While you eat meals, the physique’s digestive system breaks down dietary protein into particular person amino acids, that are absorbed and utilized by cells to construct different proteins and some different macromolecules, resembling DNA. Let’s comply with the trail that proteins take down the gastrointestinal tract and into the circulatory system.
Eggs are a great dietary supply of protein and will likely be used as our instance as we focus on the processes of digestion and absorption of protein. One egg, whether or not uncooked, hard-boiled, scrambled, or fried, provides about six grams of protein.
Within the picture under, comply with the numbers to see what occurs to the protein in our egg at every web site of digestion.
1 – Protein digestion within the mouth
Except you might be consuming it uncooked, step one in digesting an egg (or another stable meals) is chewing. The enamel start the mechanical breakdown of enormous egg items into smaller items that may be swallowed. The salivary glands secrete saliva to assist swallowing and the passage of the partially mashed egg by way of the esophagus.
2 – Protein digestion within the abdomen
The mashed egg items enter the abdomen from the esophagus. As illustrated within the picture under, each mechanical and chemical digestion happen within the abdomen. The abdomen releases gastric juices containing hydrochloric acid and the enzyme, pepsin, which provoke the chemical digestion of protein. Muscular contractions, referred to as peristalsis, additionally assist in digestion. The highly effective abdomen contractions churn the partially digested protein right into a extra uniform combination, which is known as chyme.
Fig. 6.18. Protein digestion within the abdomen
Due to the hydrochloric acid within the abdomen, it has a really low pH of 1.5-3.5. The acidity of the abdomen causes meals proteins to denature, unfolding their three-dimensional construction to disclose simply the polypeptide chain. This is step one of chemical digestion of proteins. Recall that the three-dimensional construction of a protein is important to its operate, so denaturation within the abdomen additionally destroys protein operate. (This is the reason a protein resembling insulin can’t be taken as an oral remedy. Its operate is destroyed within the digestive tract, first by denaturation after which additional by enzymatic digestion. As a substitute, it needs to be injected in order that it’s absorbed intact into the bloodstream.)
Fig. 6.19. Within the abdomen, proteins are denatured due to the acidity of hydrochloric acid.
As soon as proteins are denatured within the abdomen, the peptide bonds linking amino acids collectively are extra accessible for enzymatic digestion. That course of is began by pepsin, an enzyme that’s secreted by the cells that line the abdomen and is activated by hydrochloric acid. Pepsin begins breaking peptide bonds, creating shorter polypeptides.
Fig. 6.20. Enzymatic digestion of proteins begins within the abdomen with the motion of the enzyme pepsin.
Proteins are massive globular molecules, and their chemical breakdown requires time and mixing. Protein digestion within the abdomen takes an extended time than carbohydrate digestion, however a shorter time than fats digestion. Consuming a high-protein meal will increase the period of time required to sufficiently break down the meal within the abdomen. Meals stays within the abdomen longer, making you are feeling full longer.
3 – Protein digestion and absorption within the small gut
The chyme leaves the abdomen and enters the small gut, the place the vast majority of protein digestion happens. The pancreas secretes digestive juices into the small gut, and these include extra enzymes to additional break down polypeptides.
The 2 main pancreatic enzymes that digest proteins within the small gut are chymotrypsin and trypsin. Trypsin prompts different protein-digesting enzymes referred to as proteases, and collectively, these enzymes break proteins right down to tripeptides, dipeptides, and particular person amino acids. The cells that line the small gut launch further enzymes that additionally contribute to the enzymatic digestion of polypeptides.
Tripeptides, dipeptides, and single amino acids enter the enterocytes of the small gut utilizing energetic transport methods, which require ATP. As soon as inside, the tripeptides and dipeptides are all damaged right down to single amino acids, that are absorbed into the bloodstream. There are a number of various kinds of transport methods to accommodate various kinds of amino acids. Amino acids with structural similarities find yourself competing to make use of these transporters. That’s not an issue in case your protein is coming from meals, as a result of it naturally comprises a mixture of amino acids. Nonetheless, if you happen to take excessive doses of amino acid dietary supplements, these might theoretically intrude with absorption of different amino acids.
Fig. 6.21. Abstract of protein digestion. Notice that the traces representing polypeptide chains within the abdomen encompass strings of amino acids linked by peptide bonds, regardless that the person amino acids aren’t proven on this simplified illustration.
Proteins that aren’t absolutely digested within the small gut move into the massive gut and are finally excreted within the feces. Recall from the final web page that plant-based proteins are a bit much less digestible than animal proteins, as a result of some proteins are sure in plant cell partitions.
What occurs to absorbed amino acids?
As soon as the amino acids are within the blood, they’re transported to the liver. As with different macronutrients, the liver is the checkpoint for amino acid distribution and any additional breakdown of amino acids, which may be very minimal. Dietary amino acids then turn into a part of the physique’s amino acid pool.
Assuming the physique has sufficient glucose and different sources of vitality, these amino acids will likely be utilized in one of many following methods:
If there may be not sufficient glucose or vitality accessible, amino acids will also be utilized in considered one of these methods:
With a purpose to use amino acids to make ATP, glucose, or fats, the nitrogen first needs to be eliminated in a course of referred to as deamination, which happens within the liver and kidneys. The nitrogen is initially launched as ammonia, and since ammonia is poisonous, the liver transforms it into urea. Urea is then transported to the kidneys and excreted within the urine. Urea is a molecule that comprises two nitrogens and is extremely soluble in water. This makes it perfect for transporting extra nitrogen out of the physique.
As a result of amino acids are constructing blocks that the physique reserves with the intention to synthesize different proteins, greater than 90 p.c of the protein ingested doesn’t get damaged down additional than the amino acid monomers.
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Vitamin: Science and On a regular basis Utility by Alice Callahan, PhD, Heather Leonard, MEd, RDN, and Tamberly Powell, MS, RDN is licensed beneath a Inventive Commons Attribution-NonCommercial 4.0 Worldwide License, besides the place in any other case famous.
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