All the cells of your body contain protein, and this protein is continually broken down and remodeled to meet your growth and maintenance needs. Consuming sufficient protein in your daily diet ensures you have adequate raw materials to replenish these proteins. Protein digestion, in which your body breaks down dietary protein into usable amino acids, makes this nutrient available to your cells in support of muscle maintenance, immune function, hormone synthesis, red blood cell formation and tissue repair.
The proteins you eat are large, globular molecules made up of smaller molecules called amino acids. Protein digestion works by reducing each dietary protein to a pool of amino acids within your gastrointestinal tract. The amino acids, once they are absorbed and transported to your cells, can then mix and match in a variety of ways to create the different proteins your body needs at any given point in time. The process begins in your mouth and ends in your small intestine, with the help of digestive juices and enzymes.
Mouth to Stomach
Chewing your protein-rich foods well – especially tough ones like certain cuts of meat or whole grains – increases the surface area of the food particles and allows digestion to occur more quickly. Once the food reaches your stomach, it encounters a harshly acidic environment. Your stomach acid serves several purposes. It denatures, or unfolds, the proteins in your food so digestive enzymes can act on it, it activates a protein-specific enzyme that clips the protein into smaller molecules called peptides and it inactivates any potentially harmful microorganisms you may have ingested along with your food. The peptides in your stomach now move to your small intestine.
Stomach to Small Intestine
To neutralize the acidic food particles coming from your stomach, your pancreas secretes a bicarbonate buffer into your small intestine. The neutral environment of this portion of your gut encourages the activity of more protein-digesting enzymes originating from both your pancreas and the cells lining your small intestine. These enzymes work on the food peptides, systematically breaking them down into even smaller peptides and then snipping off the amino acids one by one. At this point, the absorptive cells of your small intestine carry the single amino acids to your bloodstream and then on to the cells throughout your body.
Your body regulates protein digestion through hormones and nerve signals. Gastrin stimulates your stomach cells to produce acid, while secretin tells your pancreas to secrete both bicarbonate and digestive enzymes. Cholecystokinin is a third hormone involved in protein digestion, which also directs your pancreas to release digestive enzymes. Your nervous system assists in protein digestion by responding to the physical presence of food in your digestive tract. Stimulation of nerves in this area helps move food through the different organs of your gut.