The role of proteins in physical activity

Proteins make up 45% of body weight. The uniqueness of amino acids is that they can combine with other amino acids to form complex structures. These are enzymes that catalyze reactions; hormones such as insulin and glucagon; hemoglobin and myoglobin, which are oxygen carriers; all tissue structures, including myosin and actin, which form muscle protein. All of them are very important for motor activity.

Research shows that proteins contribute as energy sources during fasting and intense exercise, perhaps accounting for 15% of total calories during exercise.

Protein metabolism

Dietary proteins combine in the intestine with endogenous proteins of the gastrointestinal tract, are digested and absorbed in the form of amino acids. About 10% of proteins are excreted in feces, and the remaining 90% of amino acids form an amino acid pool, which also includes proteins formed during tissue breakdown.

If the body is in equilibrium during protein synthesis, it uses amino acids from the pool to support protein breakdown. If there are not enough amino acids to enter the pool (i.e., insufficient dietary protein intake), then protein synthesis cannot support protein breakdown and body proteins are broken down to meet the pool's need for amino acids.

As a result, tissue repair slows down, leading to a decrease in physical performance. Otherwise, if dietary protein intake exceeds requirements, amino acids are deaminated (removal of the amino group) and the excess nitrogen is excreted mainly as urea, but also as ammonia, uric acid and creatine. The structure left after deamination is called alpha-keto acid. It can be oxidized for energy or converted to fat in the form of triglycerides.

Nitrogen balance

The controversial issue of protein requirements is caused by the discrepancy in the methods of assessing protein biosynthesis in the body. Nitrogen balance is one of the most widely used criteria for assessing protein metabolism, but it is not the most perfect. Nitrogen balance measures the ratio of nitrogen excreted from the body to nitrogen entering the body (food block). Negative nitrogen balance is established when nitrogen excretion exceeds its intake. Positive nitrogen balance is noted when intake exceeds protein excretion, usually during growth (adolescence, pregnancy). With normal nitrogen balance, nitrogen intake and excretion are equal. Nitrogen balance measurements are not considered decisive, since they take into account nitrogen losses only with urine and, partially, with feces. Nitrogen losses can occur through sweating and other body secretions, for example, skin peeling, hair loss, etc. Since protein transformations cannot be accurately tracked and measured after its intake, nitrogen balance does not take into account all aspects of protein metabolism. Nitrogen balance suggests that what is not excreted is used for protein synthesis.

Thus, if protein intake is changed (increased or decreased), it is important to consider that there is a mandatory adaptation period to the new regimen during which daily nitrogen excretion will be unreliable. This is an important point to remember when assessing the reliability and validity of nitrogen balance studies as a measure of protein status. The Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO) have specified a minimum of 10 days of adaptation to determine protein intake requirements when nitrogen intake changes.