Protein digestion

A protein from food is a long chain of amino acids, folded in on itself like a ball of yarn. To get any use out of it, the body has to take it apart, and that starts in the stomach and does not finish until well into the intestine, in the jejunum.

The first step is not cutting, it is unfolding. Hydrochloric acid in the stomach denatures the protein, meaning it unwinds it, leaving the chain exposed. Only then does pepsin, an enzyme made by the chief cells of the stomach, go to work, cutting the chain into pieces called oligopeptides, which move on to the duodenum, the first part of the small intestine. First the ball of yarn is unwound, then the thread is cut.

Waiting there is a second set of scissors, the pancreatic enzymes, which split those pieces into smaller and smaller ones: tripeptides, dipeptides and free amino acids. The cells of the intestine take them up.

Most of those amino acids pass into the blood and are carried throughout the body. Nine of them are called essential, not because they matter more, but because the body cannot make them.