Proteins account for up to 45% of body weight. The uniqueness of amino acids lies in the fact that they can combine with other amino acids, forming complex structures. These are enzymes that catalyze reactions; hormones such as insulin and glucagon; hemoglobin and myoglobin, which are carriers of oxygen; all tissue structures, including myosin and actin, forming the muscle protein. All of them are very important for motor activity.
Studies show that proteins contribute as energy sources during fasting and intense exercise, perhaps 15% of all kilocalories during exercise.
Metabolism of proteins
Nutritional proteins combine in the intestine with endogenous proteins of the gastrointestinal tract, digested and assimilated in the form of amino acids. About 10% of proteins are excreted with feces, and the remaining 90% of amino acids form an amino acid pool, which also includes proteins formed during tissue cleavage.
If during the synthesis of proteins the body is in equilibrium, it uses amino acids from the pool to maintain the breakdown of proteins. If amino acids are not sufficient to be included in the pool (ie, insufficient intake of dietary protein), then protein synthesis can not support their breakdown and the body proteins are split to meet pool needs in amino acids.
As a result, the restoration of tissues slows down, which leads to a decrease in physical performance. Otherwise, if the consumption of food protein exceeds the need, deamination of amino acids (removal of the amino group) occurs and excess nitrogen is released mainly in the form of urea, as well as ammonia, uric acid and creatine. The structure remaining after deamination is called alpha-keto acid. It can be oxidized to produce energy or turn into fat in the form of triglycerides.
The controversial issue of the need for proteins is caused by the divergence of methods for evaluating protein biosynthesis in the body. Nitrogen balance is one of the most used criteria for evaluating protein metabolism, but not the most perfect one. Nitrogen balance measures the ratio of nitrogen removed from the body to nitrogen, which has entered the body (food block). A negative nitrogen balance is established when the excretion of nitrogen exceeds its supply. A positive nitrogen balance is noted when the intake exceeds the excretion of the protein, usually during the period of growth (adolescence, pregnancy). Under normal nitrogen balance, the intake and release of nitrogen are equal. Nitrogen balance measurements are not considered determinative, since they take into account nitrogen losses only in urine and, in part, with feces. Nitrogen losses can occur with perspiration and other body secretions, for example, skin peeling, hair loss, etc. Since protein conversions can not be accurately traced and measured after it enters the body, the nitrogen balance does not take into account all aspects of protein metabolism. Nitrogen balance suggests that what has not been isolated is used for protein synthesis.
So, if protein intake changes (increases or decreases), it is important to consider that there is an obligatory period of adaptation to the new regime, during which the daily excretion of nitrogen will be unreliable. This is an important point to be borne in mind when assessing the validity and validity of nitrogen balance studies as a measure of the state of protein metabolism. The Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) designated a minimum of 10 days of adaptation to determine the need for protein consumption when changing nitrogen consumption.