Library.

“Most digestive enzymes produced by the small-intestinal wall are bound to the apical (brush-border) cell membrane of intestinal epithelial cells rather than being freely secreted into the intestinal lumen.”

The evidence supports the core claim: small-intestinal digestive enzymes are predominantly brush-border membrane proteins, especially disaccharidases and many peptidases. A minor fraction can be shed into the lumen, and not every enzyme in intestinal cells is brush-border bound, but those caveats do not overturn the main point.

“Reduced perfusion of the small-intestinal mucosa can impair enterocyte renewal and contribute to villous blunting (villous atrophy), reducing absorptive surface area.”

Evidence supports this mechanism. Reduced small-intestinal mucosal perfusion can impair epithelial renewal, promote enterocyte loss, and contribute to villous blunting, which lowers absorptive surface area. The strongest data come from ischemia, shock, and sustained hypoperfusion models, but the claim is appropriately cautious in saying this can occur and can contribute.

“After proteases finish digesting food in the duodenum, they move with chyme into the mid and distal small intestine and begin digesting themselves and other enzymes into amino acids.”

The evidence supports that pancreatic proteases continue moving with chyme beyond the duodenum and may be degraded during transit. But the claim misstates both timing and mechanism: protein digestion is not simply finished in the duodenum, and free amino acids are produced mainly through brush-border and intracellular peptidases, not primarily by proteases digesting themselves and other enzymes.

“Most intestinal juice in the small intestine comes from the contents released when small-intestinal epithelial cells rupture and slough off, rather than from fluid actively secreted by intestinal glands.”

The evidence does not support the claim. Authoritative physiology sources describe intestinal juice as mainly water and electrolytes actively secreted by crypt epithelium and glands, not fluid released from ruptured, sloughed cells. The main cited support concerns protein content in cellular debris from an older animal study, which does not establish that most intestinal juice volume comes from cell shedding.

“Under normal conditions, only a very small fraction of dietary amino acids in the small-intestinal lumen are diverted by other substances or are consumed by colonic bacteria before being absorbed, so amino-acid loss is very low.”

Most dietary amino acids are absorbed in the small intestine under normal, healthy conditions, so losses before absorption are generally low. Human studies and reviews commonly report ileal amino-acid digestibility above 90%, often above 95%. The wording is somewhat broad because digestibility differs by protein source and some proteinaceous material still reaches the colon, but that does not overturn the main conclusion for typical diets.