is recommended that adults get 30g of fiber a day, but on average, people eat much less than this, about 20g a day. Good sources of fiber include wholegrain cereals, fruits, vegetables, nuts, seeds, and legumes. To get enough fiber in the diet, it is important to include a variety of fiber-providing foods regularly, such as wholemeal bread, wholegrain breakfast cereals, fruits, vegetables, nuts, seeds, and legumes.
