Humans cannot produce enzymes that break down most plant fibers on their own, unlike herbivores. Instead, bacteria in the human gut partially break down these fibers to produce short-chain fatty...
Mechanism
Synthesis from 1 study
Humans can't digest plant fiber because we lost the enzymes needed to break it down, thanks to our ancestors eating mostly meat. Our stomachs and gut bacteria are now optimized for meat, not plants, so most fiber just passes through with only a tiny bit turned into energy by bacteria that aren't...
Most probable mechanism
Humans don't make the enzyme needed to break down plant fiber like cellulose, so instead of digesting it themselves, they rely on gut bacteria to slowly turn small parts of it into energy-rich fats. Their bodies evolved to focus on digesting meat, so they made stronger stomach acid, more meat-digesting enzymes, and gut bacteria that prefer protein over plants. This means fiber mostly passes through undigested, with only a little broken down by bacteria.
The human genome lacks functional genes encoding cellulase enzymes, preventing direct hydrolysis of cellulose and other plant polysaccharides.
Evolutionary pressure from a meat-based diet led to reduced selective maintenance of enzymes for plant carbohydrate digestion, including salivary and pancreatic amylase, and loss of fiber-degrading enzyme systems.
High protein intake and low fiber availability promote gastric acid secretion and activation of proteolytic enzymes such as pepsin, trypsin, and chymotrypsin, optimizing digestion of animal proteins.
The gut microbiome shifts from fiber-fermenting bacteria to proteolytic bacteria that thrive on amino acids from dietary and sloughed intestinal proteins.
Proteolytic bacteria ferment amino acids into short-chain fatty acids, ammonia, and other nitrogenous compounds, but only a minor fraction of plant polysaccharides are fermented due to limited substrate availability and absence of specialized cellulolytic taxa.
Undigested plant polysaccharides pass through the gastrointestinal tract largely intact, with minimal energy extraction compared to herbivores that possess endogenous cellulase and specialized fermentation chambers.
Evidence from Studies
Supporting (1)
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Human Digestive Physiology and Evolutionary Diet: A Metabolomic Perspective on Carnivorous and Scavenger Adaptations
Contradicting (0)
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