Proteins are complex macromolecules that are needed for the proper functioning of the body and are required for every chemical reaction that takes place in the body’s cells and tissues. The word protein is derived from the Greek word proteos, which means to take first place, indicating the primary role this nutrient plays in the functioning of the body and the importance it has in nutrition.
Proteins are constructed from a combination of smaller units, called amino acids, which are composed of carbon, oxygen, hydrogen, nitrogen and sometimes sulfur atoms. Each individual protein consists of amino acids that are strung together, forming a polypeptide chain. Each chain will vary in length depending upon the specificity of the protein, which is inherent in the sequence of the amino acid chains and how they fold onto one another. It is within this complex, three-dimensional folding structure, that forms the protein and determines its role within the body.
The Function and Roles of Protein
Structural proteins are found in muscle, blood, bone, hair, nails, skin, and connective tissue. Functional proteins are required in the formation of a variety of hormones, digestive enzymes and antibodies. Proteins are also found in the nuclei of cells, which influence most of a person’s hereditary characteristics. Proteins are the body’s foundation, responsible for the structure, growth, development, and regulation of organs and tissues. Proteins are very diverse in function. Their responsibilities range from aiding in the composition of all bones, organs, muscle, cartilage and connective tissue to being the small functional unit that drives every biological change within the body, such as enzyme function, messenger/transport systems, storage and antibody functions. For example, as an enzyme, proteins are critical in all of the biochemical reactions in cells and for the encoding and synthesis of the genetic material in the formation of DNA. This makes their role even more pronounced in that they are not only involved in the building of new tissue, but they are responsible for the repair and removal of damaged tissue. As a messenger, proteins assist in the transmission of signals between cells and transporting hormones from one cell to another. When functioning as an antibody, proteins protect the body by binding to foreign substances, such as bacteria or viruses, to aid the body in their excretion. On a larger scale, proteins are the basis of how the body moves, functions and regenerates.
Two values are commonly used to measure protein quality:
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
The PDCAAS score measures the “completeness” of a protein. It is now a federally accepted standard for determining protein quality for preschool aged children. If a protein contains each amino acid essential for life, it is called a “complete protein” and is given a high score. The highest score is 1.
Biological Value (BV)
Biological Value measures the amount of protein retained from protein consumed through diet. BV is an internationally recognized measure of protein quality. When measuring the BV of a protein source, two nitrogen studies are done. The first study determines how much nitrogen is lost from the body even when no protein is ingested. This amount of nitrogen loss is assumed to be inevitable, as the body will naturally lose it regardless of the amount of nitrogen consumed through dietary protein. The second part, measures an ingested amount of protein that is slightly below the body’s requirement. Biological Value is derived from measuring the intake of protein, then determining the nitrogen uptake versus nitrogen excretion. The theoretical highest BV of any food source is 100%. This rating system shows how well and how quickly an individual can actually use the protein he/she consumes.
Types of Protein
Whey protein is a fast absorbing protein, the primary reason it is a favorite amongst bodybuilders. During post workout, the body needs protein for repair, making whey a suitable choice because it is highly bioavailable, and achieves the highest biological value of any protein source.
Casein protein (milk protein) is a slow absorbing form of protein. Strategically, casein protein could be very useful in helping sustain elevated amino acid levels over a longer period of time. It is a rich source of protein, which can provide a reliable, steady flow of protein during the day or at night. Since casein protein slowly enters the blood stream, it has very little impact on protein synthesis. Muscle growth is dependent on the balance of protein synthesis and protein breakdown. Therefore, one should try to increase protein synthesis and decrease protein breakdown. To achieve this, one can supplement with both whey and casein protein together.
Egg protein is considered to be one of the best forms of natural protein. It is commonly referred to as the “perfect protein”. The white of eggs contains the proteins, while the yolk is mostly fat (6 grams of fat). Egg protein scores 100 on the biological value because the body retains all of its protein.
The Importance of Protein in the Diet
In addition to the many cellular roles of protein, in regards to structure and function, ingested protein is required for the formation of new proteins. The body is constantly synthesizing new proteins, thus, a constant supply of amino acids is required. Because the body cannot manufacture each amino acid essential for protein synthesis, a continuous supply of protein nutrients is required in the diet. The body requires roughly 56 grams of daily protein in men and about 46 grams of daily protein in women (RDA). When proteins are deficient in the diet, the body will begin to break down its own muscle tissue (catabolism) to obtain the amino acids it needs. This can lead to stunted growth, poor muscle formation, reduced muscle tone, an inadequate immune system and unhealthy hair and skin amongst a host of other conditions.
A balanced diet, and a good variety of protein sources, is imperative in achieving a healthy body. The best protein sources come from meat, fish, poultry, eggs and dairy products. Vegetarians, primarily, get their protein intake from legumes and nuts. However, vegetables and grains contain only small amounts of protein.
The Digestive Flow of Proteins
Once in the body, proteins are broken down and “prepped” for synthesis, so they can be absorbed into the bloodstream and transported to various tissues. This breakdown process is cyclical in nature, in that proteins in the form of enzymes (proteases), are involved in the initial stages of digestion. The pre-existence of proteins in the body is required to help break down ingested proteins. Once in the stomach, hydrochloric acid and digestive enzymes (protease pepsin) break down the ingested proteins by means of denaturation. Protein denaturation is commonly defined as any noncovalent change in the structure of a protein. This is a process, whereby, the three-dimensional structure of protein is unfolded into linear chains, so they can be transported to the duodenum (first section of the small intestine). In the duodenum, the chains are broken down further into single amino acid units or dipeptides (two amino acids). From this point, the amino acids are absorbed into the jejunum (the middle section of the small intestine), where they are assimilated throughout the body to begin the process of developing new proteins.
Proteins for Athletic Performance
Athletes should consume twice the amount of protein as non-athletes, because of the continuous destruction of muscle and other tissues. The adequation of additional protein enables athletes to grow, repair and recover. In addition, athletes will utilize both readily available and stored protein during exercise. Utilized protein must be replaced or the athlete will suffer a loss of muscular strength and size, due to the catabolic mechanisms that occur under conditions of inadequate proteins. This catabolic process of proteins can occur during strenuous, prolonged exercise when the body is depleted of carbohydrates and glycogen. High intensity or strength exercises can also lead to a catabolic breakdown.
Who Malnutrition Effects
According to the World Health Organization, approximately 300 million children worldwide suffer from growth retardation due to protein deficiency and malnutrition. Children with protein malnutrition have a 40% mortality rate due to increased susceptibility to infections.
In developed countries, protein malnutrition is most prevalent in people who have suffered a severe trauma, due to the increased need for recovery. Others who are impacted are individuals with a medical condition or eating disorder. Elderly individuals are also at risk of protein malnutrition.