Maintenance of the Body
Digestion of Proteins
Proteins digested in the GI tract include not only dietary proteins
(typically about 125 g per day), but also 15–25 g of enzyme pro-
teins secreted into the GI tract lumen by its various glands and
(probably) an equal amount of protein derived from sloughed and
disintegrating mucosal cells. Healthy individuals digest much of
this protein all the way to its
Protein digestion begins in the stomach when pepsinogen
secreted by the chief cells is activated to
(actually a group
of protein-digesting enzymes) (Figure 23.32). Pepsin functions
optimally in the acidic pH range found in the stomach: 1.5–
2.5. It preferentially cleaves bonds involving the amino acids
tyrosine and phenylalanine, breaking the proteins down into
polypeptides and free amino acids.
Pepsin, which hydrolyzes 10–15% of ingested protein, is in-
activated by the high pH in the duodenum, so its proteolytic
activity is restricted to the stomach.
(the enzyme that
coagulates milk protein) is not produced in adults.
Because the colon does not secrete digestive enzymes, diges-
in the small intestine. As noted earlier, how-
ever, resident colon bacteria do break down and metabolize the
residual complex carbohydrates and some proteins further, add-
ing much to their own nutrition but essentially nothing to ours.
In some people, intestinal lactase is present at birth but then be-
comes deﬁcient due to genetic factors. When people with
consume lactose, the undigested disaccharides cre-
ate osmotic gradients that prevent water from being absorbed
in the intestines and also pull water from the interstitial space
into the intestines. Te result is diarrhea. Bacterial metabolism
of the undigested solutes produces large amounts of gas that
result in bloating, ﬂatulence, and cramping pain. For the most
part, the solution to this problem is simple—add lactase enzyme
“drops” to your milk or take a lactase tablet before consuming
Proteins and protein fragments
are digested to amino acids by
pancreatic proteases (trypsin,
chymotrypsin, and carboxy-
peptidase), and by brush border
aminopeptidase, and dipeptidase)
of mucosal cells.
The amino acids are then
absorbed by active transport into
the absorptive cells, and move to
their opposite side.
The amino acids leave the villus
epithelial cell by facilitated
diffusion and enter the capillary
via intercellular clefts.
Protein digestion and absorption in the small intestine.