Cellulose
An insoluble fibre that forms plant cell walls. Humans cannot digest it, and it adds bulk to stools and helps bowel transit.
Cellulose is the structural component of plants (the “skeleton” of plants) and the most abundant organic molecule on Earth. Chemically, it is a glucose polymer, but with (beta) bonds that human enzymes cannot break. Hence its status as an insoluble fibre.
The original angle (why we can’t digest it but cows can): a biological curiosity: we eat cellulose without getting any calories from it, for lack of an enzyme (cellulase). Ruminants (cows, sheep), on the other hand, host micro-organisms in their rumen that produce this enzyme and ferment cellulose into energy. That is why a cow can live on grass and we cannot.
Our colonic microbiome ferments cellulose very little (unlike soluble fibres). It therefore passes through the digestive tract almost intact, playing a mechanical role:
- It increases stool bulk and softens stools
- It speeds up transit (helping prevent constipation)
- It provides fullness through its bulk
- It “sweeps” the intestine
A little-known industrial use: cellulose is also a food additive (E460 to E469). It is added as a texturising agent, as an anti-caking agent (in industrial grated cheese!) or to boost the fibre content of processed products. “Microcrystalline cellulose” is common.
Natural sources: wheat bran, vegetables (especially the fibrous parts: stalks, skins), wholegrain cereals, pulses. A diet rich in whole plant foods naturally provides cellulose and a varied range of fibres.