Why Humans Digest Glycogen but Not Cellulose: Enzyme Specificity and Structure

Structural Difference

Polysaccharide	Linkage	Architecture	Human Enzyme
Glycogen	α-1,4 (linear) + α-1,6 (branch points every 8–12 residues)	Highly branched, soluble granule	Glycogen phosphorylase, debranching enzyme, amylases
Cellulose	β-1,4	Linear, hydrogen-bonded microfibrils → crystalline	None (no cellulase)

Metabolic Consequence

• Glycogen: Rapid mobilization → glucose-1-P → glycolysis (liver exports glucose; muscle uses locally).
• Cellulose: Passes as insoluble fiber → fermented by colonic microbiota → SCFAs (butyrate, propionate, acetate) → absorbed for colonic health, minimal caloric contribution.

Evolutionary Perspective

Herbivores host cellulase-producing symbionts (rumen, cecum). Humans lost cellulase; our large brain energy budget favored cooking and enzymatic digestion of starch (amylase gene amplification) over fiber fermentation.