Digestion of food is a fairly complex process and differs somewhat according to the type. Most carbohydrates are quickly digested and turned into glucose, whereas protein takes much longer to be reduced into its components, amino acids. Amino acids are needed to keep your skin, fingernails and hair looking healthy and marvelous. To best understand the process, protein digestion can be divided into five stages.
Chewing protein-rich foods such as beef, fish or cheese doesn’t change them chemically, but it physically breaks them up into smaller pieces -- exposing more surface area -- and prepares them for further digestion in your stomach and intestines. Saliva contains enzymes that begin digesting certain carbohydrates and fats while they are in your mouth, but it doesn’t contain any enzymes that act on protein. Saliva is also important for lubrication, which helps protein and other nutrients flow easily into your stomach.
The second stage of protein digestion occurs in your stomach, although it’s the first place that protein is chemically changed. As long as it’s acidic enough -- pH measurement of 3.0 or less -- your stomach juices cause the long peptide chains in protein to unfold. The acidity also activates an enzyme called pepsin, which breaks down collagen tissue and reduces the long polypeptides into peptide fragments and clumps of amino acids. Protein spends about three or four hours in your stomach undergoing these chemical changes before it enters your small intestine. Drinking lots of liquid with a protein-rich meal is not a good idea because it dilutes stomach acid and compromises digestion.
The third stage of protein digestion occurs in the initial section of the small intestine -- called the duodenum. The partially digested protein comes into contact with different enzymes secreted from the pancreas, namely trypsin and chymotrypsin. These proteolytic enzymes cleave the peptide fragments and clumps into single amino acids, which are small enough to be absorbed through the intestinal wall. In contrast to the acidic environment needed in the stomach, alkaline conditions -- pH of over 7.0 -- are needed in the small intestine for this process to occur, so bicarbonate and other alkaline substances play an important role here.
Although some amino acids are absorbed in the duodenum, the majority are absorbed further along the small intestine in segments called the jejunum and ileum. The amino acids pass through the intestinal wall and directly into the blood, where they are then transported to the liver for processing, storage and distribution. Amino acids are used to build and repair protein-based tissues such as muscle, as well as compounds such as enzymes.
The material left over from the digestion and absorption of protein-rich food is mainly fiber and perhaps some gristle, which flows through the large intestine toward the bowel. Insoluble fiber attracts water and bulks up the stool, which tends to clean the inside of the large intestine and stimulate bowel movements. The time needed for complete protein digestion ranges between one and three days, depending on your metabolism, intestinal motility, hydration and activity level.
- Human Metabolism: Functional Diversity and Integration; J. Ramsey Bronk
- Human Biochemistry; Charles Dreiling
- Encyclopedia of Human Nutrition; Benjamin Caballero et al.
Sirah Dubois is currently a PhD student in food science after having completed her master's degree in nutrition at the University of Alberta. She has worked in private practice as a dietitian in Edmonton, Canada and her nutrition-related articles have appeared in The Edmonton Journal newspaper.