During the First Few Hours of Life, Proteins Are Not Digested but Are Absorbed Intact
In general, a major function of digestion is to break down proteins by hydrolysis. Under most circumstances, this process is a benefit to the animal not only from a nutritional and digestive standpoint, but also from a toxicologic and allergic standpoint; potentially toxic or allergenic proteins are broken down before they arc absorbed into the body.
In the special case of some neonates, however, there is a need to absorb proteins intact. In most livestock species, including horses, cattle, sheep, and swine, essentially no antibodies are passed through the placenta from the dam to fetus. Thus, young livestock are born without the immunological protection of their mothers antibodies. In these species, antibodies from the mother must be acquired through ingestion of colostrum, the special mammary secretion present at birth. In these animals the digestive tract at birth is altered from the adult state, so the antibody proteins are absorbed intact rather than after digestion. There are three primary alterations, as follows:1. Acid secretion from the stomach is delayed for several days after birth.
2. A similar delay appears in the development of pancreatic function, and thus acid and trypsin digestion of proteins is avoided.
3. A specialized intestinal epithelium present at birth only is capable of engulfing soluble proteins in the intestinal lumen and discharging them into the lateral spaces.
The fetal intestinal epithelium has the same villous structure as the mature epithelium, but the villi are covered with special enterocytes capable of protein absorption. Immediately after birth, this special epithelium starts to disappear, and it is essentially gone after 24 hours. The loss of the protein-absorptive function in the neonate is referred to as gut closure.