What inactivates pepsin in infants?

Question

In infants, rennin helps in digestion of milk. Pepsin is also present in their stomach. Why do infants need rennin for milk digestion, at the first place? Why does pepsin not act on the milk proteins in infants? Does the production quantity of rennin play a role in inhibiting the action of pepsin?

Answer 1

Why is rennin needed for milk digestion? Because it helps the milk remain in the baby's stomach long enough to digest the milk and absorb the nutrients from it.

In a baby, the stomach's under-developed, immature tissues need additional time to break down the milk proteins. However, milk is a liquid, and therefore will tend to pass through the baby's stomach too quickly to be digested. Rennin helps to curdle or coagulate the milk protein (casein) into an insoluble, semi-solid gel (paracasein), allowing the milk to remain in the baby's stomach a little longer. This allows enough time for the critical nutrients in the milk to be absorbed.

So, why does pepsin not act on the milk proteins in infants? Because pepsin requires a highly acidic environment to be effective, and rennin does not.

Because a baby's stomach is underdeveloped and immature, it does not produce as much hydrochloric acid as an adult. So, whereas an adult might have a very acidic stomach (pH 2), a baby will have a stomach that is only slightly acidic (pH 5 to 7). Pepsin requires a highly acidic environment to digest proteins effectively, and since milk is alkaline, the combination of the low acid production and the alkaline milk produces an environment where the pH is too high (i.e., too alkaline) for the pepsin to work properly, Rennin, on the other hand, works just fine at these alkaline (i.e., higher pH) levels found in a baby's stomach.

To be clear, the quantity of rennin produced has nothing to do with causing this. It is the alkaline pH that inhibits the action of pepsin.

So, where does all this end up? As the baby matures and starts to eat solid foods, the parietal cells in the stomach start to secrete more hydrochloric acid, as much as an adult. This acidic environment makes pepsin useful to the child (finally!), but now rennin is no longer necessary. So, the production of rennin naturally declines as pepsin completely takes over the role of protein digestion. Then, since rennin is not needed anymore, the production of rennin stops altogether.