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The digestion process is defined as the “process by which macromolecules in food are broken down into their small-molecule subunits.”
This breakdown would occur incredibly slowly without the participation of digestive enzymes present in the digestive tract.
Enzymes are often named by adding the ending -as- to the name of the substance they work on. So the enzymes that break peptide bonds are called peptidases (enzymes that digest proteins).
Although all amino acids are linked by the same peptide bond, the type of R group in the amino acids on both sides of the bond affects the action of peptidases so much that it generally takes several different enzymes to digest a protein molecule fully.
Protein digestion usually is very efficient, and practically all the protein (98%) that an adult ingests is completely digested into amino acids and absorbed.
However, the digestive systems of newborns are somewhat less efficient, which has important functional implications.
Breast milk contains antibodies, a type of protein necessary for providing immunity against infections in babies.
If the child’s digestive system were as efficient as an adult’s, these antibodies would be digested like any other protein.
However, a slightly less efficient digestive system allows the antibodies to remain intact and pass from the intestine into the bloodstream, providing essential immunity for the baby.
Other proteins, such as those in cow’s milk, can also pass into the baby’s bloodstream undigested and can be the source of allergies or food intolerances later in life.
Formula or formula are designed to minimize this risk, so this is an important reason to maintain breastfeeding for up to six months.
Importance of protein digestibility
Protein is a complicated component of the human diet. Without protein, cells would not function properly, our organs and tissues would not be able to perform their tasks, and our bodies would age.
There are many sources of protein available to humans. We can find it in tissues and animal products such as chicken and milk, in vegetable sources such as grains such as beans, and nuts such as walnuts and almonds, and it can be isolated in the case of whey and protein—complete vegetables.
When choosing which types of fuel to use in the daily diet, you must select highly digestible proteins; in this way, the body can use as much protein as possible.
Highly digestible proteins provide amino acids to cells and allow muscles to develop, organs to function, and in general, for the body to function.
Protein digestion
To understand why highly digestible proteins are essential, we must first understand how the digestive system breaks down proteins.
Complex chains of amino acids create proteins. The human body is made up of proteins and can synthesize some of them, except essential amino acids.
To obtain these amino acids, they must be consumed. This is why finding a complete protein source is so important.
Animal proteins are very similar to human proteins; they are a quick and easy source of these essential amino acids. However, many people can’t assimilate animal proteins.
Many of the proteins we eat begin to break down into amino acids in the stomach.
Protein digestion begins in the stomach, the walls of which secrete hydrochloric acid.
This stomach acid denatures proteins and begins the process of breaking them down. Along with acid, the stomach is packed with enzymes designed to transport specific proteins to the small intestine safely. These are called peptides.
An enzyme called pepsin, produced by cells lining the stomach wall, begins by attacking some peptide bonds and splits long protein chains into shorter polypeptides.
The digestion of virtually all proteins in food into individual amino acids is completed by more peptidases released directly from the small intestine cells.
Then more peptidases are released from the pancreas into the small intestine. They split the polypeptide chains into even shorter lengths and begin to remove individual amino acids from the ends of the chains.
There are many different types of protein, and they are all broken down in different ways and different parts of the digestive tract.
Throughout this chain of protein digestion, the stomach, liver, pancreas, and small intestine work in concert to break down complex proteins into the simple amino acids that compose them.
Amino acids are transported through the small intestine wall into the bloodstream. The blood carries them to all cells in the body, where they can be absorbed and used by each type of cell to produce its particular types of protein by putting them back together. In the order determined by the DNA on the chromosomes.
When absorbed into the bloodstream and carried to other parts of the body, these individual amino acids can recombine as the body needs them to repair muscle tissues and cell walls, create hormones and red blood cells, and strengthen the immune system.
Measurement of protein digestibility
It is not difficult to find protein in a diet. However, not all protein is digestible.
This is a problem because people may think they eat a high protein diet. However, many processes can affect the digestibility of protein.
The processing of proteins can affect how they are separated or not broken down by the stomach and intestines.
Genetic modification has been shown to change and impair the digestibility of plant proteins, even when the protein was not the target cell for the genetic mutation.
When choosing your preferred source of protein, it is essential to consider whether the protein is digestible or not.
While it is impossible to look at a slice of chicken or a plate of beans and know if the protein is digestible, there is a way to determine the digestibility of proteins.
It is a score called the Indispensable Digestible Amino Acid Score; this score is determined by the number of amino acids that are digested in the ileum or small intestine rather than throughout the digestive tract.
Protein digestion disorders
Dietary proteins are made of amino acids. The body can use dietary protein for energy, muscle incorporation, or incorporation into nitrogen-containing compounds.
Protein digestion begins in the stomach with an enzyme called pepsin. It continues in the small intestine, where enzymes in the pancreas and intestinal lining break down the protein into smaller peptides.
These peptides break down into tripeptides and dipeptides that can pass through the intestinal lining.
Protein digestion disorders can occur when these processes are disturbed or abnormal.
Milk protein digestion
Proteins take longer to digest in the stomach than carbohydrates, and milk contains some of the slowest-digesting proteins.
Casein proteins are soluble in milk but form insoluble curds once they reach the stomach, making it difficult for digestive enzymes to break them down into more minor compounds.
Slower digestion is also associated with the delayed release of amino acids from protein into the bloodstream.
Milk offers two complete protein sources that help increase satiety and provide a constant source of essential amino acids.
Dairy foods that travel from the mouth to the intestines are like drivers going to work on a multi-lane road.
Some dairy proteins enter the fast lane and are digested quickly, while others stay in the slow lane and take longer to reach their final destination.
The casein proteins in milk become more challenging to digest, forming insoluble little balls of curds when they reach the stomach.
Digestive enzymes have to work hard to separate these curds, resulting in longer digestive times and a slower release of protein nutrition.
These properties of milk caseins may have evolved to benefit mammalian babies, but slow digestion offers the benefit of increased satiety for milk drinkers of all ages.
Eating foods, like milk, that take the slow route may mean eating less food overall.
At the most superficial level, proteins are chains of amino acids linked by peptide bonds.
But the food we eat contains protein in its most complex form – those chains of amino acids are rolled into balls, which must unroll and separate for the intestines to absorb and then transfer individual amino acids into the bloodstream.
Stomach acids help unfold all the twists and turns of amino acid chains, allowing digestive enzymes produced by the stomach wall to work by breaking peptide bonds.
The easier it is to get to individual amino acids, the faster the protein can be processed. As such, proteins with less structural complexity have faster digestive rates.
As proteins advance, the individual casein proteins in milk are relatively simple in structure, lacking the high degree of coils, twists, and folds found in many other proteins.
In theory, they should rush through the stomach and transfer their amino acids into the bloodstream shortly after ingestion. In practice, however, researchers have found the exact opposite.
The casein proteins in milk form tiny spheres called micelles. The hydrophilic (water-loving) portions of the protein on the outside of the sphere and the hydrophobic (water-fearful) portions on the inside.
With hydrophilic structures on the outside, micelles are soluble in water (or milk, which is mostly water). But when the micelles reach the stomach, the digestive enzyme chymosin cuts one of the bonds in the outer protein (known as the kappa subunit), leaving only the hydrophobic subunits inside.
Without their protective coating, the now insoluble proteins form a curd.
In the cheese-making process, chymosin is also responsible for producing casein curd).
Therefore, by effectively turning a liquid into a solid, the enzyme makes casein proteins more challenging to digest.
The protein must break down into a smaller particle size to be effective.
When whey protein is not broken down into the most miniature composition, it creates large peptides that can cause discomforts, such as bloating, nausea, and cramps.
These symptoms are generally present in conditions such as allergies or food intolerances, such as lactose intolerance.
Advantages of slow or rapid protein digestion
There are apparent benefits to rapid digestion. Nutrients and energy are available quickly after consumption, feeding the body’s immediate needs.
Whey protein in milk is considered a “fast” protein because its amino acids appear in the bloodstream relatively quickly after digestion (one of the many reasons why proteins are promoted as recovery foods after physical exercise).
However, one of the downsides to express delivery is the need to replace those nutrients more frequently.
Evolutionary processes solved this problem by equipping milk with complementary “slow” casein proteins that provide protein.
In addition to making the baby feel full, slower digestion also means the slower release and subsequent absorption of the amino acids in casein.
With complete proteins that take both the slow and fast lanes, milk provides mammalian babies with an almost constant supply of amino acids necessary for their growth and development.
When a human adult drinks a glass of cow’s milk, chymosin works the same way in a baby.
Curds are produced, taking longer to break down into individual amino acids.
Milk is the only food that delivers both proteins simultaneously.