Protein is considered one of the three major macronutrients; the others include fat and carbohydrate. Dietary protein is essential to human life because it provides the substrate for the production of enzymes, hormones, antibodies, and structural components of the cell (1). Protein also helps maintain acid-base balance and provides energy. In addition, protein is required for the synthesis of new muscle tissue and can provide the means for an increase in strength and hypertrophy.
Protein provides four kilocalories per gram and is structurally different from carbohydrates and fats because it contains a nitrogen group (2). Carbohydrates and fats contain only hydrogen, oxygen, and carbon in different amounts depending on the specific nutrient and its function. Proteins, on the other hand, contain hydrogen, oxygen, carbon, and nitrogen. Amino acids are the individual subunits of protein; therefore, when protein is broken down via digestion, which we will dive into later, it is divided into each constituent amino acid.
Among the pool of amino acids that make up protein, there are 20 that are further divided into two separate categories; essential and non-essential. Essential amino acids are those that your body cannot synthesize itself; therefore, they must be consumed in the diet. Non-essential amino acids are those that can be synthesized by the body – as long as there is enough substrate (essential amino acids) present – to carry out necessary function. (2). Essential amino acids include Histidine, Lysine, Threonine, Isoleucine, Methionine, Tryptophan, Leucine, Phenylalanine, and Valine. Non-essential amino acids include Alanine, Aspartic Acid, Glutamine, Serine, Arginine, Cysteine, Glycine, Tyrosine, Asparagine, Glutamic Acid, and Proline.
Upon ingestion, protein is mechanically digested (chewing) in the mouth. It then travels down the esophagus where it enters the stomach and is broken down by an enzyme called pepsin. The function of pepsin is to catalyze the breakdown of the peptide bonds that holds together the various amino acids making up the protein. From here, the shorter chains of amino acids (polypeptides) are transferred to the small intestine where the enzymes trypsin, chymotrypsin, and carboxypeptidase further begin to break down the peptide bonds holding polypeptides together. With assistance from brush border enzymes, which are enzymes found on the microvilli of the small intestine, these polypeptides are broken down further into peptides or short chains of amino acids. These amino acids are then absorbed into the bloodstream through the walls of the small intestine where they then travel to the liver to be used for their necessary function (3).
Once at the liver, amino acids are sent to various parts of the body to begin synthesis of other products necessary for proper bodily function. Amino acids can synthesize enzymes, or catalysts, which means they cause a reaction in the body to occur. Enzymes are considered a tertiary protein structure and each is comprised of specific amino acids used to cause a reaction. In addition, amino acids can form hormones, which are needed to initiate a process in a specific cell type. Antibodies, on the other hand, are comprised of polypeptides of amino acids (University of Arizona, 2000). These structures are made by white blood cells, or ß-cells, of the immune system and are important because they identify and destroy pathogens (4).
Another important function of amino acids is the maintenance of proper acid-base balance or pH. In short, amino acids can be positively or negatively charged due to the hydrogen ions they contain. When the body becomes too acidic, as in times of intense exercise, protein will accept these excess hydrogen ions. Whereas when the body becomes too basic, these amino acids will donate their hydrogen ions to maintain equilibrium. The body’s pH is maintained around 7.35-7.45 on the pH scale. The pH scale ranges from 0 to 14; where 0 is highly acidic with a high concentration of hydrogen ions and 14 is highly basic due to the lack of hydrogen ions. As a reference, water is 7 or neutral, bleach is around 11 or highly basic, and vinegar is around 3 or highly acidic.
As mentioned previously, proteins contain 4 kilocalories per gram, therefore, they yield energy. However, protein oxidation (the use of amino acids as fuel) is dependent on various conditions. In times of high stress and low stored energy, the body will oxidize protein to provide substrate for more energy. This occurs largely when exercise has been maintained for an extended period of time without the consumption of either carbohydrates or fats. This is one of the reasons for the muscle sparing affect of a ketogenic diet. The presence of fat and ketones as energy substrate can allow for preservation of muscle proteins. Lastly, protein makes up some structural components of cells. The majority of the cell membrane is comprised of fatty acids called phospholipids but protein plays a large role in the cell membrane by being responsible for the flow of nutrients across the membrane to be used within the cell (5).
One of the most notable functions of protein is its ability to synthesize new muscle tissue. After protein is broken down, it is sent to different parts of the body to perform a specific function. One of these functions is muscle protein synthesis (MPS). MPS can be triggered by the amino acid, leucine, mechanical damage (lifting weights) or phospholipids (phosphatidic acid) (6). When MPS is activated, the body begins to take the ingested amino acids, code them to be used to make new muscle, then translate these codes into new muscle tissue.
As you can see protein is essential to many physiological functions within the body. These functions cannot occur without the presence of the necessary amino acids; therefore, it is crucial that protein is a staple in your diet. In my next article, I will discuss protein requirements for performance as they more closely relate to the ketogenic diet.