These are the unrefined carbohydrates.
Unrefined carbohydrates are carbohydrates are exactly as they appear in nature and are usually referred to as whole grains in dietary language.
Whole grains have the entire seed kernel which includes the bran, germ and endosperm. Whole grains are nutrient - dense food that contain high concentrations of vitamins, minerals. antioxidants and fiber as well as some fat.
It is the fiber and fat found in whole grains that cause these carbohydrates to require longer time in digestion. Some of the fiber is insoluble while fat take longest to digest.
Refined carbohydrates on the other hand have been stripped of most of the fiber and have no fat and so are digested very quickly.