Like all foods, dietary protein must be digested and broken down to its smallest components in order to be absorbed from the gut and distributed throughout the body. Here is a brief description of the process of transforming the proteins you eat into the amino acids that will be delivered through the bloodstream to all the tissues and organs in the body. We’ll also cover the speed at which free amino acids can be digested and absorbed, compared to the rates of protein digestion.
An Overview of Gut Structure
The gastrointestinal tract, or gut, refers to the entire system involved in the digestion of food. It is basically a tube that starts from the mouth, extends down through the pharynx, esophagus, stomach, and intestines, and ends at the rectum and anus.
The small intestine has three sections: the duodenum, the jejunum, and the ileum that are distinct in their selective absorption of various nutrients.
The duodenum is the first part of the small intestine and is the “mixing” site of many different enzymes from the stomach, liver, gallbladder, and pancreas. It is located between the stomach and the middle part of the small intestine, or jejunum.
The jejunum is the primary site of absorption, as it is lined with enterocytes, which are specialized cells for the absorption of small nutrient particles previously digested by enzymes in the duodenum.
The ileum is the final and longest segment of the intestine where bile acids and vitamin B12 get absorbed, as well as leftover products of digestion that were not absorbed by the jejunum.
The juncture at the end of the ileum (terminal ileum) begins the large intestine, which consists of the cecum, colon, rectum, and anus. These structures comprise the last part of the gastrointestinal tract, where water is absorbed and the remaining waste material is stored as feces before being removed by defecation from the colon and the rectum.
Chewing and Stomach Digestion
When we chew, we break down food mechanically, while compounds in saliva soften the food and begin the chemical breakdown of macronutrients. Muscles in the esophagus assist in the process of swallowing food on its way to the stomach.
Once in the stomach, the food is exposed to more chemicals and enzymes that further break it down to simpler nutrients and nutritional components in a mixture called chyme (partially digested food and water). Pepsin is the active protein-digesting enzyme of the stomach.
Pepsin acts on protein molecules by breaking the peptide bonds that hold the molecules together. Muscles that surround the lining of the stomach help to mix the food in with digestive enzymes, and a specialized sphincter (ring of muscle) controls the rate at which the chyme is introduced to the small intestine. There is also a sphincter at the top of the stomach (pyloric valve) that prevents food from going back up into the esophagus.
The Small Intestine
Bile from the gallbladder and pancreatic juices from the pancreas are introduced to chyme in the duodenum. Digestion of protein is completed in the small intestine by the pancreatic enzymes trypsin, chymotrypsin, and carboxypeptidase. This mixture is passed onto the jejunum.
The small intestine mucosa can only transport single amino acids or short polymers of two to three amino acids (i.e., dipeptides and tripeptides). The inner lining of the small intestine is a very thin layer which enables nutrients to readily pass into the blood. The wall of this portion of the intestine is made up of folds, each of which has many tiny finger-like projections known as villi on its surface. Furthermore, the epithelial cells that line these villi possess even more microvilli resulting in an extremely large surface area for the absorption of digestion products. The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the hepatic portal vein and the liver.
Amino Acid Absorption
The free amino acids, along with remaining dipeptides and tripeptides, are transported by a process called “secondary active transport.” Active, in this case, means that the process requires energy, and the difference between primary and secondary active transport is the source of energy.
In primary active transport, energy is derived directly from the breakdown of ATP, whereas secondary active transport gets energy from coupling transport of ions along with the amino acid. Any remaining dipeptides and tripeptides are cleaved to form individual amino acids inside the enterocyte. Individual amino acids are then passively transported past the lining and into the blood.
The Large Intestine
Layers of circular and longitudinal smooth muscle enable chyme to be pushed along the ileum by waves of muscle contractions. Gut motility is the term given to the stretching and contractions of the muscles in the gastrointestinal tract, and peristalsis is the term for the synchronized contraction of these muscles.
The semi-solid chyme that reaches the large intestine is comprised of fiber and non-digestible components of food. There are no villi in the large intestine, but there are bacteria that help in the final stages of digestion. Muscular contraction is a very important component of digestion throughout the small and large intestines, as it is necessary to move the chyme and ultimately feces through the body towards the rectum.
Free Amino Acids Versus Intact Protein
Intact proteins must be broken down to individual amino acids in order to be absorbed from the gut into the blood. Therefore, a primary difference between consuming free amino acids versus dietary proteins relates to matters of digestion.
A solution of free amino acids will be absorbed very rapidly and appear in the bloodstream within minutes, reaching peak concentrations in 30-40 minutes. Dietary proteins vary quite a bit in the rate of amino acid delivery because of other components in the food source (for example, fat content) that affect digestion rate. On average, dietary proteins will release amino acids into the blood within an hour of eating, and amino acid concentrations will continue to rise up to four hours after a meal.
There are distinct benefits to each of these patterns of digestion and absorption and, therefore, a reason to include both high-quality dietary proteins and balanced mixtures of essential amino acid supplements in your dietary regimen.