The critical importance of protein in nutrition and life of the human body was discovered by chemists in the early 19th century, they also came up with "international" name for these chemical compounds - "proteins", from the Greek protos - "first, chief".
Proteins have the incomparable value in the human diet: first of all, they serve as "building blocks" for the whole body. In addition, they are responsible for the metabolic and regulatory functions in the body; serve as a basis for the creation of tissues, such as muscle fibers; perform transport functions in metabolic systems of the body, such as hemoglobin (oxygen carrier in the blood) - a complex protein; control the functions of the body, for example, some very important protein-hormones as insulin; are involved in energy metabolism in the digestive processes, protect the body (toxins, antibodies are proteins too), and perform many other highly important functions (Webb, 2014).
In the chemical sense protein is a very large molecule consisting of the amino acid residues. One of the most common classifications of proteins is a classification by their biological role in the body. And the most important properties of proteins is determined by amino acids they consist of. Therefore, protein as food is not that valuable by itself but is valuable by amino acids which are its components (Reimers, 2014).
The amino acid is a type of organic acids. The total amount of amino acids that can be found in organisms is more than 100 species, and they are all somehow involved in metabolic processes. However, not all amino acids may be found in the composition of proteins (Reimers, 2014).
Most of the food proteins are not absorbed by the body directly. The vast majority of proteins are destroyed by the digestive juices and the body uses their constituent - amino acids. By using the amino acids derived from the fragmentation of proteins that have come with the food, the body builds the needed proteins by itself (Reimers, 2014). For animals (including, of course, humans), there are the so-called essential amino acids among the required for the construction of proteins. These amino acids can not be synthesized, thus, they must come with food. The total amount of essential amino acids is 8: lysine, methionine, tryptophan, leucine, isoleucine, valine, threonine, phenylalanine (Reimers, 2014).
Furthermore, there are amino acids, which are "dependent" of essential amino acids: cysteine and tyrosine are formed only out of the essential methionine and phenylalanine. Thus, the most important is not the amount of protein but the quality of amino acid composition (Reimers, 2014).
Protein metabolism in the human body is the very complex process. Depending on the condition of the body the necessary amount of certain proteins is constantly changing, the proteins are cleaved, synthesized, some amino acids are transformed into the other, or decay and release energy. As a result, a part of proteins, which is typically about 25-30 g of protein per day, is being lost during the functioning of the organism. Therefore, the right quantity of proteins must always be present in the diet (Webb, 2014).
The required amount of protein in human food depends on several factors: whether a person is at rest or performing heavy work, what is his emotional state and so on. The recommended protein intake is 0.75-0.80 grams of quality protein per 1 kg of body weight for an adult, ie, about 56 grams per day for an average man and 45 g for females (Webb, 2014). Children, especially very young, need more protein (up to 1.9 g per 1 kg of body weight per day), as their bodies are growing rapidly. Intense endurance loads increase the need for protein due to the increased degradation of protein in muscles. Research has established that the protein requirements during intense stress endurance can be increased to 1-1.5 g / kg body weight (Webb, 2014). Thus adequate energy consumption should be assured. In the study, male runners on 10 000m distance had shown that if energy demand is not satisfied at least for 100 kcal / day, the proteins start to be consumed for energy needs and nitrogen balance are negative even when consumed 2 g protein / kg body weight. A sufficient amount of energy from carbohydrates and fats has a protein-sparing effect. Well-trained weightlifters have positive nitrogen balance at any load which is provided by protein intake of 1.5 g / kg body weight (Webb, 2014). Higher amounts of protein neither provide any additional benefit in magnitude nor retention of nitrogen and protein in the body nor results in sports. Satisfaction in the protein needs of the general population, and power sports athletes is quite possible through the consumption of conventional foods (Webb, 2014).
Anabolism promotes muscle growth and weight gain. As a result of anabolism, small molecules combine in the polymer chains, for example, polypeptides and amino acids form new proteins in the muscles, an athlete weight and volume of muscles increases (Reimers, 2014). Catabolism is the opposite of anabolism. It is difficult splitting into simpler substances, as well as the decay of the old parts of cells and tissues of the body. As a result of catabolism, muscle tissue is destroyed: the original proteins fall into amino acids, which are partially biosynthesis and partly simply excreted (Reimers, 2014). Hormones such as cortisol, adrenaline and glucagon shift the metabolic balance in favor of predominance of catabolism and muscle breakdown. Stress, diet, lack of sleep, etc. create unfavorable hormones, which negatively affects the state of the athlete's muscular tissue. To keep anabolism and prevent catabolism athlete should be correct and balanced diet. The diet should be balanced over all the macro- and micronutrients, an athlete should split meals to 5-8 times a day. A special attention should be paid to proteins, as the main source of amino acids (Reimers, 2014).
References
Densie Webb (2014) “Athletes and Protein Intake” Today’s Dietitian. Retrieved from: http://www.todaysdietitian.com/newarchives/060114p22.shtml
Elliot Reimers (2014) “Complete Guide To Protein Anabolism And Catabolism”. Muscle and Strength. Retrieved from: https://www.muscleandstrength.com/expert-guides/protein-anabolism-catabolism
