In nutrition, fibre is often considered as a non-digestible component of food. It is true that humans lack the cellulase enzyme necessary to digest fibre, but so do ruminants such as cows, sheep and goats. These animals can however digest grass and other fibrous foods and extract energy from it by fermenting the material in their intestines. This process involves the use of bacteria who possess the ability to convert the cellulose in plant foods into short chain fatty acids such as propionate, butyrate or acetic acid. Other animals and humans also possess this ability, and it is known that a high fibre meal can provide significant energy value in the form of short chain fatty acids, which are absorbed and metabolised. Evidence from animal studies show that the exact energy value extracted depends on the type of fibre ingested. In pigs for example, lignin is undigested, whereas pectins (from fruit) are almost totally digested. Further, hemicellulose tends to be more digested than cellulose, although both are partly digested.
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