Plant fibers are the portions of plant foods that are not digested in the human small intestine. During this century, remarkable advances have been made in defining the characteristics and importance of most nutrients such as carbohydrate, proteins, fats, vitamins, and minerals. Plant fibers have largely been neglected because they considered to have no nutritive values. In the last decade, however, considerable attention has been focused on the various plant fibers because of their influence on gastrointestinal physiology. Evidence is emerging that plant fibers have profound influences on human nutrition because they alter the absorption and metabolism of many nutrients. We will review the evidence that plant fibers greatly influence the absorption and subsequent metabolism of carbohydrates and fats.