The proteins, which are practically always eaten cooked, and in contradistinction to the carbohydrates thereby rendered less digestible, because they not only become coagulated but also contracted and more concentrated in the process, undergo no digestive changes in the mouth. In the stomach, however, they are met by the gastric juice, a fluid containing pepsin and hydrochloric acid, and under its influence pass through a series of compounds termed acid-meta-proteins, proteoses, peptones, polypeptides, each one being a protein in a stage more soluble than its predecessor, until after the acidity has reached a certain stage they are propelled into the duodenum or the first part of the alimentary canal.

The mixture of food is at this stage called chyme, and by a complicated and highly interesting process its acid nature stimulates the walls of the duodenum to send an actual chemical messenger or hormone right through the blood into the liver and pancreas, thus intimating that the time has come for them to put forth their digestive efforts. This results in the outflow of bile and pancreatic fluid, the latter containing a ferment called trypsinogen, which is luckily inactive so long as it remains in the pancreas, otherwise it would soon digest it. Shortly after entering the intestinal canal, however, it meets a friend termed entero-kinase, and this converts it into trypsin, a ferment four times more powerful in its effects on protein than the pepsin of the gastric juice. It acts, however, in an alkaline medium, and so alkali-meta-protein, proteoses, peptones, and polypeptides are formed as before, but a final stage is reached termed amino-acids, and this is all-important because it is the form in which, or immediately antecedent to which, proteins are absorbed.