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Why are humans unable to digest cellulose?

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A biochemical flowchart shows the metabolic pathways for carbohydrate utilization, including glycolysis, the pentose phosphate pathway, and the tricarboxylic acid (TCA) cycle. The diagram traces the conversion of glucose, fructose, and glycerol through various intermediates, leading to energy production, carbon dioxide release, and biosynthesis of amino acids and porphyrins. Key steps and branches are labeled, with colored boxes highlighting biosynthetic outputs. — Carbohydrate Pathways for the utilization of carbohydrates.

Humans are unable to digest cellulose because the human digestive system lacks the necessary enzymes required to break it down.12

While cellulose is a polysaccharide composed of long chains of glucose units—similar to starch—the chemical bonds linking these units in cellulose cannot be hydrolyzed by human digestive juices. Most animals, including humans, do not possess cellulase, the specific enzyme needed to split these bonds.312

In contrast, some herbivorous animals are able to derive nutrition from cellulose because they harbor symbiotic microorganisms in their alimentary tract that produce the required enzymes to ferment and break down the cellulose for them. Although humans cannot digest cellulose, it remains an important component of dietary fiber, which aids in digestive health.312