Dietary fibre is defined as material of plant origin which is resistant to degradation by digestive enzymes, passing essentially unchanged through the stomach and small intestine, until reaching the large intestine. Although dietary fibre is not absorbed by the body, it still plays an important role in the normal functioning of the digestive tract. Dietary fibre (i.e. fibre rich grain products, fruit and vegetables) comprised an extensive part of the typical diet of the population in the early 20th century.
Fibre content has fallen progressively throughout the 20th century, and modern diets with a high content of processed food contain much reduced or little dietary fibre. It is not just the overall level of fibre in the diet which is important, but also the relative amounts of the main types of fibre:- cellulose, hemicellulose, lignan and pectin.