Protein is an essential part of the human diet. It’s found in a variety of foods like eggs, dairy, seafood, legumes, meats, nuts, and seeds.
Regardless of the source, the protein that we eat gets broken down and reformed into new proteins in our bodies.
These proteins do everything from fighting infections to helping cells divide. You name it, they’re doing it.
At its simplest, a protein is a chain of amino acids, bound to one another by peptide bonds. Like a string of beads.
These strings get twisted and folded into a final protein shape.
When we eat protein, it gets broken down into its individual amino acids.
Most amino acids have a central carbon atom bonded to one amino or nitrogen-containing group and one carboxylic acid group - that’s why it’s called an amino acid.
The carbon also has one hydrogen atom and a side chain which is unique to each amino acid. The exception to this is proline which has a tiny little ring structure instead.
Although there are hundreds of amino acids in nature, humans use only about 20 of them to make basically every type of protein.
They include: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. Phew, that’s 20.
One way to divide them is by defining which ones our bodies can make ourselves, and which ones we cannot.
There are 5 amino acids - alanine, asparagine, aspartic acid, glutamic acid, and serine - that we can get from foods, but we can also make ourselves. These are called nonessential amino acids.
Then, there are 6 of them that we call conditionally essential because healthy bodies can make them under normal circumstances - arginine, cysteine, glutamine, glycine, proline, and tyrosine. But we can’t make them in cases like starvation or certain inborn errors of metabolism.
Finally, there are 9 of them that we can only get from food - histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. We call these the essential amino acids.
Dietary protein provides the essential amino acids that are needed to make our own proteins, hormones, and other important molecules. A circle of life, of sorts. But to do so, we need to break the dietary protein down first, through a process called proteolysis.
When we eat a protein-containing food, proteolysis begins when the food reaches the stomach.
First, hydrochloric acid denatures the protein, unfolding it and making the amino acid chain more accessible to enzymatic action.
Then, pepsin, which is a protein itself made by gastric chief cells, enters the picture.
Pepsin cleaves any available protein into smaller oligopeptide chains which move into the duodenum, where a second set of digestive enzymes, made by the pancreas, further chop the oligopeptides into tripeptides, dipeptides, and individual amino acids.
These can all be taken up into the intestinal cells, where di- and tripeptides are then converted into amino acids.
Some amino acids remain in these cells and are used to synthesize intestinal enzymes and new cells. But most enter the bloodstream and are transported to other parts of the body.
In general, animal-based protein foods like eggs, dairy, seafood, and meat provide all nine essential amino acids in adequate amounts.
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- "Amino Acid Metabolism in Pediatric Patients" Nutrition (1998)