ISSA Nutritionist Chapter 5: Protein 1

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    Chapter Goals:

    • Be able to give definitions for the key terms related to protein and amino acids. 
    • Know how to explain the functions and physiological effects of protein and amino acids within the body. 
    • Know the differences regarding the essential and nonessential amino acids.
    • Know the digestion and absorption processes for proteins in the body.


    Protein is possibly our most vital macronutrient and it has a clear role as a component of health and fitness. 

    Studies have shown, though, that diets too high in protein may be just as detrimental as diets lacking in protein. It is also fact that active people need to intake more protein than a person who is more sedentary. 

    Some of the role’s played in the diet by protein are that of provision of structure and growth, catalyst for reactions, signaling, support for the immune system and activity in it—transportation across the cell membrane. 

    Proteins are not a primary source of energy, but they may be used as a last resort during intense exercise and when nutrition is not up to par.


    Proteins are macromolecules and polypeptides, which are compounds that have 10 – 100 or even more molecular subunits. Each subunit is going to be a small molecule known as an amino acid which are linked by a peptide bond. 

    There are four levels of structure within each protein, and the shape of the protein is going to give information regarding its role and function in the body. 

    The unique amino acid sequence of the polypeptide chain is determined through genetics to make the primary structure. 

    The local folding for polypeptide chains makes the secondary structure. 

    Combinations of chains and sheets then make up the protein’s shape. We consider the 3-d shape of the polypeptide to be the tertiary structure. 

    Some proteins are made up of several polypeptides, which we know as subunits. 

    The amino acids are the main structure creating a secondary helix.

    For nutrition, it is of importance to consider the amino acids, rather than the entire protein. 

    There are 20 amino acids that we consider to be biologically important, but many more of them exist in nature and in the body. These amino acids are the building blocks for protein, but also for their individual roles played in the body. 

    Every amino acid is made up of one central carbon atom that is called the alpha carbon. Bonded to this alpha carbon are these:

    • An amino group
    • A carboxyl group
    • A hydrogen atom
    • The R group

    When amino acids attach to each other through covalent bonds, they form the peptides. These are formed through a dehydration reaction, so water is removed.

    The Role of Protein in the Body

    Protein is required for growth, maintenance, and the repair of cells, like muscle cells; for production of the enzymes and hormones used in the body, and also for the expression of DNA.

    Proteins exist in a wide range of shapes and sizes and we divide these into two categories: simple and conjugated. 

    Proteins may fill these roles:

    • Contractile
    • Hormonal
    • Structural
    • Transporter
    • Enzymes
    • Receptor

    Amino Acids

    Naturally, amino acids are put in classifications based upon their dietary needs in the body. 

    The essential amino acids cannot be manufactured in the body, and then must come from the diet. Nonessential amino acids can be made in the body. 

    It is important that our protein sources, like most things in the diet, are varied and include the right proportions of nutritionally important amino acids. 

    Amino Acid Review

    Some amino acids act as components of proteins, and then others have expanded functions as biochemical intermediates or precursors of other amino acids. 


    This is a nonessential amino acid that we find in high amounts within our muscle tissue and it plays a role with the glucose-alanine cycle when the body is low on other fuel sources.


    This is a nonessential amino acid that has a vital role in the stimulation for the release of GH and insulin-like growth factor. 


    This is a nonessential amino acid that is involved in the proper function of the CNS.

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    Aspartic Acid

    This can be referred to as L-aspartic acid or L-aspartate, and it is a nonessential amino acid that is used to help reduce the levels of ammonia in the blood post-exercise.

    Branched Chain Amino Acids

    These include the amino acids of isoleucine, leucine, and also valine.

    These together are three amino acids that make up 35% of the amino acids within the muscle cells and it can be used as energy by the body. The body utilizes these amino acids in times of high stress, injury, or exercise.

    Some benefits of these branched-chain amino acids as a supplement include:

    • Increases in endurance during exercise
    • Reduction in the fatigue associated with exercise
    • Improvements in mental performance
    • Increased levels of energy
    • Stimulation of protein synthesis
    • Improvements in the function of the immune system
    • Increases in the levels of lean body mass
    • Increases in overall strength


    This is a nonessential amino acid playing a role in the urea cycle for the removal of the ammonia in the blood.


    This is a nonessential amino acid that bears sulfur and plays a vital role in the production of energy and maybe used to manufacture methionine and serine. 

    Glutamic Acid

    This may be also known as glutamate, and it is a nonessential amino acid that acts as an intermediary in the Krebs cycle and is vital for its uses in carb metabolism. 


    This is one of the most plentiful nonessential amino acids in the body. People under stress from disease or injury may have decreased levels of this amino acid.  


    This is a nonessential amino acid that is made from serine, and it is important for many types of substances within the body like protein, DNA, phospholipids, collagen, and creatine.


    This is an essential amino acid that is needed for the importance in growth and repair of human tissues.


    This is an essential amino acid that is needed mainly for forming the hemoglobin in the body.


    This essential amino acid is important in the production of energy when exercising.


    This essential amino acid is found in large amounts in the muscle tissue and is used for growth and repair primarily. 


    This is an essential amino acid that is involved in transmethylation.


    The essential amino acid is a precursor of the nonessential amino acid tyrosine.


    A nonessential amino acid that is found in great amounts in collagen and can be made easily from glutamic acid.


    This is a nonessential amino acid we find in the proteins and it is derived from glycine. It works to form many vital substances. 


    This nonessential amino acid works to play a major role in the brain tissues, functioning of the nervous tissues, regulation of blood pressure, and the transport of electrolytes across the cell membrane. 


    This is an essential amino acid, and it is important for collagen, tooth enamel, protein, and elastic tissues. 


    This essential amino acid is needed to produce vitamin B3 and the neurotransmitters serotonin and melatonin.

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    This nonessential amino acid is made from phenylalanine and works as a foundation for thyroid hormones.


    This is an essential BCAA and is integral for muscle tissues and may be used for energy. 

    Proteins, Amino Acids, and Energy

    In times of severe restriction of calories, the body will release some amino acids in order to keep providing energy for activity. 

    Even when well-fed, the body will still sometimes use proteins for energy, even at rest. 

    The body can utilize the three BCAAs for energy during exercise, but it uses leucine most of all. 

    The Quality of Proteins

    Not all proteins are going to be equal nutritionally for ideal growth in children.

    Complete proteins are going to be proteins with all of the essential amino acids in the proper amount needed for growth and maintenance of the body. 

    Incomplete proteins are deficient in one or more of the essential amino acids. 

    Free-form and Peptide-bonded Amino Acids

    • The terms free-form and peptide-bonded are often used to describe the amino acid content of food or supplements. 
    • Peptide-bonded amino acids are amino acids that are linked together.
    • Dipeptides are two amino acids linked together. 
    • Tripeptides are three amino acids linked together.
    • Polypeptides are four or more amino acids linked together.

    Protein Digestion

    There are several factors changing a protein’s shape and chemistry. 

    Changes in temperature, pH, high salt concentrations, alcohol, mechanical agitation, and exposure to chemicals alter the shape of a protein.

    Mouth to stomach

    Chewing will break down the larger chunks of protein into smaller pieces that can then be swallowed. None of the enzymes in saliva will act on protein.

    In the stomach, the gastric juices will break down protein further. The acidity will unfold some of the parts of proteins.

    Pepsin is secreted in the stomach lining to break the peptide chains into shorter fragments. 

    Small intestine to the bloodstream

    Most protein digestion happens in the small intestine with the use of protease. 

    Chyme enters the small intestine to the entrance of chymotrypsin and trypsin.

    Transport proteins using the energy from ATP move the amino acids through the intestinal cells and to the blood to then be sent to the liver before being sent to the other cells or before breaking down further. 

    Food Sources of Protein

    Proteins are found in both animals and plants, and both protein and fat are usually going to be found together, especially in animal products. 

    Most animal proteins tend to be higher in quality than plant proteins.

    Protein and Athletes

    Athletes typically need a diet that is protein high since they place a high demand on their body. 

    Differing types of exercise and intensities are going to require this protein in varying amounts. 

    Nutrition coaches should work to consider a few things when making their protein suggestions for their athletes.

    Protein can be broken down and used to store as fat.

    For the building of lean muscle mass, daily protein intake must maintain a positive protein balance.

    Carb intake can also help to offset the damage to muscles and the promotion of recovery via insulin and its downstream effects on the MAPK pathway, which is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell. 

    Fitness professionals should prioritize whole foods for protein in most nutrition plans but also likely include supplemental proteins for athletes if needed. 

    ISSA Nutritionist Chapter 5: Protein 2
    ISSA Nutritionist Chapter 5: Protein 3
    ISSA Nutritionist Chapter 5: Protein 4

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